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Your Source for HVAC&R Professional Development
FUNDAMENTALS OF
STEAM SYSTEM DESIGN
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(I-P EDITION)
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A Fundamentals of HVAC&R Series
Self-Directed Learning Course
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Your Source for HVAC&R Professional Development
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1791 Tullie Circle, NE • Atlanta, GA 30329-2305 USA • Tel 404.636.8400 • Fax 404.321.5478 • www.ashrae.org
Karen M. Murray
Dear Student,
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Email: kmurray@ashrae.org
Manager of Professional Development
Welcome to the ASHRAE Learning Institute (ALI) Fundamentals of HVAC&R Series of self-directed or group
learning courses. We look forward to working with you to help you achieve maximum results from this course.
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You may take this course on a self-testing basis (no continuing education credits awarded) or on an ALI-monitored basis with credits (PDHs, CEUs or LUs) awarded. ALI staff will provide support and you will have access
to technical experts who can answer inquiries about the course material. For questions or technical assistance,
contact us at 404-636-8400 or edu@ashrae.org.
Skill Development Exercises at the end of each chapter will test your comprehension of the course material.
These exercises allow you to apply the principles you have learned and develop a deeper mastery of the subject
matter. If you take this course for credit, please complete the exercises in the workbook and send copies from
each chapter to: ASHRAE Learning Institute, 1791 Tullie Circle, Atlanta, GA 30329-2305 or edu@ashrae.org.
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Please include your student ID number with each set of exercises submitted. Your student ID is the last five digits of your order number, which can be located in the top left corner of your packing slip. You can also email us
for your ID at edu@ashrae.org. We will return answer sheets to the Skill Development Exercises and maintain
records of your progress. Please keep copies of your completed exercises for your own records.
When you finish all exercises, please submit the course evaluation, which is located at the back of your course
book. Once we receive all chapter exercises and the evaluation, we will send you a Certificate of Completion
indicating 35 PDHs/LUs or 3.5 CEUs of continuing education credit. Please note: The ALI does not award partial credit for SDLs. All exercises must be completed to receive full continuing education credit.
We hope your educational experience is satisfying and successful.
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Sincerely,
Karen M. Murray
Manager of Professional Development
ASHRAE
AN INTERNATIONAL ORGANIZATION
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© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
ASHRAE Learning Institute
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(I-P Edition)
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Fundamentals of Steam System Design
Prepared by
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Dr. Ho Sung Lee
Dr. Peter Parker
Western Michigan University
ASHRAE
1791 Tullie Circle NE • Atlanta, GA 30329
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Fundamentals of Steam System Design I-P
A Course Book for Self-Directed or Group Learning
ISBN 978-1-933742-01-4
ASHRAE
Fundamentals
of
© 2006 ASHRAE
All rights reserved.
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Fundamentals of...
ASHRAE has compiled this publication with care, but ASHRAE has not investigated,
and ASHRAE expressly disclaims any duty to investigate, any product, service, process,
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like. ASHRAE does not warrant that the information in this publication is free of errors.
The entire risk of the use of any information in this publication is assumed by the user.
Psychrometrics
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Manager of Professional
Development
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Managing Editor
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Secretary/
Administrative Assistant
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Refrigeration
ASHRAE STAFF
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Steam System
Design
Heating and
Cooling Loads
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Print
No part of this publication may be reproduced without permission in writing from
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© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Table of Contents
1-1
1-2
1-3
Chapter 2. HVAC Steam Systems
2.1 Steam and Condensate
Sensible Heat and Latent Heat
Dry and Wet Saturated Steam
Superheated Steam
Gauge Pressure and Absolute Pressure
Specific Volume
Condensate
2.2 Advantages of Steam
2.3 Basics of Steam Systems in HVAC
Condensate Return and Steam Trap
Water Hammer
Steam Separator
Drip Leg, Dry Return and Feed water System
Air Vent
System Operation
Vacuum Breaker
Heating Load
2.4 Operating Pressures of Steam Systems
2.5 Steam Heating Systems
Steam Distribution
One-Pipe Steam Heating Systems
Two-Pipe Steam Heating Systems
2.6 Steam Condensate Systems
2-1
2-2
2-2
2-3
2-3
2-3
2-3
2-5
2-6
2-7
2-7
2-7
2-8
2-8
2-8
2-11
2-11
2-12
2-12
2-13
2-13
2-13
2-15
2-16
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Chapter 1. Introduction to Steam System Design
1.1 Introduction
1.2 Review of Thermodynamic Fundamentals
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Table of Contents
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3-1
3-2
3-2
3-5
3-7
3-8
3-9
3-9
3-12
3-12
3-13
3-13
3-13
4-1
4-2
4-3
4-4
4-5
4-9
4-10
4-12
4-14
4-15
4-15
4-16
4-17
4-18
Chapter 5. Boilers
5.1 Boilers
5-1
5-2
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Chapter 4. Terminal Units II
4.1 Unit Heaters
Type of Unit Heater
Location for Proper Heat Distribution
Rating of Unit Heaters
Piping Connections
4.2 Unit Ventilators
Heating Capacity Requirements
4.3 Fan-Coil Units
4.4 Cabinet Heaters
4.5 Induction Units
4.6 Air-Handling Units
Humidifiers
Steam Coils
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Chapter 3. Terminal Units I
3.1 Natural Convection Units
Radiators
Convectors
Baseboard Units
Finned-Tube Units
3.2 Ratings of Heat –Distributing Units
3.3 Corrections for Nonstandard Conditions
3.4 Applications
Radiators
Convectors
Baseboard Radiation
Finned-Tube Radiation
2-16
2-16
2-17
2-19
2-19
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Vented Return Systems (Dry and Wet Returns)
Nonvented Return Systems
2.7 Boiler Connections
Gravity Return
Pumped Return
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
5-3
5-3
5-4
5-6
5-8
5-8
5-8
5-9
5-13
5-14
5-15
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5.2 Boiler Classifications
Working Pressure and Temperature
Construction Materials
Type of Draft
Condensing or Noncondensing
5.3 Fuels and Combustion
Fuels
Combustion
5.4 Efficiency
5.5 Cost of Producing Steam
5.6 Boiler Sizing
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Table of Contents
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Chapter 6. Steam Valves, Steam Traps, Flash Tanks, and
Condensate Receiver Tanks
6.1 Fundamentals of Valves
6.2 Manual Valves
Globe Valves
Gate Valves
Ball Valves
Plug Valves
Check Valves
6.3 Self-Contained Valves
Pressure-Reducing Valves (PRV)
Installation
Valve Size Selection
Temperature Control Valves (Temperature Regulators)
6.4 Safety Devices
6.5 Steam Traps
Mechanical Traps
Float and Thermostatic Traps
Inverted Bucket Traps
Thermostatic Traps
Bellows Thermostatic Traps
Bimetallic Thermostatic Traps
Kinetic Traps
Thermodynamic Traps (Disc Traps)
6-1
6-2
6-3
6-3
6-4
6-4
6-5
6-5
6-6
6-6
6-7
6-9
6-10
6-11
6-13
6-14
6-14
6-15
6-15
6-16
6-16
6-17
6-17
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6-17
6-18
6-20
6-24
Chapter 7. Steam Piping Design
7.1 Pipe Sizing
Initial Pressure and Pressure Drop
Maximum Velocity
Equivalent Length of Run
Sizing Steam Pipes
Sizing Condensate Pipes
7.2 Piping
Supply Piping Design Considerations
Return Piping Design Considerations
Terminal Equipment Piping Design Considerations
7.3 Pipe Materials
7.4 Insulation
Types of Insulation
Forms of Insulation
Insulation Materials
Insulation Thickness
7.5 Pipe Expansion
Expansion Loops
Expansion Joints
7.6 Pipe Supporting Elements
Pipe Supports
Hangers
Roller Supports
Pipe Guides
Anchors
7-1
7-2
7-3
7-3
7-4
7-7
7-10
7-15
7-17
7-20
7-21
7-21
7-23
7-24
7-24
7-25
7-25
7-27
7-27
7-27
7-29
7-31
7-31
7-31
7-32
7-32
Appendix A
Table 1: Temperature Table
Table 2: Pressure Table
Table 3: Properties of Superheated Steam
A:1
A:3
A:6
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Orifice Traps
Steam Trap Selection and Sizing
6.6 Flash Steam
6.7 Condensate Receiver Tanks
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Table of Contents
Student Answer Sheets
Evaluations
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Preface
This material is designed as an introduction to the use of steam for residential and
commercial heating, humidification, and, to a much lesser extent, air conditioning. Each chapter is introduced with the learning objectives for the chapter, followed by the material for the chapter. A set of bibliographic references follows the
text material. These references are included as sources of further information on
the material in the chapter. A set of skill exercises follows the bibliographic references.
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Symbol and abbreviation usage in the inch-pound system presents some problems.
For example, there is great disparity in the use of the symbol “lb”. Some authors use
this abbreviation to mean a measure of mass, others use it as a measure of force (due
to the numeric equivalency between a pound (mass) and a pound (weight or force),
and some authors let the reader determine the appropriate interpretation from the
context. Similarly, “h” and “hr” are abbreviations used for “hour”. In this text, we
will use lbm to indicate a pound (mass), lbf to indicate a pound (force), and “hr” to
indicate “hour”. However, some of the tables and figures that we include come
from older references and will use lb for either lbm or lbf. Likewise, “h’ is occasionally used for “hour”. In these cases, we believe the context is sufficiently unambiguous
that the symbols are easily interpreted.
The properties of water and steam are extensively studied and tabulated. Periodically, the published tables are revised as measurement methods are refined. Major revisions occurred in 1967 and again in 1997. The steam tables in the appendix and the
data used in the examples use the 1997 version of the steam tables. When compared
to older tables, the properties of water and steam may differ somewhat, but the difference is normally quite small (e.g. < 0.1%) and generally insignificant for engineering purposes.
This book is the result of the efforts of many. In addition to the authors, the
manuscript was reviewed by Roy Ahlgren, Richard Hegberg, Kelly Paffel, and Doug
Reindl. They provided important insights and suggestions, although final
responsibility for the material rests with the authors. Bruce Kimball from the
ASHRAE Learning Institute was the Managing Editor.
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© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 1
Contents of Chapter 1
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Instructions
Study objectives of Chapter 1
1.1 Introduction
1.2 Thermodynamics Review
Summary
The Next Step
Bibliography
Skill Development Exercises
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•
•
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Introduction to Steam
System Design
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
1:2 · Fundamentals of Steam System Design
Instructions
Study Objectives of Chapter 1
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Read the material of Chapter 1. Re-read the material emphasized in the summary. As
necessary, review the material in Fundamentals of Thermodynamics and Psychrometrics to ensure a thorough comprehension of thermodynamics as applied to open and closed systems. At the end of the chapter, complete all of the skill development exercises.
Chapter 1 is intended to give an overview of steam systems used for HVAC and to review the thermodynamic principles necessary for the analysis of steam systems used in
HVAC. After studying Chapter 1, you should be able to define a system, determine if it
is open or closed, and write the appropriate material and energy balance for the system. You should then be able to describe the basics of HVAC steam systems and classify them ( or parts thereof) as open or closed systems.
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1.1 Introduction
Steam system heating is an important component of building design. Designing and
specifying efficient and economic space conditioning systems takes a significant effort.
Similarly, air conditioning ( the control of the temperature and humidity of the air in
the process or building space) may be critical for proper process operation or important
for employee comfort. Steam is an ideal medium for supplying energy or moisture for
many applications. The proper design of a steam HVAC system depends upon a thorough understanding of the system requirements, the properties of steam, and the ability
of a source to supply that steam.
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The design of a building’s steam heating systems first requires determining the heating
load. The heating load is a result of many factors including the building’s construction,
the local weather distribution, the building occupancy patterns, and indoor environmental requirements. The heating load determines the size of the required heating
source. In determining the heating load, reference should be made to chapters 28 (Residential Heating Loads) and 29 (Load Calculation Procedures) of ASHRAE’s 2005 Fundamentals Handbook and to ASHRAE’s Cooling and Heating Load Principles. Typical examples of heating loads are:
Solar Radiation
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Infiltration
Ventilation Air
People
Appliances
Lighting
Water, in both its liquid and vapor form, is a rather unique substance that makes it particularly suitable for HVAC uses. It is abundant and thus relatively cheap. The conden-
1.1 Introduction
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 1: Introduction to Steam System Design · 1:3
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sation of the vapor liberates a rather large amount of energy on a per unit mass basis,
which makes it ideal for transporting heat energy from a source to a destination. It
changes from the liquid to the vapor state at relatively low temperatures (e.g. 212°F at 1
atmosphere). The liquid form is relatively easy to pump. Both the vapor and liquid
forms are non-toxic, non-corrosive, and non-flammable. Thus, systems that utilize
steam have been developed to heat work spaces, humidify air, and provide an energy
source for heat-driven refrigeration.
A simple schematic of a steam heating system is shown in Figure 1.1:
1002 BTU Output
Steam condensing
in radiator
Volume of 1 pound
of steam: 27 cubic feet
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180° F
Condensate Return
Volume of 1 pound of liquid
water: approximately 1 pint
32 BTU to heat water from 180° to 212° F
970 BTU to vaporize one pound of water
Electrical input to boiler: 293.7 watt-hours
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Heater
Figure 1.1 Steam Heating System Schematic
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1.2 Thermodynamics Review
In the previous paragraphs, we have used the "system" rather loosely. To ensure proper
analysis, it is important to properly define what is in a "system" and what is not in a system. A "system" is that collection of components and processes that is identified for
study. Everything else is the "surroundings" or the "environment". Consider the oldfashion radiator used for space heating. As shown in Figure 1.2, a relatively complete
collection of components consists of the radiator, the inlet and return piping, the boiler,
and a steam storage drum. If we were interested in studying the performance of this
group of components for space heating, we might consider the entire collection as the
system. However, if we were more interested in the heat transfer processes in the radiator, we might single out the radiator for study. Identifying single or multiple components for inclusion in the system is normally done by drawing a circle around the components to be included in the scope of the system.
1.2 Thermodynamics Review
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
1:4 · Fundamentals of Steam System Design
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Radiator
Boiler
Figure 1.2 Radiator System
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Figure 1.3a includes all the components as the system, while Figure 1.3b isolates the
radiator as the system. In the case of Figure 1.3b, everything except the radiator is
included in the surroundings; only the radiator is in the system.
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Radiator
Boiler
Radiator
Radiator
Boiler
Boiler
a
a
Figure a
Figure b
Figure 1.3 Example Systems
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Figure 1.3 also serves to illustrate the difference between "closed" and "open" systems. A
system is closed when there is no exchange of mass with the surroundings. As is shown
in Figure 1.3a, there is no flow of steam across the system boundary, so the system is
closed. An open system, on the other hand, can exchange mass with the surrounding.
In Figure 1.3b, the heating fluid enters and leaves the system (the radiator), so the system is open. Energy flows, such as heat, can cross the system boundaries for both open
and closed systems. Steam HVAC systems are normally “closed” since supply steam
should not be allowed to “leak” into the building structure and condensate must be collected and returned to the boiler to conserve energy and minimize water treatment
costs.
1.2 Thermodynamics Review
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 1: Introduction to Steam System Design · 1:5
It is important to properly define a system for study. Too narrow a definition leaves out
critical parts for the analysis; too inclusive a definition may add unnecessary complexity
and hide the important components or processes.
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While the transient (time-varying) behavior of systems is often of interest, the steady state behavior of the system is usually used as the design basis. Thus, the mass and energy balances for
steady flow are critical steps in the analysis. The mass balance for a closed or open system is:
Mass:
0 =
∑ m· – ∑ m·
in
(1-1)
out
Where m· represents the flow rate.
Note that flows into the system are considered positive and flows leaving the system are
considered negative.
Explicitly showing the heat and work energy flows in Figure 1.2, we have Figure 1.4.
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The closed system energy balance1 for the system of Figure 1.4 is:
⎛ dE
-------⎞
= Q· – W·
⎝ dt ⎠ sys
where
energy of the system
flows of heat energy into (or out of) the system
rate of work applied to the system
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E =
Q· =
W· =
(1-2)
The open system, steady-state energy balance is similar to the closed system energy
balance, but has additional terms related to the flow of energy through the system:
2
2
υ
υ
·
·
∑ m· ⎛⎝ h + -----2 + gz⎞⎠ – ∑ m· ⎛⎝ h + -----2 + gz⎞⎠ = Q – W
out
(1-3)
in
where
h
υ
z
1.
=
=
=
specific enthalpy of the material in stream
velocity of material in stream
elevation above a common reference plane
The sign of the work term in the energy balance must be considered carefully. ASHRAE and other
mechanical engineering societies consider work done by the system as positive, while chemical
engineers and the International Union of Pure and Applied Chemistry consider work done by the
system as being negative. When consulting multiple references, always check for the sign convention
for the work term.
1.2 Thermodynamics Review
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
1:6 · Fundamentals of Steam System Design
Radiator
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Heat
Boiler
Work
Figure 1.4 Energy Flows
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g = acceleration due to gravity
The energy term E on the left hand side of Equation (1-2) includes the internal energy as
well as kinetic and potential energy terms:
2
υ
E = m ⎛ u + ----- + gz⎞
⎝
⎠
2
where:
mass of the system
specific internal energy
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m =
u =
(1-4)
A variety of other energy terms, such as chemical and nuclear energies have been
neglected.
For most HVAC systems, changes in kinetic and potential energy are negligible compared to the changes in the other energy forms and can be neglected. The closed and
open system energy balance generally reduce to equations (1-5) (closed system) and (1-6)
(open system)
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m ( u2 – u1 ) = Q – W
∑ m· h – ∑ m· h
out
= Q· – W·
(1-5)
(1-6)
in
where the subscripts on the internal energy terms in (1-5) represent two points in time
and Q and W are the total amounts of energy and work supplied in the time interval
under study.
The signs on both work and heat flow are critical, as they show the direction of flow.
Universally, the flow of heat, Q· is considered positive when flowing from the surround-
1.2 Thermodynamics Review
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 1: Introduction to Steam System Design · 1:7
ing into the system. Thus, in Figure 1.4, the flow of heat to the boiler is positive and the
flow of heat from the radiator is negative.
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As noted in the footnote above, there are two conventions for the sign of the work term.
This text will use the ASHRAE convention that work done by a system is positive, while
work done on the system is negative. Thus, in Figure 1.4, the work supplied to the
pump (in the form of electrical energy) is considered negative.
The specific internal energy, u, and the specific enthalpy, h, are properties of the material. These two energy terms are related to each other by (1-7)
h ≡ u + ( P ⋅ v )v
where
P
u
v
=
=
=
(1-7)
pressure of the system
internal energy per unit mass
specific volume of the material.
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For most fluids of interest, these properties are tabulated as functions of temperature
and pressure. The values for h and u do not have a true zero; rather a zero value is assigned at some arbitrary temperature and pressure. For example, the steam tables2 assign a value of 0 Btu/lbm to the internal energy of liquid water at the triple point (the
point where ice, liquid, and water vapor coexist: 32.02°F, 0.089 psia) For most applications, we will be concerned about the difference between two states, so the arbitrary zero
value state is of no concern.
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To illustrate this relation between h, u, P, and υ , consider saturated steam at 212°F and
1 atm. At these conditions, the steam tables list the properties as:
u = 1077.4 Btu/lb m
P = 1 atm
3
v = 26.78 ft /lb m
Applying equation (1-7), we would compute
3
h = 1077.4 Btu/lb m + 1 atm ⋅ 26.78 ft ⁄ lb m = 1150.3 Btu/lb m
and the steam table gives an identical value. (The conversion factor for the P·v term is:
2.7195 Btu/atm·ft3.)
Changes between two states are simply the difference between the properties at the two
states:
2.
Steam tables are tabulated properties of water and water vapor. Properties normally include the
specific volume, enthalpy, and entropy of both the vapor and the liquid states as functions of
temperature and pressure. Appendix A is an abbreviated copy of the steam tables.
1.2 Thermodynamics Review
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
1:8 · Fundamentals of Steam System Design
∆u = u 2 – u 1
∆h = h 2 – h 1
Using the steam tables, the enthalpy difference between steam at 212°F and 1 atm and
water at the same conditions is:
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h fg = ∆h = 1150.3 Btu ⁄ lb m – 180.2 Btu ⁄ lb m = 970.1 Btu ⁄ lb m
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This enthalpy change from the liquid state to the vapor state is also called the heat of vaporization3and is usually given the symbol h fg .
The heat of vaporization for water is large compared to other materials. Water is also
abundant, non-toxic, and relatively non-corrosive. This makes it ideal for transporting
energy from high temperature sources (e.g. a boiler) to spaces that need heat, such as
rooms, buildings, or processes.
Example 1-1:
Solution:
pE
Consider the radiator in Figure 1.4. Let’s assume that this radiator must heat a room
and calculations show that the heating demand is 10,000 Btu/hr. How much steam
(available at 20 psi, saturation) must be condensed (and returned to the boiler) in the radiator to supply this energy flow?
A picture of our system is given in Figure 1.5. We assume that there is a steady flow of
steam to the radiator and that changes in kinetic energy and potential energy are negligible, especially compared to the energy given up by the condensing steam. Thus the
open system energy balance (Equation 3) becomes:
ro
u
∑ m· h – ∑ m· h
out
= Q· – W·
in
There is no work done by or to the system, so
W· = 0
G
As is evident from the picture of the system there is only a single input and a single output, reducing Equation 6 to:
3.
m· ( h water – h steam ) = Q·
The energy required to vaporize liquid water at constant temperature and pressure is variously called
the latent heat of vaporization, enthalpy of vaporization, latent heat of evaporation, and enthalpy of
evaporation. These terms all refer to the same thing – the energy required to vaporize a unit mass of
water at constant temperature and pressure.
1.2 Thermodynamics Review
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 1: Introduction to Steam System Design · 1:9
Radiator
·
Q
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Steam
t
Q
Water
Figure 1.5 Example Problem
where the outlet flow is identified as “water” and the inlet flow identified as “steam”.
From the problem statement, Q· = – 10, 000 Btu ⁄ h , where the negative sign indicates
that the energy is leaving the system (the radiator).
Using the steam tables, we find the enthalpy of steam and water at 20 psia as:
pE
h steam = 1156.2 Btu ⁄ lb m
h water = 196.3 Btu ⁄ lb m
Substituting these values into the energy balance and solving for the flow rate of steam
gives:
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u
10, 000 Btu ⁄ h
m· = ----------------------------------------------------------------------------------- = 10.4 lb m ⁄ h
1156.2 Btu ⁄ lb m – 196.3 Btu ⁄ lb m
Thus, the radiator must condense 10.4 pounds of steam per hour to supply the necessary heat to the room.
G
Summary
This introductory chapter reviewed material in previous courses. Open systems have a
material transport across the system boundary, while closed systems do not. Open systems and closed systems are defined with respect to the material flows. Energy flows
across the system boundary can occur in either open or closed systems. Any analysis
should begin by drawing a picture of the system under study and clearly identifying what
is (and what is not) in the system. Once the system is defined, the appropriate material
and/or energy balances are written. Closed systems will typically be non-steady state,
while open systems are typically analyzed at steady state.
For steam HVAC systems, material flows are normally steam or hot water. The thermodynamic properties of water and steam are given in the steam tables, which are tabular
values of enthalpy, entropy and volume as a function of temperature and pressure. The
1.2 Thermodynamics Review
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
1:10 · Fundamentals of Steam System Design
large heat of vaporization of water (and its large heat capacity) makes steam an ideal medium for transferring thermal energy from point to point.
G
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•
•
From a written problem, draw a picture of a system and show the system
boundaries.
Write the appropriate material and energy balances for a given system.
Extract the necessary properties of steam from a steam table, given the
system temperature and pressure.
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•
t
You should now be able to:
1.2 Thermodynamics Review
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 1: Introduction to Steam System Design · 1:11
Bibliography
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t
ASHRAE Handbook. 2004. HVAC Systems and Equipment. ASHRAE, Atlanta, GA.
Clifford, G. F. 1997. Modern Heating and Ventilating Systems Design. Prentice Hall,
Upper Saddle River, NJ.
Himmelblau, D., J. B. Riggs. 2003. Basic Principles and Calculations in Chemical
Engineering. Prentice Hall, Upper Saddle River, NJ.
Howell, R., H. Sauer, and W. Coad. 2005. Principles of Heating, Ventilating and Air
Conditioning. ASHRAE, Atlanta, GA.
Johnson, R. 2004. Fundamentals of Thermodynamics. ASHRAE, Atlanta, GA.
McQuiston, F. C., J. D. Parker and J. D. Spitler. 2005. Heating, Ventilating, and Air
Conditioning: Analysis and Design. John Wiley & Sons, New York, NY.
Pedersen, et. al. 1998. Cooling and Heating Load Calculation Principles. ASHRAE,
Atlanta, GA.
Sonntag, R., C. Borgnakke, and G. Van Wylen. 2002. Fundamentals of Thermodynamics.
John Wiley & Sons, New York, NY.
Bibliography
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
1:12 · Fundamentals of Steam System Design
Skill Development Exercises For Chapter 1
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Use the following scenario to answer all of the questions:
t
Please write you answers on the sheets provided in the back section of this book, and submit the
answers to the ASHRAE Learning Institute.
A teakettle is sitting on a stove burner. The stove burner is currently turned off and
the teakettle is approximately ¾ full of water.
1-1. Neglecting movement of water by evaporation, is the system open or closed?
1-2. Is the system at steady state?
1-3. The burner is turned on, but the water has not yet begun to boil. Is the
system at steady state with respect to material flow?
pE
1-4. The burner is turned on, but the water has not yet begun to boil. Is the
system at steady state with respect to energy flow?
1-5. The teakettle is boiling vigorously. Is the system open or closed?
1-6. The teakettle is boiling vigorously. Is the system at steady state with respect
to mass flow?
1-7. The teakettle is boiling vigorously. Is the system at steady state with respect
to energy flow?
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1-8. The teakettle, boiling vigorously, boils off 1 cup (approximately ½ pound)
of water in 20 minutes. What is the rate of heating energy applied to the
teakettle in BTU/h?
Skill Development Exercises For Chapter 1
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 2
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•
Introduction
Study Objectives of Chapter 2
2.1 Steam and Condensate
2.2 Advantages of Steam
2.3 Basics of Steam Systems in HVAC
2.4 Operating Pressures of Steam Systems
2.5 Steam Heating Systems
2.6 Steam Condensate Systems
2.7 Boiler Connections
Summary
Bibliography
Skill Development Exercises for Chapter 2
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•
•
•
•
•
•
•
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Contents of Chapter 2
t
HVAC Steam Systems
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
2:2 · Fundamentals of Steam System Design
Introduction
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Study Objectives of Chapter 2
t
Read the material in Chapter 2. Re-read the parts of the chapter emphasized in the summary. At the end of chapter, complete the skill development exercises without consulting the text. Re-read parts of the text as needed to complete the exercises.
Chapter 2 is an overview of steam system design. The material provides design concepts of
the steam system gradually from the fundamentals to some design aspects including
some layouts and pictures. However, the detailed layouts of any particular system will be
discussed in later chapters.
After studying Chapter 2 and working the study problems, you should be able to:
•
•
Use the steam tables.
Describe the functions of steam system components.
Calculate the heating load in the steam systems.
Understand the various phenomena such as water hammer, vacuum breaker
and flash steam taking place in the steam condensate systems.
Describe the one-pipe and two-pipe steam heating systems.
Describe the dry and wet returns in steam condensate systems.
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•
•
•
•
2.1 Steam and Condensate
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Steam is colorless and invisible vapor and at one atmosphere has a volume approximately 1600 times higher than that of liquid water . Steam is the vapor phase of water and is
generated by adding more heat than required to maintain its liquid phase at a given
pressure, causing the liquid to change to vapor without any further increase in temperature.
Sensible Heat and Latent Heat
G
If liquid water is heated in a vessel open to the atmosphere, its temperature will rise until it reaches a temperature of about 212°F. This is the boiling point of water at atmospheric pressure. Adding more heat will not produce temperature changes, but steam
will exit the vessel, producing a white cloud, which is not steam, but small droplets of
water.
To raise the temperature of 1 lbm (pound-mass) of water from 32°F (freezing point) to
212°F (boiling point) requires 180 Btu (British Thermal Unit) of heat, which is known as
sensible heat (enthalpy of liquid), which is seen in Column 6 of Table 2-1. To change 1 lbm
of water at atmospheric pressure into steam at 212°F from water at the same temperature
requires 970 Btu of heat, which is known as latent heat (or enthalpy of evaporation) (Column 7). It is latent heat that permits large quantities of heat to be transmitted efficiently
with little change in temperature. The total heat (enthalpy of steam - column 8) of 1 lbm
2.1 Steam and Condensate
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 2: HVAC Steam Systems · 2:3
of saturated steam at atmospheric pressure is 1150 Btu that is the combined heat of the
sensible heat and the latent heat.
Dry and Wet Saturated Steam
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Properties of water in the liquid and vapor forms are tabulated in “steam tables”, such as
the one in Appendix A, a portion of which is shown in Table 2-1. The properties of
steam are often presented in graphical form as well as tabular form. The Mollier Diagram
shows the properties of steam as a function of enthalpy and entropy. The Mollier diagram is commonly used to analyze steam heating cycles and can be found in most texts
on thermodynamics.
Saturated steam is a unique state of water vapor such that, at constant pressure, a small
loss of heat energy will cause some of the vapor to return to the liquid state, or condense. Saturated steam can be complete free of liquid droplets or can contain droplets
of liquid water. Steam that carries no moisture (liquid water) is called dry saturated
steam, while steam that contains moisture (water) is called wet saturated steam. The percentage of water in steam determines the steam quality or the dryness of the steam. If the
steam contains 5% of water by weight, the steam is 95% dry, or a quality of 0.95.
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Superheated Steam
When there is no longer any water present, the steam itself absorbs any additional heat
and becomes superheated steam. Superheated steam at a given pressure can be at any temperature above that of saturated steam. From this point on, the steam no longer follows
the relationship shown in the saturated steam tables, but a separate table is required,
known as superheated steam tables. Superheat is the number of degrees above the saturated steam temperature at the corresponding pressure.
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Gauge Pressure and Absolute Pressure
Steam pressure is described as the force it exerts per unit area and measured as pounds
force per square inch (psi or lbf /in2) and may be given as gauge pressure or absolute
pressure. Gauge pressure is that shown on an ordinary pressure gauge and is the pressure above the atmospheric pressure. Absolute pressure is the pressure measured from
the datum of a perfect vacuum (zero absolute pressure). Zero psig on the gauge pressure
scale is therefore 14.7 psia on the absolute pressure scale. Vacuum pressure is the pressure
below the atmospheric pressure.
Specific Volume
Specific volume, the reciprocal of density, is the volume of a unit mass and indicates the
volumetric space that 1 lbm of steam or water occupies. Dry saturated steam at atmospheric pressure takes up a lot of space. The volume of 1 lbm of steam at atmospheric
pressure is 26.8 ft3. As steam pressure rises, so steam becomes more compact. At 15
psig, 1 lbm occupies only 13.9 ft3. At 50 psig, the volume has halved again to 6.9 ft3. By
the time the pressure has risen to 200 psig, the volume of 1 lbm of saturated steam has
shrunk to 2.14 ft3. This is a valuable point. From the steam table we see that steam at
this pressure has only 4.5% more heat per pound than atmospheric steam, but we can
get more than nine times the amount of steam at the higher pressure into the same
2.1 Steam and Condensate
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
2:4 · Fundamentals of Steam System Design
Absolute
Pressure
(psia)
Steam
Temp.
(°F)
0
5.3
10.3
15.3
20.3
25.3
30.3
35.3
40.3
45.3
50.3
55.3
60.3
65.3
70.3
75.3
80.3
85.3
90.3
95.3
100.3
0.2
0.5
1
2
3
4
5
6
7
8
9
10
14.71
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
53.1
79.6
101.7
126.0
141.4
152.9
162.2
170.0
176.8
182.8
188.2
193.2
212.0
227.9
240.0
250.3
259.3
267.2
274.4
281.0
287.1
292.7
298.0
302.9
307.6
312.0
316.3
320.3
324.1
327.8
331.4
334.8
338.1
Specific
Specific
Heat of Sat.
Volume of Volume of Liquid Enthalpy
Sat. Liquid Sat. Steam of Sat. Liquid (hf)
(Btu/lbm)
(ft3/lbm)
(ft3/lbm)
0.0160
0.0161
0.0161
0.0162
0.0163
0.0164
0.0164
0.0165
0.0165
0.0165
0.0166
0.0166
0.0167
0.0168
0.0169
0.0170
0.0171
0.0172
0.0172
0.0173
0.0173
0.0174
0.0174
0.0175
0.0175
0.0176
0.0176
0.0177
0.0177
0.0177
0.0178
0.0178
0.0179
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2.1 Steam and Condensate
1525.932
641.324
333.508
173.717
118.704
90.628
73.523
61.979
53.649
47.345
42.404
38.423
26.781
20.092
16.306
13.748
11.900
10.500
9.402
8.517
7.788
7.176
6.656
6.207
5.816
5.473
5.169
4.897
4.653
4.432
4.232
4.050
3.882
21.20
47.62
69.73
94.02
109.39
120.89
130.16
137.99
144.79
150.83
156.27
161.22
180.18
196.25
208.51
218.93
228.03
236.15
243.50
250.23
256.45
262.24
267.66
272.76
277.59
282.18
286.55
290.73
294.73
298.57
302.27
305.84
309.29
pE
Gauge
Pressure
(psig)
Latent Heat Total Heat
of Steam
Enthalpy of
Enthalpy of
Evaporation
Sat. Vapor
(hfg)
(hg)
(Btu/lbm)
(Btu/lbm)
1063.24
1084.44
1048.30
1095.92
1035.71
1105.44
1021.74
1115.76
1012.80
1122.19
1006.05
1126.94
1000.56
1130.72
995.90
1133.89
991.82
1136.61
988.17
1139.00
984.86
1141.13
981.84
1143.06
970.11
1150.29
959.94
1156.19
952.04
1160.55
945.21
1164.14
939.16
1167.19
933.68
1169.83
928.66
1172.16
924.01
1174.24
919.66
1176.11
915.57
1177.81
911.70
1179.36
908.03
1180.79
904.53
1182.12
901.16
1183.34
897.94
1184.49
894.82
1185.55
891.82
1186.55
888.92
1187.49
886.11
1188.38
883.37
1189.21
880.71
1190.00
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Table 2-1 Properties of Saturated Steam
t
space. At higher pressure, smaller piping can be used to transport the same amount of
energy. But the latent heat of the saturated steam also drops as the pressure increases.
Thus, at 200 psig, the latent heat of 1 lbm of saturated steam is only 86% of that at atmospheric pressure.
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 2: HVAC Steam Systems · 2:5
Table 2-1 Properties of Saturated Steam (Continued)
105.3
110.3
115.3
120.3
125.3
130.3
135.3
140.3
145.3
150.3
155.3
160.3
165.3
170.3
175.3
180.3
185.3
190.3
195.3
200.3
120
125
130
135
140
145
150
155
160
165
170
175
180
185
190
195
200
205
210
215
341.3
344.4
347.3
350.2
353.0
355.8
358.4
361.0
363.6
366.0
368.4
370.8
373.1
375.3
377.5
379.7
381.8
383.9
385.9
387.9
0.0179
0.0179
0.0180
0.0180
0.0180
0.0181
0.0181
0.0181
0.0182
0.0182
0.0182
0.0182
0.0183
0.0183
0.0183
0.0184
0.0184
0.0184
0.0184
0.0185
3.729
3.587
3.455
3.333
3.220
3.114
3.015
2.922
2.834
2.752
2.675
2.601
2.532
2.466
2.404
2.344
2.288
2.234
2.183
2.134
312.62
315.85
318.98
322.03
324.99
327.87
330.68
333.42
336.10
338.71
341.27
343.77
346.21
348.61
350.96
353.27
355.53
357.75
359.94
362.08
t
Steam
Temp.
(°F)
Total Heat
Latent Heat
of Steam
Enthalpy of
Enthalpy of
Evaporation
Sat. Vapor
(hfg)
(hg)
(Btu/lbm)
(Btu/lbm)
878.12
1190.74
875.60
1191.45
873.14
1192.12
870.73
1192.76
868.38
1193.37
866.07
1193.94
863.81
1194.49
861.60
1195.02
859.42
1195.52
857.28
1195.99
855.18
1196.45
853.12
1196.89
851.09
1197.30
849.09
1197.70
847.13
1198.09
845.18
1198.45
843.27
1198.80
841.39
1199.14
839.52
1199.46
837.69
1199.77
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Absolute
Pressure
(psia)
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Gauge
Pressure
(psig)
Specific
Specific
Heat of Sat.
Volume of Volume of Liquid Enthalpy
Sat. Liquid Sat. Steam of Sat. Liquid (hf)
(Btu/lbm)
(ft3/lbm)
(ft3/lbm)
Condensate
Steam gives up the latent heat due to heat transfer, becoming condensate (liquid). The latent heat that is given up by 1 lbm of steam in condensing is the same as the amount that
was put into it in during vaporization. The condensate that is formed is at the temperature of the saturated steam and contains all its sensible heat. When condensing at atmospheric pressure, the volume is reduced by a factor of 1600 . This volume reduction can
create a vacuum. Therefore, it is common practice that a vacuum breaker is installed to
relieve the vacuum to prevent the back flow or damage to the equipment.
Example 2-1:
Water at 202°F is pumped into a boiler in which the pressure is 150.3 psig. How much heat
must be supplied by the fuel to evaporate each pound of water into dry saturated steam?
Solution:
The total heat of steam required to evaporate 1 lbm of water from 32°F to 366°F at a
pressure of 150.3 psig is found to be 1196.0 Btu/lb m (Table 2.1). Since the water
2.1 Steam and Condensate
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
2:6 · Fundamentals of Steam System Design
pumped to the boiler at a temperature of 202°F has 170.1 Btu/lbm of the heat of saturated liquid, the total amount of heat to be supplied by the fuel to evaporate 1 lbm of water into dry saturated steam is:
t
1196.0 – 170.1 = 1025.9 Btu/lbm
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Example 2-2:
If the steam in Example 2-1 contained 3% moisture by weight, calculate the heat which must
be supplied by the fuel to evaporate each pound of water into the wet saturated steam.
Solution:
The total heat of steam of 1195.6 Btu/lbm at 150.3 psig consists of the sensible heat of
338.7 Btu/lbm and the latent heat of 857.3 Btu/lbm. The 3% of the water carries over
without converting to steam. So, the total heat to evaporate 1 lbm of water from the liquid at 202°F to steam that is 3% wet at 150.3 psig (366°F) is:
338.7 + 0.97 (857.3) – 170.1 = 1000.2 Btu/lbm
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Example 2-3:
If a pressure gauge in a steam system reads 4 psi, in a location where the atmospheric
pressure is 14.7 psi, determine the absolute pressure of the steam system.
Solution:
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Absolute pressure = Gauge pressure + Atmospheric pressure
Absolute pressure = 4 + 14.7 = 18.7 psia
Example 2-4:
What is the volume of 1 lbm of steam at 155 psia, if it is 20% wet?
Solution:
Dry steam at a pressure of 155 psia has a specific volume of 2.92 ft3/lbm, while the specific volume of the water is 0.0181 ft3/lbm. The volume of 1 lbm of steam that is 20%
wet, or 80% dry, at a pressure of 155 psia is:
2.92 (0.8) + 0.0181 (0.2) = 2.34 ft3
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Note that the volume of the liquid water in the wet steam is insignificant.
2.2 Advantages of Steam
Steam has many advantages over its alternatives such as electricity, direct-fired heat and
hot water. Steam offers the following advantages:
2.2 Advantages of Steam
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 2: HVAC Steam Systems · 2:7
•
•
•
•
t
•
Because of the low density of steam, steam can be used in tall buildings
where water systems create excessive static pressure.
Steam can travel great distances, such as in facilities with scattered building
locations.
Steam flows through the system unaided by external energy sources such as
pumps.
Steam is pressure-temperature dependent. Therefore, the system
temperature can be controlled by varying either steam pressure or
temperature.
Steam can be distributed throughout a heating system with little change in
temperature.
Steam is non-toxic and non-flammable.
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2.3 Basics of Steam Systems in HVAC
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Steam generated in a boiler is distributed to the various terminal units, where it is condensed, giving up latent heat, and the condensate is returned to the boiler. A typical
low-pressure steam system is schematically shown in Figure 2.1. The motive force for the
steam is the pressure generated in the boiler. The condensate flows to the vicinity of the
boiler, where a condensate pump raises the pressure to return the liquid to the boiler.
Steam systems are very efficient in transporting energy, especially when the distance is
large.
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In heating systems today, steam is infrequently used for directly for space heating with
radiators or steam-to-air coils. Instead, steam is often used to heat water, which is then distributed and used in terminal units such as water-to-air coils. However, there are other applications in hospitals, various industrial plants, and the process industry that still utilize
direct steam. The emphasis here is on HVAC applications, where steam is available at
low pressure (less than 15 psig).
Condensate Return and Steam Trap
Figure 2.1 is a schematic of a low-pressure steam system. A properly operating condensate
return circuit is critical to the efficient operation of the steam system. Saturated steam will
condense whenever it comes in contact with a surface at a temperature less than the
steam’s saturation temperature. Therefore, even before the steam reaches the terminal
devices, small amounts of condensate will form in the piping. Devices known as steam
traps are installed to remove this condensate. A steam trap will allow liquid to pass
through to the condensate return but will prevent the steam from passing through the
trap. Every terminal device requires a proper steam trap.
Water Hammer
It is very important that condensate not be allowed to collect in the steam piping, because of the possibility of water hammer. A slug of condensate may form, completely filling the pipe, and moving at the same high velocity as the steam. When the slug reaches
an obstruction or change in direction, high impact forces are exerted on the piping, pro-
2.3 Basics of Steam Systems in HVAC
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
2:8 · Fundamentals of Steam System Design
t
ducing the hammer effect (noise) and, possibly, damage. Another type of water hammer
is caused by a bubble of flash (or live) steam in the condensate line. Rapid condensation
of the steam bubble (or imploding) may cause water hammer with as much or greater intensity than the type described above. Therefore, it is very important to remove condensate from the system as quickly as possible.
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Steam Separator
The steam leaving the boiler may have some condensate suspended in it. A steam separator is used to separate the two phases, the condensate being removed through a trap.
Two typical steam separators are shown in Figure 2.2. They are also used for process requiring very dry steam and on secondary steam lines where large amounts of condensate
can form. Steam separators are never used in systems where superheated steam is generated in the boiler.
Drip Leg, Dry Return and Feed water System
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Steam piping is usually inclined downward in the direction of flow to ensure removal of
condensate. A small pocket or drip leg should be provided to collect the condensate
above the drip trap (Figure 2.3). A strainer is usually installed upstream of the trap to
collect dirt, scale, and other solid contamination (Figure 2.4). The condensate usually returns to some central point (condensate receiver tank) by gravity and is then pumped
into the boiler or feed water system with a centrifugal pump, specially designed for this
purpose. The gravity part of the return may not be completely filled with condensate
and in that case behaves like open channel flow at atmospheric pressure; it is then referred to as a dry return (discussed later in the section of Steam Condensate Systems).
The remainder of the space is filled with flash steam and possibly some air. If the boiler
is located at a higher elevation than the terminal devices, the condensate is collected at a
lower level and pumped up to the boiler feedwater system (Figure 2.1). When the boiler is
lower than the terminal devices, the condensate may flow by gravity directly into the
boiler feedwater system. It is common practice to slope the condensate return in the direction of flow to a collection point not only to enhance removal of the condensate but
also to clear the line of contamination. (Figure 2.1).
Air Vent
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Air, in the presence of steam, is detrimental to heat transfer. The air displaces steam and
does not have any significant energy to give up. Further, air may collect in the heating
device and drastically reduce the heat transfer surface. Some air may exist in a gravity return system; it is vented to atmosphere and release from the condensate-collecting reservoir before the condensate is pumped into the boiler as feedwater. In general, automatic
air vents (Figure 2.5) should be placed at any point in the steam supply piping where air
may collect. The ends of main lines are usually fitted with an air vent as shown in Figure
2.1. For all heat transfer devices, a proper air vent should be installed on the steam inlet
or on the provide port on the heat transfer device. Never assume that any steam trap
will be able to vent the heat transfer device properly of air and non-condensables. The
air vent devices should work as an air vent and vacuum breaker. DO NOT USE a swing
check valve installed backwards as a vacuum breaker on heat transfer.
2.3 Basics of Steam Systems in HVAC
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Chapter 2: HVAC Steam Systems · 2:9
Steam Main
Steam
Separator
Air Vent
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Air Vent
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Steam trap
Steam Trap
Boiler
Make up
Shell-and-Tube
Heat Exchanger
Air Vent
Air-Handling Unit
Feedwater System
Baseboard Unit
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Steam Trap
Condensate Return
Condensate Receiver Tank
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Figure 2.1 Schematic of a Low-Pressure Steam System.
Figure 2.2 Steam Separators
2.3 Basics of Steam Systems in HVAC
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2:10 · Fundamentals of Steam System Design
Drip Leg
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28" minimum
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Steam Main
Strainer
Condensate
Return
Steam Trap
Drain
Drain
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Figure 2.3 Trap Draining Drip Leg on Steam Main
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Figure 2.4 (a) Float and Thermostatic Trap (F&T steam trap) and (b) Strainer
Figure 2.5 Thermostatic Air Vent for Steam Radiators and Convectors
2.3 Basics of Steam Systems in HVAC
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Chapter 2: HVAC Steam Systems · 2:11
Strainer
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Room Thermostat
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Steam Supply
Unit Heater
On-Off
Control Valve
Strainer
Steam Trap
Check
Valve
Steam Trap
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Condensate Return Line
Figure 2.6 Steam Heating with Unit Heater
System Operation
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When a steam heating system is started up after being idle for some time, it is filled with
air. As the boiler begins to produce steam, the air is gradually forced out through the
piping as the system comes into equilibrium with the steam. Therefore, during startup
the capacity of the air vents and traps has to be greater than at the full load design condition. This should be taken into accounting during the design and sizing phase. At the
full design load, steam containing little air is supplied to the heating device through a
two-way control valve, where it is condensed; the condensate leaves through a steam trap
in the bottom of the device and flows by gravity in a dry return to a condensate-collecting reservoir.
Vacuum Breaker
Figure 2.6 shows typical piping and fittings for a heating coil. If there is a higher pressure
in the heating device than in the return, condensate flows freely. At some point, when
the steam is throttled as the control valve responds to reduce load, the pressure in the
heating device may fall below the atmospheric pressure in the condensate return. Then,
there is little or no potential for condensate to flow through the trap. This situation has
unpredictable results. To remedy it, the device may be vented to the atmosphere, allowing air to enter and mix with the steam and later be removed by the air vent or steam
trap air venting capabilities.. Also, a connection can be made between the gravity return
and the device just above the trap (check valve). A vacuum breaker is installed in the line
to prevent bypass of steam into the return when the pressure in the device is greater
than atmospheric. Figure 2.6 shows this piping arrangement as a dashed line.
2.3 Basics of Steam Systems in HVAC
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2:12 · Fundamentals of Steam System Design
Heating Load
(2-1)
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q· = m· ⋅ h fg
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After the steam distribution system is laid out and the heating load for each heating device is known, the various elements of the system can be sized, including the boiler. The
pressure level will be less than or equal to 15 psig in a low-pressure system. The boiler capacity is given by
where
q· = heating load, Btu/hr
m· = mass flow rate, lbm/hr
hfg = latent heat (enthalpy of vaporization), Btu/lbm
Example 2-5:
Solution:
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How much steam flow would be required at a steam heating coil if you had a heat load
of 100,000 Btu/hr, and steam is available at 5.3 psig.
Enter the steam table with a pressure of 20 psia (5.3 + 14.7). The latent heat at 20 psia is
found to be 959.9 Btu/lbm, which is the energy required to change saturated vapor
(steam) to saturated liquid (condensate). Now simply divide the heat load of 100,000
Btu/hr by the latent heat of 959.9 Btu/lbm. The result is 104.2 lbm/hr, which is the required flow rate of the steam.
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2.4 Operating Pressures of Steam Systems
Because of the various codes and regulations governing the design and operation of boilers, pressure vessels, and systems, steam systems are classified according to operating
pressure. Low-pressure systems operate up to 15 psig, and high-pressure systems operate over
15 psig. There are many sub classifications within these broad classifications, especially
for heating systems such as one- and two-pipe, gravity, vacuum, or variable vacuum return systems. However, these classifications relate to the distribution system or temperature-control method. Regardless of classification, all steam systems include a source of
steam, a distribution system, and terminal equipment, where steam is used as the source
of power or heat.
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One of the most important decisions in the design of a steam system is the selection of
the generating, distribution, and utilization pressure. Considering investment cost, energy efficiency, and control stability, the pressure should be held to the minimum values
above atmospheric pressure that accomplish the required heating task, unless detailed
economic analysis indicates advantages in higher pressure generation and distribution.
The first step in selecting pressure is to analyze the load requirements. Space heating
and domestic water heating can best be served, directly or indirectly, with low-pressure
steam less than 15 psig or 250°F. Other systems that can be served with low-pressure
2.4 Operating Pressures of Steam Systems
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Chapter 2: HVAC Steam Systems · 2:13
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steam include single-stage absorption units (10 psig), cooking, warming, dishwashing,
and snow melting heat exchangers. Thus, from the standpoint of load requirements,
high-pressure steam (above 15 psig) is required only for loads such as dryers, presses,
molding dies, power drives, and other processing, manufacturing, and power requirements. The load is the most critical factor that establishes the pressure requirement.
2.5 Steam Heating Systems
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When the source is close to the load(s), the generation pressure should be high enough
to (1) meet the load design pressure, (2) overcome friction losses between the generator
and the load, and (3) satisfy the control range. Losses are caused by flow through the
piping, fittings, control valves, and strainers. If the boiler is located remote from the
loads, there could be some economic advantage in distributing the steam at a higher
pressure to reduce the pipe size. When an increase in the generating pressure requires a
change from below to above 15 psig, the generating system equipment changes from
low-pressure class to high-pressure class and there are significant increases in both investment and operating cost.
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Steam heating systems are classified as one-pipe or two-pipe systems, according to the piping
arrangement that supplies steam to and returns condensate from the terminal equipment. These systems can be further subdivided by (1) the method of condensate return
(gravity flow or mechanical flow by means of a condensate pump or vacuum pump) and
(2) by the piping arrangement (upfeed or downfeed and parallel or counterflow for onepipe systems).
Steam Distribution
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Steam supply piping should be sized so that the pressure drops in all branches of the
same supply main are nearly uniform. Return piping should be sized for the same pressure drop as supply piping for quick and even steam distribution. Because it is impossible to size piping so that the pressure drops are exactly same, the steam flows first to
those units that can be reached with the least resistance, resulting in uneven heating.
Units farthest from the source of steam will heat last, while other spaces are overheated.
This problem is most evident when the system is filling with steam. It can be severe on
systems in which temperature is controlled by cycling the steam on and off. The problem can be alleviated or eliminated by the use of balancing valves or inlet orifices.
One-Pipe Steam Heating Systems
The one-pipe system has a single pipe through which steam flows to and condensate is returned from the terminal equipment (Figure 2.7). These systems are designed as gravity
return, although a condensate pump can be used where there is insufficient height above
the boiler water level to develop enough pressure to return condensate directly to the
boiler. A one-pipe system with gravity return does not have steam traps; instead it has air
vents at each terminal unit and at the ends of all steam mains to vent the air so the system can fill with steam (Figure 2.7). In a system with a condensate pump, there must be
an air vent at each terminal unit and steam traps at the ends of each steam main.
2.5 Steam Heating Systems
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2:14 · Fundamentals of Steam System Design
Figure 2.7 One-Pipe System
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The one-pipe system with gravity return has low initial cost and is simple to install, because it requires a minimum of mechanical equipment and piping. One-pipe systems are
most commonly used in small facilities such as small apartment buildings and office
buildings. In large facilities, the larger pipe sizes required for two-phase flow, problems
of distributing steam quickly and evenly throughout the system, the inability to zone the
system, and difficulty in controlling the temperature make the one-pipe system less desirable than the two-pipe system.
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The heat input to the system is controlled by cycling the steam on and off. In the past,
temperature control in individual spaces has been problem. Many systems have adjustable vents at each terminal unit to help balance the system, but these are seldom effective. A practical approach is to use a self-contained thermostatic valve in series with the air
vent that provides limited individual thermostatic control for each space.
Many designers do not favor one-pipe systems because of their distribution and control
problems. However, when a self-contained thermostatic valve is used to eliminate the
flow and control problems, one-pipe systems can be considered for small facilities,
where initial cost and simple installation and operation are prime factors.
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Most one-pipe gravity return systems are in facilities that have their own boiler. Because
returning condensate must overcome boiler pressure, these systems usually operate from
a fraction of 1 psig to a maximum of 5 psig. The boiler hookup is critical and the Hartford loop (discussed later in the section on Boiler Connections) is used to avoid problems
that can occur with boiler low-water conditions. All piping and radiators must be located above the boiler in a one-pipe system since gravity is used to return condensate to the
boiler.
2.5 Steam Heating Systems
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Chapter 2: HVAC Steam Systems · 2:15
Figure 2.8 Two-Pipe System*
* ASHRAE, 2000 Handbook - Systems and Equipment, Chapter 10, Fig. 15, pg. 10.12
Two-Pipe Steam Heating Systems
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Two-pipe systems use separate pipes to deliver the steam and return the condensate from
each terminal unit (Figure 2.8). Thermostatic traps are installed at the outlet of each terminal unit to keep the steam in the unit until it gives up its latent heat, at which time
the trap cycles open to pass the condensate and permits more steam to enter the unit. If
orifices are installed at the inlet to each terminal unit (discussed in the section on Steam
Distribution) and if the system pressure is precisely regulated to deliver only the amount
of steam each unit is capable of condensing, the steam traps can be omitted. However,
omitting steam traps is generally not recommended for an initial design.
The two-pipe systems can have either gravity or mechanical returns; however, gravity returns are restricted to use in small systems and are generally outmoded. In larger systems
that require higher system pressures to distribute the steam, some mechanical means,
such as a condensate pump or vacuum pump, must be used to return condensate to the
boiler. A vacuum return system is often used on larger systems and has the following advantages:
•
The system fills quickly with steam. In contrast , the steam in a gravity return
system must push the air out of the system, resulting in delayed heat-up and
condensate return that can cause low-water problems.
2.5 Steam Heating Systems
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
2:16 · Fundamentals of Steam System Design
•
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•
The steam supply pressure can be lower, resulting in more efficient
operation.
Condensate can be returned from lower elevations by condensate lift fittings
and the use of smaller pipe sizes as compared to standard pressure systems.
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Variable vacuum or subatmospheric systems are a variation of the vacuum return system
in which a controllable vacuum is maintained in both the supply and return sides. This
permits using the lowest possible system temperature and prompt steam delivery to the
terminal units. The primary purpose of variable vacuum systems is to control temperature.
Unlike one-pipe systems, two-pipe systems can be simply zoned where piping is arranged
to supply heat to individual sections of the building having similar heating requirements. Heat is supplied to meet the requirements of each section without overheating
other sections. The heat also can be varied according to factors such as the hours of use,
type of occupancy, and magnitude of the thermal load.
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2.6 Steam Condensate Systems
The majority of steam systems used in heating applications are two-pipe systems, in
which the two pipes are the steam pipe and the condensate pipe. This discussion is limited to the sizing of the condensate lines in the two-pipe systems. Gravity one-pipe airvent systems in which steam and condensate flow in the same pipe, frequently in opposite directions, are considered obsolete and are no longer being installed.
Vented Return Systems (Dry and Wet Returns)
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To ensure flow through steam traps, the condensate return line is usually vented to the
atmosphere (1) near the point of entrance of the flow streams from the load traps, (2) in
proximity to all connections from drip traps, and (3) at transfer pumps or feedwater receivers. With this design, the only driving force for flow in the return system is gravity.
Return lines that are above the liquid level in the condensate receiver tank or boiler
have both liquid and gas (air and vapor) in the pipe and are called dry returns (Figure
2.9A); those below the liquid level are thus filled with liquid and are called wet returns
(Figure 2.9B). The dry return lines in a vented return system have flowing liquid in the
bottom of the line and gas in the top (Figure 2.9A). It is common practice to slope the
return lines in the direction of flow to a collection point for effective removal of condensate and sediment or solids in the lines.
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Nonvented Return Systems
Those systems, in which there is a continual steam pressure difference between the
point where the condensate enters the line and the point where it leaves (Figure 2.9C),
are called nonvented return systems. When saturated condensate at pressure above the return system pressure enters the return (condensate) mains, some of the liquid flashes to
steam. This occurs typically at drip traps into a vented return system or at load traps leaving process load devices that are not valve-controlled and typically have no subcooling. If
the return main is vented, the vent lines will relieve any excessive pressure and prevent a
2.6 Steam Condensate Systems
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 2: HVAC Steam Systems · 2:17
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backpressure phenomenon that could restrict
the flow through the traps from valved loads. If
the return line is not vented, the flash steam results in a pressure rise at that point. The passage
of the fluid through the steam trap is a throttling or constant enthalpy process. The resulting fluid on the downstream side of the trap can
be a mixture of saturated liquid and steam.
Thus, in nonvented returns, it is important to
understand the condition of the fluid as it enters the return line from the trap
2.7 Boiler Connections
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Figures 2.10 and 2.11 show recommended boiler connections for pumped and gravity return
systems; local codes should be checked for specific legal requirements. Small boilers usually
have one steam outlet connection sized to reduce steam velocity to minimize carryover of liquid water into supply lines. Large boilers can
have several outlets that minimize boiler water
entrainment. Condensate in boilers can be returned by a pump or a gravity return system. Return connections shown in Figures 2.10 and
2.11 for a multiple-boiler gravity return installation may not always maintain the correct water
level in all boilers. Extra controls or accessories
may be required.
Figure 2.9 Types of Condensate Return Systems*
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* ASHRAE, 2000 Handbook - Systems and Equipment,
Chapter 10, Fig 16, pg. 10.12
2.7 Boiler Connections
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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2:18 · Fundamentals of Steam System Design
Figure 2.10 Typical Boiler Connection (gravity)*
* ASHRAE, 2000 Handbook - Systems and Equipment, Chapter 35, Fig. 12, pg 35.17
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Figure 2.11 Typical Boiler Connection (pumped returns)
2.7 Boiler Connections
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Chapter 2: HVAC Steam Systems · 2:19
Supply Main
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Dry Return
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Equalizer
A
Boiler Water Line
2 to 4 in.
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Hartford
Loop
Riser
Wet Return
Figure 2.12 Detailed Boiler Connections with Gravity Return1
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1 ASHRAE, 2001. ASHRAE Handbook—Systems and Equipment, Chapter10, Fig, 2, pg 10.4
Gravity Return
Recommended piping connections for systems using gravity return are detailed in Figure 2.12. Dimension A must be at least 28 in. for each 1 psig maintained at the boiler to
provide the static pressure required to return the condensate to the boiler. The Hartford
loop protects against a low water condition, which can occur if a leak develops in the wet
return portion of the piping. The Hartford loop takes the place of a check valve on the
wet return; however, certain local codes require check valves. Because of hydraulic pressure limitations, gravity return systems are only suitable for systems operating at a boiler
pressure between 0.5 and 1 psig. However, since these systems have minimum mechanical equipment and low initial installed cost, they are appropriate for many small systems.
Pumped Return
Recommended piping connections for steam boilers with pump-returned condensate
are shown in Figure 2.11. Common practice provides an individual condensate or boiler feedwater pump for each boiler. Pump operation is controlled by the boiler water level
control on each boiler. However, one pump may be connected to supply the water to
2.7 Boiler Connections
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
2:20 · Fundamentals of Steam System Design
each boiler from a single manifold by using feedwater control valves regulated by the individual boiler water level controllers. When such systems are used, the condensate return pump runs continuously to pressurize the return header.
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Summary
•
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•
•
Understand the characteristics of steam and condensate.
Use the steam tables.
Describe the basic functions of steam system components.
Calculate the steam supply requirements based on the heating load for a
steam system.
Understand the various phenomena such as water hammer, vacuum breaker
and flash steam taking place in the steam condensate systems.
Describe the one-pipe and two-pipe steam heating systems.
Describe the dry and wet returns in steam condensate systems.
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•
•
•
•
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This chapter has introduced the basic concepts and skills about steam system design. After studying Chapter 2, you should be able to:
2.7 Boiler Connections
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 2: HVAC Steam Systems · 2:21
Bibliography
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Armstrong International, Inc. 2000. Solution Source for Steam, Air and Water Systems.
Armstrong International, Inc., Three Rivers, Michigan.
ASHRAE Handbook. 2005. ASHRAE Handbook-Fundamentals (I-P Version). ASHRAE,
Atlanta, GA.
Goodall, P.M. 1980. The Efficient Use of Steam. Butterworth-Heinemann, England.
Howell, R., H. Sauer, and W. Coad. 2005. Principles of Heating, Ventilating and AirConditioning. ASHRAE, Atlanta, GA.
McCauley, James F. 2000. Steam Distribution Systems Deskbook. Prentice Hall.
McQuiston, F.C., J. D. Parker, and J. D. Spitler. 2005. Heating, Ventilating, and Air
Conditioning: Analyis and Design. John Wiley & Sons, New York, NY.
Northcroft, L.G. and W. M. Barber. 1968. Steam Trapping and Air Venting (4th Edition)
Hutchinson & Co. LTD, London.
Paffel, Kelly. 2003. Steam System Training Manual. Plant Support & Evaluations, Inc.,
Naples, FL.
U.S. Department of Energy, Improving Steam System Performance, a Sourcebook for Industry.
Office of Industrial Technologies (OIT), U.S. Department of Energy.
Bibliography
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
2:22 · Fundamentals of Steam System Design
Skill Development Exercises for Chapter 2
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2-1. Describe saturated steam and superheated steam, respectively.
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Please write you answers on the sheets provided in the back section of this book, and submit the
answers to the ASHRAE Learning Institute.
2-2. Water at 193.2° F is pumped into a boiler in which the pressure is 20.3
psig. How much heat must be supplied by the fuel to evaporate each pound
of water into dry saturated steam?
2-3. If the steam in Problem 2-2, contained 5% moisture by weight, calculate
the heat which must be supplied by the fuel to evaporate each pound of
water into the wet saturated steam.
2-4. A vacuum gauge connected to a steam system reads 5.8 psi at a location
where the atmospheric pressure is 14.5 psi. Determine the absolute pressure
in the steam system.
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2-5. What is the volume of 1 lbm of steam at 25 psia, if it is 16% wet?
2-6. Describe the advantages of steam compared to its competitors such as
electricity and direct-fired heat.
2-7. Describe two sources causing water hammer.
2-8. What are the functions of drip leg?
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2-9. A water heater using saturated steam at a pressure of 5.3 psig has a capacity
of 50,000 Btu/hr. Determine the mass flow rates of both the steam required
and the condensate that flows into the return line.
2-10. Describe low-pressure and high-pressure steam HVAC systems.
2-11. Describe the advantages and disadvantages of the one-pipe and two-pipe
steam heating systems.
2-12. Describe the dry and wet returns in steam condensate systems.
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2-13. Why is the Hartford loop installed?
2-14. Describe both the gravity return and pumped return in boiler
connections.
Skill Development Exercises for Chapter 2
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Chapter 3
Introduction
Study Objectives of Chapter
3.1 Natural Convection Units
3.2 Ratings of Heat –Distributing Units
3.3 Corrections for Nonstandard Conditions
3.4 Applications
Summary
Bibliography
Skill Development Exercises for Chapter 3
References
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•
•
•
•
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Contents of Chapter 3
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Terminal Units I
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3:2 · Fundamentals of Steam System Design
Introduction
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Study Objectives of Chapter 3
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Read the material in Chapter 3. Re-read the parts of the chapter emphasized in the summary. At the end of chapter, complete the skill development exercises without consulting the text. Re-read parts of the text as needed to complete the exercises.
Chapter 3 provides the descriptions of terminal units that transfer heat mainly by natural convection and some radiation. The units are radiator, convector, baseboard unit,
and finned-tube unit. Some layouts and pictures are provided for students to experience
recently available products.
After studying Chapter 3 and working the study problems, you should be able to:
Describe the operation of natural convection units.
Sketch the layouts of the natural convection units.
Calculate the heating load of the units.
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•
•
•
3.1 Natural Convection Units
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Radiators, convectors, and baseboard and finned-tube units are heat-distributing devices
(terminal units) used in steam heating systems. They supply heat through a combination
of radiation and convection and maintain the desired air temperature and/or mean radiant temperature in a space without fans. Figures 3.1 and 3.2 show sections of typical
heat –distributing units. Units are inherently self-adjusting in the sense that heat output
is based on temperature differentials; cold spaces receive more heat and warmer spaces
receive less heat.
Radiators
Radiators are heat-distributing devices that consist of hollow metal coils or tubes. Steam
or hot water passes through the coils or tubes. Heat from the steam or hot water is transferred to the tubes, which emit heat. The air in a building space is heated by radiation
and convection.
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The term radiator, while generally confined to sectional cast-iron column, large-tube, or
small-tube units, also includes flat panel types and fabricated sectional types. Small-tube radiators, with a length of only 1.75 in. per section, occupy less space than column and largetube units and are particularly suited to installation in recesses. (See Table 3-1 on page
3:10) A typical cast-iron radiator is shown in Figure 3.3. Column wall-type, and large-tube
radiators are no longer manufactured, although many of these units are still in use.
3.1 Natural Convection Units
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Chapter 3: Terminal Units 1 · 3:3
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Figure 3.1 Terminal Units
Figure 3.2 Typical Radiators
3.1 Natural Convection Units
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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3:4 · Fundamentals of Steam System Design
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Figure 3.1 Typical Cast-Iron Radiator
3.1 Natural Convection Units
Figure 3.2 Typical Tubular Steel Radiator
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 3: Terminal Units 1 · 3:5
The following are the most common types of radiators.
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Sectional radiators are fabricated from welded steel metal sections (generally
2, 3, and 4 tubes wide), and resemble freestanding cast-iron radiators.
Panel radiators consist of fabricated flat panels (generally 1, 2, or 3 deep),
with or without an exposed extended fin surface attached to the rear for
increased output. These radiators are most common in Europe.
Tubular steel radiators (Figure 3.4) consist of supply and return headers
with interconnecting parallel steel tubes in a wide variety of lengths and
heights. They may be specially shaped to coincide with the building
structure. Some are used to heat bathroom towel racks.
Specialty radiators are fabricated of welded steel or extruded aluminum and
are designed for installation in ceiling grids or floor-mounting. An array of
unconventional shapes is available.
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•
Convectors
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A convector is heat-distributing unit that operates with gravity-circulated air (natural
convection). It has a heating element with a large amount of secondary surface and contains two or more tubes with headers at both ends (Figure 3.5). The heating element is
surrounded by an enclosure with an air inlet opening below and an air outlet opening
above the heating element (Figure 3.6 (a)).
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Convectors are made in a variety of depths, sizes, and lengths and in enclosure or cabinet
types. Three typical outlet styles of convectors are shown in Figure 3.6 (b). The heating elements are available in fabricated ferrous and nonferrous metals (aluminum-finned-copper tubes). The air enters the enclosure below the heating element, heated in passing
through the element, and leaves the enclosure through the outlet grille located above the
heating element. Factory-assembled units comprising a heating element and an enclosure
have been widely used. These may be wall-hung, freestanding, or recessed and may have
outlet grilles or louvers and arched inlets or inlet grilles or louvers, as desired.
The followings are three common types of convectors.
•
•
•
Wall-hung convectors: These units keep floor or carpeting free of dustgathering corners. They are available in front outlet, sloping top outlet and
extruded grille top outlet styles.
Freestanding (Floor-mounted) convectors: The outlet styles of floor model
are the same as the wall-hung convectors, which are shown in Figure 3.6 (b).
The front panel can be removed easily to simplify cleaning (Figure 3.7 (b)).
Recessed convectors: Recessed units take heating equipment completely out
of the room. With fronts finished to blend smoothly with wall surfaces,
3.1 Natural Convection Units
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3:6 · Fundamentals of Steam System Design
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semi-recessed and fully-recessed units provide uninterrupted compatibility
with modern interiors (Figure 3.7 (c)).
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Figure 3.3 A Heating Element with Three Tubes and Headers at Both Ends
Figure 3.4 (a) A Heating Element in a Convector, (b) Three Outlet Styles of Convectors*
* Trane, Cabinet Heaters FIN-PRC005-EN, Trane Company, a Division of American Standard Inc. Trane,
Residential Customer Relations, PO Box 9010, Tyler, TX 75711-9010
3.1 Natural Convection Units
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Chapter 3: Terminal Units 1 · 3:7
Figure 3.5 (a) Wall-Hung Convector, (b) Freestanding Convector, and (c) Recessed Convector
Baseboard Units
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Baseboard (or baseboard radiation) units are designed for installation along the bottom
of walls in place of the conventional baseboard. Various baseboard units are shown in
Figure 3.1. They may be made of cast iron, with a substantial portion of the front face
directly exposed to the room, or with a finned-tube element in a sheet metal enclosure.
They operate with gravity-circulated room air.
Figure 3.6 A Typical Finned-Tube Baseboard Unit*
* Rosemex products catalog, Ray-Vector Wall-Fin Enclosures, Hydronic Heating, page 25.
Baseboard heat-distributing units are divided into three types: radiant, radiant convector, and finned tube. The radiant unit, which is made of aluminum, has no openings
for air to pass over the wall side of the unit. Most of this unit’s heat output is by radiation.
3.1 Natural Convection Units
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3:8 · Fundamentals of Steam System Design
The radiation convector baseboard is made of cast iron or steel. The units have air
openings the top and bottom to permit the circulation of room air over the wall side of
the unit, which has extended surface to provide increased heat output. A large portion
of the heat emitted is transferred by convection.
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The finned-tube baseboard has a finned-tube heating element concealed by a long, low
sheet metal enclosure or cover, as shown in Figure 3.8. A major portion of the heat is
transferred to the room by convection. The output varies over a wide range, depending
on the physical dimensions and the materials used. A unit with a high relative output
per unit length compared to overall heat loss (which would result in a concentration of
the heating element over a relative small area) should be avoided. Optimum comfort for
room occupants is obtained when units are installed along as much of exposed wall as
possible. Low-cost, sloping-top baseboard enclosure is ideal for many light commercial
applications such as banks, offices, and hospitals.
Finned-Tube Units
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Finned-tube (or fin-tube) units are fabricated from metallic tubing, with metallic fins
bonded to the tube (Figure 3.1). They operate with gravity-circulated room air. Finnedtube elements are available in several tube sizes, in either steel or copper - 1 to 2 in. nominal steel or ¾ to 1 ¼ in. nominal copper - with various fin sizes, spacing, and materials.
A typical finned-tube unit is shown in Figure 3.9. The resistance to the flow of steam or
water is the same as that through the standard distribution piping of equal size and type.
Figure 3.7 Finned-Tube Unit
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Finned-tube elements installed in occupied spaces generally have covers or enclosures
in a variety of designs. When human contact is unlikely, they are sometimes installed
bare or provided with an expanded metal grille for minimum protection. A cover has a
portion of the front skirt made of solid materials. The cover can be mounted with
clearance between the wall and the cover, and without completely enclosing the rear of
the finned-tube element. A cover may have a top, front, or inclined outlet. An enclosure is a shield of solid material that completely encloses both the front and rear of the
finned-tube element. An enclosure may have an integral back or may be installed tightly against the wall so that the wall forms the back, and it may have a top, front, or inclined outlet.
3.1 Natural Convection Units
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Chapter 3: Terminal Units 1 · 3:9
3.2 Ratings of Heat –Distributing Units
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For convectors, baseboard units, and finned-tube units, an allowance for heating effect
may be added to the test capacity (the heat extracted from the steam or water under standard test conditions).
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The heat output ratings of heat-distributing units are expressed in Btu/h, MBh (1000
Btu/h), or in square feet equivalent direct radiation (EDR). By definition, 240 Btu/h =
1 ft2 EDR with 1 psig steam. This heating effect reflects the ability of the unit to direct
its heat output to the occupied zone of a room. The application of a heating effect factor
implies that some units use less steam or hot water than others to produce an equal
comfort effect in a room.
3.3 Corrections for Nonstandard Conditions
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The heating capacity of a radiator, convector, baseboard, finned-tube heat-distributing
unit, or radiant panel is a power function of the temperature difference between the air
in the room and heating medium in the unit, shown as
q = c(ts– ta)n
where
= heating capacity, Btu/h
= constant determined by test
= average temperature of heating medium, F.
= room air temperature, °F. Air temperature 60 in. above the floor is generally
used for radiators, while the entering temperature is used for convectors,
baseboard units, and finned-tube units.
= exponent that equals 1.3 for cast-iron radiators, 1.4 for baseboard radiation, 1.5
for convectors, 1.0 for ceiling radiant panels (water systems only), and 1.1 for
floor radiant panels (water systems only). For finned-tube units, n varies with air
and heating medium temperatures. Correction factors to convert heating
capacities at standard conditions to heating capacities at other conditions are
given in Table 3-2. Equation (3-1) may also be used to calculate heating capacity
at nonstandard conditions.
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q
c
ts
ta
(3-1)
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n
3.2 Ratings of Heat –Distributing Units
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3:10 · Fundamentals of Steam System Design
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Table 3-1 Small-Tube Cast-Iron Radiators
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Table 3-2 Correction Factors c for Various Types of Heating Units
3.3 Corrections for Nonstandard Conditions
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Chapter 3: Terminal Units 1 · 3:11
Example 3-1:
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A small cast-iron radiator is used as a supplementary heater in a room. The radiator requires a heating load of 3840 Btu/h with a minimum width at a standard condition of
radiators, steam at 215°F (1-psig steam) and room temperature of 70°F. Design the radiator for the requirements using the information given in Table 3-1.
Solution:
The rating of radiators in a steam system is determined by steam pressure and room (or
air) temperature. Since Table 3-1 is based on a standard condition of radiators (215 °F
steam and 70 °F), 3-tubes-per-section in the table may be selected in order to match both
the heating load of 3840 Btu/h and the minimum width required. Making ten (10) sections yields a total rated capacity and a total length as: Catalog rating 384 Btu/h × 10
Sections = 3840 Btu/h and Spacing 1.75 in. × 10 Sections = 17.5 in. length. The castiron radiator designed conforms to a total rated capacity of 3840 Btu/h and a total
length of 17.5 in.
Example 3-2:
Solution:
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What would be the new output rating of the same cast-iron radiator designed in Example 3-1, if it is now operated at 6-psig steam and a room temperature of 60°F.
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Operating conditions other than a standard condition of radiators can be considered by
multiplying correction factors provided in Table 3-2. A correction factor of 1.23 is selected in the table for steam at 6 psig (steam temperature of 230°F) and air (or room) temperature at 60°F. Therefore, the new output rating at 6-psig steam and room temperature of 60°F is calculated as: 3840 Btu/h × 1.23 = 4723 Btu/h.
Example 3-3:
What would be an amount of steam per hour that must be supplied to the radiator designed in Example 3-2?
Solution:
The amount of steam per hour can be expressed as a mass flow rate of steam and may be
calculated by using Equation (2-1). The mass flow rate of steam for the radiator is ob·
tained by dividing the output rating by the latent heat at 6 psig as m· = q ⁄ h fg . The latent heat (hfg) at 6-psig steam is found from Table 2-1 to be approximately 959 Btu/lbm.
Substituting gives:
m· = ( 4723 Btu ⁄ h ) ⁄ ( 959 Btu ⁄ lb m ) = 4.92 lb m ⁄ h
the result is 4.92 lbm / h, which must be supplied to the radiator.
3.3 Corrections for Nonstandard Conditions
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3:12 · Fundamentals of Steam System Design
Example 3-4:
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A large classroom has a winter design heat loss of 68,000 Btu/h with installed steam
baseboard radiators. The baseboard units house copper tubing with aluminum fins and
operates with steam at 15 psig and entering air (or room) temperature at 65°F. Commercial catalog data are given in Figure 3.10. Using Tables 3-2 and Figure 3.10, specify
the length of the baseboard units with 1-in. tubing for the requirements.
Figure 3.10 Capacities and dimensions of baseboard units.
* ROSEMEX, Baseboard, BB-8, BB-10, BB-12, pg 3
Solution:
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The catalog data in Figure 3.10 provide ratings of three baseboard units with both steam
and hot water. These ratings with steam are based on a standard condition of baseboard
units (65°F entering air temperature and 1 psig (or 250 °F) steam). A correction factor is
found to be 1.32 for steam at 15 psig in Table 3-2.The rating of the baseboard with 1-in.dia. tubing is 1414 Btu/h/ft from the catalog data.
(1414 Btu/h/ft ) ×r 1.32 = 1866.5 Btu/h/ft
(68000 Btu/h)/(1866.5 Btu/h/ft) = 36.43 ft
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The result is approximate 37 ft in length, which is required to meet the heat loss of the
classroom with the baseboard units chosen in the catalog provided.
3.4 Applications
Radiators
Radiators can be used with steam or hot water. They are installed in areas of greatest
heat loss-under windows, along cold walls, or at doorways. They can be installed free-
3.4 Applications
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 3: Terminal Units 1 · 3:13
standing, semi-recessed, or with decorative enclosures or shields, although the enclosures or shields affect the output.
Convectors
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Unique and imported radiators are generally not suitable for steam applications, although they have been used extensively in low-temperature systems with valves and connecting piping left exposed. Various combinations of supply and return locations are
possible, which may alter the heat output. Although long lengths may be ordered for linear applications, lengths may not be reduced or increased by field modification. The
small-cross sectional areas often inherent in unique radiators require careful evaluation
of flow requirements, water temperature drop, and pressure drop.
Convectors can be used with steam or hot water. Like radiators, they should be installed
in areas of greatest heat loss. They are particularly applicable where wall space is limited,
such as in entryways and kitchens.
Baseboard Radiation
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Baseboard units can be used with either steam or hot water. When used with one-pipe
steam systems, tube sizes of 1.25 in. NPS must be used to allow drainage of condensate
counterflow to the steam flow.
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The basic advantage of the baseboard unit is that its normal placement is along the cold
walls and under areas where the greatest heat loss occurs. Other advantages are (1) it is
inconspicuous, (2) it offers minimal interference with furniture placement, and (3) it
distributes the heat near the floor. This last characteristic reduces the floor-to-ceiling
temperature gradient to about 2 to 4 °F and tend to produce uniform temperature
throughout the room. It also makes baseboard heat-distributing units adaptable to
homes without basements, where cold floors are common.
Heat loss calculations for baseboard heating are the same as those used for other types of
heat-distributing units.
Finned-Tube Radiation
The finned –tube unit can be used with either steam or hot water. It is advantageous for
heat distribution along the entire outside wall, thereby preventing downdrafts along the
walls in buildings such as schools, churches, hospitals, offices, airports, and factories. It
may be the principal source of heat in a building or a supplementary heater to combat
downdrafts along the exposed walls in conjunction with a central conditioned air system. Normal placement of a finned tube is along the walls where the heat loss is the
greatest. If necessary, the units can be installed in two or three tiers along the wall. Its
placement under or next to windows or glass panels helps to prevent fogging or condensation on the glass.
Many enclosures have been developed to meet building design requirements. The wide
variety of finned-tube elements (tube size and material, fin size, spacing, fin material,
and multiple tier installation), along with the various heights and designs of enclosures,
3.4 Applications
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3:14 · Fundamentals of Steam System Design
give great flexibility of selection for finned-tube units that meet the needs of load, space,
appearance.
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Summary
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This chapter has introduced the basic concepts and skills about the natural-convection
units such as radiator, convector, baseboard unit, and finned-tube unit. Some manufacturer’s catalog data are provided for students to experience recent commercial products.
Basic heat calculations were also introduced with some examples.
After studying Chapter 3, you should be able to:
•
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•
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Describe functions of various terminal units that transfer most heat by
natural convection.
Calculate the heating capacities of various terminal units.
Design various terminal units for specific requirements.
3.4 Applications
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 3: Terminal Units 1 · 3:15
Bibliography
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Armstrong International, Inc. 2000, Solution Source for Steam, Air and Water Systems.
Armstrong International, Inc., Three Rivers, Michigan.
ASHRAE. 1988 ASHRAE Handbook -- Equipment. Atlanta, GA.
ASHRAE. 2001 ASHRAE Handbook -- Systems and Equipment, I-P Edition. Atlanta, GA.
Curtiss, Peter and Newton Breth. 2002. HVAC Instant Answers. McGraw-Hill.
Kreider, Jan F., Handbook of Heating, Ventilation, and Air Conditioning, CRC Press.
McQuiston, F. C., J. D. Parker and J. D. Spitler. 2005. Heating, Ventilating, and Air
Conditioning: Analysis and Design. John Wiley & Sons, New York, NY.
Northcroft, L.G. and W. M. Barber. 1968. Steam Trapping and Air Venting (4th Edition)
Hutchinson & Co. LTD, London.
Howell, R., H. Sauer, and W. Coad. 2005. Principles of Heating, Ventilating and Air
Conditioning. ASHRAE, Atlanta, GA.
Swenson, S. Don, 1992, HVAC, American Technical Publishers, Inc. Homewood,
Illinois 6043.
Bibliography
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
3:16 · Fundamentals of Steam System Design
Skill Development Exercises for Chapter 3
Complete these questions by writing your answers on the worksheets at the back of this book.
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3-1. What are the mechanisms of heat transfer of natural convection units?
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3-2. List the terminal units that are defined as natural convection units.
3-3. Describe the operation of radiator.
3-4. Describe the operation of convector.
3-5. What is the recessed convector?
3-6. Describe the operation of finned-tube baseboard unit.
3-7. What is the difference between a convector and a baseboard unit?
3-8. What is the heating effect on natural convection units?
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3-9. A small cast-iron radiator is used as a supplementary heater in a room. The
radiator requires a heating load of 2300 Btu/h with a minimum width at a
standard condition of radiators, steam at 215°F (1-psig steam) and room
temperature of 70°F. Design the radiator for the requirements using the
information given in Table 1.
3-10. What would be the new output rating of the same cast-iron radiator
designed in Exercise 3-9, if it is now operated at 15-psig steam and a room
temperature of 65°F.
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3-11. What would be an amount of steam per hour that must be supplied to
the radiator designed in Exercise 3-10?
3-12. A large classroom has a winter design heat loss of 46,000 Btu/h with
installed steam baseboard radiators. The baseboard units house copper
tubing with aluminum fins and operates with steam at 6 psig and entering
air (or room) temperature at 60°F. A commercial catalog data is given in
Figure 3.10. Using Tables 3-2 and Figure 3.10, specify the length of the
baseboard units with 1-in. tubing for the requirements.
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3-13. What is the advantage of baseboard unit in applications?
Skill Development Exercises for Chapter 3
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 4
Introduction
Study Objectives of Chapter
4.1 Unit Heaters
4.2 Unit Ventilators
4.3 Fan-Coil Units
4.4 Cabinet Heaters
4.5 Induction Units
4.6 Air-Handling Units
Summary
Bibliography
Skill Development Exercises for Chapter 4
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Contents of Chapter 4
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Terminal Units II
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4:2 · Fundamentals of Steam System Design
Introduction
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Study Objectives of Chapter 4
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Read the material in Chapter 4. Re-read the parts of the chapter emphasized in the summary. At the end of chapter, complete the skill development exercises without consulting the text. Re-read parts of the text as needed to complete the exercises.
Chapter 4 provides the descriptions of forced convection units in connection with
steam. The units include unit heater, unit ventilator, fan-coil unit, cabinet heater, induction unit, and air-handling unit for an HVAC system.
After studying Chapter 4 and working the study problems, you should be able to:
•
•
•
Understand the concepts and applications of various forced convection
units.
Describe functions of various terminal units that use forced convection.
Calculate the heating capacities of various terminal units in connection with
steam as a heating medium.
Design various terminal units according to specific requirements.
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•
4.1 Unit Heaters
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A unit heater is an assembly of elements, the principal function of which is to heat a
space. The essential elements are a fan and motor, a heating element, and an enclosure.
Filters, dampers, directional outlets, duct collars, combustion chambers, and flues may
also be included. Some types of unit heaters are shown in Figure 4.1. Unit heaters can
be used with either steam or hot water.
Unit heaters have the following principal characteristics:
•
•
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Relatively large heating capacities in compact casings
Ability to project heated air in a controlled manner over a considerable
distance
Relatively low installed cost per unit of heat output
Application where sound level is permissible
They are, therefore, usually placed in applications where the heating capacity requirements, the physical volume of the heated space, or both, are too large to be handled adequately or economically by other means. Through the elimination of extensive duct installations, the space is freed for other use.
Unit heaters are principally used for heating commercial and industrial structures such
as garages, factories, warehouses, showrooms, stores, and laboratories, as well as corridors, lobbies, vestibules, and similar auxiliary spaces in all types of buildings. Unit heat-
4.1 Unit Heaters
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Chapter 4: Terminal Units II· 4:3
ers may often be used to advantage in specialized applications requiring spot or intermittent heating, such as at outside doors in industrial plants or in corridors and vestibules.
Cabinet unit heaters may be used where heated air must be filtered.
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Unit heaters may be applied to a number of industrial processes, such as drying and curing, in which the use of heated air in rapid circulation with uniform distribution is of
particular advantage. They may be used for moisture absorption applications, such as removing fog in dye houses, or to prevent condensation on ceilings or other cold surfaces
of buildings in which process moisture is released. When such conditions are severe,
unit ventilator or makeup air unit may be required.
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Figure 4.1 Typical Unit Heaters
Type of Unit Heater
Propeller fan units (Figure 4.1 A and B) are generally used in nonducted applications
where the heating capacity and distribution requirements can best be met by units of
moderate output and where heated air does not need to be filtered. Horizontal-blow
units are usually installed in buildings with low to moderate ceiling heights. Downblow
units are used in space with high ceilings and where floor and wall space limitations dictate that the heating equipment be kept out of the way. Downblow units may have an
adjustable diffuser to vary the discharge pattern from a high-velocity vertical jet (to
achieve the maximum distance of downward throw) to a horizontal discharge of lower
velocity (to prevent excessive air motion in the zone of occupancy). Revolving diffusers
are also available.
Industrial centrifugal fan units (Figure 4.1 C) are applied where heating capacity and
space volume are large or where filtration of the heated air or operation against static resistance is required. Downblow or horizontal-blow units may be used, depending on the
requirements.
4.1 Unit Heaters
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4:4 · Fundamentals of Steam System Design
Cabinet unit heaters (Figure 4.1 D) are used for applications in which a more attractive
appearance is desired. They are suitable for free-air delivery or low static pressure duct
applications. They may be equipped with filters, and they may be arranged to discharge
either horizontally or vertically up or down.
Location for Proper Heat Distribution
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Duct unit heaters are used where the air handler is remote from the heater. These heaters sometimes provide an economical means of adding heating to existing cooling or
ventilating systems with ductwork. They require flow and temperature limit controls.
Units must be selected, located, and arranged to provide complete heat coverage and, at
the same time, maintain acceptable air motion and temperature at an acceptable sound
level in the working or occupied zone. Proper application depends on size, number, and
type of units; direction of airflow and type of directional outlet used; mounting height;
outlet velocity and temperature; and air volumetric flow. Many of these factors are interrelated.
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The mounting height may be governed by space limitations or by the presence of equipment such as display cases or machinery. The higher a downblow heater is mounted, the
lower the temperature of the air leaving the heater must be to force the heated air into
the occupied zone. Also, the distance that air leaving the heater travels depends largely
on the air temperature and initial velocity. A high temperature reduces the area of heat
coverage.
Unit heaters for high-pressure steam or high-temperature hot water should be designed
to produce approximately the same leaving air temperature as would be obtained from a
lower temperature-heating medium.
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To obtain the desired air distribution and heat diffusion, unit heaters are commonly
equipped with directional outlets, adjustable louvers, or fixed or revolving diffusers. For
a given unit with a given discharge temperature and outlet velocity, the mounting height
and heat coverage can vary widely with the type of directional outlet, adjustable louver,
or diffuser.
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Other factors that may substantially reduce heat coverage include obstructions (such as
columns, beams, or partitions) or machinery in either the discharge air stream or the approach area to the unit. The presence of strong drafts or other air currents also reduces
coverage. Exposures such as large glass areas or outside doors, especially on the windward side of the building, require special attention; units should be arranged so that
they blanket the exposures with a curtain of heated air and intercept the cold drafts.
For area heating, horizontal-blow unit heaters in exterior zones should be placed so that
they blow either along the exposure or toward it at a slight angle. When possible, multiple units should be arranged so that the discharge airstreams support each other and create a general circulatory motion in the space. Interior zones under exposed roofs or skylights should be completely blanketed. Down-blow units should be arranged so that the
heated areas from adjacent units overlap slightly to provide complete coverage.
4.1 Unit Heaters
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Chapter 4: Terminal Units II· 4:5
For spot heating of individual spaces in larger unheated areas, single unit heaters may be
used, but allowance must be made for the inflow of unheated air from adjacent spaces
and the consequent reduction in heat coverage. Such spaces should be isolated by partitions or enclosures, if possible.
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Horizontal unit heaters should have discharge outlets located well above head level.
Both horizontal and vertical units should be placed so that the heated air stream is delivered to the occupied zone at acceptable temperature and velocity. The outlet air temperature of free-air delivery unit heaters used for comfort heating should be 50 to 60°F
higher than the design room temperature. When possible, units should be located so
that they discharge into open spaces, such as aisles, and not directly on the occupants.
Manufacturers’ catalogs usually include suggestions for the best arrangements of various
unit heaters, recommended mounting heights, heat coverage for various outlet velocities, final temperatures, directional outlets, and sound level ratings.
Rating of Unit Heaters
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Heating capacity must be determined at a standard condition. Variations in entering
steam temperature, entering air temperature, and steam flow affect capacity. The followings are typical standard conditions for rating of steam unit heaters: dry saturated steam
at 2 psig pressure at the heater coil, air at 60°F entering the heater, and the heater operating free of external resistance to airflow.
Air quantity of a unit heater is conveniently computed using the sensible heating load:
q& = m& c p (t f − ti )
(4-1)
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where m& = ρ air Q& : mass flow rate (lbm/hr)
ρ air : air density (0.075 lbm/ft3 @70 ° F )
Q& : volumetric flow rate (ft3/hr), 1 cfm (cubic feet per minute) = 60 ft3/hr
c p : specific heat of air (0.24 Btu/lbm.°F @70 ° F )
t f : final air temperature leaving units (°F)
ti : inlet (or entering) air temperature (°F)
Equation (4-1) can be expressed in a convenient way:
q& = ρ air Q& c p (t f − ti ) = 0.075 ⋅ 60 ⋅ cfm@60°F ⋅ 0.24 ⋅ (t f − ti ) = 1.08 ⋅ cfm ⋅ (t f − ti ) (4-1a)
4.1 Unit Heaters
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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4:6 · Fundamentals of Steam System Design
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Figure 4.2 Typical Propeller Fan Unit Heater and Louver Cone Diffuser*
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Table 4-1 VS-Model Vertical Unit Heater Steam Capacities*
4.1 Unit Heaters
* ROSEMEX,
Unit Heaters for Steam and Hot Water
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 4: Terminal Units II· 4:7
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Table 4-2 BTU Correction Factors
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Table 4-3 Air Distribution Correction Factors
ASHRAE 2000 Handbook - Systems and Equipment, Chapter 31, pg 31.1
4.1 Unit Heaters
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
4:8 · Fundamentals of Steam System Design
Example 4-1:
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(a) Select an adequate unit heater
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A propeller fan unit heater shown in Figure 4.2 is to be installed on the ceiling of a
warehouse. A heating load of 139 MBh with an entering air temperature at 70°F is required. Steam at 10-psig pressure is available as a heating medium and the mounting
height of 30 feet is desirable. Using the catalog data of Tables 4-1 to 4-3 given below,
(b) Determine the unit capacity at actual conditions
(c) Determine an amount of condensate in pounds per hour
(d) Calculate the final air temperature leaving the unit heater
(e) Determine whether a cone diffuser is needed
Solution:
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Since the operating condition is not the same as the standard condition, it is first necessary to convert the heating load to the standard conditions. Then a design is selected
and its performance evaluated at the design condition. Using Table 4-2, a correction
factor of 1.04 is found, considering the vertical projection with 10 psig steam and 70°F
air. Then, the equivalent standard heat load is computed as:
Equivalent capacity: 139 MBh/1.04 = 134 MBh (=134,000 Btu/hr)
(a) Now, select Model VS-137 from Table 4-1 that shows the closest required
capacity of 137 MBh.
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(b) Actual unit capacity: 137 MBh · 1.04 = 142.5 MBh, which is slightly higher
than the required heating load.
(c) From Equation (2-1), m& = q& h fg = (142,500 Btu/hr)/(952.5 Btu/lbm) = 149.5
lbm/hr. The result is the amount of condensate in pounds per hour. The
latent heat ( h fg ) of 952.5 Btu/lbm for 10-psig steam is found from Table 4-2.
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(d) From Equation (4-2), knowing the volume flow rate (cfm) is found to be
2640 cfm from Table 4-1,
t f = ti + q& (1.08 ⋅ cfm) = 70°F + 142,500Btu/h (1.08 ⋅ 2640) = 120°F , which
is the final air temperature leaving the unit.
(e) From Table 4-1, the heights without and with cone diffuser are 23 and 36
feet, respectively. However, considering the air distribution correction factor
of 0.98 from Table 4-3 at actual condition (10 psig and 70°F),
Height = 23 (0.98) = 22.5 feet (without diffuser)
= 36 (0.98) = 35.3 feet (with louver cone diffuser)
4.1 Unit Heaters
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Chapter 4: Terminal Units II · 4:9
So, the louver cone diffuser shall be used.
Piping Connections
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The piping of unit heaters must conform strictly to the system requirements, while at
the same time allowing the heaters to function as intended. Basic piping principles for
steam systems are discussed in Chapter 2.
Steam unit heaters condense steam rapidly, especially during warm-up periods. The return piping must be planned to keep the heating coil free of condensate during periods
of maximum heat output, and the steam piping must be able to carry a full supply of
steam to the unit to take the place of condensed steam. Adequate pipe size is especially
important when a unit heater fan is operated under on-off control because the condensate fluctuates rapidly.
Recommended piping connections for unit heaters are shown in Figure 4.3. In steam
systems, the branch from the supply main to the heater must pitch toward the main and
be connected to its top in order to prevent condensate in the main from draining
through the heater, where it might reduce capacity and cause noise.
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The return piping from steam unit heaters should provide a minimum drop of 10 in. below the heater, so that the pressure of water required to overcome resistances of check
valves, traps, and strainers will not cause condensate to remain in the heater.
Dirt pockets at the outlet of unit heaters and strainers with 0.063 in. perforations to prevent rapid plugging are essential to trap dirt and scale that might affect the operation of
check valves and traps. Strainers should always be installed in the steam supply line if
the heater has steam-distributing coils or is valve controlled.
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Adequate air vent is required for low-pressure closed gravity systems. The vertical pipe
connection to the air vent should be at least ¾ in. NPT to allow water to separate from
the air passing to the vent. If thermostatic instead of float-and-thermostatic traps are
used in vacuum systems, a cooling leg must be installed ahead of the trap.
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In high-pressure systems, it is customary to continuously vent the air through a petcock
(as indicated in Figure 4.3 B), unless the steam trap has a provision for venting air. Most
high-pressure return mains terminate in flash tanks that are vented to the atmosphere.
When possible, pressure-reducing valves should be installed to permit operation of the
heaters at low pressure. Trap must be suitable for the operating pressure encountered.
4.1 Unit Heaters
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4:10 · Fundamentals of Steam System Design
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Figure 4.3 Steam Connections for Unit Heaters*
* ROSEMEX, Unit Heaters for Steam and Hot Water
4.2 Unit Ventilators
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A heating unit ventilator is an assembly whose principal functions are to heat, ventilate,
and cool a space by introducing outdoor air in quantities up to 100% of its rated capacity. The heating medium may be steam, hot water, gas, or electricity. The essential components of a heating unit ventilator are the fan, motor, heating element, damper, filter,
automatic controls, and outlet grille, all of which are encased in a housing.
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An air-conditioning unit ventilator is similar to a heating unit ventilator; however, in addition to the normal winter function of heating, ventilating, and cooling with outdoor
air, it is also equipped to cool and dehumidify during summer. It is usually arranged and
controlled to introduce a fixed quantity of outdoor air for ventilation during cooling in
mild weather. The air-conditioning unit ventilator may be provided with a variety of
combinations of heating and air-conditioning elements. Some of the more common arrangements include
Combination hot and chilled water coil (two-pipe)
Separate hot and chilled water coils (four-pipe)
Hot water or steam coil and direct-expansion coil
Electric heating coil and chilled water or direct-expansion coil
Gas-fired furnace with direct-expansion coil
4.2 Unit Ventilators
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 4: Terminal Units II· 4:11
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Unit ventilators are used primarily in schools, meeting rooms, offices, and other areas
where the density of occupancy requires controlled ventilation to meet local codes. The
typical unit is equipped with controls that permit heating, ventilation, and cooling to be
varied while the fans operate continuously. In normal operation, the discharge air temperature from a unit is varied in accordance with the room requirements. The heating
unit ventilator can provide ventilation cooling by bringing in outdoor air whenever the
room temperature is above the room set point. Air-conditioning unit ventilators can
provide refrigeration when the outdoor air temperature is too high to be used effectively
for ventilation cooling.
Unit ventilators are available for floor mounting, ceiling mounting, and recessed applications. They are available with various airflow and capacity ratings, and the fan can be
arranged so that air is either blown through or drawn through the unit. With direct-expansion refrigerant cooling, the condensing unit can either be furnished as an integral
part of the unit ventilator assembly or be remotely located.
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Figure 4.4 (a) illustrates a typical air-conditioning unit ventilator with separator coils,
one used for heating and the other used for cooling with a four-pipe system (each coil
has two pipes, inlet and outlet). The heating coil may be hot water, steam, or electric.
The cooling coil can be either chilled water coil or a direct-expansion refrigerant coil.
Heating and cooling coils are sometimes combined in a single coil by providing separate
tube circuits for each function. In such cases, the effect is the same as having two separate coils. Unit ventilators have an opening to bring in air from outdoors for ventilation.
The outdoor air inlet has an adjustable damper on the inside for regulating the amount
of air admitted to the unit ventilator. Figure 4.4 (b) shows commercially available horizontal and vertical unit ventilators.
Figure 4.4 (a) Layout of a Unit Ventilator with Separate Coils*, (b) Horizontal and
Vertical Unit Ventilators**
* ASHRAE 2000 Handbook - Systems and Equipment, Chapter 31, pg 31.2
** Trane, Cabinet Heaters CAB-PRC001-EN, Trane Company
4.2 Unit Ventilators
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4:12 · Fundamentals of Steam System Design
Heating Capacity Requirements
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Manufacturers publish the heating and cooling capacities of unit ventilators. Table 4-4
lists typical nominal capacities. Because a unit ventilator has a dual function of introducing outdoor air for ventilation and maintaining a specified room condition, the required heating capacity is the sum of the heat required to bring outdoor ventilation air
to room temperature and the heat required to offset room losses. The ventilation cooling capacity of a unit ventilator is determined by the air volume delivered by the unit
and the temperature difference between the unit discharge and the room temperature.
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Table 4-4 Typical Unit Ventilator Capacities*
* ASHRAE 2000 Handbook - Systems and Equipment, Chapter 31, pg 31.1
Example 4-2:
A room has a heat loss of 24,000 Btu/hr at a winter outdoor design condition of 0° F
and an indoor design of 70° F, with 20% outdoor air. Minimum air discharge temperature from the unit is 60° F. To obtain the specified number of air changes, a 1250-cfm
unit ventilator is required.
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(a) Determine the ventilation heat requirement.
(b) Determine the total heating requirement.
(c) Determine the ventilation cooling capacity of this unit with outdoor air
temperature below 60°F.
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Solution:
(a) Ventilation heat requirement: Considering 20% of the rated capacity of
ventilation, which contributes to heat ventilating air from outdoors in
Equation (4-1a),
q& v = 1.08 ⋅ cfm ⋅ ( 20 / 100) ⋅ (t r − to )
where q&v : heat required to heat ventilating air, Btu/hr
cfm : volumetric flow rate, ft3/min
t r : required room air temperature (°F)
4.2 Unit Ventilators
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Chapter 4: Terminal Units II· 4:13
to : outdoor air temperature (°F)
(b) Total heating requirement:
q& t = q& v + q& s
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where q& t = total heat required, Btu/hr
t
q& v = 1.08 ⋅ 1250( 20 / 100) ⋅ (70 − 0) = 18,900 Btu / h
q& s = heat required to make up heat losses, Btu/hr
q& t = 18,900 + 24,000 = 42,900 Btu/hr
(c) Ventilation cooling capacity:
From Equation (4.1a) for this calculation,
q& c = 1.08 ⋅ cfm ⋅ (t r − td )
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where q& c : ventilation cooling capacity of the unit, Btu/hr
t r : required room air temperature (°F)
td : unit discharge air temperature (°F)
q& c = 1.08 ⋅ 1250 ⋅ (70 − 60) = 13,500 Btu / h
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Example 4-3:
A building is occupied by 35 people. The Uniform Building Code calls for 30 cfm of
ventilation air per person. The difference between the indoor and outdoor temperature
is 40°F. Find the heat loss from ventilation air.
Solution:
To find the volumetric flow rate of ventilation air, the volumetric flow rate per person is
multiplied by the number of people that occupy the building.
Q& = 35 people · 30 cfm = 1050 cfm
To find the heat loss from ventilation air, again using Equation 4-1a:
q& = 1.08 · 1050 · 40 = 45,360 Btu/hr
4.2 Unit Ventilators
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4:14 · Fundamentals of Steam System Design
4.3 Fan-Coil Units
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Fan-coil unit systems include cooling as well as heating, normally move air by forced
convection through the conditioned space, filter the circulating air, and introduce outside ventilation air. Fan-coil units with chilled water coils, heating coils, blowers, replaceable air filters, drain pans for condensate, etc., are designed for these purposes. These
units are available in various configurations to fit under windowsills, above furred ceilings, and in vertical pilasters built into walls. These units must be properly controlled by
thermostats for heating and cooling temperature control, by humidistats for humidity
control, by blower control or other means for regulating air quantity, and they must
have a method for adding ventilation air into the building. Fan-coil units, with a dampered opening for connection to apertures in the outside wall, are available. These units
are used extensively in hotels and other residential buildings, but they are not suitable
for commercial building because wind pressure allows no control over the amount of
outside air that is admitted.
Figure 4.5 Typical Fan-Coil Unit*
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(d)
(e)
(f)
(g)
(h)
(i)
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Figure 4.6 Typical Fan-Coil Unit or Cabinet Heaters**
4.3 Fan-Coil Units
* ASHRAE 2000 Handbook - Systems and Equipment, Chapter 3, pg 3.3
** Trane, Cabinet Heaters UNT-IOM-6
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 4: Terminal Units II· 4:15
4.4 Cabinet Heaters
4.5 Induction Units
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Cabinet heaters are basically the same as fan-coil units and are often used in entranceways and vestibules that can have high intermittent heat loads. Cabinet heaters are also
addressed in Section 4.1 with a name of cabinet unit heater.
Induction units are similar to fan-coil units, but the air is supplied by a central air system
rather than individual fans in each unit. Induction units are most commonly used as perimeter heating for facilities with central systems.
Figure 4.7 shows a basic arrangement for an induction unit terminal. Centrally conditioned primary air is supplied to the unit plenum at medium to high pressure. The
acoustically treated plenum attenuates part of the noise generated in the unit and duct.
A balancing damper adjusts the primary air quantity within limits.
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Medium- to high- velocity air flows through the induction nozzles and induces secondary
air from the room through the secondary coil. Thus the primary air provides the energy
required to circulate the secondary air over the coil in the terminal unit. This secondary
air is either heated or cooled at the coil, depending on the season, the room requirement, or both. Induction units are usually installed under a window at a perimeter wall,
although units designed for overhead installation are available. During heating season,
the floor-mounted induction unit can function as a convector during off-hours, with hot
water to the coil and without a primary supply.
Figure 4.7 Induction Unit*
* ASHRAE 2000 Handbook - Systems and Equipment, Chapter 3, pg 3.2
4.4 Cabinet Heaters
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4:16 · Fundamentals of Steam System Design
4.6 Air-Handling Units
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The function of an air-handling unit (AHU) or Air Handler is to supply conditioned air
to one or several building zones. The air-handling unit supplies air at a specific flow rate
and temperature to the zone in order to meet its heating or cooling load. The fundamental equipment components include dampers, air filters, heating & cooling coils,
and fans. The system may also include humidifiers, dehumidifiers and heat recovery.
Centralized systems are installed in building mechanical rooms and provide heating and
cooling to the zones through extensive ductwork. Generally, plant equipment for centralized systems includes chillers, cooling towers, and boilers.
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As shown in Figure 4.8, during operation of a typical AHU, the outdoor air mixes with
return air and passes through an air filter and a variety of air conditioning devices. The
reheat coil raises the temperature of the cooled supply air if the cooling coil over-cools
the air in order to remove humidity. Figure 4.9 depicts the horizontal and vertical types
of typical packaged air handling units. The humidifier in a commercial air handler is
usually a steam type such as direct steam injection humidifier. (For more details, see the
Humidifier section below.)
Figure 4.8 Equipment Layout for Air Handling Unit*
(j)
(k)
(l)
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(m)
(n)
Figure 4.9 Typical Packaged Air Handlers**
* ASHRAE 2000 Handbook - Systems and Equipment, Chapter 2, pg 2.3
** Trane, Cabinet Heaters UNT-PRC003-EN
4.6 Air-Handling Units
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 4: Terminal Units II· 4:17
Humidifiers
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Air-handling units are available as packaged equipment in many configurations using
any desired method of cooling, heating, humidification, filtration, etc. In large systems
(over 50,000 cfm), air-handling units are usually custom-designed and fabricated to suit
a particular application. Air handlers may be either centrally located or decentralized.
Many office buildings locate air-handling unit at each floor. This saves the space required for distribution ductwork. The reduced size of equipment as a result of duplicated systems allows the use of less expensive packaged equipment.
Most air handling systems requiring humidification use steam. This can be centrally generated as part of the heating plant, where potential contamination from the water treatment of the steam is more easily handled (making it nontoxic) and therefore of less concern. Low humidity increases evaporation from the membranes of the nose and throat,
the skin and hair, leading to respiratory illness. Low humidity also increases static electricity, resulting in unpleasant sparks and destruction of electromagnetic data stored on
disks and tapes. Extreme humidity is the most detrimental to human comfort, productivity, and health. 30%-60% relative humidity at normal room temperature provides the
best condition to human occupancy.
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The capacity of the humidifying equipment should not exceed the expected peak load
by more than 10%. Many humidifiers add some sensible heat that should be accounted
for the psychrometric evaluation.
Figure 4.10 (a) Jacketed Steam Humidifier, (b) Panel Steam Dispersion Humidifier*
* ASHRAE 2000 Handbook - Systems and Equipment, Chapter 20, pg 20.7
Humidifiers can generally be classified as either residential or industrial. Residential humidifiers include pan humidifiers, wetted element humidifiers, and atomizing humidifi-
4.6 Air-Handling Units
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4:18 · Fundamentals of Steam System Design
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ers. Industrial humidifiers include heated pan humidifiers, direct steam injected humidifiers, electrically heated steam humidifiers, and atomizing humidifiers. The direct steam
humidifiers used mostly in central air systems are addressed here. Steam can be introduced into the air stream through a jacked steam humidifier or a panel steam dispersion
humidifier, which are shown in Figure 4.10 (a) and (b). Jacketed steam humidifiers (Figure 4.10 (a)) contain the steam jacket to prevent any condensate droplets from being introduced into the air stream. Panel steam dispersion humidifiers (Figure 4.10 (b)) does
not contain jacket but a panel to disperse steam into the air stream. Units must be installed where the air can absorb the discharged vapor before it comes into contact with
components in the air stream, such as air coils, dampers, or turning vanes. Otherwise,
condensation can occur in the duct. Absorption distance varies according to the design
of the humidifier distribution device and the air conditions within the duct.
Steam Coils
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For air heating coils, steam is the preferred medium for heat transfer throughout much
of industry. It affords advantages over liquids because it is easy and inexpensive to move
from the boiler to the point of use and because it gives up so much energy at a constant
temperature when it condenses. Process control is easily and quickly accomplished with
essentially no lag time as is experienced with liquids.
The selection of coil construction and materials is a multi-step process that must take a
number of factors into consideration. Steam coils can be oriented vertically or horizontally, with air flow transverse to the coil axis.
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Standard coils (Figure 4.11 (a)) are used for most applications where entering air temperatures are above 35°F and steam is at a constant pressure. It is used extensively in
high-pressure process applications and for “reheat” in HVAC systems such as in air-handling units. It is not, however, recommended where constant outlet air temperatures are
required immediately after the coil, such as in multi-zone heating systems, or where a
modulating steam control valve is used to control temperature. (Standard coils can be
used with a modulating steam control valve under certain circumstances.)
Steam Distributing coils (Figure 4.11 (b)) can be used where air is below freezing and/
or modulating control is used. It is recommended where a single row delivers the required performance, a modulating steam control valve is used, even outlet air temperatures are required over the whole coil face, and stainless steel tubes are used.
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A steam distributing coil is designed with an inner steam distribution tube that is inserted in an outer finned tube. The center tube is fed with steam, which travels up this distribution tube and is then discharged into the outer tube through diffuser openings.
The steam then travels back between the outside wall of the distribution tube and the
inside wall of the finned tube to the condensate header (Figure 4.7 (b)). The inner tube
provides an even distribution of steam along the total length of the outer finned tube.
4.6 Air-Handling Units
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Chapter 4: Terminal Units II· 4:19
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Figure 4.11 (a) Standard Coils and (b) Steam Distributing Coils
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Summary
This chapter has introduced the basic concepts and skills about the forced-convection
units such as unit heater, unit ventilator, cabinet heater, induction unit, fan-coil unit,
and central air-handling unit. Some manufacturer’s catalog data are provided for students to experience recent commercial products. Basic heat calculations were also introduced with some examples.
After studying Chapter 4, you should be able to:
• Describe functions of various steam terminal units that transfer most heat by
forced convection.
• Calculate the heating capacities of various terminal units.
• Design various types of terminal units according to specific requirements.
4.6 Air-Handling Units
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
4:20 · Fundamentals of Steam System Design
Bibliography
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Armstrong International, Inc. 2000. Solution Source for Steam, Air and Water Systems.
Armstrong International, Inc., Three Rivers, MI.
ASHRAE, 1988 ASHRAE Handbook - Equipment. ASHRAE, Atlanta, GA.
ASHRAE, 2000 ASHRAE Handbook - Systems and Equipment. ASHRAE, Atlanta, GA.
Curtiss, Peter and Breth, Newton. 2002. HVAC Instant Answer, McGraw-Hill, New
York.
Howell, R., H. Sauer, and W. Coad. 2005. Principles of Heating, Ventilating and Air
Conditioning. ASHRAE, Atlanta, GA.
Kreider, Jan F. 2000. Handbook of Heating, Ventilation, and Air Conditioning. CRC Press,
Boca Raton, FL.
McQuiston, F. C., J. D. Parker and J. D. Spitler. 2005. Heating, Ventilating, and Air
Conditioning: Analysis and Design. John Wiley & Sons, New York.
Northcroft, L. G. and W. M. Barber. 1968. Steam Trapping and Air Venting, 4th Edition.
Hutchinson & Co. LTD, London.
ROSEMEX, Baseboard, Products Catalog BB-8, BB-10, BB-12, Mecar Metal Inc., 1560,
Marie-Victorin Blvd, Saint-Bruno (Quebec), J3V 6B9.
Swenson, S. Don, 1992, HVAC, American Technical Publishers, Inc. Homewood, IL.
Trane. Blower Coil Air Handler UNT-PRC003-EN. Trane Company, a Division of
American Standard Inc.
Bibliography
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 4: Terminal Units II · 4:21
Skill Development Exercises for Chapter 4
Complete these questions by writing your answers on the worksheets at the back of this book.
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4-1. Describe the functions and applications of unit heaters.
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4-2. A propeller fan unit heater shown in Figure 4.2 is to be installed in a
laboratory. The heating load of 67 MBh with an entering air temperature at
70°F is required. Steam at 5-psig pressure is available as a heating medium
and the mounting height of 20 feet is desirable. Using the catalog data of
Tables 4-1 to 4-3 given in the text,
(a) Select an adequate unit heater.
(b) Determine the unit capacity at actual conditions.
(c) Determine an amount of condensate in pounds per hour.
(d) Calculate the final air temperature leaving the unit heater.
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(e) Determine whether a cone diffuser is needed.
4-3. Why should the return pipe from steam unit heaters provide a minimum
drop of 10 in. below the heaters?
4-4. Describe the functions and applications of unit ventilators.
4-5. List the heating medium other than steam for unit ventilators.
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4-6. A room has a heat loss of 18,000 Btu/hr at a winter outdoor design
condition of 5°F and an indoor design of 65°F, with 15% outdoor air.
Minimum air discharge temperature from the unit is 55°F. To obtain the
specified number of air changes, a 1000-cfm unit ventilator is required.
(a) Determine the ventilation heat requirement.
(b) Determine the total heating requirement.
(c) Determine the ventilation cooling capacity of this unit with outdoor
air temperature below 55°F.
4-7. A building is occupied by 70 people. The Uniform Building Code calls for
30 cfm of ventilation air per person. The difference between the indoor and
outdoor temperature is 50°F. Find the heat loss from ventilation air.
(Exercises continued on next page)
Skill Development Exercises for Chapter 4
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4:22 · Fundamentals of Steam System Design
4-8. Sketch the layout of a typical unit ventilator.
4-9. What is the difference between fan-coil unit and unit ventilator?
4-10. What are the distinguishing features of a induction unit?
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4-11. Describe the distinguishing function and applications of an air-handling
unit.
4-12. What is the function of humidifier in an air-handling unit?
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4-13. Describe two types of steam coils with brief explanations.
Skill Development Exercises for Chapter 4
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Chapter 5
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Contents of Chapter 5
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Boilers
Introduction
Study Objectives of Chapter 5
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5.1 Boilers
5.2 Boiler Classifications
5.3 Fuels and Combustion
5.4 Efficiency
5.5 Cost of Producing Steam
5.6 Boiler Sizing
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Summary
Bibliography
Skill Development Exercises for Chapter 5
References:
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5:2 · Fundamentals of Steam System Design
Introduction
Study Objectives of Chapter 5
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Read the material in Chapter 5. Re-read the parts of the chapter emphasized in the summary. At the end of chapter, complete the skill development exercises without consulting the text, except as necessary to acquire data from tables or figures. Reread parts of
the text as needed to complete the exercises.
In this chapter, you will learn about boilers, including boiler types, energy source alternatives, fuels used for combustion, efficiency concepts, and sizing/selection of boilers
for steam heating applications. We will not discuss the internal design of boilers and
their construction.
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After studying Chapter 5 and working the study problems, you should be
able to:
Classify the boilers in terms of boiler pressure, material, and draft type.
Describe differences between condensing and noncondensing boilers.
Classify the hydrocarbon fuels commonly used in boilers.
Describe the difference between the higher and lower heating values for
fuels.
Calculate the air/fuel ratio for the combustion reactions.
Describe the flue gas analysis (Orsat gas analyzer) for the combustion
products.
Calculate the dew point of the combustion products.
Calculate the excess air for combustion.
Explain combustion, boiler, and seasonal efficiencies.
Select and size boilers.
Calculate the steam value or the fuel cost for a boiler.
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5.1 Boilers
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A boiler is device designed to transfer thermal energy from a heat source to a working
fluid. In a boiler, heat is generated by an energy source from either the byproduct of a
combustion process or an electric resistance heating element. In boilers of interest to the
ASHRAE community, the working fluid leaving the boiler is water in the form of a hot
liquid or steam. The hot water or steam is then circulated to terminals where it is used
to meet heating loads. A device used to heat air is more commonly known as a “furnace”, not a boiler; however, you may see the firebox, or combustion chamber, of some
boilers referred to as a furnace.
Excluding special and unusual fluids, materials, and methods, a boiler consists of a castiron, steel, aluminum, or copper pressure vessel heat exchanger designed to (1) burn fossil fuels (or use electric current) and (2) transfer the released heat to water and steam.
5.1 Boilers
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Chapter 5: Boilers· 5:3
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5.2 Boiler Classifications
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Boiler heating surface is the area of fluid-backed surface exposed to the products of combustion, or the fire-side surface. Various codes and standards define allowable heat
transfer rates, in terms of heating surface. Boiler designs provide for connections to a
piping system, which delivers heated fluid to the point of use and returns the cooled fluid to the boiler.
Boilers may be grouped into classes based on such criteria as working pressure and temperature, fuel used, material of construction, combustion chamber design (fire-tube vs
water-tube), type of draft (natural or mechanical), and whether their flue gases are condensing or noncondensing. They may also be classified according to shape and size, application (such as heating or process), and the state of the output medium (steam or water). Boiler classifications are important to the specifying engineer because they affect
performance, first cost, and space requirements. Excluding designed-to-order boilers, significant class descriptions are given in boiler catalogs or are available from the boiler
manufacturer. The following basic classifications may be helpful.
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Working Pressure and Temperature
With few exceptions, boilers are constructed to meet the ASME Boiler and Pressure
Vessel Code.
Low–pressure boilers are constructed for a maximum working pressure of 15 psig
steam. Operating and safety controls and relief valves, which limit temperature and pressure, are ancillary devices required to protect the boiler and prevent operation beyond
design limits.
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High-pressure boilers are designed to operate above 15 psig steam. Similarly, operating
and safety controls and relief valves are required.
Steam boiler rates are often expressed in pounds of steam per hour (lbm steam/hr) or
the energy content of the steam (Btu/hr). Steam boilers are generally available in standard sizes of up to 50,000 lbm steam/hr (60,000 to 50,000,000 Btu/hr), many of which
are used for space heating in both new and existing systems. On larger installations, they
may also provide steam for auxiliary uses, such as hot-water heat exchangers, absorption
cooling, laundry, and sterilizers. In addition, many steam boilers provide steam at various temperatures and pressures for a wide variety of industrial processes.
Steam boiler rates are also expressed in boiler horsepower as a result of early boilers used
to drive engines with one engine horsepower or one boiler horsepower (BHP), equivalent to 34.5 lbm steam/hr (pounds of water evaporated at 212°F). This equals 33,475
Btu/hr, which is based on 970 Btu of enthalpy of evaporation to change 1 lbm of water
into steam at 212°F.
Every steam boiler is rated at the maximum working pressure determined by the ASME
Boiler Code Section under which it is constructed and tested. When installed, it must
5.2 Boiler Classifications
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5:4 · Fundamentals of Steam System Design
also be equipped with safety controls and pressure relief devices mandated by such code
provisions.
Example 5-1
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If a commercial boiler with a rate of 400 BHP is considered for a steam system design,
what will be the equivalent gross output for the boiler in lbm steam/hr and in Btu/hr,
respectively, at 212°F?
Solution:
Gross output in lbm steam/hr at 212°F: 400 BHP · 34.5 lbm steam/hr/BHP=13,800 lbm
steam/hr at 212°F.
Gross output in Btu/hr at 212°F: 400 BHP · 33,475 Btu/hr/BHP=13,390,000 Btu/hr
(or 13.39 MMBtu/hr)
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Tips: The gross output of a boiler will vary if the steam pressure and average feedwater
temperature are other than at the atmospheric pressure and 212 °F. However, the boiler
output is often used as a good estimation for other than at atmospheric pressure and
212 °F.
Construction Materials
Most noncondensing boilers are made with cast iron or steel. Some small boilers are
made of copper or copper-clad steel. Condensing boilers are typically made of stainless
steel or aluminum.
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Cast-iron boilers are designed according to ASME Boiler Code requirements and range
in size from 35,000 to 13,000,000 Btu/hr gross output. They are constructed of individually cast sections, assembled into blocks (assemblies) of sections. Push or screw nipples,
gaskets, and/or an external header join the sections pressure-tight and provide passages
for the water, steam and products of combustion. The number of sections assembled determines the boiler size and energy rating. Sections may be vertical or horizontal, the vertical design being more common (Figure 5.1A and Figure 5.1C).
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The boiler may be dry-base (the combustion chamber is beneath the fluid backed sections), as in Figure 5.1B; wet-base (the combustion chamber is surrounded by fluidbacked sections, except for necessary openings), as in Figure 5.2A; or wet-leg (the combustion chamber top and sides are enclosed by fluid-backed sections) as in Figure 5.2B.
The three types of boilers can be designed to be equally efficient. Testing and rating
standards apply equally to all three types. The wet-base design is easiest to adapt for combustible floor installations. Applicable codes usually demand a floor temperature under
the boiler no higher than 90°F above room temperature. A steam boiler at 215°F may
not meet this requirement without appropriate floor insulation. Large cast-iron boilers
are also made as water-tube units with external headers (Figure 5.2C).Steel boilers generally range in size from 50,000 Btu/hr to the largest boilers made. Designs are constructed to ASME Code requirements. They are fabricated into one assembly of a given size
and rating, usually by welding. The heat exchange surface past the combustion chamber
5.2 Boiler Classifications
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Chapter 5: Boilers· 5:5
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Figure 5.1 Residential Boilers*
Figure 5.2 Cast-Iron Commercial Boilers**
* ASHRAE, 2000 Handbook - Systems and Equipment, Chapter 27, pg 27.2
** ASHRAE, 2000 Handbook - Systems and Equipment, Chapter 27, pg 27.3
5.2 Boiler Classifications
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5:6 · Fundamentals of Steam System Design
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is usually an assembly of vertical, horizontal, or slanted tubes. Boilers of the fire-tube design contain flue gases in tubes completely submersed in fluid (Figure 5.1D and Figure
5.1E show residential units, and Figure 5.3A through Figure 5.3E show commercial
units). Water-tube boilers contain fluid inside tubes with the tube pattern arrangement
providing for the combustion chamber (Figure 5.4A and Figure 5.4B). The internal configuration may accommodate one or more flue gas passes. As with cast-iron boilers, drybase, wet-leg, or wet-base designs may be used. Most small steel boilers are of the drybase, vertical fire-tube type (Figure 5.1D).
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Larger boilers usually incorporate horizontal or slanted tubes; both fire-tube and watertube designs are used. A popular horizontal fire-tube design for medium and large steel
boilers is the scotch marine, which is characterized by a central fluid-backed cylindrical
combustion chamber, surrounded by fire-tubes accommodating two or more flue gas
passes, all within an outer shell (Figure 5.3A through Figure 5.3E). However, the design
in Figure 5.3E uses a dry base and wet-leg, while others in Figure 5.3A through Figure
5.3D use wet base. A fire-tube boiler is a steam boiler in which hot gaseous products of
combustion pass through tubes surrounded by boiler water. Fire-tube boilers are usually
designed for smaller steam capacities and low pressure heating. Water-tube boilers are
more complex and contain about 75% less water than comparable fire-tube boilers. Water-tube boilers are used for operations requiring larger capacities and pressures.
Copper boilers are usually some variation of the water-tube boiler. Parallel finned copper tube coils with headers, and serpentine copper tube units are most common (Figure
5.1F and Figure 5.1G). Some are offered as wall-hung residential boilers. The commercial bent water-tube design is shown in Figure 5.4A. Natural gas is the usual fuel for copper boilers.
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Stainless steel boilers usually are designed to operate with condensing flue gases. They
are often limited to operating temperatures of 210°F or less to avoid problems that
might result from stress cracking.
Aluminum boilers are also usually designed to operate with condensing flue gases. Typical design incorporate either cast aluminum boiler sections or integrally finned aluminum tubing.
Type of Draft
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Draft is the pressure difference that causes air and/or fuel to flow through a boiler or
chimney. A natural draft boiler is designed to operate with a negative pressure in the
combustion chamber and in the flue connection. The pressure difference is created by
the tendency of hot gases to rise up a chimney or by the height of the boiler up to the
draft control device. In a mechanical draft boiler, a fan or blower or other machinery
creates the required pressure difference. These boilers may be either forced draft or induced draft. A forced draft fan is located at the entrance to the boiler furnace to maintain a positive pressure in the combustion chamber, while an induced draft fan is located at outlet of the boiler furnace to maintain a negative pressure in the combustion
chamber. Some boilers having both forced draft and induced draft fans include dampers
to balance them and to operate the furnace at a pressure slightly less than atmospheric,
reducing the leakage of the flue gas out of the small opening in the furnace. This configuration is called a balanced draft boiler.
5.2 Boiler Classifications
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Chapter 5: Boilers· 5:7
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Figure 5.3 Scotch Marine Commercial Boilers*
Figure 5.4 Commercial Fire-Tube and Water-Tube Boilers**
* ASHRAE, 2000 Handbook - Systems and Equipment, Chapter 27, pg 27.3
** ASHRAE, 2000 Handbook - Systems and Equipment, Chapter 27, pg 27.4
5.2 Boiler Classifications
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5:8 · Fundamentals of Steam System Design
Condensing or Noncondensing
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Until recently, boilers were designed to operate without condensing the water vapor
from the combustion process in the flue gas in the boiler. This precaution was necessary
to prevent corrosion of cast-iron or steel parts. Hot water units were often operated at
140°F minimum return water temperature to prevent rusting when natural gas was
used.
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Because a higher boiler efficiency can be achieved with a lower water temperature, the
condensing boiler allows the flue gas water vapor to condense and drain. Figure 5.5
shows a typical relationship of overall condensing boiler efficiency to return water temperature. The dew point of 130°F shown in the figure varies with the percentage of hydrogen in the fuel and oxygen-carbon dioxide ratio, or excess air, in the flue gases. A
condensing boiler is shown in Figure 5.1H. Condensing boilers can be of the fire-tube,
water-tube, or cast aluminum section design. Due to the relatively low condensate temperature, condensing boilers are limited primarily to hot water systems.
Figure 5.5 Effect of Inlet Water Temperature on Efficiency of Condensing Boilers*
* ASHRAE, 2000 Handbook - Systems and Equipment, Chapter 27, pg 27.4
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5.3 Fuels and Combustion
Fuels
Boilers may be designed to burn coal, wood, various grades of fuel oil, waste oil, various
types of fuel gas, or to operate as electric boilers. A boiler designed for one specific fuel
type may not be convertible to another type of fuel. Some boiler designs can be adapted
to burn coal, oil or gas. Several designs allow firing with oil or gas by burner conversion
or by using a dual fuel burner.
5.3 Fuels and Combustion
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Chapter 5: Boilers· 5:9
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Generally, hydrocarbon fuels are classified according to physical state (gaseous, liquid,
or solid). Different types of combustion equipment are usually needed to burn fuels in
different physical states. Gaseous fuels can be burned in premix or diffusion burners
that take advantage of the gaseous state. Liquid fuel burners must include a means of atomizing or vaporizing fuel into small droplets or to a vapor for burning, and must provide adequate mixing of fuel and air. Solid fuel combustion equipment must (1) heat
fuel to vaporize sufficient volatiles to initiate and sustain combustion, (2) provide residence time to complete combustion, and (3) provide space for ash containment.
The heating value of a fuel can be determined by measuring the heat evolved during
combustion of a known quantity of fuel in a calorimeter. Higher heating value (HHV)
is determined when water vapor in fuel combustion products is condensed and the latent heat of vaporization is included in the fuel’s heating value. Lower heating value
(LHV) is obtained when latent heat of vaporization is not included. When the heating
value of a fuel is specified without designating higher or lower, it generally means the
higher heating value in the United States. (Lower heating value is mainly used for internal combustion engine fuels.)
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Gaseous Fuels. Heating applications are presently limited to natural and liquefied petroleum gases (propane, butane, etc.). Natural gas used as fuel typically contains methane,
CH4 (70 to 96%); ethane, C2H6 (1 to 14%); propane, C3H8 (0 to 4%); butane, C4H10
(0 to 2%); pentane, C5H12 (0 to 0.5%); hexane, C6H14 (0 to 2%); carbon dioxide, CO2
(0 to 2%); oxygen, O2 (0 to 1.2%); and nitrogen, N2 (0.4 to 17%). Typical heating values
for gaseous fuels are listed in Table 5-1.
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Liquid fuels. Liquid fuels, with few exceptions, are mixtures of hydrocarbons refined
from crude petroleum. Fuel oils for heating are broadly classified as distillate fuel oils
(lighter oils) or residual fuel oils (heavier oils). ASTM has established specifications for
fuel oil properties, which subdivide the oils into various grades. Grades No.1 and 2 are
distillate fuel oils. Grades No. 4, 5 (Light), 5 (Heavy), and 6 are residual fuel oils. Typical
gravity and heating values of standard grade oil are shown in Table 5-2.
Solid Fuels. Solid fuels include coal, coke, wood, and waste products of industrial and
agricultural operations. Of these, only coal is widely used for heating applications.
Chemically, coal consists of carbon, hydrogen, oxygen, nitrogen, sulfur, and a mineralmatter-free basis. Higher heating values of coals are frequently reported with their proximate analysis.
Combustion.
The combustion of fuel in a boiler is a chemical reaction and is governed by the principles of stoichiometry. This section discusses the combustion of natural gas (for our purposes assumed to be 100% methane) in boilers as an example of fuel burning for heat
production. The stoichiometric, complete combustion of the methane (CH4) is
5.3 Fuels and Combustion
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5:10 · Fundamentals of Steam System Design
Table 5-1 Heating Values of Gaseous Fuelsa
Btu/ft3
*Btu/lbm
MJ/m3
Specific Gravity
Air=1.0
Natural
1,030
22,889
38.4
0.6
Propane (C3H8)
2,500
21,786
93.1
1.53
Butane(C4H10)
3,175
21,167
118.3
2.00
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Gas
a. ASHRAE, 2000 Handbook - Systems and Equipment, Chapter 18
*Calculated with air density=0.075 lbm/ft3 @14.7 psi and 68°F
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Table 5-2 Typical API Gravity, Density, and Heating Value of Standard Grades of Fuel Oila
API Gravity
Density
lbm/gal
Heating Value
Btu/gal
1
38 to 45
6.950 to 6.675
137,000 to 132,900
2
30 to 38
7.269 to 6.960
141,800 to 137,000
4
20 to 28
7.787 to 7.396
148,100 to 143,100
5L
17 to 22
7.940 to 7.686
150,000 to 146,800
5H
14 to 18
8.080 to 7.890
152,000 to 149,400
6
8 to 15
8.448 to 8.053
155,900 to 151,300
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a. ASHRAE, Handbook-Systems and Equipment, Chapter 18, pg. 18.6
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Stoichiometric (ideal):
CH4 + 2(O2 + 3.76 N2) → CO2 + 2 H2O + 7.52 N2
(5-1)
For incomplete combustion, carbon monoxide is formed according to the reaction:
CH4 + (3/2) * (O2+3.76 N2) → CΟ + 2 Η2Ο + 5.64 Ν2
(5.1a)
Boiler efficiency and heating calculations are based on the assumption of complete combustion of the fuel.
5.3 Fuels and Combustion
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Chapter 5: Boilers· 5:11
20% excess air (or 120% theoretical air):
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Air is generally composed of approximate 21% oxygen and 79% nitrogen by volume.
The 3.76 in front of N 2 is calculated from the oxygen nitrogen ratio of air: 0.79/
0.21=3.76. Practically, excess air is required to complete the combustion. If 20% excess
air is used in the following example, the combustion equation in this case will be
(5-2)
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CH4 + (1.2)(2)(O2 + 3.76N2) → CO2 + 0.4O2 + 2H2O + (1.2)(2)(3.76)N2
The air/fuel mass ratio is often required for system design. This ratio is determined
from Equation 5-1 and the molecular weights of the various compounds involved in the
combustion process. Using the following molecular weights (as gram/mole or lbm/ lbmole):
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CH4 = 16
O2 = 32
N2 = 28
CO2 = 44
H2O = 18
Equation 5-2 becomes
[16]+[(1.2)(2)(32+3.76·28)] → [44]+[(0.4)·32]+[2·(18)]+[(1.2)·2·(3.76)·28]
From this, the air/fuel ratio by mass is calculated as:
A/F = mair/mfuel = [(1.2)(2)(32+3.76·28)]/[16] = 20.6 lbair/lbfuel
(5-3)
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This indicates that 20.6 lbm of air is required to burn 1 lbm of methane for complete
combustion with 20% excess air.
Flue gas analysis is a method of determining the amount of excess air in a combustion
process. This information can be used to find an approximate value of boiler efficiency.
Periodic regular analysis can provide a trend of boiler efficiency with time, indicating
possible problems with the burner or combustion equipment in a boiler. Flue gas analysis is often expressed as the volumetric fraction of the flue gases (oxygen, nitrogen, and
carbon monoxide). A sample of the flue gases is collected and cooled to room temperature and pressure so that the water vapor in the flue gases is not included in the volumetric analysis (Orsat gas analyzer). If the three values (oxygen, nitrogen, and carbon
monoxide) are known, the excess air can be found from*
Excess Air, % =
100[O2 − 0.5CO ]
0.264 N 2 − (O2 − 0.5CO )
(5-4)
* Carbon monoxide, resulting from incomplete combustion of the fuel, is more easily measured than
CO2. With the amount of CO known, the amount of oxygen required for complete combustion to
CO2 can be computed.
5.3 Fuels and Combustion
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5:12 · Fundamentals of Steam System Design
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The amount of excess air varies with the fuel and with the design of the boiler. The recommendation of the manufacturer should be followed. The optimum excess air fraction
is usually between 10 and 50%. On well-designed natural gas-fired systems, an excess level of 10% is attainable. An often-stated rule of thumb is that boiler efficiency can be increased by 1% for each 15% reduction in excess air or 40°F reduction in stack gas temperature. The relationship between excess air (%) and combustion efficiency with flue
gas temperature is shown in Table 5-3. Note that the temperature difference between
the flue gas and the combustion air is used in this table, rather than the actual flue gas
temperature.
Example 5-2
The volumetric analysis of flue gas from combustion of methane in a gas boiler is measured to be
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10.5% carbon dioxide
3.0% oxygen
86.5% nitrogen
0% carbon monoxide
Find the amount of excess air and estimate the combustion efficiency with a flue gas less
combustion air temperature of 400°F. Is the excess air within the recommended range
suggested above?
Table 5-3 Combustion Efficiency for Natural Gasa
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Excess %
Air
Oxygen
9.5
2.0
15.0
3.0
28.1
5.0
44.9
7.0
81.6
10.0
Combustion Efficiency (%)
Flue gas temperature less combustion air temperature, °F
200
300
400
500
600
85.4
83.1
80.8
78.4
76.0
85.2
82.8
80.4
77.9
75.4
84.7
82.1
79.5
76.7
74.0
84.1
81.2
78.2
75.2
72.1
82.8
79.3
75.6
71.9
68.2
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a. Department of Energy, Improving Steam System Performance: A Sourcebook for Industry, Office of Industrial
Technology, Energy Efficiency and Renewable Energy, U.S. Department of Energy, Appendix B Steam Tip Sheet
Number 4.
Note: Assumes complete combustion with no water vapor in the combustion air.
Solution:
Equation (5-4) will be used as follows:
Excess Air,% =
5.3 Fuels and Combustion
100[3.0 − (0.5 ⋅ 0)]
= 15.1%
0.264 ⋅ 86.5 − (3.0 − 0.5 ⋅ 0)
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Chapter 5: Boilers· 5:13
The excess air is 15.1%, within the 10-50% range above.
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From Table 5-3, the combustion efficiency with 15.1% excess air and the flue gas temperature of 400°F is found to be 80.4%.
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Dew Point is the temperature at which condensation begins. Water vapor in flue gas is
mainly produced from the combustion of hydrocarbons in the fuel. The amount of water vapor in the combustion products may be calculated from the fuel burned by assuming an ideal-gas partial pressure equation:
Pv =
Nv
Pprod
N prod
(5-5)
where Pv and Pprod are the partial pressure of water vapor and the total pressure of the
combustion products, respectively. N v N prod is the mole number ratio of water vapor
and products (or volume fraction of water vapor in flue gas).
Example 5-3
Solution:
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Determine the dew point at a total pressure of 14.7 psia for the combustion products in
Equation (5-2), assuming that the excess air is dry and the fuel contains no water.
2moles
⋅ 14.7 psi = 2.37 psi
(1 + 0.4 + 2 + 1.2 ⋅ 2 ⋅ 3.76)moles
Using the saturated steam table, Table 2, Appendix A,
Using Equation (5-5), Pv =
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Dew point = T@2.37psi = 131° F
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5.4 Efficiency
The efficiency of fuel-burning boilers is defined in three ways: combustion efficiency,
overall (boiler) efficiency, and seasonal efficiency.
Combustion efficiency is the higher heating value of the fuel minus stack (flue gas outlet) loss, divided by the higher heating value of fuel, and generally ranges from 75 to
86% for most noncondensing boilers. Condensing boilers generally operate in the range
of 88 to 95% combustion efficiency. A stoichiometric amount of air is required to completely react with a given quantity of fuel. In practice, combustion conditions are never
ideal, and additional or excess air must be supplied to completely burn the fuel. Approximate combustion efficiency can be determined under any operating condition by measuring flue-gas temperature and percentage of CO2 or O2 and consulting a chart or table
for the fuel being used (see Table 5-3 and Figure 5.5). Inadequate excess air results in unburned combustibles (fuel, soot, smoke, and carbon monoxide) while too much results
in heat lost due to the increases flue gas flow, thus lowering the combustion efficiency.
5.4 Efficiency
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5:14 · Fundamentals of Steam System Design
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Overall efficiency (fuel-to-steam efficiency, or boiler efficiency) is gross energy output
divided by energy input. Gross output is measured in the steam leaving the boiler and
depends on the characteristics of the individual installation. Overall efficiency of electric
boilers is generally 92 to 96%. Overall efficiency is lower than combustion efficiency by
the percentage of heat loss from the outside surface of the boiler (radiation loss). Overall
efficiency can be determined only under controlled laboratory test conditions.
5.5 Cost of Producing Steam
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Seasonal efficiency is the actual operating efficiency that the boiler will achieve during
the heating season at various loads. Because most heating boilers operate at part load,
the part-load efficiency, including heat losses when the boiler is off, has a great effect on
the seasonal efficiency. The difference in seasonal efficiency between a boiler with an
on-off firing rate and one with modulating firing rate can be appreciable if the airflow
through the boiler is modulated along with the fuel input to maintain a constant excess
air level
Example 5-4:
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The cost of steam generation is generally dependent upon fuel type, unit fuel cost, boiler
efficiency, feed water temperature, and steam pressure. In other words, the cost depends
on combustion efficiency and energy required to produce steam. The energy varies with
different operating pressures and feed water temperatures. Table 5-4 shows the heat input required to produce one pound of saturated steam at different operating pressures
and varying feed water temperatures.
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A boiler, fired with natural gas costing $4/MMBtu, produces 15,000 lbm/hr of 15 psig
steam (rated full load) and is supplied with 150°F feed water. The boiler operates at 28%
excess air with a flue gas less combustion air temperature of 400°F. If the boiler operates
at the rated full load for 5000 hours per year, calculate the cost of producing steam
(steam cost) per 1000 lbm of steam and also estimate the annual fuel cost for the boiler
operation.
Solution:
Energy required to produce one pound of steam with 15 psig and 150°F feed water is
found to be 1046 Btu/lbm from Table 5-4.
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Combustion efficiency with 28% excess air and flue gas less combustion air temperature
of 400°F is obtained to be 79.5% from Table 5-3.
Steam cost:
1046 Btu/lbm · 100/79.5 · $4/1,000,000 Btu = $5.26/1000 lbm
Annual fuel cost:
15,000 lbm/hr · $5.26/1000 lbm · 5000 h = $394,500
5.5 Cost of Producing Steam
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 5: Boilers· 5:15
Tips: The steam value, as a first cost, can be calculated in terms of energy (Btu):
$4/MMBtu · 100/79.5 = $5/MMBtu.
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In fact, boilers seldom operate at the rated full load over a year.
Operating
Pressure, psig
15*
150
450
600
50
1,145
1,178
1,187
1,184
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Table 5-4 Energy Required to Produce One Pound of Saturated Steama (Btu/lbm)
Feed Water Temperature, °F
100
150
200
1,096
1,046
995
1,128
1,078
1,028
1,137
1,087
1,037
1,134
1,084
1,034
250
945
977
986
984
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a. Department of Energy, Improving Steam System Performance: A Sourcebook for Industry, Appendix B
Steam Tip Sheet Number 15
* Calculated from steam tables on the difference between the enthalpies of saturated steam and feed water, the
table based on Reference 8.
5.6 Boiler Sizing
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Boiler sizing is the selection of boiler output capacity to meet connected load. The boiler
gross output is the rate of heat delivered by the boiler to the system under continuous
firing at rated input. Net rating (net IBR** rating) is gross output minus a fixed percentage (called the piping and pickup factor) to allow for an estimated average piping heat
loss, plus an added load for initially heating up the water in a system (sometimes called
pickup). This IBR piping and pickup factor ranges from 1.27 to 1.33 for steam boilers,
with smaller number applying as the boilers get larger. The net rating is calculated by dividing the gross output by the appropriate piping and pickup factor.
Piping loss is variable. If all piping is in the space defined as load, loss is zero. If piping
runs through unheated spaces, heat loss from the piping may be much higher than accounted for by the fixed net rating factor. Pickup loss is also variable. When the actual
connected load is less than design load, the pickup factor may be unnecessary.
On the coldest day, extra output (boiler and radiation) is needed to pick up the load
from a shutdown or low night setback. If night setback is not used, or if no extended
shutdown occurs, no pickup load exists. Standby capacity for pickup, if needed, can be
in the form of excess capacity in baseload boilers or in a standby boiler.
** Historically, the Institute of Boiler and Radiator Manufacturers, now part of the Hydronics Institute
of the Gas Appliance Manufacturers Association. Boiler ratings are still published as part of an IBR
list.
5.6 Boiler Sizing
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5:16 · Fundamentals of Steam System Design
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If piping and pickup losses are negligible, the boiler gross output can be considered the
design load. If piping loss and pickup load are large or variable, those loads should be
calculated and equivalent gross boiler capacity added. Boiler capacity must be matched
to the terminal units and system delivery capacity. That is, if the boiler output is greater
than the terminal output, the boiler will cycle on the high-limit control, delivering an average output that is much lower than the boiler gross output. Simply adding all of the
peak heating unit capacities of all the zones in a building can result in an oversized boiler since the zones do not all require peak heating simultaneously. Significant oversizing
of the boiler may result in much lower overall boiler efficiency.
One method of avoiding the poor efficiency due to oversizing of boiler would be to use
two (or more) smaller boilers, the combined capacity of which would total the needed
load. Properly chosen, the smaller boilers would have operated more nearly at full load
more of time resulting in higher seasonal efficiency. However, smaller boilers cost more
than one boiler with the capacity equal to the total of the smaller boilers. Multiple boiler systems also offer standby security and maintenance without entire shutdown; if one
boiler should fail or shutdown for maintenance, the other could carry at least part of the
load.
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Boilers are often sized by their sea-level input fuel ratings. If a gas boiler is not to be located at sea level, the effect of altitude must be accounted for in the rating. Some boiler
designs use a forced draft burner to force additional combustion air into the firebox to
offset part of the effect of altitude. Also, enriched or pressurized gas may be provided at
high altitude so that the heating value per unit volume is the same as at sea level. If no
accommodation to altitude is made, the output of a gas boiler drops by approximately
4% per 1000 ft of altitude above sea level. For example, a gas boiler located in Denver,
Colorado (5000 ft) will have a capacity of only 80% of its sea level rating.
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Boiler selection should be based on a competent review of the following parameters:
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All Boilers
5.6 Boiler Sizing
•
•
•
ASME Code Section, under which the boiler is constructed and tested
Net boiler output capacity, Btu/hr
Steam quality
•
•
Total heat transfer surface area, ft2
Water content of the boiler, lbm
•
•
•
•
•
•
•
Auxiliary power requirements
Internal water-flow patterns
Cleaning and service access provisions for fireside and waterside heat
transfer surfaces
Part-load and full-load efficiency
Space requirements and piping arrangement
Water treatment requirements
Regulatory requirements for emissions, fuel usage/storage
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Chapter 5: Boilers· 5:17
Combustion space (furnace volume)
Internal flow patterns of combustion products
Combustion air and venting requirements
Fuel availability/capability
Electric Boilers
•
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Fuel-Fired Boilers
Electric boilers are a separate class of boiler. Because no combustion occurs,
no boiler heating surface and no flue gas venting are necessary. The heating
surface is the surface of the electric elements or electrodes immersed in the
boiler water. The design of electric boiler is largely determined by the shape
and heat release rate of the electric elements used.
Example 5-5
Solution:
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Estimate the net output of a commercial boiler, which has a gross output of
80,000 Btu/hr.
For small boilers, a piping and pickup factor of 1.33 is applied.
Estimated net rating: 80,000 Btu/hr /1.33 = 60,150 Btu/hr ≈ 60,000 Btu/hr
Example 5-6
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A gas boiler, with an 80% boiler efficiency at the rated peak load, is used to supply
space heat to a building. The boiler energy analysis performed over a year shows that
62,000 MMBtu of natural gas were used for the year to meet the annual heating load of
31,000 MMBtu, which indicates that the overall annual boiler efficiency of 50% resulted from the partial loads or burner on-off operations over the year. If the single boiler is
replaced with two smaller boilers that meet the exact heating load of the building and
the overall annual boiler efficiency is improved to 75%, what would be the annual cost
saving with the replacement? Assuming that natural gas costs $4/MMBtu.
Solution:
Knowing that boiler efficiency = energy output/energy input = steam energy/fuel energy, we seek the solution with no change in the annual heating load of 31,000 MMBtu.
Annual cost saving :
(ann. fuel consum. at 50% comb. eff. – ann. fuel consum. at 75% comb. eff.) · fuel cost
Fuel consumption at 50% boiler efficiency = 62,000 MMBtu
Fuel consumption at 75% boiler efficiency:
62,000 MMBtu · 0.5/0.75 = 41,333 MMBtu, or 31,000 MMBtu/0.75 = 41,333 MMBtu
5.6 Boiler Sizing
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5:18 · Fundamentals of Steam System Design
then,
Annual cost saving = (62,000MMBtu − 41,333MMBtu) · $4/MMBtu = $82,667
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Summary
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In this chapter, we have discussed the types of boilers, boiler efficiencies, fuels and combustion, and selection and sizing of boilers mainly for steam heating applications.
After studying Chapter 5, you should be able to:
Classify the boilers in terms of boiler pressure, material, and draft type.
Describe the condensing and noncondensing boilers.
Classify the hydrocarbon fuels commonly used in boilers.
Describe the high and low heating values of the fuels.
Understand the chemical reactions for the combustions of fuel with air.
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Calculate the air/fuel ratio for the reactions.
Describe the flue gas analysis (Orsat gas analyzer) for the combustion products.
Calculate the dew points of the combustion products.
Calculate the excess air from the combustion equations.
Explain the combustion, boiler, and seasonal efficiencies.
Select and size the boilers.
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Calculate the steam value or the fuel cost for a boiler.
5.6 Boiler Sizing
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 5: Boilers· 5:19
Bibliography
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ASHRAE. 2000 ASHRAE Handbook - Systems and Equipment. Atlanta, GA. (Particularly
chapters 18 and 35.)
ASME. 1995. ASME Boiler and Pressure Vessel Code. American Society of Mechanical
Engineers., New York.
Clifford, G. 1990. Modern Heating, Ventilating & Air Conditioning. Prentice Hall.
Çengel, Y. A. and M. A. Boles. 2002. Thermodynamics: An Engineering Approach, 4th Ed.
McGraw Hill.
Department of Energy. Improving Steam System Performance, a Sourcebook for Industry.
Office of Industrial Technology, Energy Efficiency and Renewable Energy, U.S.
Department of Energy. (available at http://www1.eere.energy.gov/industry/
bestpractices/pdfs/steamsourcebook.pdf)
Howell, R., H. Sauer, and W. Coad. 2005. Principles of Heating, Ventilating and Air
Conditioning. ASHRAE, Atlanta, GA.
Kreider, Jan F., Handbook of Heating, Ventilation, and Air Conditioning. CRC Press, Boca
Raton, FL.
Bibliography
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
5:20 · Fundamentals of Steam System Design
Skill Development Exercises for Chapter 5
Complete these questions by writing your answers on the worksheets at the back of this book.
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5-1. What are the working pressure ranges for both the low-pressure and highpressure boilers?
5-2. Calculate the gross outputs at 212° F both in Btu/hr and BHP for a boiler
producing 16,000 lbm steam/hr of 15 psig?
5-3. What is the unique feature of the cast-iron boilers compared to other boilers
such as steel boilers, copper boilers, etc.?
5-4. Describe the difference between fire-tube boilers and water-tube boilers.
5-5. Describe why a higher efficiency can be achieved with condensing boilers.
5-6. What types of boilers (material) are adequate for the condensing boilers?
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5-7. Describe the difference between the high heating value and the low heating
value of a fuel.
5-8. List three gaseous fuels with their heating values.
5-9. The combustion equation for propane gas is shown. Determine the air/fuel
ratio in mass.
C3H8 + (1.5)(5)(O2 + 3.76N2) → 3CO2 + 2.5O2 + 4H2O + (1.5)(5)(3.76)N2
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5-10. The volumetric analysis of flue gas from combustion of propane in a gas
boiler is measured to be 8.9% carbon dioxide, 7.4% oxygen, 83.7%
nitrogen, 0% carbon monoxide. Find the amount of excess air and estimate
the combustion efficiency with a flue gas less combustion air temperature of
500°F. Is the excess air within the recommended range suggested?
5-11. Determine the dew point at atmospheric pressure for the combustion
products in Problem 6-10, assuming that the excess air is dry and the fuel
contains no water.
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5-12. Determine the combustion efficiency for the boiler operating at 45%
excess air with a flue gas less combustion air temperature of 500° F.
5-13. A boiler, fired with natural gas costing $3.5/MMBtu, produces 34,000
lbm/hr of 150 psig steam (rated full load) and is supplied with 200° F feed
water. If the boiler operates at the rated full load for 8000 hours per year and
the combustion efficiency is 75.2%, calculate the steam value per MMBtu
and also estimate the annual fuel cost for the boiler operation.
Skill Development Exercises for Chapter 5
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Chapter 5: Boilers· 5:21
5-14. Estimate the net output of a commercial boiler, which has a gross output
of 50,000 lbm steam/hr. (hint: large boiler)
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5-15. A boiler operates at 45% excess air with a flue gas less combustion air
temperature of 400°F and consumes 200,000 MMBtu of natural gas per
year. Tuning the boiler reduces the excess air to 9.5% with a flue gas less
combustion air temperature of 300° F. Estimate the annual cost saving of
the boiler, assuming that natural gas costs $4/MMBtu.
Skill Development Exercises for Chapter 5
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© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 6.
Contents of Chapter 6
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Steam Valves, Steam
Traps, Flash Tanks, and
Condensate Receiver
Tanks
Introduction
Study Objectives of Chapter 6
6.1 Fundamentals of Valves
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6.2 Manual Valves
6.3 Self-Contained Valves
6.4 Safety Devices
6.5 Steam Traps
6.6 Flash Steam
6.7 Condensate Receiver Tanks
Summary
Bibliography
Skill Development Exercises for Chapter 6
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•
•
•
•
•
•
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6:2 · Fundamentals of Steam System Design
Introduction
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Study Objectives of Chapter 6
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Read the material in Chapter 6. Re-read the parts of the chapter emphasized in the summary. At the end of chapter, complete the skill development exercises without consulting the text. Re-read parts of the text as needed to complete the exercises.
In this chapter, we will discuss the design, selection, and sizing of system components
used in steam systems.
•
•
•
•
•
After studying Chapter 6 and working the study problems, you should be
able to:
Describe the common manual and automatic control valves in steam
systems.
Understand the installation and operation of safety relief valves in steam
systems
Select and size steam traps.
Understand flash steam and size the flash tanks.
Size condensate receiver tanks.
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•
6.1 Fundamentals of Valves
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The valve is one of the most basic and indispensable components of a steam system. By
definition, a valve is a device that controls the flow of a fluid or vapor. In a broad sense,
valves are fluid-controlling elements in a piping system that can be manually or automatically-actuated. They are constructed to withstand a specific range of temperature, pressure, corrosion, and mechanical stress. It is the designer’s responsibility to select and
specify the proper valve for each application to give the best service for the economic requirements.
Valves have some of the following primary functions:
Starting, stopping, and directing flow
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Regulating, controlling, or throttling flow
Preventing backflow
Relieving or regulating pressure
The following service conditions should be considered before specifying or selecting a
valve:
1. Type of fluid: liquid, vapor, or gas
-Is it a pure fluid or does it contain solids?
6.1 Fundamentals of Valves
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 6: Valves, Tanks & Traps· 6:3
-Does it remain a liquid throughout its flow or does it vaporize?
2. Pressure and temperature
-Will these vary in the system?
3. Flow considerations
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-Should worst case (maximum or minimum values e.g. pressure, temperature,
flow) be considered in selecting correct valve materials?
-Is the valve to be used for simple shutoff or for throttling flow?
-Is the valve needed to prevent backflow?
-Is the valve to be used for directing flow?
4. Frequency of operation
-Will the valve be operated frequently?
-Will operation be manual or automatic?
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6.2 Manual Valves
Each valve style has advantages and disadvantages for a given application. In some cases,
the design documents provide inadequate information, so that selection is based on economics and local stock availability by the installer and not on what is really optimal
from a design viewpoint. Good submittal practice and approval by the designer are required to prevent unwanted and / or undesirable substitutions. The questions listed in
Section 6.1 must be evaluated carefully.
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Globe Valves
Figure 6.1 Globe Valve
Source: ASHRAE, 2000 Handbook - Systems and Equipment. Chapter 42, Page 42.1
Globe valves are most frequently used in smaller diameter pipes but are available in sizes
up to 12 in. They are used for flow regulation by throttling. The principle types are the globe
6.2 Manual Valves
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6:4 · Fundamentals of Steam System Design
valve, the Y valve (45 degree), the angle valve (right angle), and the needle valve (fine
control).
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In a globe valve, flow is controlled by the position of a circular disk forced against or
withdrawn from an annular ring, or seat, that surrounds an opening through which
flow occurs (Figure 6.1). The direction of movement of the disk is parallel to the direction of the flow through the valve opening (or seat) and normal to the axis of the pipe in
which the valve is installed. Due to this change in flow direction and the annular opening, there is a significant pressure loss across the valve, even in the fully open position.
Gate Valves
Gate valves are intended to be fully open or completely closed. They are designed to permit flow or stop flow and should not be used to regulate or control flow. Valves in inaccessible locations may be provided with a chain wheel or extension spindle.
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A gate valve blocks flow by means of a wedge disk fitting against machined seating faces
(Figure 6.2). The straight-through opening of the valve is as large as the full bore of the
pipe, and the gate movement is perpendicular to the flow path. Various types wedges
are available for specific applications. The pressure loss across a gate valve is minor in
the fully open position.
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Figure 6.2 Gate Valve
Source: ASHRAE, 2000 Handbook - Systems and Equipment, Chapter 42, Page 42.1
Ball Valves
Ball valves are used to control flow. By their design, the valve moves from the fully
closed position to the fully open position in a ¼ turn of the valve stem. A ball valve
contains a precision ball held between two circular seals or seats (Figure 6.3). Ball valves
6.2 Manual Valves
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 6: Valves, Tanks & Traps· 6:5
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have various port sizes. The design of the ball valve is such that the diameter of the opening in the ball is nearly identical to the inside pipe diameter. In the fully open position, the flow is straight through the valve with no change in direction (as in a globe
valve) and hence there the pressure drop across the valve is negligible.
Plug Valves
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A plug valve is a manual fluid flow control device (Figure 6.4). Like the ball valve, it operates from fully open to complete shutoff with a 90° turn.
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The cutaway view of a plug valve shows a valve with an orifice that is considerably smaller than the full size of the pipe. Lubricated plug valves are usually furnished in gas applications. A plug valve is selected as an on/off control device because (1) it is relatively inexpensive; (2) when adjusted, it holds its position; and (3) its position is clearly visible to
the operator. The pressure drop across a plug valves is dependent upon the ratio of the
area of the orifice to the area of the pipe in which it is installed. The effectiveness of this
valve as a flow control device is reduced if the orifice of the valve is fully ported.
Figure 6.3 Ball Valve
Figure 6.4 Plug Valve
Source: ASHRAE, 2000 Handbook - Systems and Equipment, pg 42.4
Check Valves
Check valves prevent reversal of flow, controlling the direction of flow rather than stopping or starting flow. Some basic types include swing check and lift check. Most check
valves are available in screwed and flanged body styles.
Swing Check Valves have hinge-mounted disks that open and close with flow (Figure
6.5). It is most suited to low velocities with infrequent flow changes. It is commonly
used in pipelines containing gravity flow or pumped liquids.
Composition discs can be used to prevent noise nuisance or where positive low pressure shutoff is required. The valve can also be equipped with an outside lever and
weight for faster closure or to keep the valve open until a predetermined reverse pressure is reached.
6.2 Manual Valves
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6:6 · Fundamentals of Steam System Design
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Lift Check Valves depend upon upward flow pressure raising a disc (Figure 6.6). When
flow ceases or reverses, the disc is forced back on its seat by backpressure and gravity.
Figure 6.5 Swing Check Valves*
Figure 6.6 Lift Check Valve**
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* ASHRAE, 2000 Handbook - Systems and Equipment, pg 42.12
** McCauley, James F., 2000, Steam Distribution Systems Deskbook, Fairmont Press, pg 162.
6.3 Self-Contained Valves
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These valves do not require an external power signal. They include pressure regulating
and pressure-reducing valves. These valves are operated by the steam or condensate in
the piping system. Temperature regulating valves convert a change in temperature into a
pressure signal by the expansion or contraction of a heat sensitive fluid contained in a
sensor.
Pressure-Reducing Valves (PRV)
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Where steam is supplied at pressures higher than required, one or more pressure-reducing valves may be used to reduce this pressure. The pressure-reducing valve reduces pressure to a safe point and regulates pressure to that required by end-use equipment. These
valves may be direct acting or pilot operated (Figure 6.7). To maintain a set pressure, the
6.3 Self-Contained Valves
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 6: Valves, Tanks & Traps· 6:7
downstream pressure must be sensed either through an internal sensing port or an external line.
Figure 6.7 Pressure-Reducing Valves ,
Cap
Pilot Valve
(a) direct-acting, (b) pilot-operated
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Bellows
Spring
Piston Ring
Stem
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Strainer
Sensing Port
Valve & Seat
Valve & Seat
Strainer
Sensing Port
Ports
(a)
(b)
Source: Armstrong International, Inc., 2000,
Solution Source for Steam, Air and Water
Systems, Armstrong International, Inc., Three
Rivers, Michigan, PTC-10 and PTC-21.
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As shown in Figure 6-7a, the downstream pressure is sensed directly by the bellows or diaphragm that controls the movement of the valve stem. The construction of the direct
acting PRV is such that it is designed primarily for low to moderate flows and an accuracy of about 10%. The pilot operated PRV, shown in Figure 6-7b is basically two valves
stacked on top of each other. The top, or pilot, valve is similar to the direct acting PRV
in that the downstream pressure is sensed by the bellows or diaphragm of the pilot valve.
These bellows then move the stem of the main valve. This design permits a greater flow
capacity for a given line size and improved pressure control accuracy.
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The amount of pressure drop below the set pressure that causes the valve to react to a
load change is called droop. As a general rule, pilot-operated valves have less droop than
direct-acting types. Pilot-operated valves are capable of larger capacity and greater accuracy than direct-acting valves. The installation of a typical pilot operated pressure-reducing
valve is shown in Figure 6-8. Direct-acting valves can be used for reduced pressure up to
50 psig, providing they can pass the required steam flow without excessive deviation in
reduced pressure. To properly size these valves, only the mass flow of steam, the inlet
pressure, and the required outlet pressure must be known. Valve line size can be determined by consulting manufacturers’ capacity charts.
Due to their construction, simplicity, accuracy, and ease of installation and maintenance, these valves have been specified for most steam-reducing stations.
Installation
Pressure-reducing valves should be readily accessible for inspection and repair. There
should be a bypass around each reducing valve equal to the area of the reducing-valve
seat ring. Steam pressure gages, graduated up to the initial pressure, should be installed
on the low-pressure side and the high-pressure side. The low-pressure gage should be
ahead of the shutoff valve because the reducing valve can be adjusted with the shutoff
6.3 Self-Contained Valves
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6:8 · Fundamentals of Steam System Design
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valve closed. Strainers should be installed on the inlet of the pressure-reducing valve. A
typical service connection is shown in Figure 6.8.
Figure 6.8 Typical Pressure-reducing Valve Connection
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Source: Armstrong International, Inc., 2000, Solution Source for Steam, Air and Water Systems, Three
Rivers, Michigan, PTC-10 and PTC-21.
When the pressure turndown ratio is greater than that of a single valve, a two-stage reduction can be made by arranging two pressure-reducing valves in series (Figure 6.9) and
it is advisable to install a pressure gage immediately before the reducing valve of the second-stage reduction to set and check the operation of the first valve. A drip trap should
be installed between the two reducing valve.
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Where pressure-reducing valves are used, one or more relief devices or safety valves must
be provided, unless the equipment on the low-pressure side meets the requirements for
the full initial pressure. The relief or safety devices are adjoining or as close as possible to
the reducing valve. The combined relieving capacity must be adequate to avoid exceeding the design pressure of the low-pressure system if the reducing valve does not function properly.
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Safety valves should be set at least 5 psi higher than the reduced pressure if the reduced
pressure is under 35 psig and at least 10 psi higher than the reduced pressure if the reduced pressure is above 35 psig or the first-stage reduction of a double-reduction. The
outlet from relief valves should not be piped to a location where the discharge can jeopardize persons or property or is a violation of local codes.
When making a two-stage reduction (e.g., 150 to 50 psig and 50 to 2 psig), allow for the
expansion of steam on the low-pressure side of each reducing valve by increasing the
pipe area to about double the area of a pipe the size of the reducing valve. This also allows steam to flow at a more uniform velocity. It is recommended that the valves be separated by a distance of a minimum of twenty outlet-pipe diameter to reduce excessive
6.3 Self-Contained Valves
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Chapter 6: Valves, Tanks & Traps· 6:9
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hunting action of the first valve, although this should be verified with information obtained from the valve manufacturer.
Figure 6.9 Two-Stage Pressure-
Reducing Valve
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Source: Armstrong International,
Inc., PTC-10 and PTC-21.
Valve Size Selection
Pressure-reducing valves should be sized to supply the maximum steam requirements of
the heating system or equipment. Consideration should be given to rangeability, speed
of load changes, and reducing accuracy required to meet system needs, especially with
temperature control systems using intermittent steam flow to heat building.
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The reducing valve should be selected carefully. Piping to and from the reducing valve
should be adequate to pass the desired amount of steam at the desired maximum velocity. A common error is to make the size of the reducing valve the same as the service or outlet pipe
size; this makes the reducing valve oversized and causes wiredrawing or erosion of the valve and
seat because of the high-velocity flow caused by the small lift of the valve.
The pressure reducing valve size should be selected by calculating the valve flow coefficient (Cv) to provide the design steam flow at a pressure drop ∆p across the valve.
When p2 > 0.5 p1 ,
ws = 2.1
Cv
K
∆p( p1 + p2 )
(6-1)
where
ws = steam flow, lb/h
K = 1+0.0007 x (°F of superheat)
6.3 Self-Contained Valves
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6:10 · Fundamentals of Steam System Design
Cv = flow coefficient, gpm at ∆p =1 psi
p1 = entering steam absolute pressure
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∆p = steam pressure drop across valve, p1 - p2
t
p2 = leaving steam absolute pressure
When the downstream pressure is 58% or less of the absolute inlet pressure, steam
reaches critical or sonic velocity so that increasing the pressure drop produces no further
increase in flow.
When p2 ≤ 0.58 p1 , the following critical pressure drop formula is used:
ws = 1.61C v p1
(6-2)
pE
If a pressure-reducing valve with a flow coefficient of 2.5, reducing the saturated steam
from 15 psia to 9 psia, is recommended by the valve manufacturer, the steam flow rate
can be estimated as:(Since 9 > 0.5 · (15) = 7.5, using Equation (6-1))
ws = 2.1 ×
2.5
× (15 − 9) × (15 + 9) = 63 lb/h
1
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On installations where the steam requirements are large and variable, wiredrawing and
cycling can occur during low load conditions e.g. mild weather. To overcome this condition, two reducing valves are installed in parallel, with sizes selected on a 70 and 30%
proportion of maximum flow. During mild weather, the larger valve is set for 2-3 psi
lower reduced pressure than the smaller one and remains closed as long as the smaller
one can meet the demand. During the remainder of the heating season, the valve setting
may be reversed to keep the smallest one closed, except when the larger one is unable to
meet the demand.
Temperature Control Valves (Temperature Regulators)
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Self-contained temperature control valves do not require an outside energy source such
as compressed air or electricity (Figure 6.10) to make them functional. They depend on
a temperature-sensing bulb and capillary tube filled with either an oil or a volatile liquid.
In an oil-filled system, the oil expands as the sensing bulb is heated. This expansion is
transmitted through the capillary tube to an actuator bellows in the valve top, which
6.3 Self-Contained Valves
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 6: Valves, Tanks & Traps· 6:11
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causes the valve to close. The valve opens as the sensing bulb cools and the oil contracts;
a spring provides a return force on the valve stem.
Figure 6.10 Self-Contained Temperature
Control Valve
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Source: ASHRAE, 2000 Handbook - Systems and
Equipment, Page 10.11
A volatile liquid control system is known as a vapor pressure or vapor tension system.
When the sensing bulb is warmed, some of the volatile liquid vaporizes, causing an increase in the sealed system pressure. The pressure rise is transmitted through the capillary tube to expand the bellows, which then moves the valve stem and closes the valve.
Thermal systems can actuate the control valve either directly or through a pilot valve.
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6.4 Safety Devices
The terms safety valve, relief valve, and safety relief valve are sometimes used interchangeably, and although the devices generally provide a similar function (safety), they
have important differences in their modes of operation and application in HVAC systems.
Safety valves open rapidly (Pop-action). They are used for steam.
Relief valves open or close gradually in proportion to excessive pressure. They are used
for liquids.
Safety relief valves perform a dual function: they open rapidly for steam and gradually
for liquids. Typical application is for heating water.
Application of these safety devices must comply with building codes and the ASME
Boiler and Pressure Vessel Code. For the remainder of this discussion, the term “safety
valve” is used generically to include any or all of the three types described.
Safety valve construction, capacities, limitations, operation, and repair are covered by
the ASME Boiler and Pressure Vessel Code. For pressures above 15 psig, refer to Sec-
6.4 Safety Devices
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6:12 · Fundamentals of Steam System Design
tion I. Section IV covers steam boilers for pressures less than 15 psig. Unfired pressure
vessels (such as heat exchange process equipment or pressure-reducing valves) are covered by Section VIII.
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The capacity of a safety valve is affected by the equipment on which it is installed and
the applicable code. Valves are chosen based on accumulation, which is the pressure increase above the maximum allowable working pressure of the vessel during valve discharge. Section I valves are based on 3% accumulation. Accumulation may be as high as
33.3% for Section IV valves and 10% for Section VIII. To properly size a safety valve,
the required capacity and set pressure must be known. On a pressure-reducing valve station, the safety valve must have sufficient capacity to prevent an unsafe pressure rise if
the reducing valve fails in the open position.
The safety valve set pressure should be high enough to allow the valve to remain closed
during normal operation, yet allow it to open and reseat tightly when cycling. A minimum differential of 5 psi or 10% of inlet pressure (whichever is greater) is recommended. When installing a safety valve, consider the following:
pE
Install the valve vertically with the drain holes open or piped to drain.
The seat can be distorted if the valve is overtight or the weight of the discharge piping is
carried by the valve body. A drip-pan elbow on the discharge of the safety valve will prevent the weight of the discharge piping from resting on the valve (Figure 6.11).
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Use a moderate amount of pipe thread lubricant (first 2 to 3 threads) on male threads
only.
6.4 Safety Devices
Figure 6.11 Safety/Relief Valve
Source: Armstrong International, PTC39.
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 6: Valves, Tanks & Traps· 6:13
Install clean flange connections with new gaskets, properly aligned and parallel, and
bolted with even torque to prevent distortion.
t
Wire cable or chain pulls attached to the test levers should allow for a vertical pull and
their weight should not be carried by the valve.
6.5 Steam Traps
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Testing of safety valves varies between facilities depending on operating conditions. Under normal conditions, safety valves with a working pressure under 400 psig should be
tested manually once per month and pressure-tested once each year. For higher pressures, the test frequency should be based on operating experience. When steam safety
valves require repair, adjustment, or set pressure change, the manufacturer or approved
stations holding the ASME V, UV, and/or VR stamps must perform the work. Only the
manufacturer is allowed to repair Section IV valves.
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Steam traps are an essential part of all steam systems, except one-pipe systems. Traps discharge condensate, which forms as steam gives up some of its heat, and direct the air
and non-condensable gases to a point of removal. Condensate forms in steam mains
and distribution piping because of unavoidable heat losses through less-than-perfect insulation, as well as in terminal equipment such as radiators, convectors fan-coil units,
and heat exchangers, where steam gives up heat during normal operation. Condensate
must always be removed from the system as it accumulates for the following reasons:
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Although condensate contains some valuable heat, the condensate accumulated in the
terminal equipment reduces the rate of heat transfer at the surfaces. It also causes other
operating problems because it retains air and non-condensable gases such as CO 2 ,
which further reduces heat transfer and causes corrosion.
Steam moves rapidly in piping, so when condensate accumulates to the point where the
steam can push a slug of it, serious damage can occur from resulting hydraulic shock or
hammer.
Ideally, the steam trap should remove all condensate promptly, along with air and noncondensable gases that might be in the system, with little or no loss of live steam. A
steam trap is an automatic valve that can distinguish between steam and condensate or
other fluids. Traps are classified as follows:
Mechanical traps rely on the difference in the densities of steam and condensate.
Thermostatic traps react to the difference in the temperatures of steam and condensate
to manage the separation and removal of the condensate.
Kinetic traps rely on the difference in the flow characteristics of steam and condensate.
The following points apply to all steam traps:
6.5 Steam Traps
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6:14 · Fundamentals of Steam System Design
No single type of steam trap is best suited to all applications, and most systems require
more than one type of trap.
Mechanical Traps
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Steam traps, regardless of type, should be carefully sized for the application and condensate load expected, because both undersizing and oversizing can cause serious problems.
Undersizing can result in undesirable condensate backup and excessive cycling, which
can lead to premature trap failure. Oversizing might appear to solve this problem and
make selection much easier because fewer different sizes are required, but if the trap
fails, excessive steam can be lost leading to an inefficient system.
Mechanical traps are buoyancy operated, depending on the difference in density between steam and condensate.
Float and Thermostatic Traps
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The float and thermostatic trap is commonly called the F&T trap and is actually a combination of two types of traps in a single trap body (Figure 6.12 (a)): (1) a float portion,
which is buoyancy operated, and (2) a bellows thermostatic element operating on temperature difference, which provides automatic venting. Float traps without automatic
venting should not be used in steam systems.
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Figure 6.12 Mechanical Traps, (a) Float and Thermostatic Trap and
(b) Inverted Bucket Trap
Source: ASHRAE, 2000 Handbook - Systems and Equipment, Pg 42.11
On start-up, the float valve is closed and the thermostatic element is open for rapid air
venting, permitting the system or equipment to rapidly fill with steam. When steam enters the trap body, the thermostatic element closes and, as condensate enters, the float
6.5 Steam Traps
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Chapter 6: Valves, Tanks & Traps· 6:15
rises and it continuously discharges the condensate at the rate at which it reaches the
trap.
Inverted Bucket Traps
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The F&T trap has large venting capabilities, continuously discharges condensate without backup, handles intermittent loads very well, and can operate at extremely low pressure differentials. Float and thermostatic traps are suited for use with temperature-regulated steam coils. They also are well suited for steam mains and riser drip legs on lowpressure steam heating systems. Although F&T traps are available for pressures to 250
psig or higher, they are susceptible to water hammer, so other traps are usually a better
choices for high-pressure applications.
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Inverted bucket traps eliminate the size and venting problems associated with open
bucket traps (Figure 6.12 (b)). Steam or air entering the submerged inverted bucket causes it to float and close the discharge port. As more condensate enters the trap, it forces
air and steam out of the vent on top of the inverted bucket into the trap body where the
steam condenses by cooling. When the mass of the bucket exceeds the buoyancy effect,
the bucket drops, opening the discharge port, and steam pressure forces the condensate
out, and the cycle repeats.
Unlike most cycling-type traps, the inverted bucket trap continuously vents air and noncondensable gases. Although it discharges condensate intermittently, there is no condensate backup in a properly sized trap. Inverted bucket traps are made for all pressure
ranges and well suited for steam main drip legs and most HVAC applications. Although
inverted bucket traps can be used for temperature-regulated steam coils, the F&T trap is
usually better because it has the high venting capability desirable for such applications.
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Thermostatic Traps
In thermostatic traps, a bellows or bimetallic element operates a valve that opens in the
presence of subcooled condensate and closes in the presence of steam. Because condensate is initially at the same temperature as the stream from which it was condensed, the
thermostatic element must be designed and calibrated to open at a temperature below
the steam temperature; otherwise, the trap would blow live steam continuously. Therefore, the condensate is subcooled by allowing it to back up in the trap and in a portion
of the upstream drip leg piping, both of which are left uninsulated. Some thermostatic
traps operate with a continuous water leg behind the trap so there is no steam loss; however, this prohibits the discharge of air and non-condensable gases, and can cause excessive condensate to back up into the mains or terminal equipment, thereby resulting in
operating problems. Devices that operate without significant backup can lose steam before the trap closes.
6.5 Steam Traps
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6:16 · Fundamentals of Steam System Design
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Although both bellows and bimetallic traps are temperature –sensitive, their operations
are significantly different.
Figure 6.13 Thermostatic Traps, (a) Bellows Type and (b) Bimetal Type
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Source: ASHRAE, 2000 Handbook - Systems and Equipment, pg 10.8
Bellows Thermostatic Traps
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The bellows thermostatic trap has a fluid with a lower boiling point than water. See Figure 6.13 (a). When the trap is cold, the element is contracted and the discharge port is
open. As hot condensate enters the trap, it causes the contents of the bellows to boil and
vaporize before the condensate temperature rises to steam temperature. Because the
contents of the bellows boil at a lower temperature than water, the vapor pressure inside
the bellows element is greater than the steam pressure outside, causing the element to
expand and close the discharge port.
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Assuming the contained liquid has a pressure-temperature relationship similar to that of
water, the balance of forces acting on the bellows element remains relatively constant,
no matter how the steam pressure varies. Therefore, this is a balanced pressure device
that can be used at any pressure within the operating range of the device. However, this
device should not be used where superheated steam is present, because the temperature
is no longer in step with the pressure and damage or rupture of bellows element can occur.
Bellows thermostatic traps are best suited for steady light loads on low-pressure service.
They are most widely used in radiators and convectors in HVAC applications.
Bimetallic Thermostatic Traps
The bimetallic thermostatic trap has an element made from metals with different expansion coefficients. See Figure 6.13 (b). Heat causes the element to change shape, permitting the valve port to open or close. Because a bimetallic element responds only to tem-
6.5 Steam Traps
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 6: Valves, Tanks & Traps· 6:17
perature, most traps have the valve on the outlet so that the steam pressure is trying to
open the valve. Therefore, by properly designing the bimetallic element, the trap can operate on a pressure-temperature curve approaching the steam saturation curve, although
not as closely as a balanced pressure bellows element.
Kinetic Traps
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Unlike the bellows thermostatic trap, bimetallic thermostatic traps are not adversely affected by superheat steam or subject to damage by water hammer, so they can be used
for high-pressure applications. They are best suited for steam tracers, jacketed piping,
and heat transfer equipment, where some condensate backup is tolerable. If they are
used on steam main drip legs, the element should not back up condensate.
Numerous devices operate on the difference between the flow characteristics of steam
and condensate and on the fact that condensate discharging to a lower pressure contains
more heat than necessary to maintain the liquid phase. This excess heat causes some of
the condensate to flash to steam at the lower pressure.
Thermodynamic Traps (Disc Traps)
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The thermodynamic trap is a simple device with one moving part (Figure 6.14 (a)).
When air or condensate enters the trap on start-up, it lifts the disc off its seat and is discharged. When steam or hot condensate (some of which flashes to steam upon exposure
to a lower pressure) enters the trap, the increased velocity of this vapor flow decreases
the pressure on the underside of the disc and increases the pressure above the disc, causing it to snap shut. Pressure is then equalized above and below the disc, but because the
area exposed to pressure is greater above than below it, the disc remains shut until the
pressure above is reduced by condensing or bleeding, thus permitting the disc to snap
open and repeat the cycle. This device does not cycle open and shut as a function of condensate load; it is a time cycle device that opens and shuts at fixed intervals as a function
of how fast the steam above the disc condenses. Because disc traps require a significant
pressure differential to operate properly, they are not well suited for low-pressure systems or for systems with significant back pressure. They are best suited for high-pressure
systems and are widely applied to steam main drip legs.
Orifice Traps
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Orifice traps have no moving parts (Figure 6.14 (b)). All other traps have discharge ports
or orifices, but in the traps described previously, this opening is oversized, and some
type of closing mechanism controls the flow of condensate and prevents the loss of live
steam.
6.5 Steam Traps
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6:18 · Fundamentals of Steam System Design
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The orifice trap has no such closing mechanism, and the flow of steam and condensate
is controlled by two-phase flow through an orifice.
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Figure 6.14 Thermodynamic Traps, (a) Disc Traps and (b) Orifice Traps
Source: McCauley, Page 227
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A simple explanation of this theory is that an orifice of any size has a much greater capacity for condensate than it does for steam because of the significant differences in
their densities and because “flashing” condensate tends to choke the orifice. An orifice
is selected larger than required for the actual condensate load; therefore, it continuously
passes all condensate along with the air and non-condensable gases, plus a small controlled amount of steam. Steam loss is usually comparable to that of most cycling-type
traps.
Orifice traps must be sized more carefully than cycling-type traps. On light condensate
loads, the orifice size is small and tends to clog. Orifice traps are suitable for steady pressure and load conditions such as steam main drip legs.
Steam Trap Selection and Sizing
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Proper trap sizing is the most important factor in obtaining efficient trap operation.
Even though the correct type of trap was selected, and the installation was perfect, incorrect trap sizing will cause either condensate backup or excessive steam loss. Trap sizing is
not based on the selection of piping size, but rather sizing of the internal discharge orifice. Basic considerations for trap selection and sizing are (1) condensate load, (2) safety
factor, (3) pressure differential, and (4) maximum allowable pressure.
Condensate Load. The condensate load is a flow rate or capacity of the terminal equipment at a point of trapping. It could be given as a mass flow rate (lbm/hr) or a heating
unit (MMBtu/hr) of steam or condensate.
6.5 Steam Traps
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 6: Valves, Tanks & Traps· 6:19
Safety Factor. A trap must handle more than the capacity of the terminal equipment for
best overall performance. The safety factor usually takes care of varying condensate rates,
occasional drops in pressure differential, and system design factors. The recommended
safety factors for various equipment are listed in Table 6-1.
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Pressure Differential. Pressure differential is the difference between the pressures before and after the trap. Maximum differential is the difference between boiler or steam
main pressure and return line pressure. Modulated control of the steam supply causes
wide changes in pressure differential.
Maximum Allowable Pressure. The trap must be able to withstand the maximum allowable pressure of the system or design pressure.
Example 6-1:
Size a steam trap for a 3,500-CFM fan heater that produces an 80°F air-temperature rise.
Steam pressure is constant at 60 psig. The specific heat of air is 0.24 Btu/lbm·°F and the
density of air is 0.075 lbm/ft3.
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Table 6-1 Recommended Steam Trap Selections and Safety Factors
Trap Selection
Inverted Bucket, F&T
Inverted Bucket, F&T,
Steam Mains & Branch Lines
Thermostatic
Steam Separator
Inverted Bucket
Tracer Lines
Inverted Bucket, Thermostatic
Unit Heaters & Air Handlers
Inverted Bucket, F&T
Inverted Bucket, F&T,
Finned Radiation & Pipe Coils
Thermostatic
Processor Air Heaters
Inverted Bucket, F&T
Shell and Tube Heat
Inverted Bucket, F&T
Exchangers
Flash Tank
Inverted Bucket, F&T
Safety Factor
1.5
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Application
Boiler Header
3
3
2
3
3
2-3
2
3
Source: ASHRAE, 2000 Handbook - Systems and Equipment, pg 10.8
Solution:
Knowing that: from Equation (4-1), q& = m& ⋅ c p ⋅ (t f − ti )
Heating load = [mass flow rate][specific heat of fluid][temperature rise]
Mass flow rate = [density][volume flow rate]
Heating load = [0.075 lbm/ft3][3,500 ft3/min · 60 min/1 hr][0.24 Btu/lbm·° F][80° F]
6.5 Steam Traps
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6:20 · Fundamentals of Steam System Design
Heating load = 302,400 Btu/hr
Using Equation (2-1), q& = m& ⋅ h fg
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Extrapolating, we find the latent heat ( h fg ) of 904.5 Btu/lbm at 60 psig from Table 2-1
(steam table).
Condensate (or steam) mass flow rate or capacity of the trap = (302,400 Btu/hr)/
(904.5 Btu/lbm) = 334.3 lbm/hr, which is the capacity of the fan heater.
From Table 6-1, we obtain the safety factor of 3 for unit heaters.
Trap discharge capacity = safety factor · capacity of terminal equipment or heating load
= 3 · 334.3 lbm/hr = 1002.9 lbm/hr
Answer: trap capacity = 1002.9 lbm/hr
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6.6 Flash Steam
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When hot condensate or boiler water, under a high pressure, is released to a lower pressure, part of it is re-evaporated, becoming what is known as flash steam. The heat content of flash steam is identical to that of live steam at the same pressure, although this
valuable heat is wasted when allowed to escape through the vent in the receiver. Use of
the flash steam for heating is an effective use for the enthalpy of the liquid condensate.
With proper sizing and installation of a flash tank, the latent heat content of flash steam
may be used for any heat exchange device to heat air, water, or other liquid or directly in
processes with lower pressure steam requirements. The amount of flash steam can be
calculated by the following equation:
% Flash Steam =
( h fH − h fL ) ⋅ 100
h fgL
where
h fH =Enthalpy of liquid at the higher pressure
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h fL =Enthalpy of liquid at the lower pressure
h fgL =latent heat of evaporation at the lower pressure
6.6 Flash Steam
(6-3)
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Chapter 6: Valves, Tanks & Traps· 6:21
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Figure 6.15 provides a graph for calculating the amount of flash steam as a function of
system pressure.
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Figure 6.15 Flash Steam
Armstrong International, Inc., Solution Source, CG-17-22
Although flash steam can be generated directly by discharging high-pressure condensate
to a lower pressure system, most designers prefer a flash tank to control flashing. Flash
tanks can be mounted either vertically or horizontally, but the vertical arrangement
shown in Figure 6.16 is preferred because it provides better separation of steam and water, resulting in the highest possible steam quality.
The most important dimension in the design of vertical flash tanks is the internal diameter, which must be large enough to ensure a low upward velocity of flash to minimize
water carryover. If this velocity is low enough, the height of the tank is not important,
but it is good practice to use a height of at least 2 to 3 ft. The graph in Figure 6.17 can be
used to determine the internal diameter of the tank and is based on a steam velocity of
10 ft/s, which is the maximum velocity in most systems.
Installation is important for proper flash tank operation. Condensate lines should pitch
towards the flash tank. If more than one condensate line discharges to the tank, each
line should be equipped with a swing check valve to prevent backflow. Condensate lines
6.6 Flash Steam
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6:22 · Fundamentals of Steam System Design
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and the flash tank should be well insulated to prevent any unnecessary heat loss. A thermostatic air vent should be installed at the top of the tank to vent any air that accumulates. The tank should be trapped at the bottom with an inverted bucket or float and thermostatic
trap sized to triple the condensate load.
Figure 6.16 Vertical Flash Tank
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Source: ASHRAE, 2000 Handbook - Systems and Equipment, pg 10.15
The demand load must always be greater than the amount of flash steam available to
prevent the low-pressure system from becoming over-pressurized. A safety relief valve
should always be installed at the top of the flash tank to preclude such a condition.
Because the flash steam available is generally less than the demand for low-pressure
steam, make-up steam ensures that the low-pressure system maintains design pressure.
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Flash tanks are considered pressure vessels and must be constructed in accordance with
ASME and local codes.
6.6 Flash Steam
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Chapter 6: Valves, Tanks & Traps· 6:23
Figure 6.17 Flash Tank Diameters
ASHRAE, 2000 Handbook - Systems and Equipment, pg 10.15
Example 6-2:
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Condensate at a steam pressure of 100 psig is discharged to atmospheric pressure (0
psig). What percentage will flash to steam?
Solution:
In Table 2-1, the condensate at 100 psig has a heat content of 309.1 Btu/lbm. If this is
discharged to the atmospheric pressure, the heat content drops to 180.2 Btu/lbm and
the surplus of 128 Btu/lb m re-evaporates or flashes a portion of the condensate to
steam. The latent heat of evaporation at 0 psig is 970.1 Btu/lbm. Using Equation (6-3),
the percentage that will flash to steam can be calculated:
% Flash Steam =
(309.1 − 180.2) ·100
970.1
Example 6-3:
Determine the condensate load of the flash tank with 5000 lbm/hr of 100 psig condensate entering the flash tank held at 10 psig.
Solution:
Using Table 2-1
Enthalpy of condensate at 100 psig = 309.1 Btu/lbm
6.6 Flash Steam
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6:24 · Fundamentals of Steam System Design
Enthalpy of condensate at 10 psig = 207.8 Btu/lbm
Latent heat of evaporation at 10 psig = 952 Btu/lbm
Condensate load = 5000 −
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% Flash steam = (309.1–207.8) ·100/952 = 10.6%
t
Using Equation (6-1),
5000 ⋅ 10.6
= 4,470 lbm/hr
100
Example 6-4:
Size the steam trap for the flash tank illustrated in Example 6-3.
Solution:
The safety factor for flash tanks that is recommended in the preceding paragraph is 3.
Therefore,
Example 6-5:
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Steam trap capacity = 3 · 4470 lbm/hr = 13,410 lbm/hr
Size the flash tank illustrated in Example 6-4.
Solution:
Flash steam = 5000 lbm/hr · 0.106 =530 lbm/hr
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Using Figure 6.17, the internal diameter of the flash tank is approximately 8 in. The
length of the tank would be at least 2 ft, as recommended earlier in this section.
6.7 Condensate Receiver Tanks
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Condensate receiver tanks are used to collect the condensate coming from steam users
or flash vessels. The schematic of a condensate return system is shown in Figures 2.1
and 2.9 in Chapter 2. The schematic of a typical condensate receiver tank is shown in
Figure 6.18. From the tank, the condensate may be pumped into a feedwater tank by a
level-controlled condensate pump, or directly into the boiler for a small steam system.
The tank material is typically a heavy walled or pressure vessel. . In some cases, the tank
is coated with a corrosion-resistance material. It is recommended that the tank be
stamped ASME, even if the tank is vented to atmosphere, to provide a more desirable
tank construction for industrial applications. It is always better to elevate the tank over
the condensate pump to provide a net positive suction head (NPSH) to the condensate
pump. The net positive suction head is defined as a suction pressure minus vapor pressure expressed in feet of liquid at the pump suction. The suction pressure or the elevation of the tank over the pump should be high enough to avoid cavitation in the pump
(consult with pump manufacturer for the details about the NPSH). Condensate receivers
that are used to collect condensate and transfer it to the boiler feed receiver should be sized to hold
6.7 Condensate Receiver Tanks
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 6: Valves, Tanks & Traps· 6:25
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1 – 2 minutes of condensate flow. Boiler feed receivers should be sized to hold 10 to 15 minutes of
condensate flow.
Figure 6.18 Condensate
Source: 2000 Handbook - Systems
and Equipment 10.15
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Receiver Tank
Example 6-6:
Size the condensate receiver tank with the condensate load of 4,000 lbm/hr.
Solution:
Storage tank size = 4000 lbm/hr · 1 hr/60 min. · 15 min = 1000 lbm
Convert the mass of condensate to volume in gallon using the density of 59.82 lbm/ft3
at 212° F as an approximation.
Knowing that a volume equal to the mass divided by the density,
Storage tank size (volume) = 1000 lbm / [density 59.82 lbm/ft3 · 0.13368 ft3/gallon] =
125 gallon, which can be conveniently obtained using a conversion factor of 480 as:
Storage tank size = [4000 lbm/hr / (480 lbm / hr )] · 15 min = 125 gallon
gpm
6.7 Condensate Receiver Tanks
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6:26 · Fundamentals of Steam System Design
Summary
After studying Chapter 6, you should be able to:
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Describe the common manual and automatic control valves in steam systems.
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Understand the installation and operation of safety relief valves in steam systems
Select the types of steam traps and size the steam traps in steam systems.
Understand flash steam and size the flash tanks.
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Size condensate receiver tanks.
6.7 Condensate Receiver Tanks
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 6: Valves, Tanks & Traps· 6:27
Bibliography
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Armstrong International, Inc. 2000. Solution Source for Steam, Air and Water Systems.
Armstrong International, Inc., Three Rivers, MI.
Armstrong International, Inc. http://www.armstrong-intl.com/products/valves/prv/
prv.php3. Accessed May 23, 2006
ASHRAE. 2000 ASHRAE Handbook - Systems and Equipment. Atlanta, GA.
ASHRAE, 2001 ASHRAE Handbook - Fundamentals. Atlanta, GA.
McCauley, James F. 2000. Steam Distribution Systems Deskbook. Fairmont Press, Lilburn,
GA.
PSE. 2003. Steam System Training. Plant Support & Evaluations Inc., New Berlin, WI.
Bibliography
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
6:28 · Fundamentals of Steam System Design
Skill Development Exercises for Chapter 6
Complete these questions by writing your answers on the worksheets at the back of this book.
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6-1. List the manual valves often used in steam systems each with a brief
explanation.
6-2. Describe the main difference in function between globe valve and gate
valve.
6-3. Describe a common error in the valve size selection of pressure-reducing
valves in steam distribution systems.
6-4. Size a steam trap for a 6,500-CFM fan heater that produces 90° F airtemperature rise. Steam pressure is constant at 40 psig.
6-5. Condensate at a steam pressure of 50 psig is discharged to a return-line
pressure of 5 psig. What percentage will flash to steam in the return line?
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6-6. Determine the condensate load of the flash tank with 2,000 lbm/hr of
50 psig condensate entering the flash tank held at 5 psig.
Skill Development Exercises for Chapter 6
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 7.
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Contents of Chapter 7
t
Steam Piping Design
Introduction
Study Objectives of Chapter 7
•
•
•
•
•
•
7.1 Pipe Sizing
7.2 Piping
7.3 Pipe Materials
7.4 Insulation
7.5 Pipe Expansion
7.6 Pipe Supporting Elements
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Summary
Bibliography
Skill Development Exercises for Chapter 7
References
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
7:2 · Fundamentals of Steam System Design
Introduction
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Study Objectives of Chapter 7
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Read the material in Chapter 7. Re-read the parts of the chapter emphasized in the
summary. At the end of chapter, complete the skill development exercises without
consulting the text. Re-read parts of the text as needed to complete the exercises.
In this chapter, we discuss the fundamentals of steam piping design such as pipe sizing,
piping, steam traps, pipe insulation, pipe expansions, and pipe supports.
After studying Chapter 7, you should be able to:
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7.1 Pipe Sizing
Size steam and condensate piping
Design piping; steam mains, boiler headers and drip legs and traps
Learn the common materials of steam and condensate pipes
Design the insulation of steam and condensate pipes
Calculate the thermal expansions of steam and condensate pipes
Learn the types of the pipe expansions and design the guidelines.
Determine the supports or hangers spacing
Determine the pipe guides for expansion joints and expansion loops
Calculate all forces imposed on anchors
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•
•
•
•
•
•
•
•
•
Proper sizing of piping for steam systems is important, both from economic and
engineering viewpoints. Obviously, piping that is larger than necessary is wasteful, but,
in piping that is too small, the pressure drop is so great that steam pressure at the
terminal units, or working end, falls below the required figure and the return
condensate will be insufficient.
Required pipe sizes for a given load in steam heating depend on the following factors:
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The initial pressure and the total pressure drop that can be allowed between the source
of supply and the end of the return system
The maximum velocity of steam allowable for quiet and dependable operation of the
system, taking into consideration the direction of condensate flow
The equivalent length of the run from the boiler or source of steam supply to the
furthest heating unit
7.1 Pipe Sizing
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Chapter 7: Steam Piping Design · 7:3
Initial Pressure and Pressure Drop.
Maximum Velocity.
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Table 7-1 lists pressure drops commonly used with corresponding initial steam pressures
for sizing steam piping. Several factors, such as initial pressure required at the end of the
line, should be considered, but it is most important that (1) the total pressure drop does
not exceed the initial gage pressure of the system (and in practice it should never exceed
one-half the initial gage pressure); (2) the pressure drop is not great enough to cause
excessive velocities; (3) a constant initial pressure is maintained, except on systems
specially designed for varying initial pressures (e.g., subatmospheric pressure), which
normally operate under controlled partial vacuums; and (4) for gravity return systems,
the pressure drop to the heating units does not exceed the water column available for
removing condensate (i.e., the height above the boiler water line of the lowest point on
the steam main, on the heating units, or on the dry return).
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For quiet operation, steam velocity should be 8,000 to 12,000 fpm (feet per minute),
with a maximum of 15,000 fpm, but lower pressure heating systems normally have lower
velocities. The reasonable flow velocities are listed in Table 7-2 with consideration of the
economic optimization of line sizes including pipeline erosion. The lower velocities
should be used for small pipes and the upper ones for large pipes. The lower the velocity
Table 7-1 Pressure Drops Used for Sizing Steam Pipe
Pressure Drop per 100 feet
Vacuum return
2 to 4 oz/in2
Total pressure Drop in Steam
Supply Piping
1 to 2 psi
0
0.5 oz/in2
1 oz/in2
1
2 oz/in2
1 to 4 oz/in2
2
2 oz/in2
5
4 oz/in2
8 oz/in2
1.5 psi
10
8 oz/in2
1 psi
2 psi
2 to 5 psi
2 to 5 psi
2 to 10 psi
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Initial Steam Pressure, psig
15
30
50
100
150
3 psi
4 psi
5 to 10 psi
10 to 15 psi
15 to 25 psi
25 to 30 psi
Notes: Equipment, control valves, and so forth must be selected based on delivered pressures.
One pound equals 16 ounces (oz) in weight unit.
Source: ASHRAE. 2001 Handbook – Fundamentals, Chapter 35, p35.10, Atlanta, GA
7.1 Pipe Sizing
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7:4 · Fundamentals of Steam System Design
Table 7-2 Reasonable Design Velocities for Steam and Condensate Pipes
(psig)
Use
Reasonable Velocity
Feet per minute
Feet per second
67-100
0-15
Heating
2000-6000
Saturated Steam
50 up
Miscellaneous
6000-10,000
Superheated
200 up
Condensate*
0-20
*
Added based on Reference VIII.
100-167
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Saturated Steam
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Pressure
Fluid
Large turbine and
10,000-15,000
Boiler leads
Returns
Below 4500
167-250
3-7
Nayyar, M.L.,1992, Piping Handbook, 6th Edition, McGraw-Hill.
(or the larger the pipe size), the quieter the system. In lower pressure heating systems (under
15 psig), the maximum velocities should not exceed about 10000 fpm in large pipe (12 inch),
dropping to about 2000 fpm in 2 inch and smaller pipes.
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When the condensate must flow against the steam, even in limited quantity, the velocity
of the steam must not exceed limits above which the disturbance between the steam and
the counterflowing water may (1) produce objectionable sound, such as water hammer,
or (2) results in the retention of water in certain parts of the system until the steam flow
is reduced sufficiently to permit the water to pass. The velocity at which these
disturbances takes place is a function of (1) pipe size; (2) the pitch of the pipe if it runs
horizontally; (3) the quantity of condensate flowing against the steam; and (4) the
freedom of the piping from water pockets that, under certain conditions, acts as a
restriction in pipe size. Table 7-3 lists maximum capacities for various size steam lines.
Equivalent Length of Run
All tables for the flow of steam in pipes based on pressure drop must allow for pipe
friction, as well as for the resistance of fittings and valves. These resistances are generally
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stated in terms of straight pipe; that is, a certain fitting produces a drop in pressure
equivalent to the stated number of feet of straight run of the same size of pipe. Table 74 gives the number of feet of straight pipe usually allowed for the more common types of
fittings and valves. In all pipe sizing tables in this chapter, the length of run refers to the
equivalent length of run as distinguished from the actual length of pipe. A common
sizing method is to assume the length of run and to check this assumption after pipes
are sized. For this purpose, the length of run is usually assumed to be doubled the actual
length of pipe.
7.1 Pipe Sizing
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 7: Steam Piping Design · 7:5
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Table 7-3 Comparative Capacity of Steam Lines at Various Pitches for Steam and
Condensate Flowing in Opposite Directions
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ASHRAE. 2001 Handbook - Fundamentals, Chapter 35, pp35.11
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Table 7-4 Equivalent Length of Fittings to be added to Pipe Run
ASHRAE. 2001 Handbook - Fundamentals, Chapter 35, pp35.11
7.1 Pipe Sizing
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7:6 · Fundamentals of Steam System Design
Example 7-1:
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Steam flows between section a-b in the piping system of Figure 7.1, of which the pipe is
4 inch in diameter and 132 feet in measured length. Determine the equivalent length in
feet of pipe for the run.
Figure 7.1 A Piping System for Example 7-1.
Solution:
Using Table 7-4,
Measured length = 132 ft
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4 inch gate valve = 1.9 ft
Three 4-inch elbows = 3 · 9 = 27 ft
One tee = 18 ft
Therefore, equivalent length = 178.9 ft
Tip: One elbow between the tee and the drip in the piping system should not be
counted as equivalent length due to no flow in the elbow.
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Example 7-2:
What pressure drop should be used for the steam piping of a system if the length of the
longest run is 400 ft with an allowance of 400 ft for fittings? Initial pressure must not
exceed 5 psig.
7.1 Pipe Sizing
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Chapter 7: Steam Piping Design · 7:7
Solution:
t
The equivalent length of the longest run is 800 ft. From Table 7-1 the total allowable
pressure drop is given as 1.5 psi at a system pressure of 5 psig. The pressure drop per
unit length of 100 ft is
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Pressure drop per 100 ft = (1.5 psi · 100 ft) / 800ft = 0.19 ≈ 0.2 psi per 100 ft
The value is in fair agreement with the suggested value in Table 7-1. The system piping
may then be sized using 0.2 psi/100 ft, the capacity of the pipe section in lbf/hr, and the
velocity criterion cited previously.
Sizing Steam Pipes
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Figure 7.2 is the basic chart for determining the flow rate and velocity of steam in
Schedule 40 pipe for various values of pressure drop per 100 feet, based on 0 psig
saturated steam. Using the multiplier chart (Figure 7.3), Figure 7.2 can be used at all
saturation pressures between 0 and 200 psig. These charts are based on the Moody
friction factor, which considers the Reynolds number and the roughness of the internal
pipe surfaces. The allowable pressure drop depends on the boiler pressure and the
pressure at the end of the system. Determining the allowable pressure drop per 100 ft
and boiler pressure may be somewhat of an iterative process, since pressure drop and
boiler pressure are dependent. However, Table 7-1 is a guide to selecting both values.
Schedule Number. The schedule number is an index for the standard thickness of a
pipe. The schedule number is defined as
Schedule No. = 1000 · operating pressure (psig)/allowable stress (psi)
(7.1)
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The schedule numbers for a standard steam pipe are generally 40, 80, and 120. The
Schedule 40 is generally used for steam lines and the Schedule 80 is generally used for
condensate returns.
Example 7-3:
A steam-to-water heater at the end of the longest run has a capacity of 50,000 Btu/hr. If
the pressure drop per unit length is calculated to be 0.2 psi/100 ft accounting an
equivalent length of 800 ft and the initial steam pressure is 5 psi, find the size of
Schedule 40 pipe required and the velocity of steam in the pipe.
Solution:
The enthalpy of vaporization at 5 psig can be found to be 960 Btu/lbm in Table 2-1
(steam table). Then, the flow rate of steam can be calculated using Equation (2-1) as:
& ) = (50,000Btu/hr ( q& )) / 960Btu/lbm( h fg )
Flow rate of steam ( m
=52 lbm/hr
The following steps are required to size the steam pipe:
7.1 Pipe Sizing
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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7:8 · Fundamentals of Steam System Design
Figure 7.2 Flow Rate and Velocity of Steam in Schedule 40 Pipe
at Saturation Pressure of 0 psig.
ASHRAE. 2001 Handbook - Fundamentals, Chapter 35, pp 35.12
7.1 Pipe Sizing
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Chapter 7: Steam Piping Design · 7:9
Figure 7.3 Velocity Multiplier Chart for Figure 7.2
ASHRAE. 2001 Handbook - Fundamentals, Chapter 35, pp 35.15
7.1 Pipe Sizing
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
7:10 · Fundamentals of Steam System Design
Enter Figure 7.2 at a flow rate of 52 lbm/hr, and move vertically to the horizontal line at
5 psig.
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Follow along inclined multiplier line (upward and to the left) to horizontal 0-psig line.
The equivalent mass flow rate at 0 psig is about 45 lbm/hr.
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Follow the 45 lbm/hr line vertically until it intersects the horizontal line at 0.2 psi/100
ft pressure drop. The nominal pipe size is between 1 and 1-1/4 inch and the velocity
based on 0 psig is about 2400 fpm. The larger size of 1-1/4 inch pipe is selected for
safety (the larger the steam pipe, the lower the velocity). Then, the velocity drops to
2200 fpm for the 1-1/4 inch pipe.
To find the steam velocity at 5 psig, locate the value of 2200 fpm on the ordinate (y-axis)
of the velocity multiplier chart (Figure 7.3) at 0 psig. Move along the inclined multiplier
line (downward and to the right) until it intersects the vertical 5-psig pressure line. The
velocity as read from the right (or left) scale is about 2000 fpm.
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Note: The value of 2000 fpm is in agreement with the suggested maximum velocity of
2000 fpm for smaller pipes (Section 7.1) in low pressure heating systems.
Sizing Condensate Pipes
The majority of steam systems used in heating applications are two-pipe systems, in
which the two pipes are the steam pipe and the condensate pipe. The one-pipe system in
which steam and condensate flow in the same pipe, frequently in opposite directions,
are considered obsolete and are no longer being installed. This discussion is limited to
the sizing of the condensate lines in two-pipe system.
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When steam is used for heating a liquid to 215° F or less (e.g., in domestic water heat
exchangers, domestic heating water converters, or air-heating coils), the devices are
usually provided with a steam control valve. As the control valve throttles, the absolute
pressure in the load device decreases, removing all pressure motivation for flow in the
condensate return system. In order to ensure the flow of steam condensate from the
load device through the trap and into the return system, it is necessary to provide a
vacuum breaker on the device ahead of the trap. This ensures a minimum pressure at
the trap inlet of atmospheric pressure plus whatever liquid leg the designer has
provided. Then, to ensure flow through the trap, it is necessary to design the condensate
system so that it will have a pressure above atmospheric in the condensate return line.
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Vented (Open) Return Systems. To achieve this pressure requirement, the condensate
return line is usually vented to the atmosphere (1) near the point of entrance of the flow
streams from the load traps, (2) in proximity to all connections from drip traps, and (3)
at transfer pumps or feedwater receivers.
With this design, the only motivation for flow in the return system is gravity. Return
lines that are below the liquid level in the downstream receiver or boiler and are thus
filled with liquid are called wet return; those above the liquid level have both liquid and
gas in the pipes and are called dry returns.
7.1 Pipe Sizing
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 7: Steam Piping Design · 7:11
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The dry return lines in a vented return system have flowing liquid in the bottom of the
line and gas or vapor in the top, which is illustrated in Figure 2.9A. The liquid is the
condensate, and the gas may be steam, air, or a mixture of the two. The flow
phenomenon for these dry return systems is open channel flow, which is best described
by the Manning equation. Table 7-5 is a solution to the equation that shows pipe size
capacities for steel pipes with various pitches. Recommended practice is to size vertical
lines by the maximum pitch shown, although they would actually have a capacity far in
excess of that shown. As the pitch increases, hydraulic jump that could fill the pipe and other
transient effects that could cause water hammer should be avoided. Flow values in Table 7-5 are
calculated for Schedule 40 steel pipe, with a factor of safety of 3.0, and can be used for
copper pipes of the same nominal pipe size.
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Table 7-5 Vented Dry Condensate Return for Gravity Flow
a
Flow is in lbm/hr of 180°F water for Schedule 40 steel pipes.
Flow was calculated from the Manning equation and rounded.
b
The flow characteristics of wet return lines (Figure 2.9B) are best described the DarcyWeisbach equation. Table 7-6 is a solution to the equation that shows pipe size capacity
for steel pipes with various available fluid heads. Table 7-6 can also be used for copper
tubing of equal nominal pipe size. The motivation for flow is the fluid head difference
between the entering section of the flooded line and the leaving section. It is common
practice, in addition to providing for the fluid head differential, to slope the return in the direction
of flow to a collection point such as a dirt leg in order to clear the line of sediment or solids.
Nonvented (Closed) Return Systems. For those systems in which there is a continual
steam pressure difference between the point where the condensate enters the line and
7.1 Pipe Sizing
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7:12 · Fundamentals of Steam System Design
the point where it leaves (Figure 2.9C), Table 7-7 can be used for sizing the condensate
lines. Although this table expresses condensate capacity without slope, common practice
is to slope the lines in the direction of flow to a collection point similar to wet returns to
clear the lines of sediment or solids.
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When saturated condensate at pressures above the return system pressure enters the
return (condensate) mains, some of the liquid flashes to steam. This occurs typically at
drips traps into a vented return system or at load traps leaving process load devices that
are not valve-controlled and typically have no subcooling. If the return main is vented,
the vent lines will relieve any excessive pressure and prevent a back pressure
phenomenon that could restrict the flow through traps from valve loads; the pipe sizing
would be as described above for vented dry returns. If the return line is not vented, the
flash steam results in a pressure rise at that point and the piping could be sized as
described above for closed return returns, and in accordance with Table 7-7 as
applicable.
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Table 7-6 Vented Wet Condensate Return for Gravity Flow
aFlow is in lb /hr of 180°F water for Schedule 40 steel pipes.
m
ASHRAE. 2001 Handbook - Fundamentals, Chapter 35, pg 35.18
The passage of the fluid through the steam trap is a throttling or constant enthalpy
process. The resulting fluid on the downstream side of the trap can be a mixture of
saturated liquid and vapor. Thus, in nonvented returns, it is important to understand
the condition of the fluid when it enters the return line from the trap.
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The condition of the condensate downstream of the trap can be expressed by the quality
x that is the ratio of the mass of vapor to the total mass of the mixture, defined as
x=
mv
ml + m v
(7.2)
where mv and ml are the mass of saturated vapor and the mass of saturated liquid,
respectively, in the condensate.
7.1 Pipe Sizing
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Chapter 7: Steam Piping Design · 7:13
Likewise, the volume fraction Vc of the vapor in the condensate is expressed as
Vv
Vl + Vv
(7.3)
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Vc =
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where Vv and Vl are the volume of saturated vapor and the volume of saturated liquid,
respectively, in the condensate.
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Table 7-7 Flow Rate for Dry-Closed Returns
a
For these sizes and pressure losses, the velocity is above 7000 fpm. Select another combination of
size and pressure loss.
ASHRAE. 2001 Handbook - Fundamentals, Chapter 35, pp35.18
Table 7-8 presents some values for quality and volume fraction for typical supply and
return pressures in heating systems. Note that the percent of vapor on a mass basis x is
small, while the percent of vapor on a volume basis Vc is very large. This indicates that
the return pipe cross section is predominantly occupied by vapor. Table 7-8 is used to
determine the quality of the condensate entering the return line from the trap for
7.1 Pipe Sizing
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
7:14 · Fundamentals of Steam System Design
Supply Pressure, Return Pressure,
psig
psig
0
0
0
0
0
0
15
15
x, fraction Vapor,
Mass Basis
0.016
0.040
0.065
0.090
0.133
0.164
0.096
0.128
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5
15
30
50
100
150
100
150
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Table 7-8 Flash Steam from Steam Trap on Pressure Drop
t
various combinations of supply and return pressures. If the liquid is subcooled entering
the trap, the saturation pressure corresponding to the liquid temperature should be
used for the supply or upstream pressure.
Vc, Fraction Vapor,
Volume Basis
0.962
0.985
0.991
0.994
0.996
0.997
0.989
0.992
ASHRAE. 2001 Handbook - Fundamentals, Chapter 35, pp35.19
Example 7-4:
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For a steam-to-water heater that has a capacity of 50,000 Btu/hr at an initial pressure of
5 psig as illustrated in Example 7-3, size the condensate line if the condensate flows into
a vented dry return that slopes 1/8 in./ft.
Solution:
The enthalpy of vaporization or condensation at 5 psig can be found to be 960 Btu/lbm
in Table 2-1 (steam table). Then, the flow rate of condensate can be calculated using
Equation (2-1) as:
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Flow rate of steam or condensate ( m
& ) = (50,000 Btu/hr ( q& )) / (960Btu/lbm( h fg ))
= 52 lbm/hr .
Using Table 7-5 with a line slope of 1/8 in./ft and a flow rate of 52 lbm/hr, select the
nominal ½ in. pipe, which is rated at 54 lbm/hr.
Example 7-5:
Suppose that at some point the vented dry returns feed into a vented wet return, for
which the head drops 3 ft into a condensate return tank. The estimated total equivalent
length (pipe plus fittings) in the wet return is 150 ft. If the mass flow rate of the
condensate is 2750 lbm/hr, what size pipe should be used?
7.1 Pipe Sizing
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Chapter 7: Steam Piping Design · 7:15
Solution:
First, calculate the head loss per unit length of 100 ft:
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3 ft ÷ 150 ft x 100 ft = 2 ft/100 ft
Example 7-6:
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From Table 7-6 with condensate head of 2 ft/100 ft and a flow rate of 2750 lbm/hr, a 11/2 in. pipe can handle 2840 lbm/hr, which is close to 2750 lbm/hr. Therefore, select 11/2 in. pipe. Oversizing is never a problem.
A condensate system has the steam supply at 30 psig. The return line is non-vented and
at 0 psig. The return line is to have the capacity for returning 2000 lbm/hr of
condensate. Assuming the pressure drop of ¼ psi per 100 ft for the return line, size the
return line.
Solution:
7.2 Piping
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Since the system will be throttling the condensate from 30 psig to 0 psig, there will be
flash steam, and the system will be dry-closed return (not completely full of liquid and
not vented to atmosphere). In Table 7-7 with the flow rate of 2000 lbm/hr for a 30-psig
supply and a 0-psig return for ¼ in./100 ft pressure drop, the correct size lies between 11/2 in. and 2 in. pipe. However, select the larger pipe size of 2 in., which can handle
3270 lbm/hr. Oversizing is never a problem.
-Introduction
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The piping system distributes the steam, returns the condensate, and removes air and
noncondensable gases. In steam heating systems, it is important that the piping
distribute steam, not only at full design load, but at partial loads and excess loads that
can occur on system warm-up. The usual average winter steam demand is less than half
the demand at the lowest outdoor design temperature. However, when the system is
warming up, the load on the steam mains and returns can exceed the maximum
operating load for the coldest design day, even in moderate weather. This load comes
from raising the temperature of the piping to the steam temperature. Supply and return
piping should be sized according to Section 7.1.
Steam distribution systems link boilers and the terminal units actually using steam,
transporting the steam to any location in the steam system where its heat energy is
needed. The three primary components of steam distribution systems are steam mains,
boiler headers and branch lines.
Steam Mains. The steam main is the piping that connects directly to the boiler(s) and
feeds steam to the distribution piping, as shown in Figure 7.4. One of the most
common uses of steam traps is the trapping of steam mains. Inadequate trapping on
steam mains often leads to water hammer and slugs of condensate, which can damage
control valves and other equipments.
7.2 Piping
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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7:16 · Fundamentals of Steam System Design
Figure 7.4 Boiler Header and Drip Legs
Boiler Headers. A boiler header is a specialized type of steam main which can receive
steam from one or more boilers. It is most often a horizontal line, which is fed from the
top and in turn feeds the steam mains as shown in Figure 7.4. It is important to trap the
boiler header properly to assure that any carryover (boiler water and solid) is removed
before distribution into the system.
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Branch Lines. Branch lines are takeoff lines from the steam mains supplying specific
pieces of steam-using equipment. The entire system must be designed and piped to
prevent accumulation of condensate at any point.
The required trap capacity can be obtained by using a safety factor of 1.5 for boiler headers and a
safety factor of 3 for steam mains and branch lines, respectively. Anticipated carryover from boilers
to steam mains is typically 10% of the steam flow rate. The trap capacity can be computed
from:
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Required Trap Capacity =Safety Factor x Steam Flow Rate (lbm/hr) or Load Connected
to Boiler(s) x Anticipated Percent of Condensate or Carryover (typically 10%).
Example 7-7:
If a boiler header has a connected load of 12,000 lbm/hr with an anticipated carryover
of 10%, what size steam trap for the boiler header will be required?
Solution:
The safety factor for the trap capacity in boiler headers is found to be 1.5. The required
trap capacity can be obtained by using the following formula;
7.2 Piping
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 7: Steam Piping Design · 7:17
Required trap capacity=1.5 x 12000 lbm/hr x 0.10 = 1800 lbm/hr
Supply Piping Design Considerations
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1. Size pipe according to Section 7.1, taking into consideration pressure drop and steam
velocity.
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2. Pitch piping uniformly down in the direction of steam flow at 0.25 in. per 10 ft.
3. Insulate piping well to avoid unnecessary heat loss. (See Section 7.4)
4. Condensate from unavoidable heat loss in the distribution system must be removed
promptly to eliminate water hammer and degradation of steam quality and heat transfer
capability.
4-a. Install drip legs at all low points and natural drainage points in the system,
such as at the ends of mains and the bottoms of risers, and ahead of pressure regulators,
control valves, isolation valves, pipe bends, and expansion joints.
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4-b. On straight horizontal runs with no natural drainage points, space drip legs
at intervals not exceeding 300 ft when the pipe is pitched down in the direction of the
steam flow and at a maximum of 150 ft when the pipe is pitched up, so that condensate
flow is opposite of steam flow.
4-c. These distances apply to systems where valves are opened manually to
remove air and excess condensate that forms during warm-up conditions. Reduce these
distances by about half in systems that are warmed up automatically.
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5. Where horizontal piping must be reduced in size, use eccentric reducers that permit
the continuance of uniform pitch along the bottom of piping (in downward pitched
systems). Avoid concentric reducers on horizontal piping, because they can cause water
hammer.
6. Take off all branch lines from the top of the steam mains, preferable at a 45° angle,
although vertical 90° connections are acceptable.
7. Where the length of a branch takeoff is less than 10 ft, the branch line can be pitched
back 0.5 in. per 10 ft, providing drip legs as described previously.
8. Size drip legs properly to separate and collect the condensate. Drip legs at vertical
risers should be full-size and extend beyond the riser, as shown in Figure 7.5. Drip legs
at other locations should be the same diameter as the main. In steam mains 6 in. and
over, this can be reduced to half the diameter of the main, but no less than 4 in. Where
warm-up is supervised, the length of the collecting leg is not critical. However, the
recommended length is one and a half times the pipe diameter and not less than 8 in.
For automatic warm-up, collecting legs should always be the same size as the main and
should be at least 28 in. long to provide the hydraulic pressure differential necessary for
the trap to discharge before a positive pressure is built up in the steam main.
7.2 Piping
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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7:18 · Fundamentals of Steam System Design
Figure 7.5 Method of Dripping Steam Mains
ASHRAE 2000 Handbook - Systems and Equipment, Chapter 10, pp 10.5, Atlanta, GA
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9. Condensate should flow by gravity from the trap to the return piping system. Where
the steam trap is located below the return line, the condensate must be lifted (Figure
7.6). The height of lift is related to the steam pressure at the trap inlet and the return
line pressure as:
Steam Pressure at Trap Inlet=Hydraulic Pressure of Height of Lift + Return Line
Pressure
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In systems operating above 40 psig, the trap discharge can usually be piped directly to
the return system. However, back pressure at the trap discharge (return line pressure
plus hydraulic pressure created by height of lift) must not exceed steam main pressure,
and the trap must be sized after considering back pressure. To lift the condensate 2 ft
requires a steam pressure of approximately 1 psi at the trap inlet. A collecting leg must
be used and the trap discharge must flow by gravity to a vented condensate receiver,
from which it is pumped to the overhead return, in systems (1) operating under 40 psig,
(2) where the temperature is regulated by modulating the steam control valves, or (3)
where the back pressure at the trap is close to system pressure.
10. Strainers installed before the pressure-reducing and control valves are a natural
water collection point. Since water carryover can erode the valve seat, install a trap at the
strainer blow-down connection (Figure 7.7).
7.2 Piping
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Chapter 7: Steam Piping Design · 7:19
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Figure 7.6 Trap Discharging to Overhead Return
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Figure 7.7 Trapping Strainers
Source: ASHRAE 2000 Handbook - Systems and Equipment, Chapter 10
Example 7-8:
If a Schedule-40 Nominal 3-in.-dia. piping runs horizontally in steam mains and is
designed for supervised warm-up, size the drip legs of the steam mains. Repeat the sizing
of the drip legs for a Schedule-40 Nominal 6-in. piping.
Solution:
For 3-in. piping, according to the recommendation in Section 7.2 (8), the drip legs
should be the same diameter as the main, which is 3 in. For supervised warm-up, the
length of the drip leg is 1.5 times the main pipe and not less than 8 in. So the length of
the drip leg is 8 in. since 1.5 x 3 in.=4.5 in. < 8 in.
Answer: Nominal dia. 3 in. and Length 8 in.
For 6-in. piping, according to the recommendation in Section 7.2 (Supply Piping
Considerations #8), in steam mains 6 in. and over, it can be reduced to half the
7.2 Piping
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7:20 · Fundamentals of Steam System Design
diameter of the mains, but no less than 4 in. The diameter 4 in. is selected since half of
6 in. is less than 4 in. The length is 1.5 times the main pipe, which is 1.5 x 6 in =9 in. >
8 in.
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Answer: Nominal diameter 4 in. and length 9 in.
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Return Piping Design Considerations
1. Flow in the return line is two-phase, consisting of flashing steam vapor and
condensate.
2. Pitch return lines downward in the direction of the condensate flow at 0.5 in. per 10
ft to ensure prompt condensate removal.
3. Insulate the return line well, especially where the condensate is returned to the boiler
or the condensate enthalpy is recovered. In vacuum systems, the return lines are not
insulated since condensate subcooling is required.
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4. Where possible and practical, use heat recovery systems to recover the condensate
enthalpy. In vacuum systems, the return lines are not insulated since condensate
subcooling is required.
5. Equip dirt pockets of the drip legs and strainer blowdowns with valves to remove dirt
and scale.
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6. Install steam traps close to drip legs and make them accessible for inspection and
repair. Servicing is simplified by making the pipe sizes and configuration identical for a
given type and size of trap. The piping arrangement shown in Figure 7.8 facilitates
inspection and maintenance of steam traps.
7. When elevating condensate to an overhead return, consider the pressure at the trap
inlet and the fact that it requires approximately 1 psi to elevate condensate 2 ft. See also
Section 7.2.1.
Example 7-9:
A steam main pressure is maintained at 4 psig and the condensate return is vented.
Determine the maximum height of lift when the drip trap is located below the return
line.
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Solution:
As shown in Section 7.2.1, Steam Pressure at Trap Inlet=Hydraulic Pressure of Height
of Lift + Return Line Pressure. So, the hydraulic pressure is the same as the steam
pressure since the vented return line is at 0 psig. Knowing that it requires the steam
pressure of approximately 1 psi at the trap inlet to elevate the condensate 2 ft of height
of lift,
Answer: 2 ft · 4 = 8 ft, maximum height of lift
7.2 Piping
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Chapter 7: Steam Piping Design · 7:21
Figure 7.8 Recommended Steam Trap Piping
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ASHRAE 2000 Handbook - Systems and Equipment, Chapter 10, pp10
Terminal Equipment Piping Design Considerations
1. Size piping the same as the supply and return connections of the terminal equipment.
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2. Keep equipment and piping accessible for inspection and maintenance of the steam
traps and control valves.
3. Minimize strain caused by expansion and contraction with pipe bends, loops, or three
elbow swings to take advantage of piping flexibility, or with expansion joints or flexible
pipe connectors.
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4. In multiple-coil applications, separately trap each coil for proper drainage (Figure
7.9). Piping two or more coils to a common header served by a single trap can cause
condensate backup, improper heat transfer, and inadequate temperature control.
Check valves to prevent backflow of condentsate should be installed as noted in the text
in Figure 7.9.
5. Terminal equipment, where temperature is regulated by a modulating steam control
valve, requires special consideration.
7.3 Pipe Materials
Industrial piping has been standardized primarily by two societies. The American
Society for Testing Materials (ASTM) and the American Society of Mechanical
Engineers (ASME) are the two primary classifiers of industrial steam and condensate
piping. The ASTM is concerned primarily with materials used in piping. The ASME has
7.3 Pipe Materials
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7:22 · Fundamentals of Steam System Design
established a number of codes for piping, one of which is the Power Piping Code ASME
B31.1. This code prescribes minimum requirements for the construction of power and
auxiliary service piping.
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Piping is constructed by several methods; some of these are electric-fusion welding,
electric resistance welding, submerged arc welding, extruded piping, and others. Some
materials used in steam piping are carbon steel, carbon molybdenum steel, chromiummolybdenum steel, and stainless steel.
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Steel pipe, either seamless or welded, is generally used, although the piping codes permit
a variety of materials and several types of welded pipe. For welding and bending, a
carbon seamless steel pipe is recommended. Seamless or electric resistance welded steel
pipe A53 Grade B and seamless steel pipe A106 Grade B are popular selections. The
ASTM specifications for typical heating steam pipe are shown in Table 7-9.
Figure 7.9 Trapping Multiple Coils
ASHRAE 2000 Handbook - Systems and Equipment, Chapter 10, pp10
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Main steam temperature may range up to and beyond 1050°F resulting in upgrading of
main steam line material as the temperature increases as noted below(Nayyar):
Up to 775°F-carbon steel
Up to 950°F- use ½ Cr
From more than 950°F to 1050°F-use 2-1/2 Cr
7.3 Pipe Materials
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Chapter 7: Steam Piping Design · 7:23
Table 7-9 ASTM Specifications of Heating Steam Pipe
A106
A120
Welded and
seamless steel
Seamless steel
Welded and
seamless steel
Tensile Strength, psi
Scope
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Style of Pipe
Welded: 45000
Steam and condensate for coiling
Seamless: Grade A,45000: Grade B, 50000
and bending.
Seamless: Grade A,48000: Grade B, 70000 Steam and condensate for high
temperature and for coiling and
bending.
Welded: 45000
Steam and condensate not for
Seamless: Grade A,45000: Grade B, 50000
high temperature and not for
coiling and bending
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ASTM
designation
A53
McCauley, James F. 2000. Steam Distribution Systems Deskbook. Fairmont Press.
The following conditions are outlined as a guide for design:
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Pipe lighter than standard or Schedule 40 of Wrought Iron and Wrought Steel Pipe
shall not be threaded.
Schedule 40 low-carbon-steel pipe may be used for steam pressure up to approximately
400 psia, where welded joints are used and corrosion is not a problem.
Condensate return piping may be Schedule 80 steel, stainless steel, or wrought iron.
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Schedule 80 low-carbon-steel pipe either screwed or welded may be used for saturated
steam pressure up to about 600 psi.
When steel pipe is threaded and used for steam pressure of 250 psi or greater or for
water pressure in excess of 100 psig at temperatures of 220°F or higher, it shall be
seamless of a quality at least equal to ASTM Specification A53 or A106 and of a weight
at least Schedule 80 in order to furnish added mechanical strength. Refer to ASME
B31.1.
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7.4 Insulation
Insulation is defined as those materials or combinations of materials that are used
primarily to provide resistance to heat flow and perform the following functions:
Conserve energy by reducing heat loss or gain.
Control surface temperatures for personal protection and comfort.
Facilitate temperature control of a process.
Prevent vapor flow and water condensation on cold surfaces.
7.4 Insulation
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7:24 · Fundamentals of Steam System Design
Eliminate or minimize the rate of steam vapor condensation inside supply
piping.
Increase operating efficiency of heating, ventilating, and cooling.
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Prevent or reduce damage to equipment from exposure to fire or corrosive
atmosphere.
Types of Insulation
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All pipe, valves, fittings and flanges must be insulated. Pipe insulation is generally
supplied in 3 ft long sections, completely prefabricated from various insulating
materials, and covered with a waterproof jacket (aluminum jackets are also available).
There are three major types of insulation used in the steam distribution systems.
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Fibrous Insulation – composed of small diameter fibers that finely divide the air space.
The fiber may be perpendicular or horizontal to the surface being insulated, and they
may or may not be bonded together. Silica, rock wool, slag wool and alumina silica
fibers are used. The most widely used insulation of this type is glass fiber and mineral
wool.
Cellular Insulation – composed of small individual cells separated from each other. The
cellular material may be glass or formed plastic, such as polystyrene(closed cell),
polyurethane and elastomeric.
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Granular Insulation – composed of small modules that contain voids or hollow spaces.
It is not considered a true cellular material since gas can be transferred between the
individual spaces. This type may be produced as a loose or pourable material, or
combined with a binder and fibers to make a rigid insulation. Examples of these types of
insulation are calcium silicate, expanded vermiculite, perlite, cellulose, diatomaceous
earth and polystyrene.
All of the above materials use air as the primary insulation. The air spaces between
materials retard the heat transfer within the material due to the low heat transfer
coefficient of air.
The insulation value of different types of insulation is best compared by the thermal
conductivity (Btu-in/hr-ft2·°F) or equivalently “K” factor (Btu-in/hr-ft2·°F).
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Forms of Insulation
Insulation is produced in a variety of forms suitable for specific functions and
applications. The most commonly used forms are:
Rigid Forms. Rigid boards, blocks, sheets, and pre-formed shapes produced from
cellular, granular and fibrous insulations.
Flexible Forms. Cellular and fibrous insulation are both produced in flexible sheets.
Fibrous insulation is used to produce flexible blankets contoured envelopes for
enclosing valves, and other irregular shaped items. Poured or froth foam is used to fill
7.4 Insulation
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Chapter 7: Steam Piping Design · 7:25
irregular areas and voids. Foam spray is used for flat surfaces. Spaces or voids around
odd shaped items are sometimes filled with loose granular or fibrous insulation (Loose
fill).
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Insulation Materials
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The following is a general inventory of the characteristics and properties of major
insulation materials used in commercial and industrial installations.
Calcium Silicate. Calcium silicate is a granular insulation made of lime and silica,
reinforced with organic and inorganic fibers and molded into rigid forms. The thermal
conductivity is about 0.38 Btu-in/hr·ft2·°F. Service temperature covered is 100° F to
1500° F. Flexible strength is good. Calcium silicate is water absorbent. However, it can
be dried out without deterioration. The material is noncombustible and used primarily
on hot piping and surfaces. Jacketing is field applied. It is more expensive than others.
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Fiberglass. Fiberglass is available as flexible blanket, rigid board, pipe covering and other
pre-molded shapes. The thermal conductivity is about 0.26 Btu-in/hr.ft2.°F. Service
temperature ranges from –40° F to 850° F. This product is noncombustible and has
good sound absorption qualities. Fiberglass is one of the most commonly used types of
insulation for steam piping. It is low cost.
Cellular Glass. Cellular glass is available in board and block form capable of being
fabricated into pipe covering and various shape. The thermal conductivity is about 0.33
Btu-in/hr·ft2·°F. Service temperature range is –450° F to 1200° F. It has good structural
strength but poor impact resistance. The material is noncombustible, non-absorptive
and resistance to most chemicals.
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Mineral fiber, perlite, refractory fiber, and cement are also available for steam
distribution lines and equipment.
Insulation Thickness
The cost of insulation on steam piping can easily be justified by the energy conserved.
Even without this justification the safety of personnel must be considered. When
insulation is selected, the proper amount or thickness is most important. The optimum
thickness of insulation depends upon the material, steam cost, operating temperature or
pressure, and the size of pipe. However, good “rules of thumb” for insulation of steam
line are:
Pipe size up to 2 in. requires a minimum of 1-1/2 in. of insulation.
Pipe size from 2 in. to 8 in. requires a minimum of 2 in. of insulation.
Pipe size over 8 in. requires a minimum of 3-1/2 in. of insulation.
Uninsulated steam distribution and condensate return lines are a constant source of
wasted energy. Table 7-10 shows typical heat loss from uninsulated steam and
condensate return lines. Insulation can typically reduce energy losses by 80% - 90% and
7.4 Insulation
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
7:26 · Fundamentals of Steam System Design
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help ensure proper steam pressure at terminal devices. Any surface over 120°F should be
insulated, including boiler surfaces, steam and condensate return piping, and fittings
Table 7-10 Heat Loss per 100 feet of Uninsulated Steam line
1
2
4
8
12
Heat Loss per 100 feet of Uninsulated Steam Line (MMBtu/yr)*
Steam Pressure (psig)/Saturation Temperature (°F)
15 psig/250°F
150 psig/365°F
300 psig/420°F
600 psig/488°F
140
285
375
495
235
480
630
840
415
850
1120
1500
740
1540
2030
2725
1055
2200
2910
3920
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Distribution Line
Diameter (in.)
*Based on horizontal steel pipe, 75°F ambient air, no wind velocity, and 8760 operating hr/yr.
Example 7-10:
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U.S. Department of Energy. 1999. Improving Steam System Performance, Energy Tip Sheet #2. Office
of Industrial Technologies
In a steam distribution system operating at 15 psig, a bare 2-in. diameter steam pipe
runs a total length of 300 ft.
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Estimate the heat loss (MMBtu) per year of the bare pipe with quiescent air at 75°F and
an operating time of 8760 hours per year.
If the steam pipe is to be covered by insulation, size the insulation thickness of the pipe.
Estimate the annual operating cost saving of the pipe with 80% effective insulation,
assuming that the value of steam is $5/MMBtu.
Solution:
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From Table 7-10 with 2 in. pipe at 15 psig, a value of heat loss of 235 MMBtu/year/100
ft is obtained. So,
the heat loss of bare pipe = 235 MMBtu/year/100 ft · 300 ft = 705 MMBtu/year
In the text, “Pipe size up to 2 in. requires a minimum of 1-1/2 in. of insulation” is
recommended as a rule of thumb, so the insulation thickness=1-1/2 in.
Annual cost saving=total heat loss x effectiveness of insulation x steam value
Annual cost saving = 705 MMBtu/year · 0.8 · $5/MMBtu = $2,820/year
7.4 Insulation
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Chapter 7: Steam Piping Design · 7:27
Tips: If the cost to insulate the pipe is given, the payback time period can be calculated.
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7.5 Pipe Expansion
Example 7-11:
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The temperature variations that occur in steam systems cause dimensional changes in
pipe length. This physical change causes the piping to move in many different ways. The
piping system must be designed to compensate for this movement. Piping without
compensation will be subjected to high stresses that will affect all connected equipment,
supports, and anchors. Pipe movement can also be caused by vibration and equipment
that physically moves in operation. Pipe is a rigid material that does have some elasticity.
However, if this elasticity is exceeded the pipe will fracture. Table 7-11 shows the
thermal expansion for piping materials commonly used in steam distribution systems.
Solution:
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A carbon-steel steam pipe at an installation temperature of 40°F is arranged horizontally
to have a distance of 200 ft between two anchors. If the steam pipe operates at 52 psig,
calculate the total thermal expansion (in.) of the pipe between the two anchors.
From Table 7-11 with 52 psig and carbon steel, the elongations at 300°F and 40°F are
found to be 2.35 and 0.3 in./100 ft, respectively.
2.35 – 0.3 = 2.05 in./100 ft
Therefore, total thermal expansion for 200 ft pipe = 2.05 in./100 ft · 200 ft = 4.1 in.
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General practice is to never anchor a straight run of steel pipe at both ends. Piping must
be allowed to expand or contract due to thermal changes. Ample flexibility can be
attained by designing pipe loops or by including expansion joints. There are several
methods and devices used to compensate for the movement of piping systems.
Expansion Loops.
Expansion loops are also called pipe loops or U bends and are commonly used in long
runs of piping. A simple method of designing a pipe loop is to calculate the anchor-toanchor expansion and determine the loop length L necessary to accommodate this
movement. The pipe loop dimensions can be determined using W=L/5, H=2W, and
L=2H+W as shown in Table 7-12. Note that guides must be spaced no closer than twice
the height of the loop, and piping between guides must be supported, when the length
of pipe between guides exceeds the maximum allowable hanger spacing for the size pipe.
Expansion Joints.
Although the inherent flexibility of the piping should be used to the maximum extent
possible, expansion joints must be used where movements are too large to accommodate
with pipe loops or insufficient room exists to construct a loop of adequate size. Typical
situations are tunnel piping and risers for steam pipes in high-rise buildings.
7.5 Pipe Expansion
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7:28 · Fundamentals of Steam System Design
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Table 7-11 Thermal Expansion of Metal Pipe
ASHRAE. 2000 Handbook - Systems and Equipment, Chapter 41, pg 41.10 Atlanta, GA
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Packed Slip Expansion Joints. These are telescoping devices designed to accommodate
axial movement only. Some sort of packing seals the sliding surfaces. The original
packed slip expansion joint used multiple layers of braided compression packing, similar
to the stuffing box commonly used with valves and pumps. Advances in packing
technology allow self-lubricating semi-plastic packing, which can be injected under full
line pressure without shutting off the system (see Figure 7.10). The packing may cause a
significant resistive (friction) force to anchors, which is dependent upon the pipe size
and tightness of the packing. A good figure to use to calculate the resistive (or friction)
force is 600 pounds times the nominal pipe size (in.).
Flexible Ball Joints. These joints are used in pairs to accommodate lateral or offset
movement and must be installed in a leg perpendicular to the expected movement. The
packed flexible ball joints have self-lubricating semi-plastic packing that can be injected
under full line pressure without shutting off the system (see Figure 7.11)
7.5 Pipe Expansion
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 7: Steam Piping Design · 7:29
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Table 7-12 Pipe Loop Design for A53 Grade B Carbon Steel Pipe Through 400°F
Source: ASHRAE. 2000 Handbook - Systems and Equipment, Chapter 41, pg 41.12
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Metal Bellows Expansion Joints. These expansion joints have a thin-wall convoluted
section that accommodates movement by axial, lateral, and angular movements (Figure
7.12). The bellows material is typically stainless steel. Metal bellows expansion joints can
generally be designed for the low pressures and low temperatures commonly
encountered in steam systems. Metal bellows are also constructed of multi-ply metal for
high-pressure use and long life. The bellows usually cause a resistive force due to the
spring constant, for which the resistive force can be calculated by multiplying the
constant by the thermal expansion.
In all cases, expansion joints should be installed, anchored, and guided in accordance
with expansion joint manufacturers’ recommendation.
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7.6 Pipe Supporting Elements
Introduction
Piping-supporting elements consist of (1) hangers, which support from above; (2)
supports, which bear load from below; and (3) restraints, such as anchors and guides,
which limit or direct movement, as well as support loads. Pipe-supporting elements
withstand all static and dynamic conditions including the following:
Weight of pipe, valves, fittings, insulation, and fluid contents, including test
fluid if using a heavier-than-normal media
Occasional loads such as ice, wind, and seismic forces
Forces imposed by thermal expansion and contraction of pipe bends and loops
7.6 Pipe Supporting Elements
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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7:30 · Fundamentals of Steam System Design
Figure 7.10 Slip expansion Joint
Figure 7.11 Flexible Ball Joint
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Source: ASHRAE. 2000 Handbook - Systems and
Equipment, Chapter 41, pp 41.13
Figure 7.12 Metal Bellows Expansion Joints, (a) axial movement, (b) axial, lateral or
angular movement
Frictional, spring, and pressure thrust forces imposed by expansion joints in the
system
Frictional forces of guides and supports
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Other loads, such as water hammer, vibration, and reactive force of relief valves
Test load and force
In addition, pipe-supporting elements must be evaluated in terms of stress at the point
of connection to the pipe and building structure. Stress at the point of connection to
the pipe is especially important for base elbow and trunnion supports, because the
limiting and controlling parameter is usually not the strength of the structural member,
but the localized stress and the point of attachment to the pipe. Loads on anchors, castin-place inserts, and other attachments to concrete should not be more than one-fifth the
7.6 Pipe Supporting Elements
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Chapter 7: Steam Piping Design · 7:31
ultimate strength of the attachment, as determined by manufacturer’s tests. All loads on
the structure should be communicated to and coordinated with the structural engineer.
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The ASME B31 standards establish criteria for the design of pipe-supporting elements
and the Manufacturers Standardization Society of the Valve and Fittings Industry (MSS)
has established standards for the design, fabrication, selection, and installation of pipe
hangers and supports based on these codes.
MSS Standard SP-69 and the catalogs of many manufacturers illustrate the various
hangers and components and provide information on the types to use with different
pipe systems.
The loads on most pipe-supporting elements are moderate and can be selected safely in
accordance with manufacturers’ catalog data and the information presented in this
section; however, some loads and forces can be very high, especially in multistory
buildings and for large-diameter pipe, especially where expansion joints are used at a
high operating pressure. Consequently, a qualified engineer should design, or review
the design of all anchors and pipe-supporting elements, especially for the following:
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Steam systems operating above 150 psig
Risers over 10 stories or 100 ft
Systems with expansion joints, especially for pipe diameter 3 in. and larger
Pipe size over 12 in. diameter
Anchor loads greater than 10,000 lbf
Moments on pipe or structure in excess of 1000 lbf·ft
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Pipe Supports
Several methods of pipe support are shown in Figure 7.13. Figure 7.13(a) is a typical
above ground configuration. Pipe supports can be hangers that provide support from
above, or rollers that support from below. These pipe supports cannot be considered as
pipe guides, as they are designed to simply carry the dead weight of the pipelines and its
contents. Table 7-13 may be used for suggested support spacing.
Hangers
Hangers support the weight of the pipe from above and allow free movement of the pipe
(Figure 7.14 (a), (b), and (c)). Spring hangers are common. Table 7-13 shows suggested
pipe hanger spacing and Table 7-14 provides a maximum safe load for threaded steel
loads.
Roller Supports
Roller supports support the weight of the pipe from below. A typical roller support is
shown in Figure 7.14(d). The larger pipes may be mounted on rollers, which maybe
situated on every second or third support.
7.6 Pipe Supporting Elements
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7:32 · Fundamentals of Steam System Design
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Table 7-13 Suggested Hanger Spacing and Rod size for Straight Horizontal
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ASHRAE. 2000 Handbook - Systems and Equipment, Chapter 41, pg 41.7
Pipe Guides
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Pipe guides serve to keep the lines in position on their supports and to keep the pipes
aligned with the expansion joints. Typical pipe guides are shown in Figure 7.15 (a), (b),
and (c). Pipe guides prevent buckling of the pipe during movement. All expansion joints
require guides. The first guide should be located as close to the expansion joint as
possible, within a maximum of four pipe diameters. The distance from the first to the
second guide should be up to a maximum of fourteen pipe diameters. Additional guides
should follow manufacturer’s recommendation; usually graphs for the additional guides
are available from the manufacturer. A typical pipe guide installation for an expansion
joint is shown in Figure 7.16.
7.6 Pipe Supporting Elements
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 7: Steam Piping Design · 7:33
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Table 7-14 Capacities of ASTM A36 Steel Threaded Rods
ASHRAE. 2000 Handbook - Systems and Equipment, Chapter 41, pp 41.10
Anchors
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Anchors are a special type of pipe support and are usually designed to resist movement
of the pipe in any direction at the point where the anchor is installed. Anchors serve to
stabilize the pipeline and divide the pipeline into straight runs of independently
expanding-contracting segments. Piping anchors must be designed to withstand all
forces imposed on them by the attached piping. These forces can include pressure
thrusts, restrictive forces of the expansion joints, and the frictional forces created by the
pipe guides and supports. The pressure thrust is the cross sectional area of the outer diameter of
the pipe times the line pressure. The resistive forces are described in Section 7.5. The
frictional force at the anchor is the sum of the pipe weight (at each support) including
the content and its insulation multiplied by the coefficient of friction (0.03 for rollers,
0.2 for line contact, and 0.4 for face contact).
Proper anchoring of steam lines is as important as the line itself. Thermal expansion of
pipelines creates an additional force of thousands of pounds pressure and torque. If this
thrust is not controlled with adequate anchors and guides, the lines may be pulled from
their supports, expansion joints will be broken and expansion loops made useless.
An underground anchor is shown in Figure 7.17, where a steel plate is positioned over
the pipe and welded to pipe on both sides along the full circumference. Pipe, insulation
and plate are then encased in a poured concrete block. Figure 7.18 shows a clamp type
anchor, which is used where the pipeline can be run close to a member of the building
frame and where the clamp can be welded to this frame. This anchor may also be
inverted.
7.6 Pipe Supporting Elements
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
7:34 · Fundamentals of Steam System Design
Example 7-12:
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A Schedule-40 Nominal 6-in. diameter carbon steel pipeline operating at 138 psig is
designed to run through a long tunnel under ground. The distance between anchors is
200 ft. One expansion joint is installed between two anchors. Answer the followings.
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Estimate the thermal expansion (in.) of the pipeline between the two anchors.
Design the pipe guides for the pipeline providing the locations of the expansion joint
and the guides, using Figure 7.19 for the maximum guide spacing provided by the
manufacturer.
Determine the support spacing for the pipeline.
Estimate the total force imposed on the individual anchor, which is the vector sum of all
forces; the pressure thrust, friction force, and resistive force, if a packed slip expansion
joint is used. Use Table 7-15 for pipe and insulation weights.
Solution:
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Repeat the question in (d) if a metal bellows expansion joint is used instead of the
packed slip expansion joint. The spring constant of the bellows provided by the
manufacturer is 500 lbf/in.
Table 7-11, with 35 psig and the assumed minimum temperature of 10° F, provides 2.88
and 0.08 in/100 ft, respectively. 2.88 – 0.08 = 2.80 in/100 ft. For 200 ft length between
the anchors, 2.8 in /100 ft x 200 ft = 5.6 in. Answer: Thermal expansion = 5.6 in.
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The design of the pipeline is similar to Figure 7.16.
Distance between anchor and expansion joint = 4 · 6 in. = 24 in. (= 2 ft)
Distance between expansion joint and first guide = 4 · 6 in. = 24 in. (= 2 ft)
Distance between first guide and second guide = 14 · 0.5 ft = 7 ft
For other guides, using Figure 7.19, approximately 42 ft is obtained for 6 in. diameter
and a pressure at 138 psig.
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Distances between second guide and subsequent guide = 42 ft
Support interval is 21 ft taken from Table 7-13. This is required to prevent the bowing
of pipe.
For a packed slip expansion joint: Pressure thrust = (Pressure · cross-sectional area of
outer diameter) = 138 lbf /in2 · π(6.625 in.)2/4 = 4757 lbf,
Resistive force of packing gland = 600 lbf · 6 = 3600 lbf
7.6 Pipe Supporting Elements
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Chapter 7: Steam Piping Design · 7:35
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Figure 7.13 Typical Steam Pipe Supports
Figure 7.14 (a), (b), and (c) Typical Hangers and (d) a Roller Support
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Source: McCauley. Steam Distribution Systems Deskbook, pp 78-9
7.6 Pipe Supporting Elements
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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7:36 · Fundamentals of Steam System Design
Figure 7.15 Typical Pipe Guides, (a) Spider Guide, (b) Strap Guide, and (c) Tee Guide
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Source: PSE. 2003. Steam System Training. Plant Support & Evaluations Inc. pp 6-10
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Figure 7.16 Typical Pipe Guide Installation for an Expansion Joint
7.6 Pipe Supporting Elements
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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Chapter 7: Steam Piping Design · 7:37
Figure 7.17 Underground Anchor
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Source: McCauley. Steam Distribution Systems Deskbook, pg 75
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Figure 7.18 Clamp Type Anchor (This anchor may be inverted)
7.6 Pipe Supporting Elements
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7:38 · Fundamentals of Steam System Design
Total force imposed on the anchor = (4,757+3,600+2,996) = 11,353 lbf
t
From Table 7-15, frictional force of supports and guides=(pipe weight + content weight
+ insulation weight) · (coefficient of friction) · (pipe length) = (18.96 lbf /ft + 12.52 lbf /
ft + 5.97 lbf /ft) (0.4)(200 ft) = 2,996 lbf. Water is used for the content instead of the
steam for the worst case scenario.
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Tips: The anchor load is greater than 10,000 lbf, so a qualified engineer should review
the design of the anchor.
For the metal bellows expansion joint, it is the same as the question in (c) except the
resistive force that is dependent upon the spring constant of 500 lbf /in2.
Resistive force=spring constant x thermal expansion=500 lbf /in x 5.6 in.=2,800 lbf
Total force imposed on the anchor = (4,757 + 2,800 + 2,996) = 10,553 lbf
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Tips: The above calculation for the total force for the anchor is done based on the
normal operating condition. Care should be taken if there is a test operation for
inspection, for instance, a higher pressure with water in the pipeline. The calculation for
the test condition should be repeated and compared with the normal condition. The
worst result (greater force) should be considered as a total force for the anchor.
Figure 7.19 Maximum Recommended Spacing for Pipe Guides
DME Incorporated. 2004. Santa Fe Springs, CA
7.6 Pipe Supporting Elements
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 7: Steam Piping Design · 7:39
Table 7-15 Weights for Pipe and Insulationa
Inside
diameter
in.
0.5
0.75
1
1.25
1.5
2
2.5
3
4
6
8
10
12
14
16
18
20
in.
0.84
1.05
1.315
1.66
1.9
2.375
2.875
3.5
4.5
6.625
8.625
10.75
12.75
14
16
18
20
in.
0.622
0.824
1.049
1.38
1.61
2.067
2.469
3.068
4.026
6.065
7.981
10.02
11.938
13.126
15
16.876
18.814
Pipe
Weight*
lbf/ft
Water
Weight**
lbf/ft
0.85
1.13
1.68
2.27
2.72
3.65
5.79
7.75
10.78
18.96
28.53
40.45
53.48
63.25
82.71
96.8
120.3
0.13
0.23
0.37
0.65
0.88
1.45
2.08
3.20
5.52
12.52
21.69
34.18
48.52
58.66
76.60
96.96
120.51
Insulation
Thickness
Insulation
Weight***
lbf/ft
t
Outside
Diameter
in.
2
2
2
2
2
2.5
2.5
2.5
2.5
2.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
1.49
1.60
1.74
1.92
2.04
3.19
3.52
3.93
4.58
5.97
11.11
13.06
14.89
16.04
17.87
19.70
21.53
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Nominal
Size
a
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The calculations are based on Schedule 40 carbon steel pipe
*The density of 489 lbm / ft3 for carbon steel was used.
**The density of 62.42 lbm / ft3 was used for water.
***The density of 12 lbm / ft3 for Calcium Silicate was used.
7.6 Pipe Supporting Elements
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7:40 · Fundamentals of Steam System Design
Summary
Size steam and condensate piping
t
After studying Chapter 7, you should be able to:
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Design piping; steam mains, boiler headers and drip legs and traps
Learn the common materials of steam and condensate pipes
Design the insulation of steam and condensate pipes
Calculate the thermal expansion of steam and condensate pipes
Learn the types of the pipe expansions and design the guidelines.
Determine the supports or hangers spacing
Determine the pipe guides with expansion joints and expansion loops
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Calculate all forces imposed on anchors
7.6 Pipe Supporting Elements
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Chapter 7: Steam Piping Design · 7:41
Bibliography
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ASHRAE, 2005 ASHRAE Handbook - Fundamentals, Atlanta, GA.
ASHRAE. 2000 ASHRAE Handbook -- Systems and Equipment, I-P Edition. Atlanta, GA.
Çengel, Y. A. and M. A. Boles. 2002. Thermodynamics: An Engineering Approach, 4th Ed.
McGraw Hill.
Department of Energy, Improving Steam System Performance, a Sourcebook for Industry,
Office of Industrial Technology, Energy Efficiency and Renewable Energy, U.S.
Department of Energy. (available at http://www1.eere.energy.gov/industry/
bestpractices/pdfs/steamsourcebook.pdf
Engineering Appliances. 2003. Expansion Joints User Guide. http://www.engineeringappliances.com
Howell, R., H. Sauer, and W. Coad. 2005. Principles of Heating, Ventilating and Air
Conditioning. ASHRAE, Atlanta, GA.
Kreider, Jan F. 2000. Handbook of Heating, Ventilation, and Air Conditioning. CRC Press,
Boca Raton, FL.
McCauley, James F. 2000. Steam Distribution Systems Deskbook. Fairmont Press, Lilburn,
GA.
Nayyar, M.L. 1992. Piping Handbook, 6th Edition, McGraw-Hill.
PSE. 2003. Steam System Training, Plant Support & Evaluations Inc., New Berlin, WI.
U.S. Department of Energy. 1999. Improving Steam System Performance, Energy Tip Sheet
#2, a Sourcebook for Industry. Office of Industrial Technologies (OIT), U.S.
Department of Energy.
Bibliography
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
7:42 · Fundamentals of Steam System Design
Skill Development Exercises for Chapter 7
Complete these questions by writing your answers on the worksheets at the back of this book.
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7-1. Estimate the pressure drop for a steam pipeline, which has an initial
pressure at less than 15 psig and runs 250 ft long.
7-2. A 150-feet long, 3-in. piping system carries steam and contains two gate
valves, one globe valve, one tee, and four 90° elbows. Determine the
equivalent length in feet for the run.
7-3. Find the required Schedule number of a steam pipe at 1000 psig with the
allowable stress of 1300 psi.
7-4. A steam pipeline has a measured length of 200 ft and the initial steam
pressure must not exceed 15 psig. Steam flows through the pipeline to a
terminal unit with a capacity of 96,000 Btu/hr. Size the pipe for Schedule 40
and also find the velocity of steam in the pipe.
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7-5. For the terminal unit that has a capacity of 96,000 Btu/hr at an initial
pressure of 15 psig as illustrated in Problem 7-04, size the condensate line if
the condensate flows into a vented dry return that slopes 1/8 in./ft.
7-6. A 83-ft long, 1-1/4-in. diameter condensate return line is designed to have
a vented wet return with the mass flow rate of 2,100 lbm/hr. What should be
the head drop for the return?
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7-7. A condensate system has the steam supply at 15 psig. The return line is
non-vented and at 0 psig. The return line is to have the capacity for
returning 2,550 lbm/hr of condensate. Assuming the pressure drop of ¼ psi
per 100 ft for the return line, size the return line.
7-8. A steam main has a flow rate of 8,000 lbm/hr with 10% anticipated
percent of condensation. Size the steam trap for the drip leg of the steam
main.
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7-9. If a Schedule-40 Nominal 5-in.-dia. piping runs horizontally in steam mains
and is designed for supervised warm-up, size the drip legs of the steam mains.
Repeat the sizing of the drip legs for automatic warm-up.
7-10. Describe briefly two societies for steam piping.
7-11. Select a proper material for steam and condensate piping operating at 15
psig and 0 psig, respectively. The piping is subjected to bending and coiling.
7-12. Describe the three types of insulation for steam piping.
7-13. Describe the two forms of insulation for steam piping.
Skill Development Exercises for Chapter 7
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Chapter 7: Steam Piping Design · 7:43
7-14. Describe three common materials of insulation for steam piping.
7-15. In a steam distribution system operating at 150 psig, a bare 4-in. diameter
steam pipe runs a total length of 200 ft. Answer the following.
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a) Estimate the heat loss (MMBtu) per year of the bare pipe with a quiescent air
of 70° F.
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b) If the steam pipe is to be covered by insulation, size the insulation thickness of
the pipe.
c) Estimate the annual operating cost saving of the pipe with 90% effective
insulation. Assume that the value of steam is $4.5/MMBtu.
7-16. A carbon-steel steam pipe at an installation temperature of 10° F is
arranged horizontally to have a distance of 300 ft between two anchors. If
the steam pipe operates at 35 psig, calculate the total thermal expansion (in.)
of the pipe occurred between the two anchors.
7-17. Describe the advantage of expansion joints compared to expansion loops.
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7-18. Explain the resistive forces respectively for slip and bellows expansion
joints.
7-19. Explain the differences between pipe supports and pipe guides.
7-20. Describe the function(s) of anchors.
7-21. List all the forces imposed on the anchors with brief explanations.
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7-22. A Schedule-40 Nominal 4-in. diameter carbon steel pipeline operating at
20 psig is designed to run under ceiling. The distance between anchors is
180 ft. One expansion joint is installed between two anchors. Answer the
followings.
(a) Estimate the thermal expansion (in.) of the pipeline between the two
anchors.
(b) Design the pipe guides for the pipeline providing the locations of the
expansion joint and the guides, using Figure 7-19 for the maximum guide
spacing provided by the manufacturer.
(c) Determine the support (hanger) spacing for the pipeline.
(d) If a metal bellows expansion joint is used, estimate the total force imposed
on the individual anchor, which is the vector sum of all forces; the pressure
thrust, friction force, and resistive force. Use Table 7-15 for pipe and insulation
weights. The spring constant of the bellows provided by the manufacturer is 430
lbf/in.
ESkill Development Exercises for Chapter 7
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© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Appendix A
•
•
•
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Contents of Appendix A
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Steam Tables
Table 1 - Properties of Saturated Water and Steam: Temperature Table
Table 2 - Properties of Saturated Water and Steam: Pressure Table
Table 3- Properties of Superheated Steam
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The properties of steam were computed with the GPCALCS software from General Physics
Corporation (Amherst, NY) using the 1997 IFC formulation
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© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
0.01620
0.01620
0.01630
0.01630
0.95
1.28
1.69
2.23
2.89
3.72
4.75
6.00
7.52
100
110
120
130
140
150
160
170
180
0.01650
0.01650
0.01640
0.01630
0.01610
0.01610
0.01610
0.51
0.70
80
0.01610
0.01600
0.01600
0.01600
Vf
50.17
61.98
77.19
96.93
122.82
157.10
202.96
264.99
349.87
467.45
632.44
867.19
1206.07
1702.90
2443.41
Vg
3
148.01
137.99
127.98
117.97
107.98
97.99
88.00
78.02
68.04
58.05
48.07
38.08
28.08
18.07
8.03
Hf
989.87
995.90
1001.86
1007.77
1013.62
1019.44
1025.22
1030.96
1036.68
1042.37
1048.05
1053.71
1059.36
1065.01
1070.67
Hfg
1137.89
1133.89
1129.83
1125.74
1121.60
1117.43
1113.22
1108.98
1104.71
1100.43
1096.11
1091.78
1087.44
1083.07
1078.70
Hg
88.00
147.99
137.97
127.96
117.96
107.97
97.98
920.10
927.12
934.09
941.01
947.90
954.75
961.58
968.38
975.16
981.93
988.68
995.42
1002.15
1008.89
1015.64
1068.09
1065.09
1062.05
1058.97
1055.87
1052.74
1049.58
1046.40
1043.20
1039.98
1036.74
1033.49
1030.23
1026.96
1023.67
Ug
Sf
t
0.2631
0.2474
0.2313
0.2151
0.1985
0.1817
0.1647
0.1473
0.1296
0.1116
0.0933
0.0746
0.0555
0.0361
0.0162
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78.01
68.03
58.05
48.07
38.08
28.08
18.07
8.03
Uf
1.5475
1.5816
1.6168
1.6530
1.6903
1.7288
1.7686
1.8098
1.8523
1.8964
1.9420
1.9894
2.0385
2.0896
2.1428
Sfg
vapor phase internal energy (Btu/lbm)
liquid phase entropy (Btu/lbm · R)
entropy of vaporization (Btu/lbm · R)
vapor phase enthalpy (Btu/lbm· R)
Ug
Sf
Sfg
Sg
Ufg
liquid phase internal energy (Btu/lbm)
internal energy change (Btu/lbm)
Uf
Ufg
American Engineering Units
pE
gas volume (ft /lbm)
liquid phase enthalpy (Btu/lbm)
energy of vaporization (Btu/lbm)
vapor phase enthalpy (Btu/lbm)
liquid volume (ft3/lbm)
90
0.26
0.36
60
70
0.12
0.18
40
50
T (F) P (psia)
Hf
Hfg
Hg
Vf
Vg
Table 1
Properties of Saturated Water and Steam: Temperature Table
ro
u
G
1.8106
1.8290
1.8481
1.8680
1.8888
1.9106
1.9333
1.9571
1.9819
2.0080
2.0353
2.0640
2.0941
2.1257
2.1590
Sg
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Appendix A: Steam Tables · A:1
0.01750
0.01790
0.01800
0.01810
0.01820
0.01840
0.01850
118.00
134.60
153.00
173.33
195.71
220.29
247.22
340
350
360
370
380
390
400
0.01770
0.01860
0.01780
89.65
103.05
0.01760
330
77.68
0.01740
320
67.02
0.01720
310
0.01710
300
0.01700
29.84
35.44
250
260
57.57
0.01690
290
0.01680
20.79
24.98
230
240
0.01730
0.01680
41.87
0.01670
14.71
17.20
212
220
49.22
0.01670
270
0.01660
11.54
14.14
200
210
280
0.01660
Vf
9.35
190
T (F) P (psia)
1.86
2.08
2.34
2.63
2.96
3.34
3.79
4.31
4.91
5.63
6.47
7.46
8.64
10.06
11.76
13.82
16.32
19.37
23.13
26.78
27.80
33.61
40.92
Vg
375.10
364.31
353.59
342.94
332.34
321.79
311.30
300.85
290.44
280.08
269.76
259.47
249.21
238.99
228.79
218.62
208.47
198.35
188.25
180.18
178.17
168.10
158.05
Hf
Hg
Uf
826.37
835.77
844.91
853.81
862.47
870.92
879.15
887.19
895.04
902.70
910.20
917.53
924.71
931.75
938.65
945.41
952.06
958.60
965.03
970.11
971.37
977.62
983.78
1201.46
1200.08
1198.50
1196.74
1194.81
1192.71
1190.45
1188.04
1185.48
1182.79
1179.96
1177.00
1173.92
1170.73
1167.44
1164.03
1160.54
1156.95
1153.28
1150.29
1149.53
1145.71
1141.83
342.35
374.24
363.56
352.93
741.96
751.57
760.99
770.21
779.25
788.13
796.84
805.40
813.81
822.09
830.24
838.26
846.16
853.96
861.64
869.23
876.73
884.14
891.47
897.28
898.72
905.91
913.03
Ufg
1116.21
1115.13
1113.91
1112.56
1111.07
1109.47
1107.74
1105.91
1103.97
1101.92
1099.78
1097.54
1095.22
1092.81
1090.32
1087.76
1085.12
1082.42
1079.66
1077.41
1076.84
1073.97
1071.06
Ug
t
0.5667
0.5542
0.5416
0.5289
0.5162
0.5033
0.4903
0.4772
0.4640
0.4507
0.4372
0.4237
0.4099
0.3960
0.3820
0.3678
0.3534
0.3388
0.3241
0.3122
0.3092
0.2940
0.2787
Sf
gp
e
331.82
321.34
310.91
300.51
290.15
279.83
269.54
259.28
249.05
238.85
228.68
218.53
208.40
198.29
188.19
180.13
178.12
168.06
158.02
pE
Hfg
Table 1, Properties of Saturated Water and Steam: Temperature Table, continued
ro
u
G
0.9613
0.9836
1.0063
1.0291
1.0522
1.0757
1.0994
1.1235
1.1480
1.1728
1.1982
1.2239
1.2502
1.2769
1.3043
1.3322
1.3607
1.3899
1.4199
1.4443
1.4505
1.4820
1.5143
Sfg
1.5280
1.5378
1.5479
1.5580
1.5684
1.5789
1.5897
1.6007
1.6120
1.6235
1.6354
1.6476
1.6601
1.6730
1.6863
1.7000
1.7141
1.7288
1.7440
1.7565
1.7597
1.7760
1.7930
Sg
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Appendix A: Steam Tables · A:2
25
30
15.3
193.16
10
10.3
188.22
9
20
182.81
8
5.3
176.79
7
250.30
240.03
227.92
212.00
170.00
6
14.71
152.91
162.18
4
5
126.03
141.42
2
101.69
1
3
53.13
79.55
0.5
T (F)
0.2
P (psia)
0.0
P(psig)
0.01700
0.01692
0.01683
0.01671
0.01659
0.01656
0.01652
0.01649
0.01645
0.01641
0.01636
0.01630
0.01623
0.01614
0.01607
0.01603
Vf
13.748
16.306
20.092
26.781
38.423
42.404
47.345
53.649
61.979
73.523
90.628
118.704
173.717
333.508
641.324
1525.932
Vg
American Engineering Units
94.02
69.73
47.62
21.20
218.93
208.51
196.25
180.18
161.22
156.27
150.83
144.79
137.99
130.16
120.89
109.39
Hf
945.21
952.04
959.94
970.11
981.84
984.86
988.17
991.82
995.90
1000.56
1006.05
1012.80
1021.74
1035.71
1048.30
1063.24
Hfg
Uf
1164.14
1160.55
1156.19
1150.29
1143.06
1141.13
1139.00
1136.61
1133.89
1130.72
1126.94
218.84
208.43
196.19
180.13
161.19
156.24
150.81
144.77
137.97
130.14
120.88
109.38
94.01
69.73
47.62
21.20
869.00
876.70
885.66
897.28
910.79
914.29
918.12
922.36
927.12
932.57
939.00
946.93
957.47
974.01
988.98
1006.78
1087.84
1085.13
1081.85
1077.41
1071.98
1070.53
1068.93
1067.13
1065.09
t
1062.71
1059.88
1056.31
1051.48
1043.74
1036.60
1027.98
Ug
gp
e
1122.19
1115.76
1105.44
1095.92
1084.44
Ufg
0.3682
0.3534
0.3358
0.3122
0.2836
0.2760
0.2675
0.2581
0.2474
0.2349
0.2198
0.2009
0.1750
0.1326
0.0925
0.0422
Sf
vapor phase internal energy (Btu/lbm)
liquid phase entropy (Btu/lbm· R)
entropy of vaporization (Btu/lbm· R)
vapor phase enthalpy (Btu/lbm· R)
Hg
internal energy change (Btu/lbm)
Ufg
liquid phase internal energy (Btu/lbm)
Ug
Sf
Sfg
Sg
Uf
pE
gas volume (ft /lbm)
liquid phase enthalpy (Btu/lbm)
energy of vaporization (Btu/lbm)
vapor phase enthalpy (Btu/lbm)
Hf
Hfg
Hg
3
liquid volume (ft3/lbm)
Vf
Vg
Table 2
Properties of Saturated Water and Steam: Pressure Table
ro
u
G
1.6995
1.7141
1.7319
1.7565
1.7875
1.7961
1.8056
1.8164
1.8290
1.8438
1.8621
1.8858
1.9195
1.9776
2.0366
2.1156
Sfg
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Appendix A: Steam Tables · A:3
110
95.3
165
170
175
155.3
160.3
145.3
150.3
155
160
140.3
145
150
140
125.3
135.3
135
120.3
130.3
125
130
110.3
115.3
115
105
90.3
120
100
85.3
105.3
95
80.3
100.3
85
90
75.3
65.3
70.3
75
80
60.3
65
70
55.3
45.3
50.3
55
60
40.3
45
50
35.3
40
25.3
30.3
35
P (psia)
20.3
P(psig)
370.78
368.43
366.02
363.55
361.03
358.43
355.77
353.04
350.23
347.33
344.35
341.26
338.08
334.78
331.37
327.82
324.12
320.27
316.25
312.03
307.59
302.92
297.96
292.69
287.06
280.99
274.42
267.22
259.25
T (F)
0.01824
0.01821
0.01818
0.01815
0.01812
0.01809
0.01806
0.01802
0.01799
0.01796
0.01792
0.01789
0.01785
0.01781
0.01777
0.01774
0.01770
0.01766
0.01761
0.01757
0.01752
0.01748
0.01743
0.01738
0.01732
0.01727
0.01721
0.01715
0.01708
Vf
2.601
2.675
2.752
2.834
2.922
3.015
3.114
3.220
3.333
3.455
3.587
3.729
3.882
4.050
4.232
4.432
4.653
4.897
5.169
5.473
5.816
6.207
6.656
7.176
7.788
8.517
9.402
10.500
11.900
Vg
286.55
343.77
341.27
338.71
336.10
333.42
330.68
327.87
324.99
322.03
318.98
315.85
312.62
309.29
305.84
302.27
298.57
294.73
290.73
853.12
855.18
857.28
859.42
861.60
863.81
866.07
868.38
870.73
873.14
875.60
878.12
880.71
883.37
886.11
888.92
891.82
894.82
897.94
901.16
904.53
908.03
911.70
915.57
919.66
924.01
928.66
933.68
939.16
Hfg
1196.89
1196.45
1195.99
1195.52
1195.02
1194.49
1193.94
1193.37
343.18
340.70
338.16
335.56
332.90
330.18
327.39
324.52
321.58
318.55
315.44
312.22
308.91
305.48
301.92
298.24
294.42
290.44
286.27
281.92
277.35
272.53
267.45
262.05
256.27
250.07
243.36
236.02
227.92
Uf
769.50
771.62
773.83
776.07
778.33
780.64
783.02
785.45
787.94
790.48
793.06
795.74
798.50
801.31
804.25
807.26
810.35
813.57
816.94
820.42
824.07
827.88
831.87
836.11
840.60
845.39
850.53
856.11
862.22
Ufg
1112.68
1112.32
1111.99
1111.63
1111.23
1110.82
t
1110.41
1109.97
1109.52
1109.03
1108.50
1107.96
1107.41
1106.79
1106.17
1105.50
1104.77
1104.01
1103.21
1102.34
1101.42
1100.41
1099.32
1098.16
1096.87
1095.46
1093.89
1092.13
1090.14
Ug
gp
e
1192.76
1192.12
1191.45
1190.74
1190.00
1189.21
1188.38
1187.49
1186.55
1185.55
1184.49
1183.34
1182.12
1180.79
1179.36
1177.81
1176.11
1174.24
1172.16
1169.83
1167.19
Hg
pE
282.18
277.59
272.76
267.66
262.24
256.45
250.23
243.50
236.15
228.03
Hf
Table 2, Properties of Saturated Water and Steam: Pressure Table, continued
ro
u
G
0.5299
0.5269
0.5239
0.5207
0.5175
0.5141
0.5107
0.5072
0.5036
0.4998
0.4960
0.4920
0.4878
0.4835
0.4790
0.4744
0.4695
0.4644
0.4590
0.4534
0.4475
0.4412
0.4345
0.4273
0.4196
0.4113
0.4022
0.3921
0.3809
Sf
1.5572
1.5596
1.5621
1.5647
1.5673
1.5700
1.5728
1.5757
1.5787
1.5818
1.5850
1.5883
1.5918
1.5954
1.5992
1.6032
1.6073
1.6117
1.6163
1.6212
1.6264
1.6319
1.6378
1.6443
1.6512
1.6588
1.6672
1.6766
1.6873
Sfg
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Appendix A: Steam Tables · A:4
240
245
250
230.3
235.3
220.3
225.3
230
235
215.3
220
225
210.3
200.3
205.3
210
215
195.3
200
205
190.3
180.3
185.3
190
195
175.3
180
185
170.3
P (psia)
165.3
P(psig)
400.98
399.21
397.41
395.57
393.71
391.81
389.89
387.92
385.92
383.89
381.81
379.70
377.54
375.33
373.08
T (F)
0.01865
0.01863
0.01860
0.01858
0.01855
0.01852
0.01850
0.01847
0.01844
0.01841
0.01839
0.01836
0.01833
0.01830
0.01827
Vf
1.844
1.880
1.918
1.958
1.999
2.042
2.087
2.134
2.183
2.234
2.288
2.344
2.404
2.466
2.532
Vg
364.19
376.16
374.24
372.29
370.31
368.30
366.26
825.43
827.12
828.83
830.56
832.31
834.08
835.87
837.69
839.52
841.39
843.27
845.18
847.13
849.09
851.09
Hfg
375.30
373.40
371.46
369.50
367.51
365.49
363.44
361.35
359.22
357.05
354.85
352.61
350.32
347.98
345.60
Uf
741.01
742.75
744.50
746.25
748.04
749.85
751.68
753.54
755.43
757.37
759.29
761.28
763.27
765.32
767.38
Ufg
t
1116.31
1116.15
1115.96
1115.75
1115.55
1115.34
1115.12
1114.89
1114.65
1114.42
1114.14
1113.89
1113.59
1113.30
1112.98
Ug
gp
e
1201.59
1201.36
1201.12
1200.87
1200.61
1200.34
1200.06
1199.77
1199.46
1199.14
1198.80
1198.45
1198.09
1197.70
1197.30
Hg
pE
362.08
359.94
357.75
355.53
353.27
350.96
348.61
346.21
Hf
Table 2, Properties of Saturated Water and Steam: Pressure Table, continued
ro
u
G
0.5679
0.5657
0.5635
0.5612
0.5588
0.5565
0.5541
0.5516
0.5491
0.5465
0.5439
0.5412
0.5385
0.5357
0.5328
Sf
1.5270
1.5287
1.5305
1.5323
1.5342
1.5360
1.5379
1.5399
1.5419
1.5439
1.5460
1.5482
1.5503
1.5526
1.5549
Sfg
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Appendix A: Steam Tables · A:5
0.02
69.73
0.13
0.02
130.16
0.23
0.02
161.22
0.28
0.02
180.18
0.31
V
H
S
V
H
S
V
H
S
V
H
S
5.00
(162.18)
10.00
(193.16)
14.71
(212.00)
sat.
liquid
1.00
(101.69)
P (psia)
(Tsat)
Table 3
26.78
1150.29
1.76
38.42
1143.06
1.79
73.52
1130.72
1.84
333.51
1105.44
1.98
sat.
vapor
200
.....
.....
38.85
1146.39
1.79
78.16
1148.49
1.87
392.53
1150.11
2.05
250
Temperature (F)
350
400
28.40
1168.83
1.78
41.94
1170.24
1.83
84.22
1171.69
1.91
422.42
1172.83
2.08
300
450
30.50
1192.73
1.82
44.99
1193.75
1.86
90.25
1194.82
1.94
452.28
1195.66
2.12
32.58
1216.38
1.85
48.02
1217.17
1.89
96.25
1217.99
1.97
482.11
1218.64
2.14
34.64
1239.98
1.87
51.04
1240.60
1.92
t
102.25
1241.26
1.99
511.93
1241.78
2.17
gp
e
V volume (ft3/lbm)
P pressure (psia)
S entropy (Btu/lbm· R)
T temperature (F)
pE
H enthalpy (Btu/lbm)
American Engineering Units
Properties of Superheated Steam
ro
u
G
36.69
1263.62
1.90
54.04
1264.13
1.94
108.23
1264.67
2.02
541.74
1265.09
2.20
500
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Appendix A: Steam Tables · A:6
ro
u
20.09
1156.19
1.73
16.31
1160.55
1.71
13.75
1164.14
1.70
11.90
1167.19
1.69
10.50
1169.83
1.68
9.40
1172.16
1.67
0.02
196.25
0.34
0.02
208.51
0.35
0.02
218.93
0.37
0.02
228.03
0.38
0.02
236.15
0.39
0.02
243.50
0.40
V
H
S
V
H
S
V
H
S
V
H
S
V
H
S
20.00
(227.92)
25.00
(240.03)
30.00
(250.3)
35.00
(259.25)
40.00
(267.22)
45.00
(274.42)
sat.
vapor
sat.
liquid
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
250
300
......
......
......
1.69
9.78
1185.70
11.04
1186.91
1.70
12.66
1188.11
1.72
14.82
1189.28
1.73
17.83
1190.43
1.76
1.72
10.50
1211.11
11.84
1212.01
1.73
13.56
1212.90
1.75
15.86
1213.77
1.77
19.08
1214.64
1.79
23.90
1215.49
1.81
t
1.75
11.20
1235.85
12.62
1236.55
1.76
14.45
1237.24
1.78
16.89
1237.92
1.79
20.31
1238.60
1.81
25.43
1239.28
1.84
450
gp
e
......
......
......
......
......
......
......
......
......
16.56
1165.63
1.72
22.36
1191.55
1.78
Temperature (F)
350
400
20.80
1167.21
1.75
pE
200
Table 3, Properties of Superheated Steam, continued
V
H
S
P (psia)
(Tsat)
G
1.78
11.89
1260.27
13.40
1260.84
1.79
15.33
1261.39
1.80
17.91
1261.95
1.82
21.53
1262.50
1.84
26.95
1263.05
1.87
500
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Appendix A: Steam Tables · A:7
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•
•
•
•
•
•
Instructions:
Chapter 1 - Introduction to Steam System Design
Chapter 2 - HVAC Steam Systems
Chapter 3 - Terminal Units 1
Chapter 4 - Terminal Units 2
Chapter 5 - Boilers
Chapter 6 - Steam Valves, Steam Traps, Flash Tanks, and
Condensate Receiver Tank
Chapter 7 - Steam Piping Design
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Student Answer Sheets
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After reading each chapter, answer all of the questions pertaining to that chapter on the following
worksheets. Tear the sheets off at the perforations. Additional worksheets can be included if needed. Be
sure to include your name, address, student number, and e-mail. You can return the
answers through email, fax, or postal mail. Answers can be submitted one chapter at a time, or in
groups.
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet · Fundamentals of Steam System Design: 3
Skill Development Exercises For Chapter 1
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Use the following scenario to answer all of the questions:
A teakettle is sitting on a stove burner. The stove burner is currently turned off and
the teakettle is approximately ¾ full of water.
1-1. Neglecting movement of water by evaporation, is the system open or closed?
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1-2. Is the system at steady state?
1-3. The burner is turned on, but the water has not yet begun to boil. Is the
system at steady state with respect to material flow?
1-4. The burner is turned on, but the water has not yet begun to boil. Is the
system at steady state with respect to energy flow?
1-5. The teakettle is boiling vigorously. Is the system open or closed?
1-6. The teakettle is boiling vigorously. Is the system at steady state with respect
to mass flow?
Skill Development Exercises For Chapter 1
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Student Answer Sheet: 4 · Fundamentals of Steam System Design
1-7. The teakettle is boiling vigorously. Is the system at steady state with respect
to energy flow?
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Skill Development Exercises For Chapter 1
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1-8. The teakettle, boiling vigorously, boils off 1 cup (approximately ½ pound)
of water in 20 minutes. What is the rate of heating energy applied to the
teakettle in BTU/h?
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet · Fundamentals of Steam System Design: 5
Skill Development Exercises for Chapter 2
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2-1. Describe saturated steam and superheated steam, respectively.
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2-2. Water at 193.2° F is pumped into a boiler in which the pressure is 20.3
psig. How much heat must be supplied by the fuel to evaporate each pound
of water into dry saturated steam?
Skill Development Exercises for Chapter 2
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Student Answer Sheet: 6 · Fundamentals of Steam System Design
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2-3. If the steam in Problem 2-2, contained 5% moisture by weight, calculate
the heat which must be supplied by the fuel to evaporate each pound of
water into the wet saturated steam.
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2-5. What is the volume of 1 lbm of steam at 25 psia, if it is 16% wet?
Skill Development Exercises for Chapter 2
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2-4. A vacuum gauge connected to a steam system reads 5.8 psi at a location
where the atmospheric pressure is 14.5 psi. Determine the absolute pressure
in the steam system.
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Student Answer Sheet · Fundamentals of Steam System Design: 7
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2-6. Describe the advantages of steam compared to its competitors such as
electricity and direct-fired heat.
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2-7. Describe two sources causing water hammer.
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2-8. What are the functions of drip leg?
Skill Development Exercises for Chapter 2
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Student Answer Sheet: 8 · Fundamentals of Steam System Design
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2-9. A water heater using saturated steam at a pressure of 5.3 psig has a capacity
of 50,000 Btu/hr. Determine the mass flow rates of both the steam required
and the condensate that flows into the return line.
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2-11. Describe the advantages and disadvantages of the one-pipe and two-pipe
steam heating systems.
Skill Development Exercises for Chapter 2
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2-10. Describe low-pressure and high-pressure steam HVAC systems.
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Student Answer Sheet · Fundamentals of Steam System Design: 9
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2-12. Describe the dry and wet returns in steam condensate systems.
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2-13. Why is the Hartford loop installed?
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2-14. Describe both the gravity return and pumped return in boiler
connections.
Skill Development Exercises for Chapter 2
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Skill Development Exercises for Chapter 2
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Student Answer Sheet: 10 · Fundamentals of Steam System Design
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Student Answer Sheet · Fundamentals of Steam System Design: 11
Skill Development Exercises for Chapter 3
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3-1. What are the mechanisms of heat transfer of natural convection units?
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3-2. List the terminal units that are defined as natural convection units.
Skill Development Exercises for Chapter 3
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Student Answer Sheet: 12 · Fundamentals of Steam System Design
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3-3. Describe the operation of radiator.
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3-5. What is the recessed convector?
Skill Development Exercises for Chapter 3
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3-4. Describe the operation of convector.
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Student Answer Sheet · Fundamentals of Steam System Design: 13
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3-6. Describe the operation of finned-tube baseboard unit.
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3-7. What is the difference between a convector and a baseboard unit?
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3-8. What is the heating effect on natural convection units?
Skill Development Exercises for Chapter 3
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Student Answer Sheet: 14 · Fundamentals of Steam System Design
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3-10. What would be the new output rating of the same cast-iron radiator
designed in Exercise 3-9, if it is now operated at 15-psig steam and a room
temperature of 65°F.
Skill Development Exercises for Chapter 3
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3-9. A small cast-iron radiator is used as a supplementary heater in a room. The
radiator requires a heating load of 2300 Btu/h with a minimum width at a
standard condition of radiators, steam at 215°F (1-psig steam) and room
temperature of 70°F. Design the radiator for the requirements using the
information given in Table 1.
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet · Fundamentals of Steam System Design: 15
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3-12. A large classroom has a winter design heat loss of 46,000 Btu/h with
installed steam baseboard radiators. The baseboard units house copper
tubing with aluminum fins and operates with steam at 6 psig and entering
air (or room) temperature at 60°F. A commercial catalog data is given in
Figure 3.10. Using Tables 3-2 and Figure 3.10, specify the length of the
baseboard units with 1-in. tubing for the requirements.
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3-11. What would be an amount of steam per hour that must be supplied to
the radiator designed in Exercise 3-10?
Skill Development Exercises for Chapter 3
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Student Answer Sheet: 16 · Fundamentals of Steam System Design
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Skill Development Exercises for Chapter 3
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3-13. What is the advantage of baseboard unit in applications?
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Student Answer Sheet · Fundamentals of Steam System Design: 17
Skill Development Exercises for Chapter 4
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4-1. Describe the functions and applications of unit heaters.
4-2. A propeller fan unit heater shown in Figure 4.2 is to be installed in a
laboratory. The heating load of 67 MBh with an entering air temperature at
70°F is required. Steam at 5-psig pressure is available as a heating medium
and the mounting height of 20 feet is desirable. Using the catalog data of
Tables 4-1 to 4-3 given in the text,
(a) Select an adequate unit heater.
Skill Development Exercises for Chapter 4
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Student Answer Sheet: 18 · Fundamentals of Steam System Design
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(b) Determine the unit capacity at actual conditions.
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(d) Calculate the final air temperature leaving the unit heater.
Skill Development Exercises for Chapter 4
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(c) Determine an amount of condensate in pounds per hour.
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Student Answer Sheet · Fundamentals of Steam System Design: 19
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(e) Determine whether a cone diffuser is needed.
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4-3. Why should the return pipe from steam unit heaters provide a minimum
drop of 10 in. below the heaters?
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4-4. Describe the functions and applications of unit ventilators.
Skill Development Exercises for Chapter 4
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Student Answer Sheet: 20 · Fundamentals of Steam System Design
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4-5. List the heating medium other than steam for unit ventilators.
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(a) Determine the ventilation heat requirement.
(b) Determine the total heating requirement.
Skill Development Exercises for Chapter 4
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4-6. A room has a heat loss of 18,000 Btu/hr at a winter outdoor design
condition of 5°F and an indoor design of 65°F, with 15% outdoor air.
Minimum air discharge temperature from the unit is 55°F. To obtain the
specified number of air changes, a 1000-cfm unit ventilator is required.
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet · Fundamentals of Steam System Design: 21
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4-8. A building is occupied by 70 people. The Uniform Building Code calls for
30 cfm of ventilation air per person. The difference between the indoor and
outdoor temperature is 50°F. Find the heat loss from ventilation air.Sketch
the layout of a typical unit ventilator.
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(c) Determine the ventilation cooling capacity of this unit with outdoor
air temperature below 55°F.
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4-9. What is the difference between fan-coil unit and unit ventilator?
Skill Development Exercises for Chapter 4
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Student Answer Sheet: 22 · Fundamentals of Steam System Design
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4-10. What are the distinguishing features of a induction unit?
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4-12. What is the function of humidifier in an air-handling unit?
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4-13. Describe two types of steam coils with brief explanations.
Skill Development Exercises for Chapter 4
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4-11. Describe the distinguishing function and applications of an air-handling
unit.
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Student Answer Sheet · Fundamentals of Steam System Design: 23
Skill Development Exercises for Chapter 5
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5-1. What are the working pressure ranges for both the low-pressure and highpressure boilers?
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5-2. Calculate the gross outputs at 212° F both in Btu/hr and BHP for a boiler
producing 16,000 lbm steam/hr of 15 psig?
Skill Development Exercises for Chapter 5
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Student Answer Sheet: 24 · Fundamentals of Steam System Design
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5-3. What is the unique feature of the cast-iron boilers compared to other boilers
such as steel boilers, copper boilers, etc.?
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5-5. Describe why a higher efficiency can be achieved with condensing boilers.
Skill Development Exercises for Chapter 5
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5-4. Describe the difference between fire-tube boilers and water-tube boilers.
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Student Answer Sheet · Fundamentals of Steam System Design: 25
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5-6. What types of boilers (material) are adequate for the condensing boilers?
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5-7. Describe the difference between the high heating value and the low heating
value of a fuel.
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5-8. List three gaseous fuels with their heating values.
Skill Development Exercises for Chapter 5
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Student Answer Sheet: 26 · Fundamentals of Steam System Design
5-9. The combustion equation for propane gas is shown. Determine the air/fuel
ratio in mass.
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5-10. The volumetric analysis of flue gas from combustion of propane in a gas
boiler is measured to be 8.9% carbon dioxide, 7.4% oxygen, 83.7%
nitrogen, 0% carbon monoxide. Find the amount of excess air and estimate
the combustion efficiency with a flue gas less combustion air temperature of
500°F. Is the excess air within the recommended range suggested?
Skill Development Exercises for Chapter 5
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C3H8 + (1.5)(5)(O2 + 3.76N2) → 3CO2 + 2.5O2 + 4H2O + (1.5)(5)(3.76)N2
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Student Answer Sheet · Fundamentals of Steam System Design: 27
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5-12. Determine the combustion efficiency for the boiler operating at 45%
excess air with a flue gas less combustion air temperature of 500° F.
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5-11. Determine the dew point at atmospheric pressure for the combustion
products in Problem 6-10, assuming that the excess air is dry and the fuel
contains no water.
Skill Development Exercises for Chapter 5
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Student Answer Sheet: 28 · Fundamentals of Steam System Design
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5-14. Estimate the net output of a commercial boiler, which has a gross output
of 50,000 lbm steam/hr. (hint: large boiler)
Skill Development Exercises for Chapter 5
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5-13. A boiler, fired with natural gas costing $3.5/MMBtu, produces 34,000
lbm/hr of 150 psig steam (rated full load) and is supplied with 200° F feed
water. If the boiler operates at the rated full load for 8000 hours per year and
the combustion efficiency is 75.2%, calculate the steam value per MMBtu
and also estimate the annual fuel cost for the boiler operation.
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Student Answer Sheet · Fundamentals of Steam System Design: 29
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5-15. A boiler operates at 45% excess air with a flue gas less combustion air
temperature of 400°F and consumes 200,000 MMBtu of natural gas per
year. Tuning the boiler reduces the excess air to 9.5% with a flue gas less
combustion air temperature of 300° F. Estimate the annual cost saving of
the boiler, assuming that natural gas costs $4/MMBtu.
Skill Development Exercises for Chapter 5
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Student Answer Sheet: 30 · Fundamentals of Steam System Design
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Student Answer Sheet · Fundamentals of Steam System Design: 31
Skill Development Exercises for Chapter 6
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6-1. List the manual valves often used in steam systems each with a brief
explanation.
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6-2. Describe the main difference in function between globe valve and gate
valve.
Skill Development Exercises for Chapter 6
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Student Answer Sheet: 32 · Fundamentals of Steam System Design
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6-3. Describe a common error in the valve size selection of pressure-reducing
valves in steam distribution systems.
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6-5. Condensate at a steam pressure of 50 psig is discharged to a return-line
pressure of 5 psig. What percentage will flash to steam in the return line?
Skill Development Exercises for Chapter 6
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6-4. Size a steam trap for a 6,500-CFM fan heater that produces 90° F airtemperature rise. Steam pressure is constant at 40 psig.
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Student Answer Sheet · Fundamentals of Steam System Design: 33
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6-6. Determine the condensate load of the flash tank with 2,000 lbm/hr of
50 psig condensate entering the flash tank held at 5 psig.
Skill Development Exercises for Chapter 6
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Student Answer Sheet: 34 · Fundamentals of Steam System Design
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Student Answer Sheet · Fundamentals of Steam System Design: 35
Skill Development Exercises for Chapter 7
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7-1. Estimate the pressure drop for a steam pipeline, which has an initial
pressure at less than 15 psig and runs 250 ft long.
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7-2. A 150-feet long, 3-in. piping system carries steam and contains two gate
valves, one globe valve, one tee, and four 90° elbows. Determine the
equivalent length in feet for the run.
Skill Development Exercises for Chapter 7
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Student Answer Sheet: 36 · Fundamentals of Steam System Design
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7-3. Find the required Schedule number of a steam pipe at 1000 psig with the
allowable stress of 1300 psi.
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7-4. A steam pipeline has a measured length of 200 ft and the initial steam
pressure must not exceed 15 psig. Steam flows through the pipeline to a
terminal unit with a capacity of 96,000 Btu/hr. Size the pipe for Schedule 40
and also find the velocity of steam in the pipe.
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Student Answer Sheet · Fundamentals of Steam System Design: 37
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7-6. A 83-ft long, 1-1/4-in. diameter condensate return line is designed to have
a vented wet return with the mass flow rate of 2,100 lbm/hr. What should be
the head drop for the return?
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7-5. For the terminal unit that has a capacity of 96,000 Btu/hr at an initial
pressure of 15 psig as illustrated in Problem 7-04, size the condensate line if
the condensate flows into a vented dry return that slopes 1/8 in./ft.
Skill Development Exercises for Chapter 7
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Student Answer Sheet: 38 · Fundamentals of Steam System Design
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7-8. A steam main has a flow rate of 8,000 lbm/hr with 10% anticipated
percent of condensation. Size the steam trap for the drip leg of the steam
main.
Skill Development Exercises for Chapter 7
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7-7. A condensate system has the steam supply at 15 psig. The return line is
non-vented and at 0 psig. The return line is to have the capacity for
returning 2,550 lbm/hr of condensate. Assuming the pressure drop of ¼ psi
per 100 ft for the return line, size the return line.
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Student Answer Sheet · Fundamentals of Steam System Design: 39
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7-9. If a Schedule-40 Nominal 5-in.-dia. piping runs horizontally in steam mains
and is designed for supervised warm-up, size the drip legs of the steam mains.
Repeat the sizing of the drip legs for automatic warm-up.
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7-10. Describe briefly two societies for steam piping.
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7-11. Select a proper material for steam and condensate piping operating at 15
psig and 0 psig, respectively. The piping is subjected to bending and coiling.
Skill Development Exercises for Chapter 7
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet: 40 · Fundamentals of Steam System Design
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7-12. Describe the three types of insulation for steam piping.
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7-14. Describe three common materials of insulation for steam piping.
Skill Development Exercises for Chapter 7
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7-13. Describe the two forms of insulation for steam piping.
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet · Fundamentals of Steam System Design: 41
7-15. In a steam distribution system operating at 150 psig, a bare 4-in. diameter
steam pipe runs a total length of 200 ft. Answer the following.
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b) If the steam pipe is to be covered by insulation, size the insulation thickness of
the pipe.
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a) Estimate the heat loss (MMBtu) per year of the bare pipe with a quiescent air
of 70° F.
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c) Estimate the annual operating cost saving of the pipe with 90% effective
insulation. Assume that the value of steam is $4.5/MMBtu.
Skill Development Exercises for Chapter 7
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet: 42 · Fundamentals of Steam System Design
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7-17. Describe the advantage of expansion joints compared to expansion loops.
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7-18. Explain the resistive forces respectively for slip and bellows expansion
joints.
Skill Development Exercises for Chapter 7
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7-16. A carbon-steel steam pipe at an installation temperature of 10° F is
arranged horizontally to have a distance of 300 ft between two anchors. If
the steam pipe operates at 35 psig, calculate the total thermal expansion (in.)
of the pipe occurred between the two anchors.
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet · Fundamentals of Steam System Design: 43
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7-19. Explain the differences between pipe supports and pipe guides.
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7-20. Describe the function(s) of anchors.
Skill Development Exercises for Chapter 7
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet: 44 · Fundamentals of Steam System Design
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7-21. List all the forces imposed on the anchors with brief explanations.
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(a) Estimate the thermal expansion (in.) of the pipeline between the two
anchors.
Skill Development Exercises for Chapter 7
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7-22. A Schedule-40 Nominal 4-in. diameter carbon steel pipeline operating at
20 psig is designed to run under ceiling. The distance between anchors is
180 ft. One expansion joint is installed between two anchors. Answer the
followings.
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet · Fundamentals of Steam System Design: 45
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(b) Design the pipe guides for the pipeline providing the locations of the
expansion joint and the guides, using Figure 7-19 for the maximum guide
spacing provided by the manufacturer.
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(c) Determine the support (hanger) spacing for the pipeline.
Skill Development Exercises for Chapter 7
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Student Answer Sheet: 46 · Fundamentals of Steam System Design
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Skill Development Exercises for Chapter 7
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(d) If a metal bellows expansion joint is used, estimate the total force imposed
on the individual anchor, which is the vector sum of all forces; the pressure
thrust, friction force, and resistive force. Use Table 7-15 for pipe and insulation
weights. The spring constant of the bellows provided by the manufacturer is 430
lbf/in.
© 2006 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution,
or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
ASHRAE LEARNING INSTITUTE
Self-Directed Learning Course
Evaluation Form
Course Title: Fundamentals of Steam System Design
COURSE CONTENT
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On the scale of 1 to 5, circle the number that corresponds to your feeling about the
statements below. (1=strongly agree, 5=strongly disagree, 3=undecided)
Strongly
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Agree
The objectives of the course were clearly stated.
The course content supported the stated objectives.
The course material will be valuable as a future reference
The charts and diagrams helped me understand the course concepts
The course material was well-written and accurate
The material presented will be of practical use in my work.
The degree of difficulty (level) was correct to meet my needs
and expectations
8. The Skill Development Exercises were a good measure of my
understanding of the material.
9. The Skill Development Exercises were clearly written
10. Taking this course was an effective use of my time.
11. I would recommend this course to others
GENERAL
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1. Which description best characterizes your primary job function?
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or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
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General Comments regarding any aspect of the course, including suggestions for improvement:
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BackCover.fm Page 2 Tuesday, February 14, 2012 11:15 AM
ASHRAE
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02/12
Errata noted in the list dated 4/1/11 have been corrected.