Revision 1
December 2014
Heat Exchangers
and Condensers
Student Guide
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ii
Table of Contents
INTRODUCTION ..................................................................................................................... 1
TLO 1 HEAT EXCHANGER CONSTRUCTION AND OPERATING PRINCIPLES ............................ 2
Overview .......................................................................................................................... 2
ELO 1.1 Types of Heat Exchangers ................................................................................ 3
ELO 1.2 Classification by Flow Path .............................................................................. 8
ELO 1.3 Differences Between Heat Exchangers ........................................................... 13
ELO 1.4 Heat Exchanger Startup and Operation ........................................................... 16
ELO 1.5 Calculate Changes in Flow and Temperatures ................................................ 20
ELO 1.6 Tube Failure .................................................................................................... 24
TLO 1 Summary ............................................................................................................ 27
TLO 2 CONDENSER CONSTRUCTION AND OPERATING PRINCIPLES ..................................... 28
Overview ........................................................................................................................ 28
ELO 2.1 Purpose of a Condenser ................................................................................... 29
ELO 2.2 Define Terms ................................................................................................... 30
ELO 2.3 Purpose of Vacuum ......................................................................................... 33
ELO 2.4 Thermal Shock ................................................................................................ 34
ELO 2.5 Vacuum Versus Backpressure......................................................................... 36
ELO 2.6 Drawing a Vacuum ......................................................................................... 39
TLO 2 Summary ............................................................................................................ 42
HEAT EXCHANGERS AND CONDENSERS SUMMARY ............................................................ 43
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Heat Exchangers and Condensers
Revision History
Revision
Date
Version
Number
Purpose for Revision
Performed By
10/31/2014
0
New Module
OGF Team
12/10/2014
1
Added signature of
OGF Working Group
Chair
OGF Team
Introduction
A basic understanding of the mechanical components and construction of a
heat exchanger is important to understanding how they function and
operate.
Rev 1
1
A heat exchanger is a component that allows the transfer of heat from one
fluid (liquid or gas) to another fluid. Reasons for heat transfer include the
following:





To heat a cooler fluid by means of a hotter fluid
To reduce the temperature of a hot fluid by means of a cooler fluid
To boil a liquid by means of a hotter fluid
To condense a gaseous fluid by means of a cooler fluid
To boil a liquid while condensing a hotter gaseous fluid
Regardless of the function the heat exchanger fulfills, in order to transfer
heat, the fluids involved must be at different temperatures and come into
thermal contact. Heat will always flow from the hotter to the cooler fluid.
In most heat exchangers, there is no direct contact between the two fluids.
The heat is transferred from the hot fluid to the metal that is isolating the
two fluids and then to the cooler fluid.
Heat exchangers and condensers are important to power plant operations
because they transfer heat but maintain separation between the fluids of the
processes. For example, heat exchangers and condensers transfer heat
between primary and secondary systems. Specifically, this is accomplished
via steam generators. Also, heat rejected in the steam cycle at the exhaust
of the main turbines is reclaimed or removed in the main condenser.
Objectives
At the completion of this training session, the trainee will demonstrate
mastery of this topic by passing a written exam with a grade of 80 percent
or higher on the following terminal learning objectives (TLOs):
1. Describe the purpose, construction, and principles of operation for
each major type of heat exchanger.
2. Describe the purpose, construction, and principles of operation for
condensers.
TLO 1 Heat Exchanger Construction and Operating
Principles
Overview
Heat exchangers transfer heat from a system of higher energy (greater heat)
to a system of lower energy (less heat). They do this by conduction and
convection heat transfer methods. Heat exchangers allow systems to
maintain separation of their respective processes, but transfer heat between
them. Heat exchangers use different flow designs, including counter and
cross flow. They also utilize different construction designs based on their
application.
It is important to understand the design and operation of heat exchangers
and condensers because they perform fundamental functions in power plant
system operations.
2
Rev 1
Objectives
Upon completion of this lesson, you will be able to do the following:
1. Describe the construction, effectiveness, and operation of the
following types of heat exchangers and their components (tubes,
tube sheets, baffles and shells):
a. Tube and shell
b. Plate
2. Describe hot and cold fluid flow paths in the following types of heat
exchangers:
a. Parallel flow
b. Counter flow
c. Cross flow
3. Describe the difference between the following types of heat
exchangers:
a. Single-pass versus multipass heat exchangers
b. Regenerative versus nonregenerative heat exchangers
4. Describe the operation of a typical heat exchanger to include the
following:
a. Startup and shutdown
b. Control of temperature
c. Effects and control of fouling
5. Given the necessary data, calculate flow rates, and temperatures for
various types of heat exchangers.
6. Explain the consequences of heat exchanger tube failure.
ELO 1.1 Types of Heat Exchangers
Introduction
Most chemical or mechanical systems include heat exchangers. They serve
as the systems’ means of gaining or rejecting heat. Heat exchangers are
common in heating, ventilation, and air conditioning systems; radiators on
internal combustion engines, boilers, and condensers; and as preheaters or
coolers in fluid systems.
Although heat exchangers come in every shape and size imaginable, the
construction of most falls into one of two categories: tube and shell, or
plate. As in all mechanical devices, each type has its advantages and
disadvantages.
Tube and Shell
The most basic and most common type of heat exchanger construction is
the tube and shell, shown below in the figure. This type of heat exchanger
consists of a set of tubes in a container called a shell. The fluid flowing
inside the tubes is the tube-side fluid and the fluid flowing on the outside of
the tubes is the shell-side fluid. The tube sheet(s) separate the tube-side
fluid from the shell-side fluid at the ends of the tubes. Tubes can be rolled
and press-fitted or welded into the tube sheet to provide a leak-tight seal.
Rev 1
3
In systems where the two fluids are at vastly different pressures, the tubes
contain the higher-pressure fluid and the lower-pressure fluid circulates on
the shell side. This is economical because it is less costly to design heat
exchanger tubes to withstand higher pressures than the shell of the heat
exchanger. The support plates also act as baffles to direct the flow of fluid
within the shell back and forth across the tubes.
Figure: Tube and Shell Heat Exchanger
1. U-tube
2. Shell
3. Tube
4. Support/baffle
5. Vent connection
6. Tube-side inlet
7. Tube sheets
8. Shell-side drain
9. Shell-side inlet plenum
Plate
A plate heat exchanger, as shown in the figure below, consists of plates
instead of tubes to separate the hot and cold fluids. The hot and cold fluids
alternate between each of the plates. Baffles direct the fluid flow between
plates. Since each of the plates has a large surface area, the plates provide
each of the fluids with an extremely large heat transfer area. Therefore, a
plate heat exchanger is capable of transferring much more heat than a
similarly sized tube and shell heat exchanger. This is due to the larger area
the plates provide over tubes. The high heat transfer efficiency of the plates
allows plate heat exchangers to be small compared to a tube and shell heat
exchanger with the same heat transfer capacity.
4
Rev 1
Figure: Plate Heat Exchanger
Plate heat exchangers are not commonly used because it is difficult to seal
the large gaskets between each of the plates. This problem has restricted
plate type heat exchangers to small, low-pressure applications such as on oil
coolers for engines. However, new improvements in gasket design and
overall heat exchanger design have allowed some large-scale applications of
the plate type heat exchanger. As older facilities are upgraded or newly
designed facilities are built, large plate type heat exchangers are replacing
tube and shell heat exchangers and are becoming more common in power
plants.
Heat Exchanger Applications
Many industry applications use heat exchangers. These include preheater,
radiator, air conditioner evaporator and condenser, and condensers, as
discussed in the following sections.
Preheater
In large steam systems, or in any process requiring high temperatures,
multiple preheaters increase the input fluid temperature in stages instead of
heating the fluids in one-step from ambient to its final temperature.
Preheating in stages increases the plant's efficiency and minimizes thermal
shock stress to components compared to injecting ambient temperature
liquid into a boiler or other device that operates at high temperatures. In the
case of a steam system, a portion of the process steam is tapped off and
used as a heat source to preheat the feedwater in preheater stages.
The figure below is an example of the construction and internals of a U-tube
feedwater heat exchanger found in a preheater stage of a large power
generation facility. As the steam enters the heat exchanger and flows over
and around the tubes, it transfers its thermal energy and condenses.
Rev 1
5

Steam enters from the top into the shell side of the heat exchanger
where it not only transfers sensible heat (temperature change), but
also gives up its latent heat of vaporization (condenses steam into
water).
 The steam entering the heat exchanger is redirected by baffles
(impingement plate) so that the steam and any entrained moisture do
not impinge the tube bundle. Baffles are built stronger than the tubes.
 The condensed steam then exits as liquid water at the bottom of the
heat exchanger. The feedwater enters the heat exchanger on the
bottom right end and flows into the tubes. Most of these tubes will be
below the fluid level on the shell side.
Figure: Feedwater Heater
The figure above is a U-tube feedwater heat exchanger. In this heat
exchanged the feedwater gains heat first from the condensed steam, then the
feedwater travels through the tubes and back around to the top right end of
the heat exchanger. After making the 180-degree bend, the partially heated
feedwater gains more heat from the hotter steam entering the shell side.
The feedwater gains even more heat from the hot steam and then exits the
heat exchanger. In this type of heat exchanger, the shell-side fluid level is
important in determining the efficiency of the heat exchanger, as the shellside fluid level determines the number of tubes exposed to the hot steam.
Radiator
Some think of heat exchangers as only liquid-to-liquid heat transfer devices.
However, a heat exchanger is any device that transfers heat from one fluid
(gas or liquid) to another. Some equipment depends on air-to-liquid heat
exchangers. The most familiar example of an air-to-liquid heat exchanger is
a car radiator. The coolant flowing in the engine picks up heat from the
6
Rev 1
engine block and carries it to the radiator. From the radiator, the hot
coolant flows into the tube side of the radiator (heat exchanger). The
relatively cool air flowing over the outside of the tubes picks up the heat,
reducing the temperature of the coolant.
Because air is such a poor conductor of heat, it is necessary to maximize the
heat transfer area between the metal of the radiator and the air. Fins on the
outside of the tubes increase the surface area for heat transfer and maximize
heat transfer efficiency. The fins increase the efficiency of a heat exchanger
and are common on most liquid-to-air heat exchangers and in some highefficiency liquid-to-liquid heat exchangers.
Air Conditioner Evaporator and Condenser
All air conditioning systems contain at least two heat exchangers, usually
called the evaporator and the condenser. In each of these heat exchangers,
the refrigerant fluid flows into the heat exchanger and transfers heat, either
gaining or releasing it to the cooling medium. Commonly, the cooling
medium is air or water.
In the condenser, the hot, high-pressure refrigerant gas condenses to a
subcooled liquid. The condenser accomplishes this by cooling the gas,
transferring its heat to either air or water. The cooled gas then condenses
into a liquid. In the evaporator, the subcooled refrigerant flows into the heat
exchanger, but the heat flow is reversed with the relatively cool refrigerant
absorbing heat from the hotter air flowing on the outside of the tubes. This
cools the air and boils the refrigerants.
Condensers
A condenser is a type of heat exchanger used to condense a substance from
a gaseous state to a liquid state by cooling. The condenser removes the
latent heat from the condensing fluid and transfers it to the coolant.
Normally, a tube and shell heat exchanger serves as a condenser. In most
cases, baffles are added at the inlet to prevent tube impingement from the
incoming gas or steam. Industrial plants frequently employ large steam
condensers as heat sinks for the steam system.
Knowledge Check
Tube and shell type heat exchangers are more efficient
than plate type heat exchangers.
A.
True
B.
False
Knowledge Check
In a tube and shell heat exchanger, the fluid flowing
________ the tubes is called the tube-side fluid and the
fluid flowing _________ the tubes is the shell-side fluid.
Rev 1
7
A.
around; inside
B.
around; outside
C.
inside; outside
D.
outside; inside
ELO 1.2 Classification by Flow Path
Introduction
Because heat exchangers come in so many shapes, sizes, makes, and
models, they are categorized according to common design characteristics.
One common characteristic design category is the direction of flow the two
fluids have relative to each other. The three categories are parallel flow,
counter flow, and cross flow.

Hint
When performing a heat exchanger comparison
exercise, a common error is not setting the correct
temperature difference (ΔT) relationships. The
ΔTs must be correct for each flow type of heat
exchanger. Refer to the pictures to choose the
correct relationship between the inlet and outlet of
both the cooling fluid and the cooled fluid. The
ΔTs are between the two fluids.
 Compare the average temperature difference
across the heat exchanger.
 The heat exchanger with the larger average
temperature difference is the more efficient.
Parallel Flow
In a parallel-flow heat exchanger, the tube-side fluid and the shell-side fluid
flow in the same direction as shown below in the figure. In this case, the
two fluids enter the heat exchanger from the same end with a large
temperature difference. As the fluids transfer heat, hotter to cooler, the
temperatures of the two fluids approach each other. Note that the hottest
cold-fluid temperature is always less than the coldest hot-fluid temperature.
8
Rev 1
Figure: Parallel-Flow Heat Exchanger
Counter Flow
In a counter-flow heat exchanger below, the two fluids flow in opposite
directions as shown in the figure below. Each fluid enters the heat
exchanger at the opposite end from the other. Because the cooler fluid exits
the counter-flow heat exchanger at the same end where the hot fluid enters
the heat exchanger, the cooler fluid approaches the inlet temperature of the
hot fluid. Counter-flow heat exchangers are the most efficient of the three
types of heat exchangers. In contrast to the parallel-flow heat exchanger,
the counter-flow heat exchanger’s hottest cold-fluid temperature can
actually be greater than the coldest hot-fluid temperature.
Figure: Counter-Flow Heat Exchanger
Cross Flow
In a cross-flow heat exchanger, one fluid flows in the direction
perpendicular to the second fluid; that is, one fluid flows through tubes and
the second fluid passes around the tubes at a 90 degree angle as shown
below in the figure. Cross-flow heat exchangers are common in
applications where one of the fluids changes state or phase. An example is
a steam system's condenser, in which the steam exiting the turbine enters
the condenser shell side, and the cool water flowing through the tubes
absorbs the heat from the steam, condensing the steam into water. This type
of heat exchanger has the capacity to condense large volumes of vapor.
Rev 1
9
Figure: Cross-Flow Heat Exchanger
Heat Exchanger Comparison
Each of the three heat exchanger flow types has distinct advantages and
disadvantages. However, of the three, the counter-flow heat exchanger
design is the most efficient when comparing heat transfer rate per unit of
surface area. The counter-flow heat exchanger is the most efficient because
it has the highest average difference in temperature (∆T) between the two
fluids over the length of the heat exchanger.
The log mean temperature differential for a counter-flow heat exchanger is
larger than the log mean temperature differential for a similar parallel- or
cross-flow heat exchanger.
The following exercise demonstrates how the higher log mean temperature
differential of the counter-flow heat exchanger results in a larger heat
transfer rate.
Use the following equation to calculate the log mean temperature
differential for a heat exchanger.
∆Tlm =
∆T2 − ∆T1
∆T
𝑙𝑛 (∆T2 )
1
Where:

∆T2 = larger temperature difference between the two fluid streams at
either the entrance or the exit to the heat exchanger
 ∆T1 = smaller temperature difference between the two fluid streams at
either the entrance or the exit to the heat exchanger
Heat transfer in a heat exchanger is by conduction and convection. The rate
of heat transfer (Q̇) in a heat exchanger is calculated using the following
equation:
Q̇ = Uo Ao ∆Tlm
Where:


10
𝑄̇ = heat transfer rate (BTU/hr)
Uo = overall heat transfer coefficient (BTU/hr - ft2-°F)
Rev 1

A0 = cross-sectional heat transfer area (ft2)

∆Tlm = log mean temperature differential (°F)
Consider the following example of a heat exchanger operated under
identical conditions as a counter-flow and then a parallel-flow heat
exchanger.
It is important to identify the correct ΔT. They differ for
each type of heat exchanger based on the flow
arrangement.

Hint
For a parallel-heat exchanger, the hottest fluid
loses heat to the coolest fluid at the inlet.
 In a counter-flow heat exchanger, this is not the
case.
Normally a simple drawing helps determine the correct
ΔT arrangement.
Tl = hot fluid temperature
Tl in = 200°F
Tl out = 145°F
U0 = 70 BTU/hr-ft2-°F
A0 = 75 ft2
T2 = cold fluid temperature
T2 in = 80°F
T2 out = 120°F
Counter flow:
(200℉ − 120℉) − (145℉ − 80℉)
= 72℉
200℉ − 120℉
𝑙𝑛 (
)
145℉ − 80℉
∆Tlm = 72°F
∆Tlm =
Parallel flow:
(200℉ − 80℉) − (145℉ − 120℉)
= 61℉
200℉ − 80℉
𝑙𝑛 (
)
145℉ − 120℉
∆Tlm = 61°F
∆Tlm =
Inserting values from the above calculation into the heat transfer equation
for the counter-flow heat exchanger yields the following result:
70 𝐵𝑇𝑈
𝑄̇ = (
) (75 𝑓𝑡 2 )(61℉)
ℎ𝑟-𝑓𝑡 2 -℉
Rev 1
11
𝑄̇ = 3.8 × 105 BTU/hr
Inserting the above values into the heat transfer equation for the parallelflow heat exchanger yields the following result:
70 𝐵𝑇𝑈
𝑄̇ = (
) (75 𝑓𝑡 2 )(61℉)
ℎ𝑟-𝑓𝑡 2 -℉
𝑄̇ = 3.2 × 105 BTU/hr
The results demonstrate that given the same conditions, operating the same
heat exchanger in a counter-flow manner will result in a greater heat
transfer rate than operating in parallel flow.
Knowledge Check
Refer to the drawing of a lube oil heat exchanger below.
The heat exchanger is operating with the following
parameters:
Toil in = 174°F
Toil out = 114°F
Cp oil = 1.1
ṁoil = 4 x 104 lbm/hr
Twater in = 85°F
Twater out = 115°F
Cp oil = 1.0
ṁwater = ?
What is the mass flow rate of cooling water?
A.
12
8.8 x 104 lbm/hr
Rev 1
B.
7.3 x 104 lbm/hr
C.
2.2 x 104 lbm/hr
D.
1.8 x 104 lbm/hr
Knowledge Check
What are the three basic types of heat exchangers?
Select all that are correct.
A.
Parallel flow
B.
Counter flow
C.
Cross flow
D.
Reverse flow
ELO 1.3 Differences Between Heat Exchangers
Introduction
Most large heat exchangers are not purely parallel flow, counter flow, or
cross flow; they are usually a combination of two or all three types of heat
exchangers. Real heat exchangers are more complex than the simple
components shown in the idealized figures herein used to depict each type
of heat exchanger. The reason for combining the various types is to
maximize the efficiency of the heat exchanger within the restrictions placed
on the design. Size, cost, weight, required efficiency, type of fluids,
operating pressures, and temperatures all help determine the complexity of a
specific heat exchanger.
Single-Pass and Multipass Heat Exchangers
When the two fluids pass each other several times within a single heat
exchanger, heat exchanger performance improves. When a heat exchanger's
fluids pass each other more than once, a heat exchanger is a multipass heat
exchanger. If the fluids pass each other only once, the heat exchanger is a
single-pass heat exchanger.
Example
Commonly, shown in the figure below, the multipass heat exchanger
reverses the flow in the tubes with one or more sets of U-shaped bends in
the tubes. The U-shaped bends allow the fluid to flow back and forth across
the length of the heat exchanger. A second method to achieve multiple
passes is to insert baffles on the shell side of the heat exchanger. These
baffles direct the shell-side fluid back and forth across the tubes to achieve
the multipass effect.
Rev 1
13
Figure: Single-Pass and Multipass Heat Exchangers
Regenerative and Nonregenerative Heat Exchangers
The heat exchanger’s function in a particular system determines how it is
classified. One such classification is regenerative or nonregenerative. A
regenerative heat exchanger is one in which the same fluid is both the
cooling fluid and the cooled fluid. The hot fluid leaving a system gives up
its heat to regenerate or heat up the fluid returning to the system. In a
nonregenerative heat exchanger, the hot fluid is cooled by fluid from a
separate system and the energy (heat) removed is not returned to the system.
Example
Regenerative heat exchangers are common in high-temperature systems
where a portion of the system's fluid is removed from the main process and
then returned, shown in the below figure. To improve the efficiency in the
system, the heat from the fluid leaving the main system is used to reheat
(regenerate) the returning fluid instead of being rejected to an external
cooling medium. The terms regenerative and nonregenerative only refer to
how a heat exchanger functions in a system and do not indicate any single
type (tube and shell, plate, parallel flow, counter flow, etc.) of heat
exchanger.
14
Rev 1
Figure: Regenerative and Nonregenerative Heat Exchangers
Knowledge Check
In a ________________ heat exchanger, heat from the
main process flow is ______________ the system.
A.
regenerative; rejected from
B.
regenerative; returned to
C.
nonregenerative; stored in
D.
nonregenerative; returned to
Knowledge Check
In a ________________ heat exchanger, main process
flow contacts the cooling flow _________ time(s).
Rev 1
15
A.
regenerative; one
B.
multipass; one
C.
single-pass; one
D.
single-pass; two
ELO 1.4 Heat Exchanger Startup and Operation
Introduction
Use sound operating practices when operating a heat exchanger. These
practices include startup and shutdown, temperature control, and fouling
guidelines. Although heat exchangers are simple in design, some basic
operations ensure they provide design service over a range of operating
conditions.
There are two key actions for all safe and efficient
heat exchanger operations:

Good
Points
Ensure both sides of a heat exchanger are
completely filled and vented.
 Fluids should be valved in slowly to avoid thermal
shock to the tubes and other components.
Startup and Shutdown
When placing a heat exchanger in service, consider the difference in
operating temperature between the two fluids to prevent thermal shock to
the heat exchanger. Thermal shock is a severe stress produced in a body or
in a material because of a sudden, unequally distributed change in
temperature. The thermal shock will be the most severe in places where
different metals are joined. Startup and shutdown steps include the
following:

Fill the heat exchanger with fluid on both fluid sides. Admit fluids to
the heat exchanger slowly to allow uniform expansion of all types of
metals. Normally, off-service heat exchangers are maintained filled
and pressurized; after a maintenance period, this might not be the
case. Admit the colder fluid to the heat exchanger first, followed by
the hotter fluid. For example, steam admitted to an idle main
condenser with no cooling water can induce significant thermal
stresses, called thermal shock, which will be discussed in detail later
in this lesson.
 Vent prior to placing heat exchanger in service to remove air and
noncondensable gases. Pockets of air or gas decrease the cooling
surface area and the heat exchanger may not function properly. These
pockets of air or gases may disrupt flow through the heat exchanger.
Air pockets can also contribute to the oxygen content to the steam
16
Rev 1
generator feedwater, which is undesirable from a corrosion control
perspective.
 When securing a heat exchanger, first stop the hot fluid by shutting
the discharge valve. Second, stop the colder fluid flow to the heat
exchanger. A heat exchanger that contains liquid should not be
isolated in such a manner that it does not have overpressure
protection. Liquid isolated within the heat exchanger could warm up
due to surrounding air temperature. Increased temperature will lead
to expansion of the liquid and damage to the heat exchanger if not
protected from overpressure. Overpressure protection can be
provided by a relief valve or by leaving the discharge valve open.
Temperature Control
Temperature control valves control some heat exchanger outlet
temperatures automatically by controlling the cooled fluid. By slowing the
heat exchanger flow, the heat transfer time for the hotter fluid is longer and
the hotter fluid outlet temperature will decrease. If the cooled fluid flow
rate is increased, the hotter fluid outlet temperature will rise.
The flow of the cooling fluid also affects the heat exchanger outlet
temperature because increasing the flow of cooling fluid lowers the outlet
temperature of the cooled fluid. This causes a higher differential
temperature across the heat exchanger. Slowing the cooling fluid flow
causes the reverse to happen.
Fouling
Fouling refers to a condition in a heat exchanger characterized by foreign
material such as algae, scale, or debris accumulating in a heat exchanger.
Fouling of heat exchanger tubes lowers the efficiency of a heat exchanger
by decreasing the thermal conductivity of the tubes. In order to transfer
heat, tube metal must also transfer through the fouling layer. The following
are several methods to remove fouling from heat exchanger tubes:



Hydro lancing
Chemical cleaning
Operating practices (the most effective)
Maintaining a minimum flow through the heat exchanger that has an
amount of turbulent flow is one method of prevention or minimization. The
turbulent flow also aids in heat transfer by disrupting the laminar film and
allowing more of the fluid molecules to have exposure to the tubes.
Chemicals can be added in closed systems to prevent the formation of algae
and scale.
Knowledge Check
Steam has been admitted to a main condenser for 25
minutes with no cooling water. Initiating full cooling
water flow rate at this time will...
Rev 1
17
A.
reduce the stress on the condenser shell by rapidly
cooling the shell.
B.
reduce the stress on the condenser tubes by rapidly
cooling the tubes.
C.
induce large thermal stresses on the condenser shell.
D.
induce large thermal stresses on the junctions between
the condenser tubes and the tube sheet.
Knowledge Check
Refer to the drawing of an operating lube oil heat
exchanger below. Increasing the oil flow rate through
the heat exchanger will cause the oil outlet temperature
to _________ and the cooling water outlet temperature to
__________. (Assume cooling water flow rate remains
the same.)
A.
decrease; decrease
B.
decrease; increase
C.
increase; decrease
D.
increase; increase
Knowledge Check
Refer to the drawing of an operating water cleanup
system below. All valves are identical and are initially
50 percent open. To lower the temperature at point 7, the
operator should adjust valve _____ in the open direction.
18
Rev 1
A.
A
B.
B
C.
C
D.
D
Knowledge Check
Refer to the drawing of an operating lube oil heat
exchanger below. If scaling occurs inside the cooling
water tubes, cooling water outlet temperature will
__________ and lube oil outlet temperature will
__________. (Assume oil and cooling water flow rates
remain the same.)
Rev 1
19
A.
decrease; decrease
B.
decrease; increase
C.
increase; decrease
D.
increase; increase
Knowledge Check – NRC Bank
Which one of the following will occur to reduce the heat
transfer rate in a parallel-flow heat exchanger as scaling
increases on the exterior surface of the tubes? (Assume
no operator actions.)
A.
Flow through the heat exchanger tubes will decrease.
B.
Surface area of the tubes will decrease.
C.
Thermal conductivity of the tubes will decrease.
D.
The difference in temperature across the tubes will
decrease.
ELO 1.5 Calculate Changes in Flow and Temperatures
Heat Balance Calculation for Heat Exchangers
Heat transfer in a heat exchanger occurs by conduction and convection.
The rate of heat transfer (Q)̇ in a heat exchanger is calculated using the
following equation:
Q̇ = Uo Ao ∆Tlm
The heat exchanger balance is dependent on a few key characteristics such
as mass flow, specific heat capacity of the fluids, and change in
temperature.
𝑚̇1 𝐶𝑝1 ∆𝑇 = 𝑚̇2 𝐶𝑝2 ∆𝑇2
Where:


ṁ1 = mass flow rate
Cp = specific heat capacity
 ∆T = temperature change across heat exchanger
Using this heat balance equation, it is possible to calculate the change in
mass flow rate or change in temperature of either fluid in a heat exchanger.
20
Rev 1
Determine Flow or Temperature Difference of Heat Exchanger
Fluids
Step
Action
Formula
1.
Determine the heat
transferred across the
heat exchanger to or
from one of the fluids.
Q̇ = Uo Ao ∆Tlm
2.
Determine the log
mean temperature
difference between the
two fluids if necessary.
3.
Once heat transfer is
known, solve for flow
or temperature
difference of the other
fluid.
or
𝑄̇ = ṁ1 𝐶𝑝 ∆𝑇
∆𝑇𝑙𝑚 =
∆𝑇2 − ∆𝑇1
∆𝑇
𝑙𝑛 (∆𝑇2 )
1
𝑚̇1 𝐶𝑝1 ∆𝑇1
= 𝑚̇2 𝐶𝑝2 ∆𝑇2
Demonstration
Refer to the drawing below of a lube oil heat exchanger.
The heat exchanger is operating with the following parameters:

Toil in = 165°F

Toil out = 110°F

Cp oil = 1.1 BTU/lbm-°F

ṁoil = 3.0 x 104 lbm/hr

Twater in = 65°F

Twater out = 95°F

Cp water = 1.0 BTU/lbm-°F

ṁwater = ?
What is the mass flow rate of the cooling water?
Rev 1
21
Step
Formula
Solution
Solve for
Q̇oil
Q̇oil = ṁoil Cp oil ∆Toil
Q̇
= (3.0
× 104
lbm
BTU
) (1.1
) (55℉)
hr
lbm-°F
Q̇ = 1.815 × 106 BTU/hr
Solve for
ṁwater
ṁ water Cp water ∆Twater
= ṁ oil Cp oil ∆Toil
1.815 × 106 BTU/hr
BTU
= (ṁwater )(1.0
)(30°F)
lbm-°F
1.815 × 106 BTU/hr
(1.0BTU/lbm-°F)(30℉)
= (mwater )
6.05 x 104 lbm/hr = 𝑚̇𝑤𝑎𝑡𝑒𝑟
22
Rev 1
Knowledge Check
Refer to the drawing of an operating lube oil heat
exchanger below.
Given the following information:
ṁoil = 2.0 x 104 lbm/hr
 ṁwater = 3.0 x 104 lbm/hr


Cp oil = 1.1 BTU/lbm-°F

Cp water = 1.0 BTU/lbm-°F

T water in = 92°F

T water out = 125°F

Toil in = 180°F

Toil out = ?
Which one of the following is the temperature of the oil
exiting the heat exchanger (Toil out )?
Rev 1
A.
135°F
B.
140°F
C.
145°F
D.
150°F
23
Knowledge Check
Refer to the drawing of an operating lube oil heat
exchanger below. Assume the inlet lube oil and inlet
cooling water temperatures are constant and cooling
water flow rate remains the same. Decreasing the oil
flow rate through the heat exchanger will cause the oil
outlet temperature to _________ and the cooling water
outlet temperature to _________.
A.
decrease; increase
B.
increase; decrease
C.
increase; increase
D.
decrease; decrease
ELO 1.6 Tube Failure
Introduction
Although heat exchangers are simple in design and contain no moving
parts, they are subject to failure. The most common failure is a breech in
the pressure boundary between the two fluids.
Tube Failure
High flow rate or particulate in the fluids passing through can wear or erode
heat exchanger tubes over time. A vibration may develop if fouling causes
an irregular flow pattern or flow is throttled. The resulting vibration could
compromise the seal between the tubes and tube sheet or the sealing
surfaces between fluids. If tubes(s) fail, the higher pressure fluid will be
forced into the lower pressure system and the two fluids will come in
contact with each other. Instrumentation will show an equalization of
cooling fluid and cooled fluid temperatures at some midtemperature.
24
Rev 1
Additionally, the lower-pressure system level should rise and increase the
level in an expansion tank and the higher-pressure system level should
decrease.
For example, take the case of a heat exchanger with hot borated water
flowing through the tubes cooled by fresh water. The shell-side pressure is
less than the tube-side pressure. What occurs in the event of a tube failure?
Since the pressure is higher in the tubes than the shell, borated water will
flow from the tubes into the shell, raising shell pressure. As this borated
water flows into the fresh water system, the level of borated water in the
system will decrease and the level in the fresh water system will increase.
Example
Refer to the drawing of an operating cooling water system below. What
occurs when a tube fails in the heat exchanger?
Figure: Cooling Water System
If there were a leak as indicated, the high-pressure fluid from the tubes
would force into the shell side of the heat exchanger. The low-pressure
system pressure would rise and the high-pressure system surge tank level
would lower as fluid was lost. The high-pressure fluid being cooled would
also add heat to the low-pressure system.
Knowledge Check
Refer to the drawing of an operating cooling water
system below. Which one of the following effects occurs
as a result of the failed tube in the heat exchanger?
Rev 1
25
A.
High-pressure (HP) fluid inventory increases.
B.
Pressure in the low-pressure (LP) system decreases.
C.
Temperature in the low-pressure (LP) system increases.
D.
Level in the surge tank decreases.
Knowledge Check – NRC Bank
A nuclear power plant is operating normally at 50
percent power. Which one of the following will result
from a cooling water tube rupture in the main condenser?
26
A.
Increased condenser vacuum
B.
Increased conductivity of the condensate
C.
Decreased condensate pump net positive suction head
D.
Decreased condensate pump flow rate
Rev 1
TLO 1 Summary
Some important points concerning heat exchangers are as follows:










The two methods of constructing heat exchangers are plate type and
tube and shell type.
Heat exchangers can be classified by the following types of flow:
a. Parallel flow — hot fluid and the coolant flow in the same
direction
b. Counter flow — hot fluid and the coolant flow in opposite
directions
c. Cross flow — hot fluid and the coolant flow at 90 degree
angles (perpendicular) to each other
The four heat exchanger parts are as follows:
a. Tubes/plates
b. Tube sheet
c. Shell
d. Baffles
Single-pass heat exchangers have fluids that pass each other only
once.
Multipass heat exchangers have fluids that pass each other more than
once by using U-tubes and/or baffles.
Heat exchangers should be vented when starting.
Colder fluid is supplied first to a shutdown heat exchanger.
Regenerative heat exchangers use the same fluid for heating and
cooling.
Nonregenerative heat exchangers use separate fluids for heating and
cooling.
Heat exchangers are often used in the following applications:
a. Preheater
b. Radiator
c. Air conditioning evaporator and condenser
d. Steam condenser
Now that you have completed this lesson, you should be able to do the
following:
1. Describe the construction, effectiveness, and operation of the
following types of heat exchangers and their components (tubes,
tube sheets, baffles and shells):
a. Tube and shell
b. Plate
2. Describe hot and cold fluid flow paths in the following types of heat
exchangers:
a. Parallel flow
b. Counter flow
c. Cross flow
3. Describe the difference between the following types of heat
exchangers:
a. Single-pass versus multipass heat exchangers
b. Regenerative versus nonregenerative heat exchangers
Rev 1
27
4. Describe the operation of a typical heat exchanger to include the
following:
a. Startup and shutdown
b. Control of temperature
c. Effects and control of fouling
5. Given the necessary data, calculate flow rates and temperatures for various types
of heat exchangers.
6. Explain the consequences of heat exchanger tube failure.
TLO 2 Condenser Construction and Operating
Principles
Overview
A condenser is a type of heat exchanger used to condense a substance from
a gaseous state to a liquid state by cooling. The condenser removes the
latent heat from the condensing fluid and transfers it to the coolant.
Normally, a type of tube and shell heat exchanger is employed as a
condenser. In most cases, baffles added at the inlet prevent tube
impingement from the incoming gas or steam. Industrial plants frequently
employ large steam condensers as heat sinks for the steam system.
Note
Condensers are important because they provide a heat
sink for the steam plant operating cycle. They allow the
cycle to maximize the work that can be transferred from
steam in the main turbines.
Objectives
On completion of this lesson, you will be able to do the following:
1. State the purpose of a condenser.
2. State the definitions of hotwell and condensate depression.
3. State the reason(s) why condensers in large steam cycles operate at a
vacuum.
4. State the definition of thermal shock.
5. Describe the relationship between condenser vacuum and
backpressure.
6. Explain the process of forming a vacuum within a condenser.
28
Rev 1
ELO 2.1 Purpose of a Condenser
Introduction
The steam condenser is a major component of the steam cycle in power
generation facilities. It is a closed space into which the steam exits from the
turbine and is forced to give up its latent heat of vaporization.
The purpose of the condenser is to:

Provide a heat sink for the turbines to exhaust to giving up the latent
heat of vaporization.
 Operate in a vacuum to provide the lowest heat sink to maximize the
available heat energy transfer.
 Deareate condensate and feedwater to improve corrosion protection.
A condenser, shown in the figure below is a necessary component of the
steam cycle for the following two reasons:

First, a condenser lowers the operational cost of the plant by allowing
the clean and treated condensate to be reused. This is done by
converting the used steam back into water for return to the steam
generator or boiler as feedwater. It is also far easier to pump liquid
back to the boiler than steam.
 Second, the condenser increases the cycle's efficiency by allowing the
cycle to operate with the largest possible ΔT and ΔP (change in
pressure) between the source (boiler) and the heat sink (condenser).
Figure: Single-Pass Condenser
Thermodynamic Cycle
The figure below shows a steam plant thermodynamic cycle on a T-S
diagram. The condensation process is the horizontal blue line, with the area
below the blue line illustrating heat rejection from the cycle. The liquid is
delivered to the condensate pump and then the feed pump where its pressure
is raised (point 1) to the saturation pressure corresponding to the steam
Rev 1
29
generator temperature and the high-pressure liquid is delivered to the steam
generator where the cycle is repeated.
Figure: Thermodynamic Cycle
Knowledge Check
Condensers increase cycle efficiency by...
A.
allowing the cycle to operate with the largest possible
ΔT.
B.
allowing the cycle to operate with the smallest possible
ΔT.
C.
allowing the condensate to operate with the largest
possible ΔT.
D.
allowing the condensate to operate with the smallest
possible ΔT.
ELO 2.2 Define Terms
Introduction
There are different condenser designs, but the most common is the singlepass condenser. Hotwell and condensate depression are terms used to
specifically discuss condenser operations.
30
Rev 1
Hotwell
The condenser design shown in the following figure provides cooling water
flow through straight tubes from the inlet water box on one end to the outlet
water box on the other end. The cooling water flows once through the
condenser (single pass). The separation between the water box areas and
the steam condensing area is accomplished by tube sheets, to which the
cooling water tubes are attached. The cooling water tubes are supported
within the condenser by the tube support sheets. Condensers normally have
a series of baffles that deflect the steam to minimize direct impingement on
the cooling water tubes. The bottom area of the condenser is the hotwell.
This is where the condensate collects and the condensate pump takes its
suction.
Figure: Condenser Cross-Section
To prevent the condensate level from rising to the lower tubes of the
condenser, a hotwell level control system may be used. One method of
control uses a level-sensing network to vary the condensate pump speed or
pump discharge flow control valve position. Another method employs an
overflow system (makeup reject system) that directs water from the hotwell
to a surge or makeup tank when a high level is reached.
Condensate Depression
After the steam condenses, the saturated liquid continues to transfer heat to
the cooling water as it falls to the bottom of the condenser, or hotwell. This
is called subcooling, and a certain amount of it is desirable. A few degrees
subcooling helps prevent condensate pump cavitation. The difference
between the saturation temperature for the existing condenser vacuum and
the temperature of the condensate is termed condensate depression. This is
Rev 1
31
shown as a number of degrees condensate depression or degrees subcooled.
Excessive condensate depression decreases the operating efficiency of the
plant because the subcooled condensate must be reheated in the steam
generator or boiler, which in turn requires more heat from the heat source.
Condensate Depression = Tsat - Tactual
As can be seen on the T-v diagram below, condensate depression decreases
the plant’s operating efficiency because the subcooled condensate must be
reheated in the boiler, which requires more heat from the heat source.
Condensate depression increases the heat rejected from the cycle,
decreasing overall efficiency. Excessive condensate depression also allows
an increased absorption of air by the condensate and, thus, accelerated
oxygen corrosion of plant materials.
Figure: T-v Diagram for Typical Condenser
Knowledge Check
After the steam condenses, the saturated liquid continues
to transfer heat to the cooling water as it falls to the
bottom of the condenser, or hotwell. This is called
____________ and is _______________.
32
A.
subcooling; desirable
B.
subcooling; undesirable
C.
latent heat; desirable
D.
latent heat; undesirable
Rev 1
ELO 2.3 Purpose of Vacuum
Condenser Vacuum
Because condensation is taking place, the term latent heat of condensation
occurs instead of latent heat of vaporization. The steam's latent heat of
condensation passes to the water flowing through the tubes of the
condenser. The vacuum helps increase plant efficiency by extracting more
work out of the turbine. Large steam turbines designed to exhaust into a
condenser operate within a specific vacuum range. If the pressure increases
above these limits, physical damage will occur to turbine blades. As
exhausted steam is condensed, its specific volume decreases and it occupies
less space, which helps maintain vacuum, as shown below in the figure.
If noncondensable gases are allowed to build up in the condenser, vacuum
decreases and the saturation temperature at which the steam condenses
increase. Accumulating noncondensable gases also blanket the tubes of the
condenser, thus reducing the surface area for heat transfer in the condenser.
If the condensate level is allowed to rise over the lower tubes of the
condenser, then fewer tubes are exposed for heat transfer, reducing the heat
transfer area. A reduction in the heat transfer surface has the same effect as
a reduction in cooling water flow. If the condenser is operating near its
design capacity, a reduction in the effective surface area results in difficulty
maintaining condenser vacuum. The temperature and flow rate of the
cooling water through the condenser control the temperature of the
condensate. This in turn controls the saturation pressure (vacuum) of the
condenser.
Figure: Condenser Cross-Section
Operators should maintain condenser vacuum as close to 29 inches of
Mercury (Hg) as practical. This allows maximum expansion of the steam
Rev 1
33
and therefore the maximum work. If the condenser was perfectly airtight
and no air or noncondensable gases were present in the exhaust steam, it
would be necessary only to condense the steam and remove the condensate
to create and maintain a vacuum. The sudden reduction in steam volume as
it condenses creates a vacuum. Pumping the water from the condenser as
fast as it forms maintains the vacuum. It is, however, impossible to prevent
the entrance of air and other noncondensable gases into the condenser. In
addition, some method must exist to create the initial vacuum in the
condenser. This necessitates the use of an air ejector or vacuum pump to
establish and help maintain condenser vacuum.
Knowledge Check
During normal nuclear power plant operation, a main
condenser develops an air leak that decreases vacuum at
a rate of 1 inch of Hg/minimum. Which of the following
will increase because of this condition?
A.
Extraction steam flow rate
B.
Condenser hotwell temperature
C.
Low-pressure turbine exhaust steam moisture content
D.
Steam cycle efficiency
Knowledge Check
Why do large steam condensers operate at a vacuum?
A.
To allow for maximum expansion of steam
B.
To allow for minimum expansion of steam
C.
To allow the condensate to operate with the largest
condensate depression
D.
To allow the condensate to operate with the smallest
condensate depression
ELO 2.4 Thermal Shock
Introduction
Heat exchangers and condensers experience several stresses. These include
pressure stress and thermal stress due to the nature of their function of
transferring heat. To reduce the effects of these stresses, condensers should
be as close as possible to operating temperatures prior to admitting steam
34
Rev 1
from the main turbine. If equipment is not properly preheated, severe
damage can occur to the condenser tubes and the turbine.
Large temperature differences in operating temperature between the two
fluids in a heat exchanger or between a fluid and vapor in a condenser may
be good from a thermodynamic perspective; however, they should be
controlled to prevent thermal shock. Thermal shock is the severe stress
produced in a body or in a material upon experiencing a sudden, unequally
distributed change in temperature.
Operators should and must perform the following to prevent thermal shock:






Admit hotter fluids or vapors slowly to allow uniform expansion of all
metal types.
Maintain heat exchangers filled and pressurized when out of service.
Vent the steam side of condensers and fill the waterside.
Main condensers must be at operating pressures (vacuum) prior to
admitting steam.
Supply colder fluid to the heat exchanger first, followed by hotter
fluid.
Stop the hot fluid or vapor first when securing a heat exchanger or
condenser.
Operate the heat exchanger with the colder fluid for a period to cool
down the component and reduce stress.
Heat exchangers and condensers that contain liquid should not be isolated in
such a manner that it does not have relief valve protection. Liquid isolated
within the heat exchanger could warm up due to surrounding air
temperature. The increased temperature would lead to expansion of the
liquid and damage to the heat exchanger if not protected from overpressure.
Knowledge Check
Severe stress in a mechanical component induced by a
sudden, unequally distributed temperature reduction is a
description of...
A.
fracture stress.
B.
brittle fracture.
C.
thermal shock.
D.
pressurized thermal shock.
Knowledge Check
The major thermodynamic concern resulting from
rapidly cooling a reactor vessel is...
A.
Rev 1
thermal shock.
35
B.
stress corrosion.
C.
loss of shutdown margin.
D.
loss of subcooling margin.
ELO 2.5 Vacuum Versus Backpressure
Introduction
Since a main condenser operates with an internal vacuum, it is necessary to
understand the relationship between vacuum and pressure. Turbines are
designed to operate against a range of backpressures. Operators must
understand the relationship between pressure and vacuum measurement to
respond to changing conditions and maintain the turbine within its design
ranges.
Vacuum Versus Backpressure
Pressure is a measure of the force exerted per unit area on the boundaries of
a substance (or system). Collisions of the molecules of the substance with
the boundaries of the system cause the force. As molecules hit the walls of
their container or system, they exert forces that try to push the walls
outward. The forces resulting from all of these collisions produce the
pressure exerted by a system on its surroundings. Pressure is frequently
expressed in units of lbf/in.2 (pound force per square inch, psi). Pressure
can also be measured using equivalent columns of liquid, such as water
(H2O) or mercury (Hg). These scales use units of inches of H2O or Hg.
The height of the column of liquid provides a certain pressure that can be
directly converted to force per unit area.
If the pressure is below that of the atmosphere as in the case of a condenser,
it is designated as a vacuum. A perfect vacuum would correspond to
absolute zero pressure. Gauge pressures are positive if they are above
atmospheric pressure and negative if they are below atmospheric pressure.
Vacuum, although a negative pressure, is normally expressed as a positive
value. The figure below shows the relationships between absolute, gauge,
vacuum, and atmospheric pressures.
36
Rev 1
Figure: Comparison of Pressure Ranges
Pabs = Patm + Pgauge
Pabs = Patm − Pvac
Where:
Pabs = absolute pressure
Patm = atmospheric pressure
Pgauge = gauge pressure
Pvac = vacuum pressure
In addition to pounds per square inch, pressure can be measured with
reference to the force that exists in a column of fluid at a certain height.
The most common of these are inches of water (H2O) or inches of mercury
(Hg). Conversion factors are listed below.
14.7 𝑝𝑠𝑖𝑎 = 407 𝑖𝑛𝑐ℎ𝑒𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟
14.7 𝑝𝑠𝑖𝑎 = 29.9 𝑖𝑛𝑐ℎ𝑒𝑠 𝑜𝑓 𝑚𝑒𝑟𝑐𝑢𝑟𝑦
Backpressure is the pressure felt at the exhaust of the main turbine, and
demonstrates or shows as inches of vacuum or inches of Hg.
This table shows the relationship between pressure measurements
associated with a condenser. You can see that 29.9 inches of Hg pressure
equals zero (0) inches of mercury vacuum (HgV) and zero (0) inches of Hg
equals 29.9 inches of HgV.
PSIA
PSIG
Inches of HgV
Inches of Hg
14.7
0
0
29.90
13.7
-1.0
2.03
27.87
12.7
-2.0
4.06
25.84
Rev 1
37
PSIA
PSIG
Inches of HgV
Inches of Hg
11.7
-3.0
6.09
23.81
10.7
-4.0
8.13
21.78
9.7
-5.0
10.16
19.75
8.7
-6.0
12.2
17.72
7.7
-7.0
14.25
15.69
6.7
-8.0
16.27
13.66
5.7
-9.0
18.3
11.63
4.7
-10.0
20.34
9.50
3.7
-11.0
22.4
7.57
2.7
-12.0
24.4
5.54
1.7
-13.0
26.44
3.51
0.7
-14.0
28.5
1.48
0
-14.7
29.9
0
To convert between inches of Hg and inches of HgV, subtract the pressure
or vacuum value from 29.9.
For example, given that a condenser pressure is 3 inches of Hg, determine
the corresponding vacuum by:
29.9 − 3 = 26.9 𝑖𝑛𝑐ℎ𝑒𝑠 𝑜𝑓 𝐻𝑔 𝑣𝑎𝑐𝑢𝑢𝑚
Knowledge Check
A turbine has a design backpressure of 5 inches of Hg.
The main condenser is operating at 28 inches of HgV.
What is the margin to design for the turbine?
A.
38
24.9 inches of Hg
Rev 1
B.
3 inches of Hg
C.
3.1 inches of Hg
D.
24.9 inches of HgV
Knowledge Check
The trip setpoint for a main turbine trip is a backpressure
of 7.5 inches of Hg. Currently the main condenser
vacuum is 25 inches of HgV and decreasing (absolute
value). At what vacuum will the turbine trip?
A.
22.4 inches of HgV
B.
17.5 inches of Hg
C.
22.4 inches of Hg
D.
17.5 inches of HgV
ELO 2.6 Drawing a Vacuum
Introduction
Initially, the main condenser is at atmospheric pressure. To prepare the
plant for startup, draw a vacuum in the main condenser. This serves several
purposes.

Allows recirculation of feedwater and condensate in order to clean up
and deaerate these systems.
 Allows warming of the main turbine.
 Allows the identification of any condenser tube leaks prior to placing
the turbine in service.
A vacuum is simply a condition where all the molecules are removed.
Operating the main condenser at a vacuum is necessary in order for the
steam cycle to operate at its peak efficiency. It allows the steam cycle to
exhaust to the lowest possible heat sink, thereby providing the largest
enthalpy drop through the main turbine.
Drawing a vacuum consists of isolating any potential air in-leakage paths,
establishing cooling water flow, and then removing air from the condenser
shell. The mechanical vacuum pump initially removes air from the
condenser. Once vacuum is established, air removal shifts to air ejectors if
the plant is so equipped. Air removal during operation is essential to ensure
efficient operation. If air leaks into the condenser, pressure increases; since
the condenser is a saturated system, the temperature increases as pressure
increases. Overall plant efficiency decreases because the turbine exhaust
enthalpy increases.
Rev 1
39
Vacuum Pumps
A vacuum pump may be any type of motor-driven air compressor. Its
suction is attached to the condenser and it discharges to the atmosphere. A
common type uses rotating vanes in an elliptical housing. Single-stage,
rotary-vane units are used for vacuums up to 28 inches of Hg. Two-stage
units can draw vacuums to 29.7 inches of Hg. The vacuum pump has an
advantage over the air ejector in that it requires no source of steam for its
operation. Vacuum pumps are normally the initial source of vacuum for
condenser startup.
Air Ejectors
Air ejectors are essentially jet pumps or eductors, shown below in the
figure. In operation, the jet pump has two types of fluid flowing through it.
They are the high-pressure fluid that flows through the nozzle and the fluid
being pumped which flows around the nozzle into the throat of the diffuser.
The high-velocity fluid enters the diffuser where its molecules strike other
molecules. These molecules are carried along with the high-velocity fluid
out of the diffuser, creating a low-pressure area around the mouth of the
nozzle. This process is called entrainment. The low-pressure area will
draw more fluid from around the nozzle into the throat of the diffuser. As
the fluid moves down the diffuser, the increasing area converts the velocity
back to pressure. Use of steam at a pressure between 200 psi and 300 psi as
the high-pressure fluid enables a single-stage air ejector to draw a vacuum
of about 26 inches of Hg.
Figure: Air Ejector
Normally, air ejectors consist of two suction stages. The first-stage suction
is located on top of the condenser, while the second-stage suction comes
from the diffuser of the first stage. The exhaust steam from the second
stage must be condensed. This is normally accomplished by an air ejector
condenser that is cooled by condensate. The air ejector condenser also
preheats the condensate returning to the boiler. Two-stage air ejectors are
capable of drawing vacuums to 29 inches of Hg.
Example
The figure below is a typical procedure for drawing vacuum on the main
condenser.
40
Rev 1
Action Step
How
Discussion
Establish
cooling flow

Startup support cooling
systems
Startup circulation
water system
Startup condensate
system
Various systems provide
cooling for components
necessary for vacuum
pull. They include
condensate pumps, air
compressors, and
mechanical vacuum
pumps.
Complete valve lineup
checks
Close vacuum breakers
Establish steam seal on
turbines
All possible leakage paths
must be isolated to
prevent loss of condenser
pressure and temperature
control. Air leakage
across turbine shaft seals
is removed via the gland
seal and exhaust system.
Startup steam sealing
system
Startup gland exhaust
system
Turbine shafts are sealed
using labyrinth seals and
low-pressure steam. This
prevents high-pressure
steam from leaking out
and air from leaking in.
Startup the mechanical
vacuum pump (also
known as a hogger)
Shift to the air ejectors
when vacuum reaches
approximately 26
inches of HgV
The mechanical vacuum
pump draws air out of the
main condenser. As the
air molecules are
removed, pressure
decreases.


Isolate all air in
leakage paths to
the main
condenser

Establish
turbine seals




Remove air
from the
condenser


Knowledge Check
During normal nuclear power plant operation, why does
air entry into the main condenser reduce the
thermodynamic efficiency of the steam cycle?
Rev 1
41
A.
The rate of steam flow through the main turbine
increases.
B.
The condensate subcooling in the main condenser
increases.
C.
The enthalpy of the low-pressure turbine exhaust
increases.
D.
The air mixes with the steam and enters the condensate.
TLO 2 Summary
A condenser is a type of heat exchanger used to condense a substance from
a gaseous state to a liquid state by cooling. The condenser removes the
latent heat from the fluid condensing and transfers it to the coolant.





Condenser removes latent heat of vaporization, condensing the vapor
into a liquid.
Hotwell is the area at the bottom of the condenser where the
condensed steam is collected and pumped back into the system
feedwater.
Condensate depression is the amount the condensate in a condenser is
cooled below saturation (degrees subcooled).
Condensers operate at a vacuum to ensure the temperature (and thus
the pressure) of the steam is as low as possible. This maximizes the
ΔT and ΔP between the source and the heat sink, ensuring the highest
cycle efficiency possible.
Thermal shock is the stress produced in a body or in a material as a
result of undergoing a sudden change in temperature.
Summary
Now that you have completed this lesson, you should be able to do the
following:
1. State the purpose of a condenser.
2. State the definitions of hotwell and condensate depression.
3. State the reason(s) why condensers in large steam cycles operate at a
vacuum.
4. State the definition of thermal shock.
5. Describe the relationship between condenser vacuum and
backpressure.
6. Explain the process of forming a vacuum within a condenser.
42
Rev 1
Heat Exchangers and Condensers Summary
Heat Exchangers
 The type of flow classifies different heat exchangers.
a. Parallel flow — the hot fluid and the coolant flow in the same
direction
b. Counter flow — the hot fluid and the coolant flow in opposite
directions
c. Cross flow — the hot fluid and the coolant flow at 90 degree
angles (perpendicular) to each other
 Single-pass heat exchangers have fluids that pass each other only
once.
 Multipass heat exchangers have fluids that pass each other more than
once through using U-tubes and/or baffles.
 Regenerative heat exchangers use the same fluid for heating and
cooling.
 Nonregenerative heat exchangers use separate fluids for heating and
cooling.
Condensers
Condensers perform an important function in any heat cycle. They provide
a heat sink that allows the cycle to operate at maximum efficiency.

Condensers remove latent heat of vaporization, condensing the vapor
into a liquid.
 Condensers operate at a vacuum to ensure the temperature (and thus
the pressure) of the steam is as low as possible. This maximizes the
ΔT and ΔP between the source and the heat sink, ensuring the highest
cycle efficiency possible.
Summary
This module covered the types of heat exchangers and condensers, their
applications and advantages, proper methods for operation, and system
responses.
At the completion of this training session, the trainee will demonstrate
mastery of this topic by passing a written exam with a grade of 80 percent
or higher on the following terminal learning objectives (TLOs):
1. Describe the purpose, construction, and principles of operation for
each major type of heat exchanger.
2. Describe the purpose, construction, and principles of operation of
condensers.
Rev 1
43