Room Pressurization
Control
125-2412
Rev. 2, June, 2004
Rev.2.0, June, 2004
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Copyright 2001 by Siemens Building Technologies, Inc.
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Room Pressurization Control Application Guide
Table of Contents
How To Use This Application Guide ................................................................................ I
How This Guide is Organzed.......................................................................................... I
Getting Help .................................................................................................................... I
Where To Send Comments ............................................................................................ I
Chapter 1—Introduction................................................................................................... 1
Importance of Room Pressurization .............................................................................. 1
Objective of this Application Guide ................................................................................ 1
Intended Audience ......................................................................................................... 2
Chapter 2—Pressurization Applications ........................................................................ 3
Room Static Pressure.................................................................................................... 3
Building Pressurization .................................................................................................. 4
Room Pressurization Applications................................................................................. 5
Chemical Laboratories ................................................................................................ 5
Biological Laboratories................................................................................................ 5
Biosafety Level 1 (BL-1) ........................................................................................... 6
Biosafety Level 2 (BL-2) ........................................................................................... 6
Biosafety Level 3 (BL-3) ........................................................................................... 6
Biosafety Level 4 (BL-4) ........................................................................................... 7
Hospitals...................................................................................................................... 7
Animal Holding Rooms................................................................................................ 7
Clean Rooms............................................................................................................... 8
Chapter 3—Room Pressurization Design Criteria ....................................................... 11
Design Considerations................................................................................................. 11
Room Pressurization Reference Data ......................................................................... 14
Room Pressurization Factors ...................................................................................... 15
Leakage Area............................................................................................................... 15
Chapter 4—Room Static Pressure Control .................................................................. 17
Laboratories ................................................................................................................. 17
Airflow Tracking Static Pressure Control .................................................................. 17
Airflow Tracking Control Considerations ................................................................ 19
Direct Pressure Control............................................................................................. 21
Direct Pressure Control Limitations........................................................................ 23
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Room Pressurization Control Application Guide
Door Effects ............................................................................................................ 23
VAV Fume Hood Effects......................................................................................... 23
Cascaded Pressure Control ...................................................................................... 24
Dual Pressurization Laboratories.............................................................................. 25
Health Care Facilities................................................................................................... 27
Infectious Isolation Rooms ........................................................................................ 27
Protective Isolation Rooms ....................................................................................... 27
Static Pressure Control by Airflow Tracking.............................................................. 27
Infectious Isolation Room Layout .............................................................................. 28
Ventilation and Control System Application .............................................................. 28
Room Pressure Monitors .......................................................................................... 29
Protective Isolation Room Layout ............................................................................. 29
Ventilation and Control System Application .............................................................. 29
Isolation Room Changeover...................................................................................... 30
Clean Rooms ............................................................................................................... 30
Particulate Contamination ......................................................................................... 30
Clean Room Standards............................................................................................. 31
Clean Room Pressurization Applications.................................................................. 33
Airlocks...................................................................................................................... 34
Airlock Construction .................................................................................................. 38
Clean Room Static Pressure Control ........................................................................ 40
Control System Components .................................................................................... 41
Room Pressurization Control .................................................................................... 42
Chapter 5—Air Pressurization Fundamentals ............................................................. 43
Forces Exerted by Air .................................................................................................. 43
Total Pressure ........................................................................................................... 43
Static Pressure .......................................................................................................... 44
Velocity Pressure ...................................................................................................... 45
Air Velocity ................................................................................................................... 47
Units of Pressure Measurement .................................................................................. 47
Differential Pressure .................................................................................................... 48
Summary of Pressure Components ............................................................................ 48
Glossary............................................................................................................ Glossary-1
Index ........................................................................................................................ Index-1
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Room Pressurization Control Application Guide
How To Use This Application Guide
This section covers this how this application guide is organized, how to access help, and
where to send comments regarding this document.
How This Guide is Organzed
This application guide contains the following chapters:
•
Chapter 1, Introduction, introduces the topic of room pressurization by discussing the
importance of room pressurization, the objective of this application guide, and the
intended audience.
•
Chapter 2, Pressurization Applications, defines room pressurization and discusses
positive and negative room pressurization; building pressurization; and room
pressurization applications.
•
Chapter 3, Room Pressurization Design Criteria, discusses design considerations,
room pressurization reference data, room pressurization factors, and leakage area.
•
Chapter 4, Room Static Pressure Control, discusses static pressure control in rooms
in laboratories and healthcare facilities.
•
Chapter 5, Air Pressurization Fundamentals, introduces ventilation system pressure
components and summarizes important factors in relation to these components.
•
The Glossary describes terms and acronyms used in this manual.
•
The Index helps you locate information presented in this manual.
Getting Help
For more information about room pressurization, contact Greg DeLuga
Greg.Deluga@sbt.siemens.com in Systems & Advanced Technology.
Where To Send Comments
Your feedback is important to us. If you have comments about this manual, please submit
them to technical.editor@sbt.siemens.com
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I
Room Pressurization Control Application Guide
II
Siemens Building Technologies, Inc.
Chapter 1—Introduction
Chapter 1 introduces the topic of room pressurization and discusses the:
•
Importance of Room Pressurization
•
Objective of this Application Guide
•
Intended Audience
Importance of Room Pressurization
Proper room pressurization is an absolute necessity to ensure occupant health and safety as
well as preserve the purity and integrity of an increasing array of manufactured products. In
healthcare facilities, proper room pressurization is vital for protecting workers and patients
from exposure to harmful and, sometimes, deadly airborne pathogens. Room pressurization
is an important factor in guarding against occupant and worker exposure to hazardous fumes
and biological agents in many types of laboratories. Additionally, proper room pressurization
is necessary to prevent cross contamination and ensure that the required level of
environmental sterility and purity is maintained for food and drug processing, and in microelectronic and optical manufacturing industries.
With the ever-increasing focus on ensuring occupant protection and the increasingly stringent
needs for environmental purity in production facilities, maintaining proper room pressurization
is a key requirement for facility ventilation systems. This has imposed increased responsibility
on the ventilation system designer as well as the facility operational staff to ensure that
ventilation systems are able to provide and continue to provide the required levels of room
pressurization.
Objective of this Application Guide
This Application Guide is intended to serve as a comprehensive reference to room
pressurization. It provides information ranging from the fundamental concept of room
pressurization though the design and configuration of room pressurization ventilation
systems, and their associated control systems. The information in this guide is applicable to
chemical and biological laboratories, animal research facilities, hospital isolation and
treatment rooms, industrial clean rooms, pharmaceutical processing areas, and virtually any
application where maintaining a specific static pressure relationship to another internal area
or to the outdoors is desired.
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Room Pressurization Control Application Guide
Intended Audience
This Application Guide has been written to serve an audience with a wide range of interests:
2
•
Persons with overall administrative and management responsibilities in facilities that
require room pressurization will find this guide an informative tutorial on the concept
and ramifications of room pressurization. The content is intended to enable such
individuals to effectively dialogue with their facility operations and safety
professionals concerning facility safety and operational issues involving room
pressurization.
•
Persons with day-to-day operational responsibilities in virtually all types of facilities
can use this guide as a general reference on the subject of room pressurization. This
guide also provides practical information that will enable such individuals to
determine whether existing ventilation systems are capable of and are functioning in
a manner that provides the required room pressurization.
•
Safety professionals in research and industrial facilities, and supervisory personnel in
clinical treatment facilities should find the room pressurization concepts and other
basic information presented in this guide helpful in understanding the role of room
pressurization in protecting workers and occupants from airborne hazards.
•
HVAC system designers should find the descriptions of room pressurization systems
(and their associated control systems) informative and helpful in choosing one type
of system over another. This Application Guide also provides direct design
assistance for properly configuring ventilation systems and their associated controls
to meet specific room pressurization applications.
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Chapter 2—Pressurization Applications
Chapter 2 defines room pressurization and discusses these topics:
•
Room Static Pressure
•
Building Pressurization
•
Room Pressurization Applications
Room Static Pressure
When referring to room pressurization, the term static pressure is usually applied to establish
the fact that the pressure is not due to any air motion. Rather, the pressure exists
independently of any motion of the air.
Every pressure measurement or pressure value is based on a difference between two points
or locations. Therefore, a room's static pressure value is the difference in static pressure
between the room and another location. Most often the other location is an adjoining room or
corridor. However, the other location can also be:
•
The outdoors.
•
The floor above or below.
•
A stairwell.
•
A more distant area in the building.
It is important to use the terms positive and negative when referring to room static pressure
values since it is not always apparent which location is at the higher (or lower) static
pressure. Positive indicates that the location has the higher static pressure while negative
indicates that the location has the lower value. So, stating that a room is positively
pressurized indicates that the room has a higher static pressure than the reference area.
Conversely, stating that a room is negatively pressurized indicates that the room has a lower
static pressure than the reference area. Note also that a room can, and often does, have
multiple static pressure values. For example, a room may be negative 0.01 inches w.c. with
respect to an adjoining corridor and it could also be positive 0.01 inches w.c. with respect to
another room or area. Because pressure values are always referenced to another area, there
is no limit to the number of pressure values that a given room or space may have with
respect to other locations.
If the static pressure of a certain room were negative 0.02 inches w.c. with reference to an
adjacent corridor, it would be equivalent to saying that the static air pressure of the room is
0.02 inches w.c. lower than the static pressure of the corridor. It could also be stated that the
corridor is positive 0.02 inches w.c. with respect to the room.
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Room Pressurization Control Application Guide
NOTE:
In Chapter 5, Table 4. Velocity Pressure vs. Airflow Velocity shows that a
differential pressure of 0.02 inches w. c. would result in an airflow velocity from
the positive space to the negative space of about 566 feet per minute.
Given the chance, air in a positively pressurized room (higher static pressure) will flow out of
the room and into an area of lower static pressure. Conversely, air will flow into a room that is
negatively pressurized (lower static pressure) from a higher pressure (positively pressurized)
room or area. The potential direction of airflow is always from the higher pressure area
(positively pressurized) toward a lower pressure area (negatively pressurized).
Building Pressurization
A proper building ventilation system design ensures that all areas of the building are at a
slight positive pressure with reference to the outdoors to prevent outside air from entering the
building. Note that even rooms within a building that are negative with respect to the adjacent
corridor or another room, are often at a positive static pressure with respect to the outside of
a building. Without building pressurization, outside air (in accord with the positive-to-negative
airflow) will enter a building in several ways, including door clearance openings, construction
cracks, gaps, and even the porosity of the outer walls. However, by maintaining the inside of
the building at a slight positive static pressure with respect to the outside, this undesirable
inward airflow is prevented.
Without overall positive building pressurization, the inflow of outside air can pose many
problems:
4
•
Unfiltered outside air can deposit airborne dirt wherever the air enters. For example,
around window frames, electrical outlets, etc.
•
Unfiltered outside air can bring in harmful contaminants and unpleasant odors
creating health and inside air quality (IAQ) problems.
•
Cold outside air can produce drafts and cold spots especially near the outer walls,
which adversely affect an otherwise good comfort control system.
•
Humid outside air can condense on cooler interior surfaces of the building causing
dampness, wet spots and promoting growth of mold. This would very likely occur in
unseen places, such as inside wall spaces and above ceilings.
•
Air entering through door clearances or window gaps can create annoying whistling
sounds.
•
A building interior that is negative with respect to the outdoors will result in hard to
open entry and exit doors along with annoying inward drafts whenever a door is
opened.
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Room Pressurization Applications
Room Pressurization Applications
In most applications, room pressurization is applied to control the direction of room transfer
airflow. Transfer air is air that is not directly supplied to a room or exhausted from the room
by the room’s ventilation system. Rather, transfer air is air that may enter the room or leave
the room as a result of pressure differentials, through passageways other than the ventilation
system ductwork. Typically, these passageways include the clearance area around doors
and poke-throughs around electrical and plumbing services. These air passageways also
include cracks, gaps, and even the porosity that comprises the room construction. In most
situations where room pressurization is required, using close fitting doors can minimize the
amount of these air passageways. It can also be reduced by caulking and sealing of all
cutouts that were made to accommodate electrical conduits, piping, and other room
equipment. However, depending on individual needs and applications, rooms may also be
equipped with air transfer grills to facilitate transfer air movement into or out of the room.
Most room pressurization applications are intended to control the direction of the transfer air
rather than prevent air transfer from occurring. Directional airflow is used to prevent airborne
contaminants from entering or leaving a specific room or cluster of rooms. Room
pressurization for contamination prevention can be grouped into several categories
depending upon the type of room and its purpose. The following subsections describe the
most common room pressurization applications.
Chemical Laboratories
A chemical laboratory room must be maintained at a negative static pressure with reference
to adjoining non-laboratory rooms to ensure that transfer air will not flow out of the laboratory
room and into the adjoining areas. Rather, transfer airflow should be directed into a
laboratory room from the adjacent areas (corridors or other non-laboratory rooms) to prevent
laboratory room chemical fumes from migrating out of a laboratory room. Although it is
desirable to keep laboratory room air from migrating into other areas of a building as a health
safeguard, it is also important from an IAQ perspective. Room air from chemical laboratories
often contains some trace amount of chemical fumes or gasses. Although the concentration
of chemicals or gasses in the air might be extremely slight and not a health hazard, building
occupants may react to the odor and assume that they are being exposed to an unhealthy or
hazardous environment. Therefore, it is advantageous to prevent airflow out of laboratory
rooms by ensuring that the rooms are maintained at a negative static pressure with reference
to adjoining non-laboratory areas.
Biological Laboratories
Biological laboratory rooms must also be maintained at a negative static pressure to prevent
airflow out of the laboratory room. Aside from preventing chemical odors from leaving the
room, the inward directional airflow created by negative room pressurization is intended to
prevent airborne pathogens from migrating out to other building areas. Biological laboratories
are classified as Biosafety Level 1, 2, 3 or 4 with respect to the potential hazard that the
particular laboratory presents due to the substances present and the nature of the work
performed.
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Room Pressurization Control Application Guide
Biosafety Level 1 (BL-1)
Biosafety Level 1 (BL-1) is the lowest biosafety classification and applies to biological
laboratories that need no special ventilation requirements apart from an adequate room
ventilation rate. BL-1 laboratories present a low health risk to individuals and the community.
Such laboratory rooms are only required to be separated from public areas by a closed door.
Work may take place on bench tops and without containment provisions (biological safety
cabinets). BL-1 laboratories are not specifically required to be negatively pressurized,
although most ventilation designs do incorporate negative pressurization for odor control.
Biosafety Level 1 activities may also be conducted in a general chemical laboratory. In this
case, the room ventilation must also meet the requirements for a general chemistry
laboratory.
Biosafety Level 2 (BL-2)
Biosafety Level 2 (BL-2) is the classification that applies to the largest number of
microbiological and biomedical laboratories. Although some of the substances present can
result in an infection, they may still be manipulated or processed on open benches provided
that the potential for a biological aerosol release is very low and the seriousness of a
subsequent infection is also very low. BL-2 laboratories present only a small to moderate risk
to individuals in the room and only a limited risk to the community. Such laboratories require
self-closing doors and any work where biological aerosols are likely to be released must be
done within biological safety cabinets. BL-2 laboratories are required to be negatively
pressurized to prevent airborne pathogens and aerosols from leaving the room.
Biosafety Level 3 (BL-3)
Biosafety Level 3 (BL-3) is the classification that applies to laboratory rooms that work with
highly infectious agents that are transmissible as aerosols (such as, Anthrax, Tuberculosis.
etc.) and the room ventilation requirements are stringent. BL-3 rooms must use biological
safety cabinets for all work being done in the room. These laboratory rooms must also be
separated from other building areas and have a double door (airlock) entry arrangement.
Most often, these laboratories also have adjacent areas under somewhat lesser negative
pressurization for worker gowning, showering, and other support purposes. Ventilation
system provisions for BL-3 laboratories require a single pass (no-return air) system with
HEPA1 filtered exhaust. The room static pressure must be negative to ensure against
outward airflow. Maintaining an effective room barrier with minimal penetrations makes it
advantageous to use a dedicated room ventilation system inside the laboratory unit.
1
6
High Efficiency Particulate Air filter. HEPA filters are capable of entrapping biological aerosols.
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Room Pressurization Applications
Biosafety Level 4 (BL-4)
Biosafety Level 4 (BL-4) is the classification that applies to laboratories2 that present the
highest risk to individuals in the laboratory, the facility, and to nearby communities. As such,
they must be designed in accord with very strict safety requirements. BL-4 laboratories
require all of the BL-3 provisions plus use of the highest classification of biological safety
cabinet (Class III glove box) for all work performed. These labs must be geographically
isolated and functionally independent from the rest of the buildings associated with a facility
having this type of laboratory.
BL-4 laboratories require a 100% outside air, non-re-circulating dedicated ventilation system
with HEPA filtered intake and exhaust air. The laboratory itself must be at a relatively high
negative static pressure and the adjacent support areas, such as gowning, showering, etc.,
are somewhat less negatively pressurized. However, all areas associated with the laboratory
must also be at a negative static pressure (although at a somewhat lower level) with respect
to the other non-laboratory areas of the building.
Hospitals
Operating rooms, intensive care units, nurseries, and certain other areas in hospitals are
normally positively pressurized to prevent harmful pathogens (germs) from entering these
rooms. In these instances, transfer air can only flow out from the positively pressurized room
to prevent or at least retard airborne contaminants from entering the room. Certain patient
rooms, particularly those for treating AIDS patients and any patient that is at a high risk of
infection, are also maintained at a positive pressure (referred to as protective isolation rooms)
to ensure patient protection via proper directional airflow.
Hospital rooms may also be negatively pressurized (referred to a infectious isolation) to
prevent airborne pathogens from patients with a contagious disease, such as Tuberculosis,
from migrating out and infecting hospital workers or other patients. In these instances, the
room’s negative pressurization ensures that the direction of transfer airflow is always into the
infectious isolation room.
Animal Holding Rooms
In general, rooms used to house animals and perform research functions must be kept at a
positive static pressure level. A positive static pressure level prevents the animals from being
contaminated by airborne pathogens that can enter the room. In contrast, research facility
support areas, such as cage washing, or where contaminated or animal waste material is
present, should be kept at a negative static pressure. A negative static pressure level
prevents odors from emanating and prevents the airborne transmission of harmful
substances to other areas of the facility.
2
Relatively few BL-4 laboratories exist since they deal with exotic, highly dangerous (with no known cure) infectious substances.
Such laboratory facilities are usually isolated from other buildings and protected with a sophisticated security system to ensure
against any unauthorized access.
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Room Pressurization Control Application Guide
Clean Rooms
Nearly all pharmaceutical, biomedical, microelectronics, as well as the optical industry and
many others, need to prevent contamination of their products or processes by maintaining a
clean room environment. In particular, the pharmaceutical and biomedical production facilities
require very careful room pressurization control to ensure against biological and chemical
agent contamination as a condition for meeting and maintaining regulatory (FDA)
compliance.
Microelectronics fabrication and electronic component assembly areas need a high degree of
environmental purity to prevent particulate contamination in the manufacturing process. As
microelectronic chips and data storage media continue to become more miniaturized while
providing greater processing power and data storage capacity, even the smallest airborne
particle could create an undesirable circuit-to-circuit bridge. Today’s microelectronic circuit
conductors are 100 times smaller in width than a human hair and use even smaller spacing
between conductors. These applications all require the most intensive form of contamination
prevention. As some components created by the micro electronics and optics industry
become even more miniaturized, contamination concerns extend down to the single
molecular level.
In all clean room applications, infiltration of contaminants from adjacent areas and the
outdoors (dust particles, pathogens, aerosols, etc.) is prevented by maintaining the clean
room space at a substantial positive pressure with respect to the surrounding areas. In many
clean room applications the surrounding areas are also positively pressurized in order to act
as buffer zones to help ensure against accidental contamination of the most critical areas.
8
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Room Pressurization Applications
Table 1 lists various room pressurization applications and the normal static pressure
relationships required.
Table 1. Room Static Pressurization Applications.
Application
Recommended
Static
Pressurization
Level
Relationship
to Adjacent
Area(s)
Comments
Inches
Pascals
Chemical Laboratory
0.01
to
0.02
2.5
to
5
Negative
These values apply to general chemistry
laboratories. (Higher hazard laboratories
such as those handling toxic chemicals or
radioactive substances should be at an
increased negative pressurization level.
Such labs may require double door entry
provisions and a two-stage pressurization
level.)
Biological lab: BL-1 & BL-2
0.01
2.5
Negative
Although negative pressurization is not
specifically required for BL-1 labs, it is
recommended for odor control.
Biological lab: BL-3
0.01
to
0.03
2.5
to
8
Negative
Lab support areas should be at lesser
negative pressurization levels than the
laboratory itself to ensure airflow is from
the area of least risk to highest risk.
Biological lab: BL-4
0.01
to
0.05
2.5
to
12
Negative
Airlock entry/exit provisions, clothing
change areas, other support areas, and
the laboratory itself should all be under
increasingly negative pressure to ensure
airflow is toward the laboratory as the
highest hazard area.
Animal Holding Room
0.01
2.5
Positive
Air supplied to surgery rooms to meet
ventilation and pressurization
requirements should not be re-circulated
from other areas.
Animal Support Areas
0.01
2.5
Negative
These general support areas include
autopsy rooms, cage washing, and feed
storage rooms, as well as incinerator and
sterilizer rooms.
Animal Surgical Room
0.01
to
0.02
2.5
to
5
Positive
Surgical support areas such as gowning,
hand washing, etc., should be positively
pressurized at multiple pressurization
levels, keeping the surgical room as the
most positive area.
Hospital - Protective
Isolation Room
0.01
2.5
Positive
The CDC actually recommends a
minimum pressurization level of 0.001
i h
f h
h
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Room Pressurization Control Application Guide
Table 1. Room Static Pressurization Applications.
Application
Recommended
Static
Pressurization
Level
Relationship
to Adjacent
Area(s)
Comments
Inches
Pascals
Hospital - Infectious
Isolation Room
0.01
2.5
Negative
inches w.c. for these rooms; however,
most designs incorporate higher
pressurization levels. The most effective
isolation room arrangement incorporates a
lower pressurized anteroom as a buffer
zone between the isolation room and
corridor.
Hospital Surgical Suite
0.01
to
0.02
2.5
to
5
Positive
Surgical suites include gowning, hand
washing, etc. All areas of the surgical
suite should be positively pressurized
using dual or multi-staged pressurization
levels to keep the surgical room the most
positive area.
0.05
to
0.10
12.5
to
25
Positive
Pharmaceutical processing rooms should
be 0.05 inches positive with respect to
rooms leading into the processing room.
Such rooms should be at least 0.02
inches positive with respect to each other
and non-classified areas.
Delivery
Nursery
Cystoscopy
Trauma
Clean Room:
Pharmaceutical Mfg.
Clean Room:
Micro-Electronic Mfg.
Data Storage Mfg.
Optics Mfg.
10
0.05
to
0.20
12.5
to
50
Positive
Clean spaces used for highly critical
processes should be at least 0.05
inches positive to any connecting
space and so on. An anteroom having
0.02 inches less positive
pressurization should separate
adjoining spaces that are of the same
classification but used for different
purposes.
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Chapter 3—Room Pressurization Design
Criteria
Chapter 3 discusses the following topics:
•
Design Considerations
•
Room Pressurization Reference Data
•
Room Pressurization Factors
•
Leakage Area
Design Considerations
As stated previously in this guide, the positive or negative room static pressurization
relationship between two spaces determines the potential for transfer airflow between them.
Unintentional openings that allow transfer airflow in or out of a negatively or positively
pressurized room are cumulatively referred to as the room’s leakage area. However, if there
is no room leakage area (the room is sealed off), then there can be no airflow in or out of the
room, even though a static pressure difference exists between the room and other areas.
In buildings, rooms cannot generally be totally sealed off. Except for extreme situations, such
as Biological Level 4 laboratories, there is little reason to try to maintain a perfect seal or
barrier between pressurized spaces. Personnel, equipment, and contents must be allowed to
continually enter and leave such spaces. Thus, a perfect seal or barrier is not a practical
solution for the prevention of unwanted air transfer.
This consideration leads to the fundamental reason for maintaining a differential static
pressure relationship between two spaces. Aside from a perfect seal or perfect isolation, the
next best way to prevent unwanted air transfer is to maintain a differential pressure that only
allows air transfer in an acceptable or desired direction. The differential static pressure
relationship between spaces is then created and maintained by a properly designed and
controlled ventilation system.
The most commonly used means to maintain a room at a negative or positive pressure is by
airflow tracking. Airflow tracking, also referred to as volumetric airflow tracking, maintains a
fixed differential airflow or offset between the total air supplied to the room and the total air
exhausted from the room. The total air supplied to and the total air exhausted from a room to
which airflow tracking is applied is considered to be the air that is provided by a fan powered,
ducted, ventilation system.
Figure 1 illustrates the airflow relationship of a negatively pressurized room where the total
room exhaust airflow exceeds the total room supply airflow by the fixed airflow tracking offset.
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Room Pressurization Control Application Guide
AIRFLOW
TRACKING
OFFSET
LAB0192R1
ROOM
SUPPLY
AIRFLOW
TOTAL ROOM
EXHAUST
AIRFLOW
Figure 1. Negatively Pressurized Room Airflows.
For a negatively pressurized room, airflow tracking ensures that the total amount of air
exhausted from the room always exceeds the amount of air that is supplied to the room. This
creates a slight vacuum effect in the room, which causes air from adjacent areas to flow into
the room through the room’s leakage area.
For a positively pressurized room, airflow tracking ensures that the total amount of air
exhausted from the room is always less than the amount of air that is supplied to the room.
This creates an excess amount of air in the room, which tends to flow out from the room and
into adjacent areas.
Applying airflow tracking as the method of achieving room static pressurization does not
ensure that a specific room static pressure value is attained. However, it does ensure that the
room static pressure will be negative or positive as desired, and that the desired directional
airflow into or out of the room will be maintained. Since the goal of room pressurization is to
always ensure proper directional airflow, airflow tracking is a very reliable way of achieving
this goal.
Figure 2 shows a chemical laboratory room with an airflow arrangement that maintains a
negative static pressure. Both the room supply airflow and the total room exhaust airflow
must be controlled to be at specific values to achieve the required airflow tracking offset for
the room. The resulting deficiency in the room’s supply air creates the negative static
pressure relationship between the laboratory room and the corridors. Since the laboratory
room static pressure is negative with respect to the corridors, air always has a tendency to
flow into the laboratory from the corridors, thus preventing undesirable airflow from the
laboratory room to the corridors.
12
Siemens Building Technologies, Inc.
Design Considerations
SERVICE CORRIDOR
0.00 in. w.c.
TOTAL ROOM
EXHAUST AIR
TRANSFER
AIRFLOW
INTO
ROOM
LAB0193R1
ROOM
SUPPLY
AIR
LABORATORY ROOM
- 0.01 in. w.c.
(-2.5Pa)
MAIN CORRIDOR
0.00 in. w.c.
Figure 2. Laboratory Room at a Negative Static Pressure with Respect to the Adjacent
Corridors.
Siemens Building Technologies, Inc.
13
Room Pressurization Control Application Guide
Room Pressurization Reference Data
As indicated in Table 1, good ventilation system design for chemical laboratory rooms should
ensure that the rooms are at a negative static pressure of approximately 0.01 inches w.c.
with respect to adjacent non-laboratory spaces, such as a corridor3.
The specific relationship between Room Differential Pressure, Room Leakage Area, and the
Differential Airflow is expressed by the following equations4 based on inch-pound (IP) or
metric (SI) units.
(IP)
Q = 2610 A (dP)1/2
where:
Q is the differential airflow in Cubic Feet per Minute (cfm)
A is the total room leakage area in Square Feet
dP is the differential pressure in Inches of Water (inches w.c.)
(SI)
Q = 840 A (dP)1/2
where:
Q is the differential airflow in Liters per Second,
A is the total room leakage area in Square Meters
dP is the differential pressure in Pascals
The graph in Figure 3 depicts the relationship between room differential pressure, room
leakage area and differential airflow. Figure 3 also shows room leakage area in square feet
as a family of curves on the graph. The differential airflow (difference between the total room
supply and total room exhaust airflows) is shown as Cubic Feet per Minute (cfm) along the
horizontal axis of the graph. The resulting room differential (static) pressurization values are
shown as Inches of Water (inches w.c.) along the vertical axis.
To determine what room differential airflow is needed to provide a particular differential
pressure, the desired differential pressure value on the vertical axis is followed to where its
horizontal line intersects the room leakage area curves. The required differential room airflow
is then indicated along the bottom of the graph directly below the points of intersection.
3
4
14
In applications where it is necessary to prevent contamination by air flowing into the laboratory room from adjacent spaces, the
laboratory room can be maintained at a positive static pressure. However, the laboratory room must be separated from the
adjoining area (a corridor) by a vestibule room that is maintained at a negative static pressure.
Equations taken from 1999 ASHRAE Application Handbook, Fire and Smoke Management Section, Page 51.5.
Siemens Building Technologies, Inc.
Room Pressurization Factors
0.020
0.019
0.1 Ft2
0.018
ROOM LEAKAGE AREA CURVES
0.017
0.2 Ft2
0.016
0.015
0.3 Ft2
0.014
0.013
DIFFERENTIAL
PRESSURE
0.4 Ft2
0.012
0.011
0.5 Ft2
0.010
INCHES
of
WATER
0.009
0.6 Ft2
0.008
0.007
0.75 Ft2
0.006
0.005
1.0 Ft2
0.004
0.003
1.5 Ft2
LAB0194R1
0.002
0.001
0.000
0
25
50
75
100
125 150
175 200
225 250 275
300 325 350 375
400
ROOM DIFFERENTIAL AIRFLOW - CFM
Figure 3. Room Differential Airflow vs. Differential Pressure for Various Room Leakage Areas.
Room Pressurization Factors
As Figure 3 shows, a room’s static pressurization value is wholly dependent on the
differential airflow and the room’s leakage area. For instance, the graph shows that for a
room with 1.0 sq. ft. of leakage area, a differential pressure (dP) of just under 0.010 inches
w.c. occurs when the difference between the room’s supply air and the total room exhaust is
250 cfm. This relationship exists regardless of what the room’s ventilation rate is (air changes
per hour). Therefore, to maintain a specific room pressurization value, the room’s differential
airflow must be controlled and maintained at the appropriate value.
Leakage Area
Most modest sized rooms, such as a two-person laboratory with two hinged doors, will have
a total room leakage area of about 0.5 to 1.0 sq. ft. even with relatively tight construction. To
obtain a tighter room, extensive sealing and meticulous attention to poke-throughs (places
where conduit, piping, ducts and other items pass through the room’s walls, ceiling, and floor)
is required. However, room pressurization can be more easily maintained at a constant value
if the room construction is not extremely tight.
Siemens Building Technologies, Inc.
15
Room Pressurization Control Application Guide
Figure 3 shows that when a room has as little as a 0.2 sq. ft. leakage area, a small change in
differential airflow such as only 25 cfm causes a rather large variation in the resulting
differential pressure value. Whereas, the same 25 cfm differential airflow variation for a room
having a 1.0 sq. ft. leakage area would exhibit a much smaller differential pressure variation.
Except for biological laboratory rooms where highly contagious pathogens are present, and
very critical clean rooms, little is gained by attempting to make the room exceptionally tight.
Rather, a room’s static pressure can be maintained at a more stable value if the total room
leakage area is perhaps between 0.5 and 1.0 sq. ft.
Note also that if room leakage area were significantly greater than about 1.5 sq. ft. the
resulting leakage area curve would lie close to the bottom of the chart in Figure 3. Trying to
maintain a 0.01 Inch static pressure differential for that much leakage area requires very high
differential airflows. Experience indicates that a negative 0.01 inches w.c. room differential
pressure (typical for chemical laboratory rooms) cannot be maintained in rooms that have a
leakage area much greater than about 1.5 square feet due to the excessively high differential
airflow required. Thus, except for very large rooms, a reasonable room tightness of between
0.5 and 1.5 sq. ft. of leakage area is recommended when a 0.01 inches w.c. room differential
pressure is desired.
It is also very important to consider the effect that opening a room door will have on room
pressurization. Opening a single width door of average size will increase a room's leakage
area by approximately 20 square feet or more. The resulting room leakage area curve would
essentially lie horizontally along the bottom of the graph and result in a near zero differential
pressure value for the room. With an open door, no appreciable differential static pressure
value can be maintained without an excessively high amount of differential airflow.
Consequently, it should be realized that a room’s differential pressure drops to near zero
anytime a door is opened. However, the occurrence of near zero differential pressure should
not be interpreted as a failure to contain contaminants or prevent an undesirable air transfer,
since the proper directional airflow (inward for a negative room and outward for a positive
room) will still be maintained even with a (temporary) large increase in room leakage area.
16
Siemens Building Technologies, Inc.
Chapter 4—Room Static Pressure
Control
Chapter 4 discusses static pressure control in rooms in the following facilities:
•
Laboratories
•
Healthcare Facilities
The most appropriate method of controlling a room's static pressure is dependent upon the
room application. Since the purpose for maintaining a differential pressure relationship
between rooms is to prevent cross contamination, the consequences of cross contamination
must be known in order to determine the most appropriate method of control. If the main
concern is to contain unpleasant odors, the consequences might be very small. On the other
hand, very serious consequences result if occupant safety and health are involved. These
are most typically associated with laboratories involved with highly infectious biological
aerosols, highly toxic laboratory chemicals and others such as radioactivity.
This section covers room pressurization control applications for many applications and
provides detailed recommendations for achieving the desired pressurization relationship
between a particular space and adjacent areas.
Laboratories
Airflow Tracking Static Pressure Control
Airflow tracking is the preferred room static pressurization control approach for most
laboratories. Airflow tracking can maintain desired room pressurization for both constant air
volume (CAV) ventilation systems as well as variable air volume (VAV) ventilation systems.
Figure 4 shows the essential components of a typical chemical laboratory room ventilation
system using a single duct supply terminal. This room ventilation system arrangement
enables using airflow tracking for maintaining room pressurization. The Room Controller and
the Fume Hood Controllers precisely control the room airflow as described below.
The Fume Hood Controllers modulate the exhaust airflow of the FUME HOODS to always
maintain the proper amount of exhaust. For constant air volume (CAV) fume hoods, the fume
hood exhaust is maintained at a constant value regardless of the amount that the fume hood
sash is open. For variable air volume (VAV) fume hoods the fume hood exhaust is modulated
so that the fume hood face velocity (incoming airflow) is maintained at the desired constant
value.
Siemens Building Technologies, Inc.
17
Room Pressurization Control Application Guide
SUPPLY
TERMINAL
ROOM
GENERAL
EXHAUST
CFM
FUME
HOOD
CONTROLLERS
CFM
CFM
CFM
LAB0195R1
ROOM
CONTROLLER
Figure 4. Airflow Tracking Control Arrangement for a Chemical Laboratory Room.
The specific amount of air exhausted by a VAV fume hood depends on the extent that its
respective sash is open5. Aside from controlling the fume hood exhaust airflow, each Fume
Hood Controller also provides an output signal to the Room Controller that indicates the
exact amount of fume hood exhaust airflow. The Room Controller is apprised of the amount
of exhaust air that is being exhausted by each fume hood in the room.
In addition to fume hoods, the Room General Exhaust that is directly controlled by the room
controller may also draw air out of the laboratory room. A room general exhaust is often
required for VAV laboratories with VAV fume hoods for one of the following reasons:
•
Allow more air to enter the room from the supply terminal.
•
Maintain sufficient room ventilation.
•
Maintain the desired room temperature when the fume hood sashes are closed.
The Room Controller maintains direct control over the Supply Terminal that provides the
supply make-up air for the room. In a CAV laboratory, the supply airflow into the room is
normally maintained at a constant value since the fume hood exhaust normally remains
constant. However, in a VAV laboratory the total room exhaust is dependent upon the fume
hood exhaust that varies in accordance with the fume hood sash position. Thus, the Room
Controller in a VAV laboratory must modulate both the incoming supply airflow via the Supply
Terminal and the Room General Exhaust to maintain a proper balance between the total
room exhaust airflow and the incoming supply airflow.
5
18
A VAV fume hood controller uses an airflow sensor and a modulating damper in the fume hood exhaust to maintain the
required exhaust airflow in accordance with the fume hood’s total open sash area. This ensures that fume hood face velocity
remains constant for all sash positions. Each VAV fume hood controller continuously sends the fume hood exhaust airflow
value to the room controller.
Siemens Building Technologies, Inc.
Laboratories
In a VAV laboratory the Room Controller must modulate the Room General Exhaust to
ensure that there is sufficient total room exhaust to meet the required minimum room
ventilation rate (ACH). The ROOM CONTROLLER must also modulate the Room General
Exhaust to ensure that there is sufficient total room exhaust for the amount of supply airflow
necessary to maintain the desired room temperature.
To maintain the required room static pressure, the room controller modulates the total room
supply airflow to maintain the desired airflow tracking offset between the total room supply
and total room exhaust airflows. Whenever the room's total exhaust airflow changes, the
room controller adjusts the room supply airflow to maintain the airflow tracking offset.
With a VAV ventilation system there may be times when the room will require more supply
airflow to cool the room in order to maintain the desired room temperature. In such instances,
the room controller increases the amount of supply airflow to maintain the desired room
temperature. In addition, the room controller also increases the room general exhaust so that
a corresponding amount of room air is exhausted to maintain the airflow tracking offset.
Summarized, the many functions of the room controller include:
•
Maintaining the proper amount of room general exhaust to ensure that the required
minimum room ventilation rate6 is always maintained.
•
Maintaining the proper amount of room supply airflow needed to maintain a constant
airflow tracking offset between the total room exhaust and room supply airflows.
•
Increasing the supply airflow when needed to maintain the proper room temperature.
Simultaneously increasing the room general exhaust to maintain the required
constant airflow tracking offset.
Airflow Tracking Control Considerations
As stated previously, the specific airflow tracking offset necessary to maintain a certain room
static pressure value is dependent upon the room's total leakage area. However, it is not
possible to know what a particular room’s total leakage area will be before construction is
completed. When specifying the room control scenario, a ventilation system designer must
ensure that the proper room airflow tracking offset will be maintained. To address this need,
two options are suggested:
1. A specific room airflow tracking offset may be estimated by the designer based on past
experience or as a practical maximum limit. This results in a final room differential
pressure that may or may not be within the desired pressure range
2. A specific room static pressure value may be specified. This requires some trial and error
effort during the test and balancing process to determine the necessary airflow tracking
offset. This means that the room control system must be set up to maintain the required
airflow tracking offset.
6
In negatively pressurized rooms the ACH rate is determined by the total room exhaust airflow. In positively pressurized rooms
the ACH rate is determined by total room supply airflow.
Siemens Building Technologies, Inc.
19
Room Pressurization Control Application Guide
Option 1, designing the ventilation system to maintain a specific room differential airflow, is
generally recommended for applications where a lower room differential pressure (perhaps
0.01 to 0.02 inches w.c.) is sufficient. Recall that the primary purpose of room pressurization
is to create the proper directional airflow to prevent or retard undesirable transfer of air.
Therefore, it is the airflow direction rather than an arbitrary static pressure7 that should be the
design goal of a laboratory ventilation system. This approach enables the room airflow
tracking control scenario to be set up without a lengthy trial and error process.
Option 2, choosing a specific room static pressure, is usually only necessary for applications
where higher level room static pressures are required and where cross contamination must
be strictly prevented. This presents a more difficult challenge for the testing and balancing
process, and is not recommended for most laboratory applications. In actual practice it
typically requires simultaneous adjusting the airflow in many rooms to achieve the correct
airflow balance. This is particularly the case when several rooms adjoin a common corridor.
Furthermore, it may not be possible to achieve a given static pressure relationship if a room’s
leakage area is too large or the adjacent space does not have sufficient excess supply
makeup air (such as a corridor).
For example, consider using Option 2 in a situation where 10 laboratory rooms adjoin a
common corridor. Also assume that the balancing process finds that an average airflow
tracking offset of 400 cfm is necessary to maintain a specific negative room pressurization
value. The corridor will, therefore, need to have a supply makeup airflow of 10 rooms × 400
cfm (or 4000 cfm) to attain the required room static pressure. However, the total amount of
room transfer air required (in this case 4,000 cfm) would not be known during the design
process, and might exceed the actual amount of corridor supply air that is actually available.
In contrast, following Option 1 would enable an airflow tracking offset to be initially chosen
and specified for each room. Then the total amount of excess corridor air would be known.
Even when a laboratory room door is fully opened and the doorway area increases the total
leakage area of a room, no change in the amount of room transfer airflow occurs. Although
the room static pressure will diminish, the desired airflow direction (into the negatively
pressurized room) will be maintained.
Finally, a specific airflow tracking value cannot ensure that a constant positive or negative
static pressure value will be maintained over the life of a building. As seasons pass and
building conditions change over time, the actual room static pressure level will also
undoubtedly vary somewhat from what it was initially. This necessitates periodically checking
(at least annually) and, perhaps, readjusting the differential tracking value as needed.
Although there will likely be some variation in the specific static pressure level when using
airflow tracking, it should again be noted that the primary goal of maintaining a room at a
negative (or positive) static pressure is to ensure that an undesirable transfer of air does not
occur. Airflow tracking will meet this goal by maintaining the proper directional airflow.
7
20
Differential pressure should be used to ascertain if the airflow is in the right direction (into the room) and as a convenient
reference for periodic testing to ensure that the amount of differential airflow is consistent over time.
Siemens Building Technologies, Inc.
Laboratories
Direct Pressure Control
The direct pressure control method uses a room pressure controller with a static pressure
sensing arrangement. This enables the controller to modulate the room’s supply and total
exhaust airflows as needed to maintain the room static pressure set point.
Figure 5 shows the essential components of a typical chemical laboratory room ventilation
system using a single duct supply terminal. The room ventilation system may be either CAV
or VAV with the room airflow being precisely controlled by the Fume Hood Controllers and
the Room Controller.
The Fume Hood Controllers modulate the exhaust airflow of the Fume Hoods to always
maintain the proper amount of exhaust. For constant air volume (CAV) fume hoods, the fume
hood exhaust is maintained at a constant value regardless of the amount that the fume hood
sash is open. For variable air volume (VAV) fume hoods, the fume hood exhaust is
modulated so that the fume hood face velocity (incoming airflow) is maintained at the desired
constant value.
The specific amount of air exhausted by a VAV fume hood depends on the extent that the
sash is open8. Aside from controlling the fume hood exhaust airflow, each Fume Hood
Controller also provides an output signal to the Room Controller that represents the amount
of fume hood exhaust airflow. This output signal keeps the Room Controller apprised of the
amount of exhaust air that is being exhausted by each fume hood in the room.
In addition to the air that is being exhausted from the fume hoods, air may also be exhausted
from the laboratory room by the Room General Exhaust that is directly controlled by the
Room Controller. A Room General Exhaust is often required for VAV laboratories with VAV
fume hoods to allow more air to enter the room from the supply terminal to maintain sufficient
room ventilation or maintain the desired room temperature when the fume hood sashes are
closed.
The Room Controller also maintains direct control over the Supply Terminal that provides the
supply make-up air for the room. In a CAV laboratory, the supply airflow into the room is
normally maintained at a constant value since the fume hood exhaust normally remains
constant. However, in a VAV laboratory, the room exhaust depends on the fume hood
exhaust, which varies in accordance with the fume hood sash position. Thus, the Room
Controller in a VAV laboratory must modulate both the incoming supply airflow via the Supply
Terminal and the Room General Exhaust to maintain a proper balance between the total
room exhaust airflow and the incoming supply airflow.
In a VAV laboratory, the Room Controller must modulate the Room General Exhaust to
ensure that there is sufficient total room exhaust to meet the required minimum room
ventilation rate (ACH) rate. The Room Controller must also modulate the Room General
Exhaust to ensure that there is sufficient total room exhaust for the amount of supply airflow
necessary to maintain the room’s ambient temperature.
8
A VAV fume hood controller uses an airflow sensor and a modulating damper in the fume hood exhaust to maintain the
required exhaust airflow in accordance with the fume hood’s total open sash area. This ensures that fume hood face velocity
remains constant for all sash positions. Each VAV fume hood controller continuously sends the fume hood exhaust airflow
value to the room controller.
Siemens Building Technologies, Inc.
21
Room Pressurization Control Application Guide
SUPPLY
TERMINAL
ROOM
GENERAL
EXHAUST
CFM
CFM
CFM
LAB0196R1
STATIC PRESSURE
FUME
HOOD
CONTROLLERS
CFM
ROOM
CONTROLLER
ROOM STATIC
PRESSURE SENSOR
Figure 5. Direct Pressure Control Arrangement for a Chemical Laboratory Room.
The Room Controller also monitors the room static pressure via the Room Static Pressure
Sensor and controls the total room supply airflow to maintain the required room static
pressure set point.
If the room static pressure is less than the set point, the controller reduces the supply airflow
to increase the difference between the room supply and total room exhaust airflow’s and,
thus, increase the room’s negative static pressure.
If the room static pressure is greater than the set point, the controller increases the supply
airflow to decrease the difference between the room supply and total room exhaust airflow’s
and, thus, decrease the room’s negative static pressure9.
With a VAV ventilation system there may be times, particularly during the cooling season,
when the room will require more supply airflow to maintain the desired room temperature
than the amount needed to maintain the required room static pressure. In such instances, the
Room Controller will increase the amount of supply airflow to maintain the desired room
temperature while simultaneously increasing the room general exhaust as necessary so that
the room static pressure remains constant.
In a VAV laboratory room served by a single supply duct that also has a Room General
Exhaust, the many functions of the Room Controller include:
•
9
10
22
Maintaining the proper amount of room general exhaust to ensure that the required
minimum room ventilation rate10 is always maintained.
In negatively pressurized rooms the supply airflow needs to be less then the total room exhaust airflow. In positively
pressurized rooms the supply airflow needs to be greater than the total room exhaust airflow.
In negatively pressurized rooms the ACH rate is determined by the total room exhaust airflow. In positively pressurized rooms
the ACH rate is determined by total room supply airflow.
Siemens Building Technologies, Inc.
Laboratories
•
Maintaining the proper amount of room supply airflow to maintain the required room
static pressure.
•
Increasing the supply airflow when needed to cool or heat the room while
simultaneously increasing the room general exhaust to maintain a constant room
static pressure.
Direct Pressure Control Limitations
Although direct static pressure control enables precise closed loop control of the room static
pressure, its application has certain limitations since it can be adversely affected by
numerous activities routinely taking place.
Door Effects
When a room door is opened, the area of the doorway increases the leakage area of the
room. Thus, opening a single width door can add about 20 square feet of leakage area to a
room. Figure 3 shows that if a 20 square foot leakage area curve were drawn, it would lie
almost horizontally along the bottom of the graph, even if the differential airflow were
increased beyond the values on the graph. Under an open door condition, the room controller
would attempt to correct for the low room static pressure level by reducing the supply airflow.
However, no reduction in supply airflow would restore the room static pressure to its set point
while a door is open. Meanwhile, the reduction in supply airflow would eventually have an
adverse effect on the room temperature and humidity.
To minimize the effect of an open door, the room controller should incorporate a means to
limit the supply air reduction. One means is to impose a low limit on the supply airflow as part
of the room pressure control scenario. Another means is to incorporate a door switch circuit
that signals the room controller to not make any further reduction in the supply airflow
whenever any door is not fully closed. Either of these methods can prevent the undesirable
reduction in supply airflow, although the latter requires installation of a switch on each door
and the associated circuit wiring.
VAV Fume Hood Effects
NOTE:
Direct pressure sensing control is not recommended if the room exhaust airflow
can undergo rapid changes, as is the case of laboratory rooms with VAV fume
hoods that are served by VAV ventilation systems.
To maintain a constant face velocity for all sash openings, the VAV fume hood exhaust is
controlled. Thus, when a user opens or closes a fume hood sash, the associated fume hood
controller will proportionately increase or decrease the fume hood exhaust airflow to maintain
the required constant face velocity. This VAV fume hood control action can have a significant
impact on the total room exhaust.
Siemens Building Technologies, Inc.
23
Room Pressurization Control Application Guide
To obtain a reliable room static pressure value from the room static pressure sensor, the
room controller must sample the sensor output for several seconds to factor out the pressure
variations caused by personnel movement, room air currents, and even outside air wind
gusts (referred to as signal noise). This delays the room controller’s response to room static
pressure variations. The slower control response translates into more elapsed time before
the room supply airflow is properly adjusted to maintain the required room static pressure.
This can result in an unacceptable delay before adequate supply makeup airflow is attained
after a fume hood sash is repositioned11.
Cascaded Pressure Control
Although airflow tracking is the preferred method for preventing cross contamination for most
laboratory applications, there may be valid reasons to ensure that a specific room static
pressure level is always maintained. Changes in weather, inside and outside temperature
variations, and changes occurring in building equipment and structures, can affect the room
static pressure level that is maintained by airflow tracking static pressure control. To retain
the superior speed and stability of airflow tracking and ensure that the required static
pressure is always maintained, cascaded pressure control can be applied.
Cascaded pressure control combines the functionality of both direct pressure control and
airflow tracking. This control approach is mainly an airflow tracking control approach with the
addition of a room static pressure sensor and the control components are essentially the
same as those of Figure 5.
The overall control scenario uses airflow tracking as the ongoing means to maintaining room
static pressure. However, the control scenario also uses the wall mounted pressure sensor to
enable the room controller to periodically read the actual room static pressure. If need be, the
room controller will adjust the airflow tracking differential to maintain the room static pressure
at the desired level. As a result of this arrangement, the airflow tracking value does not
remain fixed as it would for regular airflow tracking, but is periodically reset (if needed) at
regular intervals based on actual room static pressure. Therefore, this cascaded control
arrangement compensates for static pressure variations that might otherwise occur with only
airflow tracking.
An additional benefit of a cascaded control arrangement is that the room static pressure
sensor enables providing a local indication and also remote monitoring of the specific room
static pressure.
Although cascaded pressure control might seem to be the best means of static pressure
control, it also has some drawbacks:
11
24
•
It costs more than airflow tracking since it requires a room static pressure sensor and
a more complex control algorithm.
•
It must also address many of the problems associated with direct pressure control,
such as a door that is left open for an extended time.
If a VAV fume hood sash is fully opened after being closed, it could require perhaps 10 or more seconds before a new stable
room static pressure value can be used for controller response. This would delay increasing the supply makeup air required to
offset the increased fume hood exhaust airflow and could therefore adversely affect fume hood containment.
Siemens Building Technologies, Inc.
Laboratories
•
It presents a greater design challenge and involves a more difficult startup and
balancing process. In particular, the areas adjacent to the room (for example,
corridors) must have the proper level of excess supply air since the laboratory
room(s) airflow tracking offset will not remain at a constant value.
Dual Pressurization Laboratories
In some instances, it is necessary to ensure that a laboratory room does not become
contaminated by airborne impurities from adjacent areas. Examples include laboratories with
microelectronics that must maintain a clean environment, or biological or pharmaceutical
laboratories that might be adversely affected by the inward airflow resulting from negative
room pressurization. When the substances present in a laboratory room do not present a
hazard to adjacent areas, positive room pressurization can be used to maintain an
uncontaminated room environment by the control methods described previously. However,
airflow tracking is generally the preferred method.
However, a more complex situation arises when laboratory rooms also use chemicals or
contain substances that are hazardous. In such circumstances, the adjacent areas (corridors
and non-laboratory areas of the building) must be protected by a design that ensures against
improper directional airflow for the laboratory room itself as well as the adjacent nonlaboratory areas.
Figure 6 shows how this can be achieved by using a dual pressurization arrangement for a
laboratory room. The same physical arrangement can be applied to virtually any type of
laboratory room regardless of its purpose or what it contains.
The major difference between Figure 6, and Figure 4 and Figure 5, is the addition of a
vestibule entryway for the laboratory. The laboratory room controller maintains the laboratory
at a positive static pressure by controlling the laboratory room supply airflow to always
exceed the laboratory’s total room exhaust by a fixed offset value. As a result, the laboratory
room is at a positive static pressure that prevents contaminants from entering the room.
However, the vestibule exhaust, in conjunction with the laboratory room's total exhaust,
exceeds the laboratory's supply airflow. As a result, the combined unit that consists of the
laboratory room and vestibule will be at a negative static pressure with respect to the
adjacent area.
The graph in Figure 6 illustrates that the combined exhaust airflow that consists of the
vestibule exhaust, plus the total room exhaust, exceeds the room supply airflow. The
resulting transfer airflows will be in the proper direction to prevent contamination of the
laboratory and also prevent any chemical fumes or other airborne agents from migrating out
from the laboratory into the adjacent area.
For example, the laboratory might have 300 cfm more supply airflow than its total exhaust.
The vestibule exhaust might be 500 cfm. This results in a net excess exhaust of 200 cfm for
the combination of laboratory and vestibule, and is therefore negative with respect to the
adjacent area. This arrangement keeps air from the adjacent areas from entering the
laboratory room and also prevents laboratory air from migrating into the adjacent area.
Siemens Building Technologies, Inc.
25
Room Pressurization Control Application Guide
With respect to the ventilation system design, the vestibule exhaust would normally be a
constant air volume (CAV) exhaust while the laboratory room could employ either a CAV or
VAV ventilation arrangement. The important element is that the laboratory room controller
must ensure that the total exhaust of the room combination is always greater than the room
supply air. Each entryway into the laboratory room requires a vestibule unless the entryway
connects to an adjoining laboratory room that is also protected against contamination by
positive pressurization. If an adjoining area is not at the same static pressure as the
laboratory room, a vestibule entry arrangement should then be used for that respective entry.
When implementing a dual pressurization arrangement, it is advisable to ensure that the
laboratory, vestibule walls, and ceilings be completely sealed to minimize the opportunity for
transfer air to pass directly into the adjacent area from the laboratory. Depending on
individual situations, an airlock type of entry for the vestibules may also be desirable. An
airlock door arrangement uses electric door releases to ensure that only one door of the
vestibule is opened at a time.
AIRFLOW
TRACKING
OFFSET
VESTIBULE
EXHAUST
TOTAL
ROOM
EXHAUST
ROOM
SUPPLY
AIRFLOW
EXHAUST
SUPPLY
TERMINAL
ROOM
GENERAL
EXHAUST
CFM
CFM
DOORS
-
LAB0197R1
TRANSFER
AIRFLOW
VESTIBULE
+
FUME
HOOD
CONTROLLER
CFM
CFM
ROOM
CONTROLLER
LABORATORY
Figure 6. Preventing Room Contamination by Room Positive Pressure & Vestibule Negative
Pressure.
26
Siemens Building Technologies, Inc.
Healthcare Facilities
Healthcare Facilities
Infectious Isolation Rooms
Infectious isolation rooms in healthcare facilities are intended to ensure that the direction of
airflow is always into the room, thus preventing the spread of disease to persons outside of
the room, particularly the healthcare workers. The need for providing negatively pressurized
infectious isolation rooms has become more focused because of the resurgence of the
contagious disease Tuberculosis (TB). Since Tuberculosis is highly contagious, the Centers
for Disease Control and Prevention published guidelines for preventing its transmission12.
These guidelines call for a minimum room negative static pressure of 0.001 inches w.c. (0.25
Pa) for infectious isolation rooms along with a minimum room ventilation rate of 6, and
preferably 12, air changes per hour (ACH).
In actual practice, a 0.001 inches w.c. room static pressure is quite low and impractical to use
as design criteria. A more practical isolation room static pressure level is at least 0.005
inches w.c. (or higher) since this level is more readily maintained and is a necessary
minimum level to enable measurement of the room static pressure as well as continuous
monitoring.13
Protective Isolation Rooms
Protective isolation rooms require positive pressurization to ensure that room airflow remains
outward from the room, therefore preventing infectious pathogens from entering the room.
Protective isolation rooms are intended to prevent a patient from being exposed to airborne
pathogens outside of their room. This type of isolation room is needed for patients and
associated medical procedures that are susceptible to infection including organ transplants;
burn; bone marrow and leukemia patients; and for treating AIDS patients who have highly
compromised immune systems.
Static Pressure Control by Airflow Tracking
Airflow tracking is the preferred room static pressurization control approach for all types of
healthcare facility room pressurization applications. This includes operating rooms, nurseries,
and medical laboratories as well as both infectious and protective patient isolation rooms.
Airflow tracking maintains proper room pressurization (negative or positive) for both constant
air volume (CAV) room ventilation systems and variable air volume (VAV) room ventilation
systems.
12
13
1994 Guidelines for preventing the transmission of Mycobacterium and Tuberculosis in Healthcare facilities.
The CDC guidelines also recommend that isolation room static pressure be monitored daily. This can be done using a visible
smoke or by a permanently mounted static pressure sensing device that provides continuous indication of room static pressure.
Siemens Building Technologies, Inc.
27
Room Pressurization Control Application Guide
Infectious Isolation Room Layout
Figure 7 shows the optimum room airflow and control arrangement for an infectious isolation
room. The room layout consists of the patient room and an anteroom. The anteroom provides
added assurance against airflow coming out from the patient room. The airflow control
arrangement maintains the patient room as the more negative of the two rooms so that
airflow is always into the anteroom and then into the patient room. The supply air diffuser and
the exhaust air grill in the patient room are located so that the airflow pattern is towards the
patient and then out of the room. This provides maximum protection for the building
occupants and in particular for workers who must enter the patient room.
CFM
CFM
ROOM
CONTROLLER
CFM
SUPPLY
CAV EXHAUST
CONTROLLER
ANTEROOM
EXHAUST
PATIENT ROOM
ROOM
PRESSURE
MONITORS
TRANSFER DOOR
AIRFLOW
LAB0198R1
DOOR
Figure 7. Infectious Isolation Room.
Ventilation and Control System Application
The ventilation system in Figure 7 provides an exhaust for both the anteroom and the patient
room, while the patient room alone has an air supply provision.
When operating, the anteroom exhaust might typically be approximately 50 cfm (24 L/sec),
while the patient room exhaust might be approximately 200 cfm (96 L/sec) greater than the
room supply airflow.
The patient room controller maintains the proper room supply and exhaust airflows, which
ensures that the proper airflow tracking offset is maintained. The patient room controller also
maintains the proper room ambient temperature. The anteroom exhaust is also maintained at
the proper airflow either by a separate CAV exhaust controller or the anteroom exhaust may
be maintained by another control function of the patient room controller.
28
Siemens Building Technologies, Inc.
Healthcare Facilities
Room Pressure Monitors
Room pressure monitors are provided for both the patient room and the anteroom. This
ensures that proper room pressurization for both rooms can be constantly monitored and
verified. It is also important that both the anteroom and the patient room controllers be
located outside of the rooms so that access to these devices does not require service
personnel to enter either the anteroom or patient room.
Protective Isolation Room Layout
Figure 8 shows an optimum room airflow and control arrangement for a protective isolation
room. The room layout consists of the patient room and an anteroom to provide added
assurance against airflow entering the patient room from adjoining areas. The airflow control
arrangement maintains the patient room positive with respect to the anteroom and the
adjacent corridor.
CFM
CFM
ROOM
CONTROLLER
SUPPLY
ANTEROOM
ROOM
PRESSURE
MONITOR
LAB0199R1
DOOR
TRANSFER DOOR
AIRFLOW
PATIENT ROOM
+
Figure 8. Protective Isolation Room.
Ventilation and Control System Application
Room supply air is HEPA filtered and the room ventilation airflow is arranged to first pass
over the patient and then proceed toward the room exhaust grill. As an option, the anteroom
may be equipped with an exhaust provision so that the anteroom is negative with respect to
the corridor, while the patient room remains positive with respect to the anteroom and
corridor. Such an arrangement provides maximum protection for the patient even when
others are present in the room. Since the patient is directly exposed to the incoming supply
airflow, the supply air diffuser and airflow velocity must be carefully chosen to not create an
uncomfortable draft for the patient.
Siemens Building Technologies, Inc.
29
Room Pressurization Control Application Guide
Isolation Room Changeover
The American Institute of Architects (AIA) guidelines14 do not permit the same room to be
alternately used for both Infectious and Protective Isolation. Note that Figure 7 and Figure 8
show a location of the supply diffuser and exhaust grills that is different for infectious and
protective isolation rooms.
Clean Rooms
The demand for high quality pharmaceutical products and the continual miniaturization of
electronic components has created an increasing need for contamination control in their
associated processing and manufacturing areas. Maintaining the required degree of purity in
these areas is highly dependent upon room pressurization to prevent contamination.
Particulate Contamination
Particulate that can contaminate products comes from a variety of sources, including:
•
Occupants within the clean room
•
Atmospheric dust
•
Condensation of vapors
•
Bacteria
•
Chemical fumes
Keeping these contaminants within acceptable levels requires a coordinated approach, which
includes:
14
30
•
Proper Room Design—High purity clean rooms cannot normally be obtained by
conversion or by upgrading existing non-clean areas. Clean rooms typically need to
be initially designed with sufficient height and ancillary space (space above and
below the clean room) to house the large amount of specialized air handling and
filtering equipment necessary.
•
Architectural & Physical Barriers—Clean rooms must be constructed of materials
that will not release airborne particulate. Physical barriers (walls, ceilings, material
joints, etc.) must also be meticulously sealed. Adjacent spaces and other support
areas are often required to be constructed similar to the clean room itself in order to
act as buffer zones between the clean and unclean spaces.
American Institute of Architects 1996 -1997 Guidelines for Design and Construction of Hospital and Health Care Facilities.
Siemens Building Technologies, Inc.
Clean Rooms
•
Construction Procedures—Clean rooms must be constructed using specialized
techniques and practices to prevent contamination of the room materials, ventilation
ductwork, and room equipment. Room components, such as ductwork, must be precleaned and sealed before installation so that no residual particulate is present.
Construction personnel must be properly trained and must follow rigid guidelines for
handling construction materials and equipment, and in carrying out construction
procedures. Typically, they must wear outer garments that protect against bringing
contaminants into the clean room during the construction process. Tools and
construction equipment must meet a specific level of cleanliness before they may be
brought into the clean area.
•
Room Utilization Procedures—Clean room access must be restricted to authorized
personnel only. Such persons must be properly trained in acceptable procedures and
work practices. Specialty outer garments are usually required. Occupants must not
consume food in the clean room. Special room cleaning, maintenance, and
decontamination processes must be used. Room entry decontamination procedures
must be followed.
•
Ventilation & Directional Airflow—Clean rooms typically require very high rates of
HEPA or ULPA15 filtered ventilation airflow to maintain the required low level of
airborne particulate in the room atmosphere. Airflow is typically from ceiling to floor in
a laminar airflow arrangement that most often uses the entire ceiling as a supply
plenum and filter bank.
•
Room Pressurization—Proper differential pressurization has been found to be very
effective and an absolutely necessity for preventing contamination in clean rooms. All
clean rooms and surrounding areas must be maintained at specific levels of
pressurization to ensure against improper directional airflow. Two, three or more
levels of positive pressurization may be used in clean room applications with the
most critical area at the highest positive pressure level and the adjacent areas or
support at lower positive pressure levels. Adequate levels of differential
pressurization and proper directional airflow are especially critical at the room entry
and exit points.
Clean Room Standards
Clean room purity is classified in accordance with the maximum allowable concentration of
airborne particulate per cubic foot or cubic meter. Two standards set clean room
classifications: FS 209E and ISO/FDIS 14644-1. FS 209E is a United States Federal
Standard that was initially issued in the 1960's and was last revised in 1992. Up until 1999 it
was the only recognized clean room standard. It is still widely used today, especially where
U.S. Governmental regulations and interests are involved. However, in 1999 the international
standard, ISO/FDIS 14644-1 was adopted and is likely to eventually replace the FS 209E
standard.
15
HEPA - High Efficiency Particulate Air filters remove at least 99.97% of particulate 0.3 microns (0.3µm) or larger in size.
ULPA - Ultra Low Penetration Air filters remove 99.999% of particulate 0.12 microns (0.12µm) or larger on size. A micron (µm)
is one millionth of a meter or approximately 0.00003937 inches.
Siemens Building Technologies, Inc.
31
Room Pressurization Control Application Guide
Table 2 provides the allowable limits for 0.1, 0.5 and 5.0 micron size airborne particles per
cubic foot and cubic meter for different clean room classifications for the current versions of
the FS 209E and ISO/FDIS 14644-1 standards. These standards also establish allowable
limits for other size particles aside from those listed in the table, however the particle sizes
listed in Table 2 provide a comparison between the two standards.
Table 2. Clean Room Standards—Particle Concentration Limits FS 209E vs. ISO 14644-1.
Clean Room
Classification
0.1 µm Particles Per
Cubic Unit
FS
FS
ISO
Cubic
Ft.
0.5 µm Particles Per
Cubic Unit
ISO
Cubic
Meter
Cubic
Meter
1
10
2
100
FS
Cubic
Ft.
Cubic
Meter
5.0 µm Particles Per
Cubic Unit
ISO
Cubic
Meter
FS
Cubic
Ft.
Cubic
Meter
ISO
Cubic
Meter
4
1
M1.
5
3
35
1,230
1,000
1
35
32
10
M2.
5
4
345
12,200
10,000
10
353
352
100
M3.
5
5
3,450
122,000
100,000
100
3,530
3,520
1,000
M4.
5
6
34,500
1,220,0
00
1,000,0
00
1,000
35,300
35,200
7
247
293
10,000
M5.
5
7
345,000
1.22 ×
107
10,000
353,000
352,000
65
2,300
2,930
100,00
0
M6.
5
8
3,450,0
00
1.22 ×
108
100,00
0
3,530,0
00
3,520,00
0
700
24,700
29,300
9
3.45 ×
107
1.22 ×
109
32
35,200,0
00
29
293,00
0
Siemens Building Technologies, Inc.
Clean Rooms
Clean Room Pressurization Applications
Figure 9 shows a simplified clean room arrangement for a pharmaceutical processing
application. The cleanest area is the aseptic16 filling area, which is most likely where the
finished product is packaged (oral tablets, medications, etc.).
FINISHED
PRODUCT
OUTLET
++
++
PREPARATION
AREA
ASEPTIC FILLING
AREA
+++
AIRFLOW
DIRECTION
ARROWS
+
+
LAB0200R1
PERSONNEL CORRIDOR
EXIT &
DE-GOWNING
AIRLOCK
ENTRY &
GOWNING
AIRLOCK
Figure 9. Potential Clean Room Arrangement for Pharmaceutical Processing.
Workers enter and exit through the respective airlock in which they either put on the required
outer garments for entry or remove them upon exiting. The airlocks are each equipped with
two sets of doors (sliding or hinged) with an electrical interlocking arrangement that allows a
door to be open only when the other door is fully closed17. By allowing only one door to be
open at a time, the amount of air that can flow out of the clean spaces through an airlock is
limited. Using airlocks for the entry and exit provisions ensures that the required level of
positive pressurization in the clean spaces is always maintained.
16
17
Aseptic refers to a space or area in which the bacterial count is contained within required limits. Although it is not a 100%
sterile space it enables pharmaceutical products to be processed with a high degree of purity. Aseptic areas are generally
Class 100 per FE 209E.
In some airlock arrangements a time delay of as much as several minutes is also incorporated to allow time to ventilate the
airlock before the other door can be opened.
Siemens Building Technologies, Inc.
33
Room Pressurization Control Application Guide
The clean spaces are positively pressurized according to their required level of purity or
cleanliness. The most critical operations are performed in the area that has the highest level
of cleanliness (aseptic area), which also has the highest level of pressurization. In Figure 9,
each plus sign indicates the area's relative level of positive pressurization. The more plus
signs, the higher the positive pressurization. Clean room designs typically use a differential
pressure of about 0.05 inches w.c. (12.4 Pascals) between each different clean room area
classification. Thus, in Figure 9,the aseptic area (+++) would typically be designed to be 0.15
inches w.c. positive with respect to a neutral area such as the Personnel Corridor. The
adjoining Preparation Area would then be maintained at about 0.10 inches w.c., and the entry
and exit airlocks would be maintained at about 0.05 inches w.c. This ensures that airflow
(shown by dotted lines and arrows) will always flow from the most critical and cleanest space
(highest positive pressure) to a lesser clean space.
Airlocks
Four different types of airlocks are often applied to enable entry and exit to clean spaces
depending upon the nature of the clean room's purpose:
34
•
Cascading Pressure Airlock—Figure 10 shows an airlock that is kept at a lower
positive pressure than the adjacent clean space, but at a higher positive pressure
than the corridor. As a result, air from the highly positive pressurized clean space
cascades through the airlock to the area of least cleanliness, which is the Non
Classified Corridor. The same quantity of air is supplied to and exhausted from the
airlock. The FDA prefers this type of airlock when absolute containment of the clean
space is not required.
•
Pressure Bubble Airlock—Figure 11 shows an airlock that is kept at a higher
positive pressure than the adjacent spaces. This type of airlock is applied when it is
necessary to separate a bio-contained clean area from other areas. Clean
conditioned supply air is used to create a high positive pressure in the airlock, which
does not have any exhaust provision. The supply airflows to the adjacent areas
through the airlock leakage area that consists primarily of door clearances. This
directional airflow prevents cross contamination between adjacent areas. The
pressure bubble airlock is often used because its positive pressure relationship to the
adjacent areas keeps contaminants from entering the airlock.
•
Pressure Sink Airlock—Figure 12 shows an airlock that is kept very negatively
pressurized with respect to all adjacent areas. All of the air that is supplied to the
airlock, plus all air that flows into the airlock from the adjacent areas, is mechanically
exhausted from the airlock. This ensures against cross-contamination between
adjacent areas. Although the pressure sink airlock prevents cross contamination
between adjacent areas, its disadvantage is that unlike the pressure bubble airlock, it
is subject to contamination from the adjacent areas.
Siemens Building Technologies, Inc.
Clean Rooms
•
Potent Compound18 Airlock—Figure 13 shows a pressure bubble airlock combined
with a pressure sink airlock. This two-compartment airlock arrangement allows
personnel to protect themselves by putting on Personal Protective Equipment (PPE),
such as outer garments and sometimes respirators, in the pressure bubble area (++)
before entering the pressure sink area (– –) and clean space area in which the potent
compound substances are present. The negative pressure of the pressure sink area
prevents contamination of the clean space by the adjacent areas such as the
corridor. All clean conditioned supply air to the pressure bubble area (++) flows into
the adjacent pressure sink and corridor. All of the clean conditioned air supplied to
the pressure sink area along with all air that flows into that area is exhausted.
CLEAN SPACE
CEILING SUPPLY*
SLIDING OR
HINGED DOORS
AIRLOCK
FLOOR
EXHAUST
RISER*
LAB0201R1
NON CLASSIFIED CORRIDOR
*CEILING SUPPLY CFM = FLOOR EXHAUST CFM
Figure 10. Cascading Pressure Airlock.
18
A potent compound is any substance that can present a danger to anyone coming into physical contact with the substance or
airborne fumes or particulate from the substance.
Siemens Building Technologies, Inc.
35
Room Pressurization Control Application Guide
CLEAN SPACE
FILTERED
CEILING
SUPPLY*
SLIDING OR
HINGED DOORS
AIRLOCK
LAB0202R1
NON CLASSIFIED CORRIDOR
*CEILING SUPPLY CFM = AIRLOCK DOOR LEAKAGE CFM
Figure 11. Pressure Bubble Airlock.
36
Siemens Building Technologies, Inc.
Clean Rooms
CLEAN SPACE
CEILING SUPPLY*
SLIDING OR
HINGED DOORS
AIRLOCK
FLOOR
EXHAUST
RISER*
LAB0203R1
NON CLASSIFIED CORRIDOR
*FLOOR EXHAUST CFM = CEILING SUPPLY + DOOR LEAKAGE CFM
Figure 12. Pressure Sink Airlock.
Siemens Building Technologies, Inc.
37
Room Pressurization Control Application Guide
FLOOR
EXHAUST
RISER*
CLEAN SPACE
CEILING SUPPLY*
CEILING SUPPLY*
LAB0204R1
SLIDING OR
HINGED DOORS
NON CLASSIFIED CORRIDOR
Figure 13. Potent Compound Airlock.
Airlock Construction
Proper airlock construction is critical to ensuring that the required pressurization levels can
be attained. All surfaces should be well sealed and covered with a highly impervious finish
such as epoxy paint.
Airlock doors can be either hinged or sliding. Hinged doors can typically be made to fit tighter
(have less peripheral leakage area) especially if the frame is equipped with a seal or gasket
and floor sweeps are used. When hinged doors are used, they should open into the dirtier
area. Table 3 provides information on door leakage areas in inch-pound units.
38
Siemens Building Technologies, Inc.
Clean Rooms
Table 3. Airlock Door Clearance Areas.
Door Size
Total Closed Door Clearance
Area
Hinged Door
Sliding Door
(See Note 1)
(See Note 2)
36 in. × 78 in.
0.229 sq. ft.
0.792 sq. ft.
36 in. × 84 in.
0.240 sq. ft.
0.833 sq. ft.
42 in. × 78 in.
0.245 sq. ft.
0.833 sq. ft.
42 in. × 84 in.
0.255 sq. ft.
0.875 sq. ft.
48 in. × 78 in.
0.875 sq. ft.
48 in. × 84 in.
0.917 sq. ft.
60 in. × 78 in.
0.958 sq. ft.
72 in. × 78 in.
1.042 sq. ft.
Note 1: Hinged door clearance area is based on
1/4 in. along the bottom and 1/8 in. along the sides
and top.
Note 2: Sliding door clearance area is based on 1/2 in.
around the entire door perimeter.
Figure 14 provides a graph of the airflow that results when a specific differential pressure is
applied across various leakage areas (curves). The information in Table 3 and the graph in
Figure 14 will help you approximate the differential airflow that results when a differential
pressure is present.
As an example, consider a 42 in. × 84 in. closed airlock hinged door. The leakage area
shown in Table 3 is 0.255 sq. ft. Figure 15 indicates that with a differential pressure of 0.0500
inches w.c. across the closed door, the resulting airflow would be approximately 150 cfm.
Similarly, a 72 in. × 78 in. closed sliding door (leakage area of 1.042 sq. ft.) with a differential
pressure of 0.02 inches w.c. across would result in an airflow rate of approximately 520 cfm.
The Differential Pressure and Differential Airflow data of Figure 14 is expressed by the
following pressurization versus leakage area equations based on inch-pound (IP) or metric
(SI) units:
(IP)
Q = 2610 A (dP)1/2
where:
Q is the differential airflow in Cubic Feet per Minute (cfm)
A is the total room leakage area Square Feet
dP is the differential pressure Inches of Water (inches w.c.)
Siemens Building Technologies, Inc.
39
Room Pressurization Control Application Guide
Q = 840 A (dP)1/2
(SI)
where:
Q is the differential airflow in Liters per Second,
A is the total room leakage area in Square Meters
dP is the differential pressure in Pascals
0.0600
0.0575
1.1 Ft2
0.0550
1.2 Ft2
0.0525
1.3 Ft2
0.0500
0.1 Ft2
0.0475
1.4 Ft2
0.2 Ft2
0.0450
1.5 Ft2
0.0425
0.4 Ft2
0.0400
1.7 Ft2
0.5 Ft2
0.0375
DIFFERENTIAL
0.0350
PRESSURE
1.8 Ft2
0.6 Ft2
1.9 Ft2
2.0 Ft2
0.7 Ft2
0.0325
INCHES
of
WATER
1.6 Ft2
0.3 Ft2
0.8 Ft2
0.0300
0.9 Ft2
00275
1.0 Ft2
0.0250
0.0225
0.0200
0.0175
0.0150
LEAKAGE AREA
CURVES
0.0125
0.0100
0.0075
LAB0205R1
0.0050
0.0025
0.0000
0
50
100
150 200
250 300 350 400
450
500 550 600 650 700
750 800
850 900 950 1000
DIFFERENTIAL AIRFLOW - CFM
Figure 14. Airflows for Door Leakage Area vs. Differential Pressure.
Clean Room Static Pressure Control
Controlling the static pressure in clean rooms typically involves controlling individual static
pressure relationships in a multiple room arrangement. Figure 15 depicts such an
arrangement that consists of three separate clean rooms with an airlock entry. Each room
has a different positive static pressure level requirement, with the cleanest space at 0.20
inches w.c. The subsequent rooms are at 0.150 inches w.c. and 0.100 inches w.c. Finally,
the airlock is at 0.050 inches w.c. The corridor for entry into this clean room arrangement is
intended to be at a neutral (neither negative or positive) pressure. A series of Airflow
Leakage Arrows indicates the relative direction of airflow, which is always from the cleanest
area through each room space and then finally out to the Personnel Corridor.
40
Siemens Building Technologies, Inc.
LAB0206R1
Clean Rooms
ROOM
EXHAUST
MAKEUP
AIRFLOW
ROOM
EXHAUST
AIRFLOW MEASUREMENT
& CONTROL
INPUTS / OUTPUTS
MAKEUP
AIRFLOW
DOOR
SWITCH
INPUTS
'CLEANEST'
SPACE
+0.200 in. w.c.
STATIC
PRESSURE
INPUTS
DOOR SWITCH
ROOM
CONTROLLER
DS
AIRFLOW
LEAKAGE
ARROWS
ROOM
EXHAUST
+0.100 in. w.c.
DS
+0.150 in. w.c.
DS
AIRLOCK
+0.050
in. w.c.
MAKEUP
AIRFLOW
EXHAUST
DS
PERSONNEL CORRIDOR (NEUTRAL)
SUPPLY
ROOM STATIC
PRESSURE SENSOR
Figure 15. Multi-room Pressurization Control Arrangement.
Control System Components
When viewing Figure 15, note the following components, which are necessary for overall
room static pressurization control:
•
A Room Controller provides overall control for all of the rooms' static pressure and,
although the control components are not shown, the room controller can also control
each room environment (ambient temperature, relative humidity, and sometimes
other factors such as particle counting).
•
The static pressure level of each room space with respect to the Personnel Corridor
is sensed by a Room Static Pressure Sensor, which provides a static pressure input
to the room controller
•
Entry and exit to each room is via a sliding door with an automatic closing
arrangement. Each door is equipped with a Door Switch (DS), which provides an
input to the room controller when the respective door is closed.
Siemens Building Technologies, Inc.
41
Room Pressurization Control Application Guide
•
The Makeup Airflow and Exhaust Airflow for each room is individually measured and
controlled by the room controller.
Room Pressurization Control
The Room Controller continuously monitors each room's static pressure level between the
respective room and the Personnel Corridor by means of the Room Static Pressure Sensor.
The room controller modulates the room supply Makeup Airflow to ensure that the laboratory
room is always maintained at its static pressure set point by a Proportional Integral Derivative
(PID) closed loop control algorithm.
The room pressurization control scenario also includes selectable MAXIMUM and MINIMUM
room airflow limits. These airflow limits prevent the room pressurization control strategy from
attempting to increase the room supply makeup airflow above a specific limit or decrease the
room exhaust below a specific limit under transient conditions, such as when a connecting
door is open as indicated by the respective door switch. These limits prevent the control
scenario from attempting to maintain the room static pressure set point by creating an
excessive airflow imbalance.
The room static pressurization control strategy also includes selectable high and low room
static pressure alarm limits and an adjustable Room Static Pressure alarm delay period.
Whenever the room static pressure exceeds a high or low static pressure alarm limit in
excess of the alarm delay period, annunciation of the alarm condition can occur at
designated locations.
In operation, the control sequence first attends to the most critical area and then each
succeeding area of lower pressurization. In other words, the higher the required
pressurization, the higher its control priority will be.
42
Siemens Building Technologies, Inc.
Chapter 5—Air Pressurization
Fundamentals
Chapter 5 presents the fundamentals of air pressurization, introduces ventilation system
pressure components, and summarizes important factors in relation to these components.
This chapter also discusses the following topics:
•
Forces Exerted by Air
•
Total Pressure
•
Static Pressure
•
Velocity Pressure
•
Air Velocity
•
Units of Measure
Forces Exerted by Air
Air can exert a force in two ways. Wind, which is air in motion, exerts a force as it strikes an
object. As wind speed increases, the greater the force that is exerted. Air that is compressed
and confined within a container, such as a compressed air tank, also exerts a force on the
inside wall of the tank. And, the more the air is compressed the greater the force that is
exerted.
The force exerted by wind (air in motion) as well as the force exerted by air that is under
compression can be expressed in terms of its effect on a given size or unit of area. When this
force is expressed in terms of force per unit area it is commonly referred to as air pressure.
Total Pressure
Air in motion occurs naturally as wind, but air can also be put into motion by mechanical
means, such as a fan. Ventilation systems move fresh air into building areas and also
remove contaminated air from areas by means of fans and ductwork. Any time air pressure
(force per unit area) occurs as a result of air that is in motion it is termed total pressure. A
piece of paper or tissue would be readily pushed through a ventilation system duct in the
direction of airflow as a result of the force or effect of the air stream's total pressure. The total
pressure of a moving air stream is at its maximum (and is always measured) in the direction
of the airflow. Figure 16 illustrates the concept of total pressure as the result of an air stream
moving through a ventilation system duct.
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43
Room Pressurization Control Application Guide
AIRFLOW
LAB0189R1
TOTAL
PRESSURE
Figure 16. Total Pressure (Force Per Unit Area) Exerted in the Direction of Airflow and the Air
Movement Force.
Static Pressure
In contrast to the force exerted by the wind or air in motion, air at rest can exert a force within
any confined or enclosed space. Compressed air within an inflated tire exerts a force on the
entire internal surface of the tire and the resulting force enables the tire to maintain its shape
even when subjected to a heavy load. The force of air that is not due to its motion is also
expressed in terms of force per unit area but it is termed static pressure. Static pressure is
not the result of air movement. There may be no movement of the air in a tire or in a
compressed air tank, yet air pressure most definitely exists. Unlike total pressure, static
pressure is exerted in all directions as illustrated in Figure 17.
44
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Forces Exerted by Air
LAB0190R1
STATIC
PRESSURE
Figure 17. Static Pressure is Force Per Unit Area Exerted Equally in All Directions and Not
Caused by Air Movement.
Now, consider what happens when air in a confined area (such as a ventilation system duct)
is put in motion. In a confined area, air in motion has both a total pressure and a static
pressure component, and each of these components can be individually measured. However,
to measure the static pressure of a moving air stream, care must be used to ensure that the
pressure measurement is made perpendicular to the direction of air movement to exclude the
effect of the total pressure component.
If airflow in a duct is totally stopped by an obstruction (such as a fully closed damper) there
can no longer be any force component due to air movement. However, even though the air is
no longer moving, it still exerts static pressure in all directions (even in its previous forward
direction). If you tried to measure the total pressure component (in the direction of normal
airflow), you would receive a value that equals the static pressure. Therefore, total pressure
equals static pressure when air is at rest. The static pressure that still exists does not result
from air movement, but rather from the force that is being imposed upon the air. In ventilation
systems the ventilation system’s fans typically create this force.
Velocity Pressure
As previously stated, air can have two distinct measurable pressure components:
•
Static pressure
•
Total pressure
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45
Room Pressurization Control Application Guide
In still (non-moving) air the total pressure equals the static pressure. The total pressure in a
moving air stream is always greater than its static pressure. If the static pressure value of a
moving air stream is subtracted from the total pressure value, the resulting difference is
called the velocity pressure. This velocity pressure component bears a relationship to the
speed or velocity of the moving air stream. Thus, its value provides a means to determine the
velocity of a moving air stream.
Figure 18 illustrates the relationship of total pressure, static pressure, and velocity pressure
in a moving air stream.
AIRFLOW IN DUCT
LAB0191R1
TOTAL
PRESSURE
MEASUREMENT
STATIC
PRESSURE
MEASUREMENT
VELOCITY
PRESSURE
PRESSURE VALUE GRAPH
Figure 18. Moving Air Stream Pressure Relationships.
46
Siemens Building Technologies, Inc.
Air Velocity
Air Velocity
As illustrated in Figure 18, pressure measurement instruments can determine both the total
pressure and static pressure of an airstream in a duct. Subtracting the static pressure value
from the total pressure value yields the velocity pressure, which, in turn, enables the velocity
of the air stream to be determined.
When air is rapidly flowing through a duct, it typically has a significant total pressure as
evidenced by the force exerted in the direction it is moving. When air flowing through a duct
is discharged into a room, its forward motion is significantly reduced while, due to the laws of
physics (conservation of energy), its static pressure is increased. Then the difference
between the total pressure and static pressure (the velocity pressure) becomes very small
and, practically speaking, becomes nearly zero. Thus, the static pressure and total pressure
become nearly equal as air enters a room, and for this reason, a room’s air pressure
component can normally be considered to be only the static pressure.
Units of Pressure Measurement
Pressure measurements of compressed air, steam, and water are commonly expressed as
pounds per square inch (psi)19. However, typical values for total pressure, velocity pressure,
and static pressures in ventilation system applications are quite small when compared to the
forgoing pressure measurements. Thus for ventilation applications, a smaller unit of pressure
measurement is used to avoid fractions or very small decimal numbers.
The unit inches of water column (typically expressed as inches w.c., inches of water, or
inches) is commonly used for this purpose. The term inches of water simply means that the
air's static pressure value is the same as the pressure exerted by a layer of water of the
stated height on a horizontal plane. In other words, a pressure value of 1 inch w.c. exerts the
same force over a given area as would a layer of water that is exactly 1 inch in depth. Note
that due to gravity, water can only exert a force in a downward direction. However, it is
always to be assumed that air at a given static pressure (that is, 1 inch w.c.) exerts its force
in all directions.
With reference to a room’s static pressure, typical values are much less than even 1 inch of
water. In fact, values of 0.01 to 0.05 inches w.c. are more typical.
NOTE:
19
20
If psi units were used instead of inches of water, 0.01 inches w.c. and 0.05 inches
w.c. would have to be expressed as 0.000361 psi and 0.001805 psi respectively.
Such small numerical values would be very cumbersome to use.20
In SI units the kiloPascal (kPa) is the normal unit of pressure measurement. (1.0 psi equals 6.89 kPa.)
A pressure of 1.0 psi is equivalent to the force produced by a column of water 27.72 inches in height over a given area.
Therefore, a column of water 1.00 inches in height would be fractionally equivalent to 1/27.72 of 1.0 psi or 0.0361 psi. A
pressure of only 0.01 inch of water then becomes 0.000361 psi.
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Room Pressurization Control Application Guide
Differential Pressure
A pressure measurement is always a measurement of the pressure difference between two
points or locations. Every pressure measurement can be termed a differential pressure
measurement. In most instances, a plainly visible barrier separates the two “locations.” For
example, when measuring automobile tire pressure, the measurement is the difference
between the pressure inside the tire and atmospheric pressure outside the tire (with the tire
wall being the “barrier”). Similarly, boiler steam pressure is a measurement of the difference
between the pressure inside the boiler and atmospheric pressure outside of the boiler. In
these and most other common pressure measurements, it is generally understood that the
pressure value is the difference between the measured pressure and atmospheric pressure.
Also, in most common pressure measurements it is usually apparent which location is at the
higher pressure. For example, the pressure inside a steam pipe or inside an inflated tire is
higher than outside of the pipe or tire.
When referring to ventilation system pressures (typically room static pressure), it is usually
necessary to indicate the two locations that comprise the pressure measurement since this is
not always apparent. It is also necessary to use a positive or negative prefix to indicate which
location is at the higher or lower pressure. Thus, when referring to the static pressures
associated with ventilation systems, it is usually necessary to add a prefix. The prefix
“positive” indicates that the point or location is at the higher pressure, while the prefix
“negative” indicates that the area or location is at the lower pressure.
Summary of Pressure Components
The following is a summary of important factors that relate to ventilation system pressure
components.
•
Total Pressure—the force per unit area exerted by air in motion. It is measured by
sensing the force exerted by a moving air stream in the direction of airflow.
•
Static Pressure—the force per unit area exerted by air in motion or at rest. When air
is in motion (such as in a duct), it is measured by sensing the force exerted by a
moving air stream perpendicular (at right angles) to the direction of airflow. When air
is at rest, the static pressure can be measured by sensing the force exerted in any
direction.
•
Velocity Pressure—the difference between total pressure and static pressure.
Velocity pressure is mathematically related to the velocity of a moving air stream.
Knowing the velocity pressure of a moving air stream enables one to determine its
airflow velocity using the following expressions:
Airflow VelocityFt/sec. = 4005 (dPinches w. c.)1/2
OR
Airflow VelocityM/sec. = 1.29 (dPPa)1/2
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Summary of Pressure Components
Table 4 lists velocity pressures and the corresponding airflow velocities for the typical range
of ventilation airflows.
Table 4. Velocity Pressure vs. Airflow Velocity21
Velocity Pressure
21
Airflow Velocity
Inches
W. C.
Pascals
Feet per
Minute
Meters
per
Second
0.0050
1.2444
283
0.0100
2.4884
0.0150
Velocity Pressure
Airflow Velocity
Inches
W. C.
Pascals
Feet per
Minute
Meters
per
Second
1.439
0.1300
32.3489
1444
7.336
401
2.035
0.1400
34.8373
1499
7.613
3.7326
491
2.492
0.1500
37.3257
1551
7.880
0.0200
4.9768
566
2.877
0.1600
39.8141
1602
8.138
0.0250
6.2210
633
3.217
0.1700
42.3024
1651
8.389
0.0300
7.4651
694
3.524
0.1800
44.7908
1699
8.632
0.0350
8.7093
749
3.806
0.1900
47.2793
746
8.868
0.0400
9.9535
801
4.069
0.2000
49.7676
1791
9.099
0.0450
11.1977
850
4.316
0.2500
62.2095
2003
10.17
0.0500
12.4119
896
4.549
0.3000
74.6514
2194
11.14
0.0600
14.9303
981
4.984
0.3500
87.0933
2369
12.04
0.0700
17.4187
1060
5.383
0.4000
99.5352
2533
12.87
0.0800
19.9070
1133
5.755
0.4500
111.9770
2687
13.65
0.0900
22.3954
1202
6.104
0.5000
124.4190
2832
14.39
0.1000
24.8838
1266
6.434
0.5500
136.8610
2970
15.09
0.1100
27.3722
1328
6.748
0.6000
149.3030
3102
15.76
0.1200
29.8605
1387
7.049
0
3
0
Data applies to standard air at. 29.92 Inches of Mercury (Hg) and 70 F which equals 0.0750 lbs./ft (101.32 kPa & 21.1 C =
3
1.20 kg/m )
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Room Pressurization Control Application Guide
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Glossary
Airflow Tracking (Flow Tracking)
A means of maintaining a positive or negative static pressure in a particular room with
respect to an adjoining room, corridor, or building exterior. Airflow tracking consists of always
maintaining a fixed amount of airflow difference between the total supply air and total exhaust
air provided by the room or space ventilation system. Although the specific room supply and
exhaust airflow may periodically change, airflow tracking maintains a constant airflow (cfm or
l/s) differential. By maintaining more supply airflow than exhaust, a room is maintained at a
positive static pressure with respect to a non-pressurized adjacent space. By maintaining
less supply airflow than exhaust, a room is maintained at a negative static pressure with
respect to a non-pressurized adjoining space.
Differential Pressure (Differential Static Pressure)
Space or room static pressure value with reference to another room, space or location.
Fume Hood Static Pressure
The (negative) static pressure measured in the exhaust duct just after its connection to a
chemical fume hood's exhaust outlet. The static pressure value is measured just a few duct
diameters (normally 2 to 5) downstream of where the exhaust duct connects to the fume
hood.
Inches of Water (Inches Water Column; Inches W.C.)
An IP unit used to express very low air pressure values. It is typically used in ventilation
system applications. One inch of water is equal to the pressure exerted by a water column 1
inch in height at 39.2°F.
Manometer
Instrument for measuring relatively low pressures as are commonly associated with
ventilation system airflow. A manometer consists of a transparent vertical or slanted tube
containing a liquid (oil, water, or mercury). A manometer is an accurate and very repeatable
means of pressure measurement since it does not have any mechanical or electrical
components. As such, it is not subject to deterioration in accuracy (drift) due to the effects of
component wear and aging as is the case with instruments utilizing springs, gears, levers,
and various electrical components. However, due to the difficulty in reading very small
pressure values, manometers are not well suited for the very low pressure measurements
that are typically associated with room pressurization (for example, 0.01 inches w.c.).
Siemens Building Technologies, Inc.
Glossary-1
Room Pressurization Control Application Guide
Pascals (Pa)
SI unit to express relatively low air pressure values as is the case in ventilation system
applications. One Pascal is much less than 1 inch w.c. (I/P unit for low pressure values) since
248.8378822 Pascals equal 1 inch w.c. In clean room pressure applications a typical room
static pressure level of 0.050 inches w.c. would equate to approximately 12.4 Pascals. For
higher pressures the Kilopascal (KPa) unit, which equals 1000 Pascals, is more commonly
used. One psi is equal to approximately 6.895 KPa.
Pitot Tube
Standard design for an air pressure measurement probe used to measure total and static
pressures of a moving air stream. For pressure measurement in ventilation system ducts, a
pitot tube provides two pressure measurement outputs. One output is the full forward
pressure of a moving air stream and is termed total pressure. The outer output is the air
stream that is perpendicular (crosswise) to the direction of airflow and is termed the static
pressure of the air stream. By connecting a differential pressure gauge (a manometer) to the
two pitot tube measurement outputs, the difference between them, termed the velocity
pressure, is obtained. Velocity pressure is mathematically related to the velocity of a moving
air stream and provides a convenient way to determine the airflow velocity in FPM (I/P units)
or m/s (SI units).
Pressure Conversion Factors
Table 5. Pressure Conversion Factors.
1 Psi
6894.76 Pascals (Pa)
1 Pascal
0.000145038 Pounds/Sq. In.
(PSI)
1 Psi
6.89476 Kilopascals (kPa)
1 Kilopascal
0.145038 Pounds/Sq. In. (PSI)
1 Psi
2.3114155 Feet of Water at 60°F
1 Pascal
0.004026653 Inches of Water
(AMCA)
1 Psi
27.7369861 Inches of Water at
60°F
1 Kilopascal
4.026653 Inches of Water
(AMCA)
1 Inches of Water
248.83788 Pascals (Pa)
(ASHRAE)
1 mm of Hg
0.535245108 Inches of Water
at16°C
1 Inches of Water
0.24883788 Kilopascals (kPa)
1 mm of Hg
0.0193368 Pounds/Sq. In. (PSI)
1 Atmosphere
33.967626 Feet of Water at 60°F
1 Atmosphere
760.000 mm of Hg at 0°C
1 Atmosphere
14.695595 Pounds/Sq. In. (PSI)
1 Atmosphere
101.3226006 Kilopascals (kPa)
Standard Air (Standard Conditions)
Standard air is considered to be at a barometric pressure of 29.92 inches of mercury (Hg)
and at either 0% RH and a temperature of 69.8°F, or at 50% RH and a temperature of
68.0°F. It is a reference point that enables establishing other physical characteristics of as
density, specific heat, etc.
22
ASHRAE conversion factor.
2-Glossary
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Glossary
Standard Air Density (Standard Atmosphere)
This has been set at 0.075 lb. per cubic foot (I/P units) and is approximately the weight of one
cubic foot of air at 70°F and 29.921 inches of mercury (Hg), where 29.921 inches of mercury
approximately equals 14.696 psi.
Static Pressure
Pressure exerted by air at rest or the force of air that is exerted perpendicular to the direction
of airflow when air movement is present. Static pressure is not a resultant of air movement.
The static pressure of an air stream will approach maximum value when the airflow velocity is
reduced to zero by an obstruction such as a closed damper in a duct.
Total Pressure
Maximum pressure of a moving air stream that is exerted in the direction of airflow. Total
pressure is measured by sampling the pressure directly in line with the normal direction of
airflow. The total pressure of a moving air stream is always higher than the static pressure,
since total pressure also adds the force produced by the moving air stream. As airflow
velocity falls, total pressure also falls until at zero velocity the total pressure equals the static
pressure.
Velocity Pressure
Mathematical difference between the total pressure and static pressure of a moving air
stream. It is the additional pressure component that the moving air stream exerts in the
direction it is moving due to its mass.
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Glossary-3
Room Pressurization Control Application Guide
4-Glossary
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Index
A
G
ACH........................ See Room Ventilation Rate
Air Velocity .....................................................47
airflow tracking ........ 11, 12, 17, 19, 20, 21, 24,
25, 28
Airflow Tracking
Control Considerations...............................19
Airlock Construction .......................................38
Airlocks...........................................................34
Animal Holding Rooms ....................................7
Glossary of Pressurization Terms ................... 1
B
Biological Laboratories.....................................5
biological laboratory rooms ............................16
Biosafety Level 1 (BL-1)...................................6
Biosafety Level 2 (BL-2)...................................6
Biosafety Level 3 (BL-3)...................................6
Biosafety Level 4 (BL-4)...................................7
buffer zones .....................................................8
Building Pressurization ....................................4
C
cascaded pressure control.............................24
Cascaded Pressure Control...........................24
Cascading Pressure Airlock...........................34
Chemical Laboratories .....................................5
Clean Room Static Pressure Control .............40
Clean Rooms .............................................8, 30
Designs.......................................................34
Pressurization Applications ........................33
Standards ...................................................32
constant air volume ........................................26
Constant Air Volume ......................................21
Control System Components .........................41
D
Differential Pressure.......................................48
Direct Pressure Control..................................21
Direct Pressure Control Limitations ...............23
Door Effects ...................................................23
door leakage areas ........................................38
Dual Pressurization Laboratories...................25
F
fixed offset value ............................................25
Forces Exerted by Air.....................................43
fume hood exhaust airflow .............................24
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H
Health Care Facilities............................... 17, 27
HEPA filters...................................................... 6
Hospitals .......................................................... 7
I
Inches of Water.............................................. 14
Infectious Isolation Room Layout .................. 28
anteroom .................................................... 28
patient room ............................................... 28
Infectious Isolation Rooms............................. 27
inside air quality ............................................... 4
Intended Audience........................................... 2
Isolation Room Changeover .......................... 30
L
Laboratories ................................................... 17
leakage area ..11, 12, 14, 15, 16, 19, 20, 23, 34,
38, 39, 40
Leakage Area ................................................ 15
M
moving air stream .......................................... 46
N
negatively pressurized room.......................... 12
P
Particulate Contamination.............................. 30
sources....................................................... 30
Pascals .................................... 9, 14, 34, 40, 50
positively pressurized room ........................... 12
potent compound ........................................... 35
Potent Compound Airlock .............................. 35
Pressure Bubble Airlock ................................ 34
Pressure Components ................................... 48
Summary .................................................... 48
Pressure Sink Airlock..................................... 34
Pressurization Applications ............................. 3
Preventing Room Contamination................... 26
Protective Isolation Rooms ............................ 27
Layout......................................................... 29
protective patient isolation rooms
See Protective Isolation Rooms
Index-1
Room Pressurization Control Application Guide
R
T
room differential pressure ............14, 16, 19, 20
room leakage ...................11, 14, 15, 16, 39, 40
Room Pressure Monitors ...............................29
Room Pressurization
Applications ..................................................5
Control ........................................................42
Design Criteria............................................11
Factors........................................................15
Reference Data ..........................................14
Room Static Pressure Control .......................17
Room Ventilation Rate ...................................22
Total Pressure ................................... 43, 44, 48
S
Static Pressure.3, 13, 17, 27, 42, 43, 44, 45, 48
static pressure relationship ............................20
Index-2
U
Units of Pressure Measurement .................... 47
V
Variable Air Volume ....................................... 21
VAV.............................See Variable Air Volume
VAV Fume Hood Effects................................ 23
Velocity Pressure..................... 4, 43, 45, 48, 50
measurable pressure components............. 45
Static pressure ........................................... 45
Total pressure ............................................ 45
ventilation airflows ......................................... 50
typical range ............................................... 50
Ventilation and Control
System Application................................. 28, 30
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Notes
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Room Pressurization Control Application Guide
Notes
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