room-pressurization-control--application-guide_a6v10308401_us-en.pdf

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 1Introduction................................................................................................... 1

Importance of Room Pressurization .............................................................................. 1 Objective of this Application Guide................................................................................ 1 Intended Audience......................................................................................................... 2

Chapter 2Pressurization 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 3Room Pressurization Design Criteria....................................................... 11

Design Considerations................................................................................................. 11 Room Pressurization Reference Data ......................................................................... 14 Room Pressurization Factors ...................................................................................... 15 Leakage Area............................................................................................................... 15

Chapter 4Room 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

i


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 5Air 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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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 [email protected] 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 [email protected]

Siemens Building Technologies, Inc. I


II Siemens Building Technologies, Inc.

Chapter 1Introduction 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 micro-electronic 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.

Siemens Building Technologies, Inc. 1


Intended Audience This Application Guide has been written to serve an audience with a wide range of interests:

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.

2 Siemens Building Technologies, Inc.

Chapter 2Pressurization 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.



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:

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.


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 rooms 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.



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) 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) 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 High Efficiency Particulate Air filter. HEPA filters are capable of entrapping biological aerosols.




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 rooms 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.



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. Todays 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.



Table 1 lists various room pressurization applications and the normal static pressure relationships required.

Table 1. Room Static Pressurization Applications.

Recommended Static

Pressurization Level

Application

Inches Pascals

Relationship to Adjacent

Area(s)

Comments

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



Table 1. Room Static Pressurization Applications.

Recommended Static

Pressurization Level

Application

Inches Pascals

Relationship to Adjacent

Area(s)

Comments

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 Delivery Nursery Cystoscopy Trauma

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.

Clean Room: Pharmaceutical Mfg.

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.

Clean Room: Micro-Electronic Mfg. Data Storage Mfg. Optics Mfg.

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.


Chapter 3Room 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 rooms 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.



AIRFLOWTRACKING

OFFSET

ROOMSUPPLY

AIRFLOW

TOTAL ROOMEXHAUSTAIRFLOW

LA

B01

92R

1

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 rooms 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 rooms 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.


Design Considerations

MAIN CORRIDOR0.00 in. w.c.

LABORATORY ROOM - 0.01 in. w.c.

(-2.5Pa)

SERVICE CORRIDOR0.00 in. w.c.

ROOMSUPPLY

AIR

TOTAL ROOMEXHAUST AIR

TRANSFERAIRFLOW

INTO ROOM

LA

B01

93R

1

Figure 2. Laboratory Room at a Negative Static Pressure with Respect to the Adjacent Corridors.



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 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.

4 Equations taken from 1999 ASHRAE Application Handbook, Fire and Smoke Management Section, Page 51.5.


Room Pressurization Factors

DIFFERENTIALPRESSURE

INCHES of

WATER

0.020

0.019

0.018

0.017

0.016

0.015

0.014

0.013

0.012

0.011

0.010

0.009

0.008

0.007

0.006

0.005

0.004

0.003

0.002

0.001

0.000

ROOM DIFFERENTIAL AIRFLOW - CFM

0.1 Ft2

0.2 Ft2

0.3 Ft2

0.4 Ft2

0.5 Ft2

0.6 Ft2

0.75 Ft2

1.0 Ft2

ROOM LEAKAGE AREA CURVES

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400

LA

B01

94R

1

1.5 Ft2

Figure 3. Room Differential Airflow vs. Differential Pressure for Various Room Leakage Areas.

Room Pressurization Factors As Figure 3 shows, a rooms static pressurization value is wholly dependent on the differential airflow and the rooms 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 rooms supply air and the total room exhaust is 250 cfm. This relationship exists regardless of what the rooms ventilation rate is (air changes per hour). Therefore, to maintain a specific room pressurization value, the rooms 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 rooms 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.



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 rooms 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 rooms 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.


Chapter 4Room 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.



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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 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 hoods 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.


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 rooms 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.



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 rooms 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 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.


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 rooms 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 rooms 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 hoods 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.



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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 airflows and, thus, increase the rooms 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 airflows and, thus, decrease the rooms 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:

Maintaining the proper amount of room general exhaust to ensure that the required minimum room ventilation rate10 is always maintained.

9 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.

10 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.


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.



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 controllers 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:

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.

11 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.


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 non-laboratory 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 laboratorys 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.



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.

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Figure 6. Preventing Room Contamination by Room Positive Pressure & Vestibule Negative Pressure.


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 1994 Guidelines for preventing the transmission of Mycobacterium and Tuberculosis in Healthcare facilities. 13 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.



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.

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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.



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.

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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.



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:

Proper Room DesignHigh 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 BarriersClean 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.

14 American Institute of Architects 1996 -1997 Guidelines for Design and Construction of Hospital and Health Care Facilities.


Clean Rooms

Construction ProceduresClean 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 pre-cleaned 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 ProceduresClean 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 AirflowClean 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 PressurizationProper 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 (0.3m) or larger in size. ULPA - Ultra Low Penetration Air filters remove 99.999% of particulate 0.12 microns (0.12m) or larger on size. A micron (m) is one millionth of a meter or approximately 0.00003937 inches.



Table 2 provides the allowable limits for 0.1, 0.5 and 5.0 micron size airborne particles per cubic foot and cub

room-pressurization-control--application-guide_a6v10308401_us-en.pdf

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