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Technology Report

February, 2003

Siemens Industry, Inc. Page 1 of 8

Room Pressurization Control Methods

Volumetric Airflow Tracking vs. Differential Pressure Sensing

Proper room pressurization is critical for preventing

unwanted air transfer. In chemical and biological

research facilities chemical fumes and airborne

biological agents must be prevented from migrating

out of laboratory rooms to non-laboratory areas. Air

must be prevented from flowing into spaces that

require a high degree of cleanliness and purity, such

as food and drug processing operations. The

absence of airborne particulate is especially critical

for microelectronics and optical manufacturing.

Proper room pressurization is vital for protecting the

medical staff and patients from exposure to harmful

and sometimes deadly airborne pathogens in

treatment facilities. This report discusses the

advantages and disadvantages of the two most

commonly accepted methods of room pressurization

control—Volumetric Airflow Tracking and Differential

Pressure Sensing.

Pressurization and Directional

Airflow

The potential direction of air transfer is always from

an area of higher static pressure (termed the

positively pressurized area) to an area of lower static

pressure (referred to as a negatively pressurized

area). The difference between the static pressure of

two rooms or spaces is commonly referred to as the

differential pressure.

Leakage Area

Although the positive or negative pressurization

relationship between spaces establishes the

potential for air transfer or airflow, there must be an

opening between the spaces for airflow to actually

occur. Typically, such openings are the combination

of unintentional construction related gaps created by

the transverse of mechanical components (pipes,

electrical conduit, ventilation ducts, etc.) and the

necessary clearance openings around doors. All of

these openings are cumulatively referred to as a

room's leakage area.

If there is absolutely no leakage area (a room is

totally and perfectly sealed off) then no airflow takes

place, even though a differential pressure exists

between the room and its adjacent space. However,

except for extreme situations (such as a Biological

Level 4 Laboratory), there is little reason to try to

attain a perfect seal or barrier between most

pressurized spaces. Personnel typically need to

freely enter and leave, and materials often need to

be transported into and out of such spaces. Thus, a

perfect seal or barrier is not a practical solution for

the prevention of unwanted air transfer. This is the

fundamental reason for maintaining a differential

pressure relationship; it is the most practical way to

prevent unwanted air transfer. The required

differential pressure relationship is created and

maintained by a properly designed and controlled

ventilation system.

Volumetric Airflow Tracking

Control

Perhaps the most commonly applied means to

maintain a room at a negative or positive static

pressure with relation to an adjacent space is by

volumetric airflow tracking, often referred to as

airflow tracking. Airflow tracking maintains the

desired differential pressure relationship between

rooms or spaces by maintaining a specific difference

(termed the Airflow Tracking Offset) between the air

supplied to and the total amount of air exhausted

from a room or space.

To maintain a room at a negative static pressure

with respect to another room or space, the airflow

tracking offset ensures that the total amount of air

exhausted from the room always exceeds the

amount of air supplied to the room. This creates a

slight vacuum effect in the room causing air from

adjacent areas to flow into the room through the

room’s leakage area.

For positively pressurized rooms, the airflow tracking

offset ensures that the total amount of air exhausted

Document No. 1

49-977

Summary of content (8 pages)

PAGE 1
Technology Report February, 2003 Room Pressurization Control Methods Volumetric Airflow Tracking vs. Differential Pressure Sensing Proper room pressurization is critical for preventing unwanted air transfer. In chemical and biological research facilities chemical fumes and airborne biological agents must be prevented from migrating out of laboratory rooms to non-laboratory areas.
PAGE 2
from the room is always less than the amount of air supplied to the room. This creates an excess amount of air in the room causing room air to flow outward to the adjacent areas. 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 airflow tracking offset.
PAGE 3
between the total room supply airflow and total room exhaust) is shown in CFM along the bottom of the graph. And, the resulting DIFFERENTIAL PRESSURE between the room and an adjacent area is shown in INCHES OF WATER along the left hand vertical axis. To determine the differential airflow required to attain a specific room differential pressure, follow the DIFFERENTIAL PRESSURE value (on the left vertical axis) to where its horizontal line intersects the appropriate ROOM LEAKAGE AREA CURVE.
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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 0.4 Ft2 DIFFERENTIAL 0.012 0.011 PRESSURE 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 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.
PAGE 5
room. The exhaust of each VAV fume hood is modulated as necessary by individual fume hood controllers (not shown) to maintain the required fume hood face velocity. In addition, the ROOM CONTROLLER also continuously monitors the specific exhaust airflow of each fume hood. To maintain sufficient room exhaust when all fume hood sashes are closed, the room may also need a ROOM GENERAL EXHAUST.
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differential airflow and thus, decrease the negative SUPPLY AIR TERMINAL pressure. ROOM GENERAL EXHAUST FUME HOOD EXHAUSTS DIFFERENTIAL PRESSURE ROOM CONTROLLER ROOM DIFFERENTIAL PRESSURE SENSOR Figure 5. Differential Pressure Sensing Control Arrangement for a Laboratory Room. Differential Pressure Sensing Considerations Although differential pressure sensing control responds directly to room differential pressure, it can be adversely affected by numerous factors during normal room usage.
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VAV Fume Hood Effects Differential pressure sensing control is not recommended whenever the room airflow can undergo rapid changes as in laboratory rooms with VAV fume hoods. VAV fume hood exhaust must be dynamically controlled to maintain a constant fume hood face velocity for all sash positions. When a user moves a fume hood sash, the fume hood controller must rapidly increase or decrease the hood exhaust to maintain the required face velocity.
PAGE 8
ROOM DIFFERENTIAL PRESSURE DUCT AIRFLOW MEASUREMENT TIME TO OBTAIN NEW VALUE 0 2 4 6 8 TIME TO OBTAIN NEW VALUE 10 0 SECONDS 2 4 6 8 10 SECONDS Figure 6. Comparison of Time Needed to Obtain New Values After Room Airflow Change. As a result of this arrangement, the airflow tracking offset value does not remain fixed as in regular airflow tracking, but is periodically reset if needed.