Basic Documentation

Siemens Industry, Inc. Page 3 of 8
Document No. 149-977
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.
Then, from the point of intersection, proceed
downward to obtain the required ROOM
DIFFERENTIAL AIRFLOW CFM.
As seen from Figure 3, a room’s differential
pressurization value with respect to an adjacent area
is totally dependent upon the room's differential
airflow and the room’s leakage area. For instance,
the graph shows that to attain a differential pressure
(dP) of 0.008 Inches of Water for a room with a 0.75
square feet leakage area, the difference between
the room supply air and the total room exhaust
needs to be 175 cfm. Therefore, to maintain a
specific room differential pressure value, the room’s
differential airflow must be controlled and maintained
at the proper value. Note that neither room size nor
its dimensions directly enter into the room
pressurization relationship.
Leakage Area Considerations
With relatively good quality construction, modest
sized rooms such as a two person laboratory with
two hinged doors will typically have a total room
leakage area of between 0.5 and 1.0 square feet.
Maintaining a pressure differential with significantly
more leakage area than about 1.5 square feet
requires a relatively high differential airflow.
Experience indicates that a negative 0.010 Inch
W.C. room differential pressure (typical utilized for
chemical laboratory rooms) would be difficult to
maintain with a leakage area greater than about 2.0
square feet due to the excessively high differential
airflow required.
It is also very important to consider the effect that an
open door will have on room pressurization.
Opening a single width door having dimensions of
about 3 feet by 7 feet will increase a room's leakage
area by approximately 20 square feet or more. The
resulting room leakage area curve (20.0 square
feet.) would essentially lie at the bottom of the graph
and correspond to a near zero differential pressure
for the room. Therefore, while a door is open no
appreciable room differential pressure can be
sustained. However, the normal opening and closing
of doors does not really compromise the goal of
room pressurization—to prevent unwanted air
transfer. Proper directional airflow (inward for a
negative room and outward for a positive room) will
still exist through the open door and other leakage
areas as long as the room's airflow tracking offset is
maintained.