Basic Documentation

Siemens Industry, Inc. Page 5 of 8
Document No. 149-977
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. The ROOM
CONTROLLER then controls both the ROOM
GENERAL EXHAUST and the SUPPLY AIR
TERMINAL to always provide the proper amount of
room exhaust and supply make-up airflow.
Whenever the room's total airflow changes (such as
when fume hood sashes are repositioned), the
ROOM CONTROLLER adjusts the room supply
airflow and the room general exhaust to maintain
proper room airflow along with the room's airflow
tracking offset.
Volumetric Airflow Tracking
Considerations
As Figure 3 illustrates, the airflow tracking offset
necessary to maintain a specific room differential
pressure is dependent upon the room's total leakage
area. However, it is not usually possible to know
where a particular room’s leakage area will be prior
to construction. Therefore, a designer may initially
specify a room airflow tracking offset based upon the
designer's experience. Later, the airflow tracking
offset may be modified during the test and balancing
phase of the project to attain a specific level of room
differential pressure.
In practice, it is seldom necessary to achieve a
specific differential pressure value for laboratory
rooms since the purpose of room pressurization is to
create and ensure proper directional airflow for the
room. It should also be noted that a given airflow
tracking offset will not ensure that a specific
differential pressure value will be maintained over an
extended period such as the life of a building. As
buildings age and modifications are made, room
leakage area usually changes somewhat and results
in some minor variation from the initial room
differential pressure value. Although eventual
variations in room pressurization are likely, it must
be emphasized again that the primary goal of room
differential pressurization is preventing undesirable
air transfer that would still be attained.
Differential Pressure Sensing
Control
Differential pressure sensing control is based upon
the input from a room differential pressure sensor.
This sensor is typically located in the wall that
separates the room from the adjacent space. Figure
5 shows the essential components of a differential
pre
ssure sensing control configuration for a
chemical laboratory room.
The ROOM CONTROLLER monitors the room
differential pressure by means of the ROOM
DIFFERENTIAL PRESSURE SENSOR. The ROOM
CONTROLLER controls the SUPPLY AIR
TERMINAL airflow and the GENERAL EXHAUST to
maintain the required room differential pressure
setpoint. If the room differential pressure is less than
the setpoint, the ROOM CONTROLLER reduces
supply airflow to increase the room differential
airflow and consequently increase the room negative
pressure. If the room differential pressure is greater
than the setpoint, the ROOM CONTROLLER
increases supply airflow to decrease the room