Basic Documentation

differential airflow and thus, decrease the negative pressure.
SUPPLY AIR
TERMINAL
ROOM
CONTROLLER
FUME HOOD
EXHAUSTS
ROOM
GENERAL
EXHAUST
DIFFERENTIAL PRESSURE
ROOM DIFFERENTIAL
PRESSURE SENSOR
Figure 5. Differential Pressure Sensing Control Arrangement for a Laboratory Room.
Differential Pressure Sensing
Considerations
Page 6 of 8 Siemens Industry, Inc.
Document No. 149-977
Although differential pressure sensing control
responds directly to room differential pressure, it can
be adversely affected by numerous factors during
normal room usage.
Door Opening Effects
When open, a standard single width door can
increase the room leakage area by about 20 square
feet causing the resulting room leakage area curve
to lie along the bottom of the graph (see Figure 3).
Even if the ROOM CONT
ROLLER decreases the
supply airflow to zero, it will be unable to attain
sufficient room differential airflow to maintain the
differential pressure setpoint with a door open.
Having no supply airflow can adversely affect the
room’s ambient temperature and humidity especially
if the door remains open for an extended period. To
minimize the effect of door openings, a differential
pressure sensing control arrangement must always
ensure that there will be a minimum supply airflow.
As an alternative, a door switch can be used to
signal the ROOM CONTROLLER to maintain the
last supply airflow value whenever a door is open.
However, the door switches and associated wiring
involves additional cost.
Adjacent Area Makeup Air
Recall that with volumetric airflow tracking the room
differential airflow (also referred to as the airflow
tracking offset) is fixed. The total of all room airflow
tracking offsets is the total quantity of air that will
flow from the adjacent area (typically a corridor) into
the laboratory rooms. Because the ventilation
system designer knows the amount of corridor
makeup airflow required, a provision for this in the
ventilation system design can be made readily.
However, with differential pressure sensing control
the amount of airflow into each laboratory room from
the corridor is a variable. Thus, corridor makeup air
requirements cannot be readily established
especially when there are many laboratory rooms.
This complicates the designer's task and often
requires additional ventilation system components
and controls to ensure that proper corridor makeup
air will be provided.