Basic Documentation

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.
Siemens Building Technologies, Inc. 23