Basic Documentation

Room Pressurization Control Application Guide
To obtain a reliable room static pressure value from the room static pressure sensor, the
room controller must sample the sensor output for several seconds to factor out the pressure
variations caused by personnel movement, room air currents, and even outside air wind
gusts (referred to as signal noise). This delays the room controller’s response to room static
pressure variations. The slower control response translates into more elapsed time before
the room supply airflow is properly adjusted to maintain the required room static pressure.
This can result in an unacceptable delay before adequate supply makeup airflow is attained
after a fume hood sash is repositioned
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Cascaded Pressure Control
Although airflow tracking is the preferred method for preventing cross contamination for most
laboratory applications, there may be valid reasons to ensure that a specific room static
pressure level is always maintained. Changes in weather, inside and outside temperature
variations, and changes occurring in building equipment and structures, can affect the room
static pressure level that is maintained by airflow tracking static pressure control. To retain
the superior speed and stability of airflow tracking and ensure that the required static
pressure is always maintained, cascaded pressure control can be applied.
Cascaded pressure control combines the functionality of both direct pressure control and
airflow tracking. This control approach is mainly an airflow tracking control approach with the
addition of a room static pressure sensor and the control components are essentially the
same as those of Figure 5.
The overall control scenario uses airflow tracking as the ongoing means to maintaining room
static pressure. However, the control scenario also uses the wall mounted pressure sensor to
enable the room controller to periodically read the actual room static pressure. If need be, the
room controller will adjust the airflow tracking differential to maintain the room static pressure
at the desired level. As a result of this arrangement, the airflow tracking value does not
remain fixed as it would for regular airflow tracking, but is periodically reset (if needed) at
regular intervals based on actual room static pressure. Therefore, this cascaded control
arrangement compensates for static pressure variations that might otherwise occur with only
airflow tracking.
An additional benefit of a cascaded control arrangement is that the room static pressure
sensor enables providing a local indication and also remote monitoring of the specific room
static pressure.
Although cascaded pressure control might seem to be the best means of static pressure
control, it also has some drawbacks:
• It costs more than airflow tracking since it requires a room static pressure sensor and
a more complex control algorithm.
• It must also address many of the problems associated with direct pressure control,
such as a door that is left open for an extended time.
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If a VAV fume hood sash is fully opened after being closed, it could require perhaps 10 or more seconds before a new stable
room static pressure value can be used for controller response. This would delay increasing the supply makeup air required to
offset the increased fume hood exhaust airflow and could therefore adversely affect fume hood containment.
24 Siemens Building Technologies, Inc.