Basic Documentation

0246810
SECONDS
TIME TO OBTAIN
NEW VALUE
0246810
SECONDS
TIME TO OBTAIN
NEW VALUE
ROOM DIFFERENTIAL PRESSURE DUCT AIRFLOW MEASUREMENT
Figure 6. Comparison of Time Needed to Obtain New Values After Room Airflow Change.
As a result of this arrangement, the airflow tracking
offset value does not remain fixed as in regular
airflow tracking, but is periodically reset if needed. A
cascaded control arrangement will compensate for
differential pressure variations that might occur over
the long term. Since a differential pressure sensor is
part of a cascaded control arrangement, it also
enables local indication as well as remote monitoring
of a room's differential pressure level.
Although cascaded pressure control might seem to
be the best of both worlds, it also poses some
drawbacks:
 It is more costly than regular airflow tracking
since it also requires installation of the room
differential pressure sensor and inclusion of a
more complex room control algorithm.
 It requires the ventilation system designer to
also address some of the vexing problems
associated with differential pressure sensing
control such as the effects of open doors and
ensuring that the proper amount of corridor
make air will be provided.
 It involves a more complex startup and
balancing process.
 Equally satisfactory results can usually be
obtained with regular volumetric airflow tracking
and having someone occasionally check the
differential pressure values of various rooms.
Conclusion
This report has presented a detailed factual
discussion on room pressurization control by both
volumetric airflow tracking and differential pressure
sensing. Due to its slower response, differential
pressure sensing control is best limited to
applications where room airflows remain very stable
as in constant air volume ventilation systems.
The vast majority of experienced ventilation system
designers find volumetric airflow tracking to be more
satisfactory for room pressurization control. In
particular, airflow tracking is well suited to and is
perhaps the only reliable approach for controlling
room pressurization in VAV laboratory ventilation
systems and whenever fast and stable
pressurization control is crucial.
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Siemens Industry, Inc. Printed in the USA
Building Technologies Division Document No. 149-977
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