Basic Documentation

Siemens Industry, Inc. Page 7 of 8
Document No. 149-977
VAV Fume Hood Effects
Differential pressure sensing control is not
recommended whenever the room airflow can
undergo rapid changes as in laboratory rooms with
VAV fume hoods. VAV fume hood exhaust must be
dynamically controlled to maintain a constant fume
hood face velocity for all sash positions. When a
user moves a fume hood sash, the fume hood
controller must rapidly increase or decrease the
hood exhaust to maintain the required face velocity.
Therefore, every VAV fume hood control action
affects the room differential airflow and consequently
the room differential pressure.
To obtain a reliable room differential pressure value
from a room differential pressure sensor, the ROOM
CONTROLLER must sample and integrate the
sensor output for a period of time—perhaps ten or
more seconds. This is necessary to factor out signal
noise caused by personnel movement, local air
currents and even the effects of wind on the
building. When a VAV fume hood sash is opened,
this delay in obtaining a new stable room static
pressure value can delay increasing the supply
makeup air necessary to balance off the increased
fume hood exhaust airflow. This can delay the
restoration of the fume hood face velocity to the
level necessary to ensure fume containment.
In contrast, airflow measurements in ducts as
utilized by volumetric airflow tracking, provides the
ROOM CONTROLLER with actual airflow values
much more rapidly since duct airflow signals are
more robust and subject to much less signal noise.
Figure 6 depicts the output signals from a room
differential p
ressure sensor versus a ducted airflow
measurement signal upon an increase in fume hood
exhaust. Because of the appreciable signal noise, it
typically takes the ROOM CONTROLLER ten
seconds or more to obtain a new reliable differential
pressure value and begin the necessary makeup
airflow control action. In contrast, ducted fume hood
exhaust airflow measurements enables makeup
airflow control action to begin within one or two
seconds.
Cascaded Pressure Control
Although airflow tracking is the preferred method for
maintaining laboratory room pressurization and
ensuring fast fume hood response, there may be
valid reasons to ensure that a specific room
differential pressure level is maintained. When this is
necessary, cascaded pressure control can be
applied to retain the superior speed and stability of
airflow tracking and also ensure that the desired
differential pressure value is maintained.
Cascaded pressure control combines the control
functionality of both volumetric airflow tracking and
differential pressure sensing. The control
configuration is essentially the same as in Figure 4
but with the a
ddition of the ROOM DIFFERENTIAL
PRESSURE SENSOR shown in Figure 5. The
control scen
ario basically uses airflow tracking as
the regular online means to ensure fast and stable
control response. The ROOM CONTROLLER also
monitors the ROOM DIFFERENTIAL PRESSURE
SENSOR, and when necessary adjusts the airflow
tracking offset to maintain the room differential
pressure at the level desired.