Basic Documentation

face velocity of 100 fpm is only 0.000623 inches of
water, which is an extremely tiny pressure difference
to accurately measure. In fact, if the measurement
were off by only 0.00010 (one ten thousandth)
inches of water (an extremely miniscule amount), the
resulting error in air velocity would already be ±8%.
Thus, trying to obtain the true average differential
pressure value between the interior and the exterior
of a fume hood presents a very difficult challenge.
Based on this factor alone, it is hard to accept that
even under the best of conditions, side wall based
control accuracy could be better than perhaps ±10%.
It should be noted that industry standards do not
allow using a single point of airflow velocity or
differential pressure measurement to determine
average fume hood face velocity. Rather, industry
standards require that fume hood average face
velocity measurement be determined by making
multiple airflow velocity measurements
4
in the actual
plane of the sash opening and then mathematically
averaging these measured values. Figure 2
illustrates the normally accepted means
to
accurately measure the average face velocity of the
air flowing through a sash opening.
INDIVIDUAL SENSORS
SASH PLANE
FUME HOOD
Figure 2. Requirements for Measuring Average
Face Velocity.
Control Accuracy Test Results
Figure 3 shows two graphs of face velocity control
accuracy tests for the same fume hood under
Page 4 of 8 Siemens Industry, Inc.
Document No. 149-975
4. ASHRAE 110, 1995 requires that the sash opening be
equally divided into 1 square foot grids and air velocity
measurements be taken at the center of each grid. These air
velocity readings must then be mathematically averaged in
order to determine the fume hood average face velocity at
any given sash opening.
identical test conditions and outfitted with both sash
position sensing and side wall sensing face velocity
controllers. Actual face velocity was measured
similarly to Figure 2. Individual sensors were placed
at 10 sep
arate locations within the open plane of the
sash (A1, A2, B1, B2, etc.) and are listed on the top
of the test graphs. These two test graphs provide a
dynamic comparison of fume hood face velocity
control accuracy for sash position sensing and side
wall sensing. Note that each face velocity
measurement was recorded for a 30 second time
duration. Thus the total elapsed time for all test
measurements was 300 seconds (5 minutes)
5
.
The test results show that face velocity was more
consistent and exhibited much less departure from
the desired setpoint value (100 fpm) for Sash
Position Sensing Based Control. In contrast, the
face velocity exhibited significant variations under
Side Wall Sensing Based Control. Note that the
graph should not be interpreted to mean that the
face velocity variations only occurred at some test
locations. Rather, all measurements, including the
major variations (such as those in locations A2, B1,
and D1 for side wall sensing), are indicative of face
velocity variations that would have been occurring
throughout the entire sash opening during those
elapsed time periods. In addition, since the air
velocity measurement duration at each location was
30 seconds, it can be seen that some of the major
face velocity variations occurred for perhaps several
seconds under side wall sensing control.
Response Time
VAV Fume hood response time is defined as the
time that it takes for the face velocity to be restored
to within 10% of its setpoint value for a change in
sash position. The test procedure consists of moving
the sash at a uniform rate of between 1.0 and 1.5
feet per second from a starting position of 25% open
until the sash is full opened
6
.
Sash Position Sensing
Whether the sash is being opened or closed, there is
virtually no time lag between when sash movement
begins and when the sash position sensor responds.
Thus, the face velocity controller begins corrective
5. This Information is taken from a Laboratory Ventilation
Seminar Presentation during the 1998 ASHRAE Winter
Meeting in San Francisco, California, titled: "Comparison of
Through the Wall VAV Position Sensors Versus Sash
Position Sensors".
6. ASHRAE 110, 1995 - 6.4 VAV Response Test