Basic Documentation

Siemens Industry, Inc. Page 7 of 8
Document No. 149-975
and renowned authorities on fume hood safety and
containment. The prevailing view is that multiple
measurements are necessary to actually obtain an
accurate indication of average face velocity and one
sensing point will not accomplish this. Note that this
does not even take into account the additional
discrepancy that is likely to result when the single
point of measurement is in the side wall rather than
at the front of the fume hood.
Most notably the American National Standard for
Laboratory Ventilation (ANSI/AIHA Z9.5) and
ASHRAE 110 require measurements of face velocity
to be made by an airflow traverse or by an array of
sensors in the plane of the sash opening (as shown
in Figure 2). Note also that the test results graphs in
Figure 3 through Figure 5 pertain to a four-foot wide
fume hood, which i
s a modest size fume hood. Most
laboratories use larger fume hoods and past
experience indicates even more discrepancy
between the airflow measurement at a single point in
the side wall versus the airflow conditions at the
actual sash opening for larger fume hoods.
Fume Hood Equipment
The test results were all obtained for a fume hood
that was totally empty of all contents. In reality, fume
hoods are frequently full of equipment, experimental
apparatus and analytical instrumentation. These
items may affect the free and unobstructed airflow
that is needed by a side wall sensor. Quite often
fume hoods contain large pieces of equipment
(ovens, coolers, etc.) that significantly interfere with
the airflow at the side wall sensor. This can be
compared to a room's temperature sensor that is
prevented from effectively sensing room air
temperature if a large cabinet were placed directly in
front of the sensor.
Internal and External Fume Hood
Activity
Side wall sensing is also subject to the activities
taking place inside and outside of the fume hood.
Chemical processes generating buoyant fumes and
convection air currents caused by heaters adversely
affect side wall sensing. Similarly, room air currents
caused by ventilation system airflow or by persons in
motion will affect side wall sensing.
Control Stability Test Results
Figure 5 shows a pair of graphs for a control stability
test for the same fume hood with both sash position
sensing and side wall sensing face velocity control
and operating under identical test conditions. The
tests show the resulting fume hood exhaust airflow
over a 300 second (5 minute) elapsed time period.
Much greater stability is evident for sash position
sensing by the test graphs of Figure 5.
The Case for Sash Position
Sensing
Sash position sensing takes a combined
mathematical and logical approach to face velocity
control. It performs its control function in a manner
similar to the way a person would solve a problem
when applying the scientific method of analysis. In
other words, given the need to establish a certain
average face velocity across a fume hood's sash
opening, the logical approach is to:
1. Determine the size of the sash opening and
add any additional fume hood openings to
determine the total fume hood open area in
square feet.
2. Multiply the average face velocity in feet per
minute (fpm) by the total fume hood open
area to determine the fume hood airflow
needed.
ft/min × ft
2
= ft
3
/min (cfm)
3. Use a precise means to measure and adjust
the fume hood exhaust to attain the required
airflow cfm through the fume hood.
Once the required fume hood exhaust airflow has
been achieved, it follows that the required average
incoming airflow velocity (face velocity) will also be
achieved over the fume hood's open area. In fact,
mathematically dividing the fume hood exhaust
airflow by the fume hood's total open area usually
provides a more accurate average face velocity
value than the direct measurement approach of
Figure 2.
The Case for Side Wall
Sensing
As a control application, side wall sensing correctly
attempts to measure the controlled variable—fume
hood face velocity—and then modulate fume hood
airflow to maintain the average face velocity
setpoint. Although there are instances where side
wall sensing based control has worked well, there
are many instances where it has not provided the