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Technology Report

February, 2003

Siemens Industry, Inc. Page 1 of 8

Fume Hood Face Velocity Control

Sash Position Sensing vs. Side Wall Sensing

A question is often raised regarding the preferred

method of controlling fume hood face velocity

particularly with variable air volume (VAV) fume

hoods. Just to recap, the objective of VAV fume

hood face velocity control is to:

 maintain the fume hood's average face velocity

at a constant value regardless of the size of the

fume hood's sash opening.

 minimize energy usage and enhance fume hood

containment.

Methods of Face Velocity Control

The two predominant methods for controlling fume

hood face velocity are sash position sensing and

side wall sensing. In sash position sensing the

controller determines the size of the sash opening

by one or more sash position sensors. The controller

then adjusts the fume hood's exhaust airflow in

relation to the size of the sash opening. This results

in a specific amount of makeup air being drawn

through the sash opening, which can be expressed

as the fume hood's average face velocity.

For instance, if an average face velocity of 100 feet

per minute (fpm) is desired and the fume hood's

open area is determined to be 4 square feet, a fume

hood exhaust of 400 cubic feet per minute (cfm) will

be maintained. This will cause 400 cfm of makeup

air to be drawn through the 4 square foot open area

resulting in an average air (face) velocity of 100 fpm.

Similarly, if the total open area were 6 square feet,

the fume hood exhaust would be increased to 600

cfm to maintain the same 100 fpm average face

velocity.

With side wall sensing, the controller senses the

velocity of the makeup air being drawn through the

open sash area and adjusts the fume hood's

exhaust as needed to maintain the makeup airflow

at the required air (face) velocity. The airflow velocity

sensor is normally located in the fume hood's side

wall so it won't interfere with fume hood usage.

Thus, this type of face velocity control is normally

referred to as side wall sensing.

Figure 1 illustrates these two different control

approaches.

Figure 1

. Sash Position Sensing and Side Wall Sensing for a Single Vertical Sash Fume Hood.

Document No. 1

49-975

Summary of content (8 pages)

PAGE 1
Technology Report February, 2003 Fume Hood Face Velocity Control Sash Position Sensing vs. Side Wall Sensing A question is often raised regarding the preferred method of controlling fume hood face velocity particularly with variable air volume (VAV) fume hoods. Just to recap, the objective of VAV fume hood face velocity control is to:  maintain the fume hood's average face velocity at a constant value regardless of the size of the fume hood's sash opening.
PAGE 2
Differences between the two methods In Figure 1, the sash position based control uses a Sash Position Sensor as well as an Exhaust Airflow Measurement Sensor as inputs to the fume hood face velocity controller. Side wall sensing based control only uses the Side Wall Air Velocity Sensor as a controller input.
PAGE 3
accurately sense and measure the controlled variable (room temperature in this example) is a fundamental requirement for any control process. Response Time Let's consider the effect of a room temperature sensor that is very accurate, but unfortunately has a slow response time. This results in slow control response and in the case of room temperature, it would probably result in room temperature swings.
PAGE 4
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.
PAGE 5
Since the ASHRAE 110 response time test states that the test begins with the sash at 25% of the maximum design opening, the starting point for the exhaust airflow is 25% of 720 cfm or about 180 cfm. Also since the ASHRAE 110 response time test concludes when face velocity is within 10% of the setpoint, the final exhaust airflow for the test is 90% of 720 cfm or approximately 650 cfm.
PAGE 6
Sash Position Sensing Based Control Side Wall Sensing Based Control Response Time Tests: 4 Foot Fume Hood Response Time Tests: 4 Foot Fume Hood 800 800 700 700 650 CFM 650 CFM FUME 600 HOOD EXHAUST 500 AIRFLOW 400 CFM FUME 600 HOOD EXHAUST 500 AIRFLOW 400 CFM 300 300 200 180 CFM 100 200 180 CFM 100 0 50 100 150 200 250 0 300 50 100 150 200 250 300 ELAPSED TIME - SECONDS ELAPSED TIME - SECONDS Figure 4. Sash Position Sensing and Side Wall Sensing–Response Time Comparison.
PAGE 7
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.
PAGE 8
desired level of control accuracy, response time and stability. Experience indicates that the response time for side wall sensing is seconds longer than for sash position sensing. Suppliers of side wall sensing control systems do not dispute this; rather they claim that the response time will still be within generally acceptable safe limits. However, given the choice, it would certainly seem preferable to opt for faster control response and better control stability.