Operating Instructions

Siemens Building Technologies, Inc. 13

Applications

Basic Operation

The Fume Hood Controller provides Direct Digital Control (DDC) technology for controlling

fume hood face velocities in a manifolded fume hood exhaust system.

Average Face Velocity Control

The Fume Hood Controller maintains the average face velocity by calculating the open area

of the fume hood sash using inputs from the sash sensors and other measured constants.

The controller then determines the exhaust airflow volume to reach the face velocity setpoint.

The controller modulates the exhaust damper or valve position to adjust the actual exhaust

flow to reach the flow setpoint, and thereby the face velocity setpoint. The OCC.UNOCC face

velocity level is controlled by either the network or DI 1.

If enabled, the average face velocity will display at the ODP. (See Display Filters for details).

Minimum and Maximum Flow Rates

To prevent the build-up of fumes and possible corrosion of the exhaust system, it is important

to maintain a minimum exhaust airflow at approximately 20% to 30% of the maximum

exhaust airflow. If, as the open sash area is decreased, the calculated exhaust airflow

setpoint falls below the minimum exhaust airflow specified the flow setpoint is set equal to the

minimum. Under this minimum flow condition, the ODP displays FLO instead of the average

face velocity.

The Fume Hood Controller will maintain a constant face velocity until the minimum flow

condition occurs. At that point, the controller will control to a constant exhaust volume. In the

minimum flow mode, the face velocity will rise above the setpoint as the open sash area

decreases toward the fully closed position.

The maximum exhaust flow setpoint is used to limit the maximum amount of air that the

Fume Hood can draw. If, as the open sash area is increased, the calculated exhaust airflow

setpoint raises above the maximum airflow specified, the flow setpoint is set equal to the

maximum. Under this maximum flow condition, the ODP displays the average face velocity

which will drop as the open sash area is further increased.