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Hazardous Exhaust Discharge Options

Bruce E. Appel, P.E., Cator, Ruma & Associates


To describe a simple, cost-effective design concept for maintaining constant exhaust plume discharge velocities in variable flow exhaust systems. To review the reasons for maintaining adequate discharge velocities in exhaust systems. To describe the control strategy for controlling the discharge damper and the design challenges which were encountered.


By using a motorized damper in the fan discharge exhaust ducts of larger laboratory fume and general exhaust systems, the plume exit velocity can be constantly maintained under variable flow operation. The damper is a specially designed damper which channels the discharge air to the center of the damper, maintaining good throw characteristics. We will review the control sequences which have resulted in a successful application of the discharge damper. We will review some advantages and disadvantages of this method of accomplishing constant exit velocities and discuss alternative concepts, which were considered for this project.

Labs21 Connection:

This design concept helps to reduce overall environmental impacts of large laboratory exhaust systems by reducing fan energy compared to fan systems which use induced air or constant flow designs. Also, maintenance personnel safety is helped by keeping exit velocities at acceptable levels. Alternative designs of exhaust systems which maintain constant discharge velocity incorporate induced air and additional bypass air at the fan intake. These strategies are successful but do not offer fan energy savings which can be significant over the life cycle of the building.


Bruce Appel has been involved in the mechanical design of laboratory HVAC systems and controls for over 20 years. He has managed numerous large and small laboratory mechanical and electrical designs for projects in the public and private sectors. Mr. Appel is a Principal in the mechanical/electrical engineering consulting engineering firm of Cator, Ruma & Associates, a firm of 55 persons involved in mechanical and electrical building systems design of laboratories, medical facilities, higher education buildings and other public and private buildings.

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