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
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.
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.