Environmental Engineering Reference
dampers to close the inlets that are not in use. The fans are usually sin-
gle speed and are not controlled by the building control system. They are
manually turned on and run 24/7 in most cases. Most exhaust fans are in
the range of 1/4-hp to 15-hp.
Since exhaust fans do not consume a lot of energy compared to other
HVAC equipment in the building, many plant operators do not pay atten-
tion to their efficient operation. In manufacturing buildings, exhaust air
needs to be made up by fresh outside air. Due to high air exhaust, some
manufacturing buildings use almost 100% outside air during winter heat-
ing season. The supply system is used to create a comfortable environ-
ment in the plant and replace the air exhausted out by the exhaust system.
The exhaust system removes contaminated air and reduces the heat con-
centration locally. The exhaust system can be divided into general exhaust
and local exhaust. Local exhaust is more effective due to the fact it is close
to the source of contamination.
Temperature and humidity are controlled to ensure worker comfort
and product quality. When excessive exhaust occurs, the supply system
would need to supply more outside air than the minimum required for
proper ventilation, resulting in more heating and cooling energy.
One industrial facility that has done exhaust system retrofit is the
Eldec Corporation, an aerospace electronic manufacturer. With the help
of the local utility, Eldec implemented a control project to reduce exhaust
air by up to 30% for the first shift and 60% for the rest of the time and
achieved great savings with one year simple payback. The project closed
the exhaust inlets with dampers and controlled the exhaust fan speeds
with variable frequency drives (VFD). The exhaust fans are now moni-
tored and controlled by the building direct digital controls (DDC) system
to ensure proper operation and save energy.
The facility has three buildings with sizes ranging from 70,000 to
80,000 square feet (SF). The HVAC systems are variable air volume (VAV)
systems. Minimum 30% relative humidity is controlled in the production
buildings. All the three buildings have a similar operating schedule. One
building is a two-story building with 70,000 SF. Two packaged units serve
the majority of the building. Both units combined have a maximum sup-
ply air of 72,000 CFM and a minimum supply air of 44,000 CFM. Total ex-
haust CFM before retrofit was 34,650 CFM.
Before the retrofit, the exhaust system was manually controlled, by
turning off fans. There were no dampers to close the inlets and no flow
controls for the fans. Since these are local exhaust, heat recovery was not