Page 301 - Mechanical Engineers' Handbook (Volume 4)

290 Energy Auditing

the energy loss in exhaust air by transferring heat between the exhaust air and the fresh air
intake.
The common types of units available are heat wheels, surface heat exchangers, and heat-
transfer-fluid loops. Heat wheels are revolving arrays of corrugated steel plates or other
media. In the heating season, the plates absorb heat in the exhaust air duct, rotate to the
intake air duct, and reject heat to the incoming fresh air. Surface heat exchangers are air-to-
air heat exchangers. Some of these units are equipped with water sprays on the exhaust air
side of the heat exchanger for indirect evaporative cooling. When a facility’s exhaust- and
fresh-air intakes are physically separated by large distances, heat-transfer-fluid loops (some-
times called run-around systems) are the only practical approach to exhaust-air heat recovery.
With the fluid loop, heat exchangers are installed in both the exhaust and intake ducts and
the fluid is circulated between the exchangers.
A key factor in estimating savings from exhaust air heat recovery is the unit’s effec-
tiveness, expressed as the percentage of the theoretically possible heat transfer that the unit
actually achieves. With a 40 F temperature difference between the exhaust and intake air in
the heating mode, a 60% effective unit will raise the intake air temperature by 24 F. In units
with indirect evaporative cooling, the effectiveness indicates the extent to which the unit can
reduce the difference between the intake air dry-bulb temperature and the exhaust air wet-
bulb temperature. The effectiveness of commercially available exhaust air heat recovery units
ranges from 50 to 80%; greater effectiveness is usually obtained at a higher price per unit
of heat recovery capacity.

Refrigeration Heat Recovery
Heat recovery from refrigerators and air conditioners can replace fuel that would otherwise
be consumed for low-temperature heating needs. Heat recovery units that generate hot water
consist of water storage tanks with an integral refrigerant condenser that supplements or
replaces the existing condenser on the refrigerator or air conditioner. These units reduce the
facility’s fuel or electricity consumption for water heating, and also increase the refrigeration
or air conditioning system’s efficiency due to the resulting cooler operating temperature of
the condenser.
The most efficient condensing temperature will vary, depending on the compressor de-
sign and refrigerant, but in most cases it will be below 100 F. In facilities requiring water
at higher temperatures, the refrigeration heat recovery unit can preheat water for the existing
water heater, which will then heat the water to the final temperature.

Boiler Heat Recovery Devices
Part of the energy conversion losses in a boiler room can be reduced by installing a boiler
economizer, air preheater, or a blowdown heat recovery unit. Both the economizer and the
air preheater recover heat from the stack gases. The economizer preheats boiler feedwater
and the air preheater heats combustion air. The energy savings from these devices are typi-
cally 5–10% of the boiler’s fuel consumption. The savings depend primarily on the boiler’s
stack gas temperature. Blowdown heat recovery units are used with continuous blowdown
systems and can either supply low-pressure steam to the deaerator or preheat makeup water
for the boiler. Their energy savings are typically 1–2% of boiler fuel consumption. The
actual savings will depend on the flow rate of the boiler blowdown and the boiler’s steam
pressure or hot-water temperature.

More Efficient Electric Motors
Replacement of integral-horsepower conventional electric motors with high-efficiency motors
will typically yield an efficiency improvement of 2–5% at full load (see Table 5). While this