Page 300 - Mechanical Engineers' Handbook (Volume 4)

3 Performing an Energy Audit—Identifying Opportunities for Saving Energy 289

Figure 4 Adjusting air-conditioner controls to provide higher chilled-water temperatures improves
chiller efficiency. 3

these periods so that the load on the compressor is reduced or eliminated. The economizer
is a system of automatic dampers on the return air duct that are controlled by return-air and
outside-air temperature sensors. When the outside air is cooler than the return air, the damp-
ers divert the return air to the outdoors and let in enough fresh outside air to supply all the
air to the building. In humid climates, economizers must be fitted with ‘‘enthalpy’’ sensors
that measure wet-bulb as well as dry-bulb temperature so that the economizer will not let in
outside air when it is too humid for use in the building.

Building Exhaust-Air Heat Recovery Units
Exhaust-air heat recovery can be practical for facilities with large outside-air flow rates in
relatively extreme climates. Hospitals and other facilities that are required to have once-
through ventilation are especially good candidates. Exhaust-air heat recovery units reduce