Smart Buildings Systems for Architects, Owners and Builders

Evaporative condensed chillers operate essentially as a smaller and more
efficient version of a water-cooled system; they use the same evaporative cool-
ing provided by a cooling tower. The cooling is achieved using a recirculating
water system, which continuously wets the condenser tubes while fans blow air
over them, evaporating the water and thereby moving (rejecting) the heat to
the atmosphere.
Although this also uses air to move heat out of the condenser just as an air-
cooled system would, an air-cooled condenser is less efficient because it draws
ambient air over the condensing surface and rejects heat at the dry-bulb tem-
perature. Evaporative condensed chillers use considerably less water than
water-cooled chillers, thus reducing the operational costs.
For each type of chiller there are four compressor subcategories: reciprocat-
ing, centrifugal, screw driven, and absorption. Reciprocating, centrifugal, and
screw-driven chillers are powered mechanically by electric motors, steam, or
gas turbines while an absorption chiller is powered by heat and uses no moving
parts. Reciprocating compressors use pistons driven by a crankshaft and are
typically used for delivering small amounts of refrigerant at high pressure.
Centrifugal compressors use centrifugal force to compress air and are used
for delivering large volumes of refrigerant at low pressure. They are widely
used in industry because they are energy efficient and have few moving parts.
Screw-driven compressors use two opposing rotating screws to compress the
air between them.

Cooling

To w e r

–95F Water

Pump

–85F Water

–40F Water

–50F Water

Compressor

Condenser

Chilled Water Loop

or Chilled

Water Circuit

(Closed Loop)

Open Recirculating Water

Loop or Cooling

Tower Circuit

(Open Loop)

Pump

Evaporator

Air Handling Units

Figure 3.6 Water-cooled AC system.

Heating, Ventilating, and Air Conditioning Systems 37