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Refrigeration systems must operate at all hours of the year, even when the
building is unoccupied. Warmer weather tends to push refrigeration equipment to
its capacity limit, thus creating a maximum operating kW and kWh.
Evaporators - must be selected to provide the required cooling at all expected ambient
conditions even with the maximum frost on the coils (i.e., just prior to
defrosting). Evaporator coils used include two types of refrigeration systems: flooded
evaporator and direct expansion. For direct expansion systems, two of the most
commonly used refrigerant liquid metering devices are the capillary tube and
the thermostatic expansion valve.
In addition, proper provisions must be made for periodic defrosting of
evaporator air-side surfaces. Defrosting may be accomplished using refrigerant
compressor discharge hot-gas, water spray, or manually as selected to meet the user's
objectives. Suitable drain connections should be provided to carry off the
water resulting from defrost operations.
Condensers - must be selected to operate at all outdoor weather conditions in the area.
Air-cooled condensers must be supplied with the proper controls to permit
operation at low outdoor ambient conditions. Water-cooled condensers may require
water regulating valves to keep condensing pressure high enough to enable the
thermal expansion valves to function. The type of condenser selected depends
largely on the size of the cooling load, refrigerant used, quality and temperature of
available cooling water (if any), and noise considerations.
Water-cooled condensers require cooling water from an external cooling tower,
or from a lake, well, river or other similar source. Once-through use of city
water for condensing purposes is prohibited in most locations. Air-cooled
condensers are the most popular since they avoid other problems of water
acquisition, treatment and disposal. The trade-off may be higher electrical consumption.
As seen here, the evaporative condenser is a combination of a water cooled condenser and an
air-cooled condenser that rejects heat through the evaporation of water into an
airstream traveling across a condenser coil.
Compressors - must be sized to meet the varying needs of each application. Provision must
be made to protect the compressor from liquid carry over from the evaporator, in
addition to the normal safety controls (high and low pressure cutout. oil
pressure, etc.). The most common type of compressor used for commercial
refrigeration systems is the reciprocating compressor. Reciprocating compressor types
include single-stage (booster or high state), internally compounded, and open,
hermetic or semi-hermetic.
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