Figure 7-14.Two-stage semihermetic centrifugal unit.
units. An example is a large hospital with wings off a
corridor. Air conditioning may be necessary in
operating rooms, treatment suites, and possibly some
recovery wards. Chilled water-producing and
water-circulating equipment is in a mechanical
equipment room. Long mains with many joints
between condensing equipment and conditioning units
increase the chance of leaks. Expensive refrigerant has
to be replaced. It may be better to provide
water-cooling equipment close to the condensing units
and to circulate chilled water to remote air-cooling
coils. Chilled water is circulated to various
room-located coils by a pump, and the temperature of
the air leaving each coil may be controlled by a
thermostat that controls a water valve or stops and
starts each cooling coil fan motor.
Types of Coolers
The two most commonly used water coolers
(evaporators) for chilled water air conditioning are
flooded shell-and-tube and dry-expansion coolers.
The disadvantage of the flooded shell-and-tube cooler
is that it needs more refrigeration than other systems of
equal size. Furthermore, water in tubes may freeze and
split tubes when the load falls off.
Flooded coolers should be controlled with a
low-pressure float control-a float valve placed so the
float is about the same level as the predetermined
refrigerant level. The float, as a pilot, moves a valve in
the liquid line to control the flow of refrigerant to the
evaporator. Automatic or thermostatic expansion
valves control the dry-expansion coolers. The
refrigerant is inside the tubes; therefore, freezing of
water on the tubes is less likely to cause damage.
The primary purpose of the condenser is to liquefy
the refrigerant vapor. The heat added to the refrigerant
in the evaporator and compressor must be transferred
to some other medium from the condenser. This
medium is the air or water used to cool the condenser.