In comfort cooling applications, actual cooling loads are
seldom at full load conditions. Capacity control is achieved in finned
coil evaporators that directly chill air by splitting the coil into
independent sections. The principal reason is to permit coil sections to
be activated and deactivated to better match coil cooling capacity with
compressor loading. The combination of smaller coil sections controlled by
correspondingly sized expansion valves improves valve performance and part
load humidity control.
Capacity control in shell and tube evaporators is usually
handled using the return water temperature. For example, if the full-load
temperature range for chilled water is from 44°F to 54°F, water returning
at 50°F indicates the cooling load is about 60%. Liquid refrigerant is
metered to the evaporator to match the load using an orifice plate system
or an expansion valve. On large chillers, the expansion valve is pilot
operated.
The refrigerant is recovered by condensing it in a heat exchanger using
air or water to reject the heat. Air cooled condensers are most common in
smaller sizes, up to about 200 ton capacity. Technically, there is no upper
limit on the size of an air cooled condenser, but operating cost issues
usually dictate water cooled units for applications over about 100 tons.
There are two water cooled designs: cooling towers and evaporative
condensers. Both work on the principal of cooling by evaporating water into
a moving air stream. The effectiveness of this evaporative cooling process
depends upon the wet bulb temperature of the air entering the unit, the
volume of air flow and the efficiency of the air/water interface.
Evaporative condensers use water sprays and air flow to
condense refrigerant vapors inside the tubes. The condensed refrigerant
drains into a tank called a liquid receiver. Refrigerant sub cooling can be
accomplished by piping the liquid from the receiver back through the water
sump where additional cooling reduces the liquid temperature even further.
Cooling towers are essentially large evaporative coolers where the cooled
water is circulated to a remote shell and tube refrigerant condenser. Notice
the cooling water is circulating through the tubes while refrigerant vapor
condenses and gathers in the lower region of the heat exchanger. Notice also
that this area "sub cools" the refrigerant below the temperature of
condensation by bringing the coldest cooling tower water into this area of
the condenser. The warmed cooling water is sprayed over a fill material in
the tower. Some of it evaporates in the moving air stream. The evaporative
process cools the remaining water.
The volume of water used by both evaporative condensers and cooling
towers is significant. Not only does water evaporate just to reject the
heat, but water must be added to avoid the buildup of dissolved solids in
the basins of the evaporative condensers or cooling towers. If these solids
build up to the point that they foul the condenser surfaces, the performance
of the unit can be greatly reduced.
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