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.
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