Sterlco GC Series Central Chiller - Manual (Page 9)

GC Series Central Chillers Chapter 2: Functional Description 9 of 79
Refrigeration Circuit
Water- and remote air-cooled refrigerant condensing differ only in the way the compressed
gas is condensed to a liquid. Shown below is a water cooled version.
The refrigerant is compressed in the
compressor and flows through the discharge
line as a gas to the condenser.
There it gives up its heat as it condenses to
a liquid in the condenser.
Liquid refrigerant from the condenser heat
exchanger flowing in the liquid line passes
through a shut-off valve into a filter/dryer
that removes moisture and other
contaminants. After the filter/dryer the
refrigerant passes through a refrigerant sight
glass provided to view the flow of liquid
refrigerant, and to view if the system is free
of moisture. The refrigerant then passes
through the electronic, modulating expansion valve, which allows the refrigerant to expand
(boil off) and cool (remove the heat from) the fluid inside of the evaporator.
From the evaporator the refrigerant gas flows through the suction line back into the
compressor.
An optional modulating electronic hot gas bypass valve can be added to control cooling
capacity during intermittent or partial load conditions. This feature contributes substantially
to chiller longevity by eliminating excessive cycling of the compressor and providing close
temperature control.
System Control
Putting this all together, the controller maintains the desired leaving fluid temperature using
multiple inputs to determine if, when and for how long the compressor(s) are on.
Single Module Operation
Once the unit’s power is enabled and the controller is turned on (see Chapter 4 for more
details on the operation of the controller) it verifies the flow through the electronic flow
switch. If the flow is not established within 10 seconds the controller will alarm and disable
the refrigeration circuit. Upon flow verification the controller uses the leaving fluid
temperature and the setpoint temperature to determine the operation of the compressors.
When the leaving fluid temperature is greater than the setpoint plus the “compressor on
differential” value the controller will enable the compressor; if equipped with two
compressors the controller will enable the one with the least amount of hours. The second
compressor will be enabled if the leaving fluid temperature remains above the setpoint plus
the “compressor on differential” value.
The optional hot gas valve is designed to trim the load of one compressor and will modulate
in order to meet the desired leaving fluid setpoint via a PID algorithm. After the compressor
starts, the hot gas valve is allowed to modulate. The compressor will shut off if the leaving
fluid temperature drops below the “compressor off differential” and the hot gas valve is at
100% for the “compressor off delay” time. The hot gas will reset to 0% when the compressor
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