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The compressor is the heart of the refrigeration circuit. It takes the cool, low-pressure gas entering the compressor and
compresses it, which creates the hot, high-pressure gas that exits the compressor. Since the compressor is not 100%
efficient, some extra heat is added to the refrigerant as it is being compressed.
The hot, high-pressure gas that exits the compressor is delivered to the condenser. In the condenser, the heat is
transferred from the refrigerant into the air or water that is passing through the condenser. As the heat is transferred, the
refrigerant changes from a gas to a liquid. The condenser has been sized to remove the heat from the process load and the
heat that was added by the compressor.
After leaving the condenser, the liquid refrigerant passes through the filter drier and sight glass. The filter drier removes
any particles or moisture from the refrigerant. The sight glass is used to monitor the stream of liquid refrigerant. The liquid
refrigerant then passes through the thermal expansion valve (TXV) which meters the flow into the evaporator where the
process starts all over again.
Capacity and temperature control is accomplished with a hot gas bypass system. If the chiller were catering to a partial
load from the process, the coolant supply temperature would normally tend to drop. The microprocessor senses this drop
in temperature, and opens the hot gas bypass solenoid valve. When this valve is open, some of the hot compressor
discharge gas is directed to the inlet of the evaporator instead of going through the condenser. This reduces the chillers
cooling capacity and puts an additional heat load on the evaporator, which brings the coolant temperature back up to set
point. The microprocessor cycles the hot gas solenoid valve as is needed to maintain the coolant temperature to within 1°F
(1°C) of the set point even with loads as low as 10% to 25% of full capacity.
If the process heat load is extremely low, or even nonexistent, the hot gas bypass system may not be able to put enough of
a load on the evaporator, and the coolant temperature will begin to drop. When the coolant temperature drops 7°F (3°C)
below the set point temperature, the controller will shut the compressor off. When the coolant temperature rises back to
the set point temperature, the compressor comes back on. The compressor will remain off for at least two and one half
minutes to prevent short cycling.
Chiller Construction
Compressor
The chiller is equipped with either a hermetic rotary vein or scroll compressor. Both the compressor and the motor are
encased together and solidly mounted in the cabinet. The compressor is unidirectional, so it is important to have power
phased correctly when operating on a three-phase power supply. The cool refrigerant suction gas cools the motor
windings, and there is an internal thermal overload to protect the windings from overheating. The compressor is lubricated
with oil that travels throughout the system with the refrigerant.
Air Cooled Condenser
(Air Cooled Units Only)
The condenser is constructed of heavy gauge copper tubing and aluminum fins for maximum heat transfer capabilities. The
condenser has been generously sized so the chiller can operate with full cooling capacities in ambient air temperatures of
up to 95°F (35°C). When the ambient air temperatures are above 95°F (35°C) the chiller will lose approximately 1% of its
cooling capacity per 1°F (0.5°C) above 95°F (35°C). The chiller is capable of operating in ambient temperatures of up to
105°F (40.5°C).
The fan draws cool air through the condenser and discharges warm air out the top of the cabinet. The unit is designed to
draw sufficient air through the chiller as long as there are no obstructions. The fans are not designed to draw air through
ductwork or discharge air through ductwork. The discharge air will be approximately 20°F (12°C) warmer than the intake
air.
Water Cooled Condenser
(Water Cooled Units Only)
The condenser is a coaxial tube-in-tube type heat exchanger constructed of a steel outer tube with copper inner tubes. The
condenser water passes through the copper inner tubes, while the refrigerant passes around the tubes, in the outer steel
tube.
