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8 TRG-TRC010-EN
notes
period one
Components
Expansion Device
An expansion device is used to maintain the pressure difference between the
high-pressure (condenser) and low-pressure (evaporator) sides of the
refrigeration system, as established by the compressor. This pressure
difference allows the evaporator temperature to be low enough for the
refrigerant to absorb heat from the water being cooled, and the condenser
temperature to be high enough for the refrigerant to reject heat to water at
normally available temperatures. High-pressure liquid refrigerant flows through
the expansion device, causing a pressure drop that reduces the refrigerant
pressure to that of the evaporator. This pressure reduction causes a small
portion of the liquid to boil off, or “flash,” cooling the remaining refrigerant to
the evaporator temperature.
The expansion device is also used as a liquid refrigerant metering system,
balancing the refrigerant flow rate with the evaporator load condition. In our
example centrifugal chiller, the expansion device used is a set of 2 orifice
plates. At full load, a large amount of refrigerant is moving through the chiller.
The column of liquid refrigerant in the liquid line pressurizes the liquid at its
base. During passage through the orifice plates, the liquid refrigerant
undergoes a pressure drop equal to the head (H
1
) before some of it flashes to
vapor.
to
to
evaporator
evaporator
orifice plates
orifice plates
H
H
1
1
expansion device
Orifice Plates
Figure 13
trgtrc010_book.book Page 8 Wednesday, September 1, 1999 10:39 AM