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36 TRG-TRC004-EN
notes
period three
The Compressor in a System
The second method bypasses refrigerant vapor from the compressor discharge
line to the suction line. This method requires the service of an additional
expansion valve, called a liquid injection valve. The remote bulb of this valve is
attached to the suction line near the compressor. When reduced suction
pressure causes the bypass valve to open, the expansion valve senses the
resulting rise in suction temperature (superheat) at its remote bulb. A rising
suction temperature causes this expansion valve to open, mixing liquid
refrigerant with the hot, bypassed refrigerant vapor. The heat content of this
refrigerant vapor causes the liquid refrigerant to evaporate, thus cooling the
mixture. This increase in the refrigerant flow rate stabilizes the compressor
suction pressure (temperature).
The principal advantage of hot gas bypass to the suction line is that the amount
of refrigerant piping is generally less than the other method. A key
disadvantage is that the refrigerant velocity in the evaporator and suction line
drops very low when the bypass valve is open. This creates a problem of oil
hanging up in the evaporator coil and suction piping. For this reason, this
method is not acceptable in applications where the evaporator is located below
the compressor.
When hot gas bypass is applied to a water chiller containing a direct-expansion
evaporator, hot gas bypass to the evaporator inlet is always used. In a direct-
expansion evaporator, liquid refriegerant flows through the tubes and water
fills the surrounding shell. Oil holdup within the tubes can be a problem at part
load when refrigerant velocity is reduced. The increased velocity brought about
by bypassing to the evaporator inlet solves this problem for water chillers.
Finally, when hot gas bypass is applied to a system, the need for condensing
pressure control must be considered. Sufficient condensing pressure must be
available to ensure adequate refrigerant flow to produce a bypass load when
the hot gas bypass valve is to be opened. If a decreasing load is accompanied
by a corresponding reduction in condensing pressure, the hot-gas-bypass valve
may not be capable of bypassing refrigerant vapor at the rate required to
stabilize the suction temperature within reasonable limits. The result is that the
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Figure 50