Trane TRG-TRC004-EN manual +RW*DV%\SDVVWR6XFWLRQ/LQH

period three

The Compressor in a System

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			Figure 50

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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