Trane TRG-TRC003-EN User Manual

32

TRG-TRC003-EN

period four

The primary purpose of the expansion device is to drop the pressure of the 
liquid refrigerant to equal the evaporator pressure. At this lower pressure, the 
refrigerant is now inside the saturation envelope where it exists as a mixture of 
liquid and vapor.

The high-pressure liquid refrigerant (

*) flows through the expansion device, 

causing a large pressure drop. This pressure drop reduces pressure and 
temperature of the refrigerant to that of the evaporator (

$). At the lower 

pressure, the temperature of the refrigerant is higher than its boiling point. This 
causes a small portion of the liquid to boil, or flash. Because heat is required to 
boil this small portion of refrigerant, boiling refrigerant absorbs heat from the 
remaining liquid refrigerant, cooling it to the evaporator temperature. Notice 
that there is no change in enthalpy during the expansion process.

The purpose of subcooling the liquid refrigerant in the condenser is to avoid 
flashing the refrigerant before it reaches the expansion device. If a valve is used 
as the expansion device, the presence of refrigerant vapor can cause improper 
operation and premature failure.

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