Step 4: Expansion Device

The vapor compression cycle is the primary cycle used in commercial refrigeration systems.

The Vapor Compression Cycle for the Mechanical PE Exam

Step 4: Expansion Device: The final step is the expansion device, which is the counterpart of the compressor. The expansion device reduces the pressure of the liquid, which causes not only the pressure to decrease but also the temperature to decrease. During this process, some of the liquid refrigerant is turned into a gas, this is called flash gas. The resultant of the expansion device is a cold partial liquid-vapor refrigerant mix. The cold refrigerant liquid-vapor mix then repeats the process at the evaporator.

The expansion device that is primarily used in air conditioning systems is called a thermostatic expansion valve (TXV. The TXV as its name describes, opens and closes, based on a thermal device. The adjustment of the opening/closing determines the amount of refrigerant that is passed through and evaporated. The TXV uses the temperature of the evaporator output as a basis for determining the amount of refrigerant.

For example, if the TXV senses that the evaporator is producing an output refrigerant temperature that is too cold, then there is too much refrigerant for the heat load (hot side of the evaporator) and the refrigerant sent to the evaporator needs to be throttled down (decrease cold side of the evaporator). If the TXV senses that the output of the evaporator is too high, then the amount of refrigerant cannot keep up with the heat load (hot side) then the TXV should allow more refrigerant to the evaporator (increase cold side).