Introduction
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The pressure–enthalpy(P–h) chart plots the properties of a refrigerant: refrigerant pressure (vertical axis) versus enthalpy, or heat content (horizontal axis). A diagram of the basic vapor-compression refrigeration cycle can be superimposed on a pressure–enthalpy chart to demonstrate the function of each component in the system.
Refrigerant enters the evaporator in the form of a cool, low-pressure mixture of liquid and vapor (A). Heat is transferred from the relatively warm air or water to be cooled to the refrigerant, causing the liquid refrigerant to boil and in some cases superheat (B). The resulting vapor (B) is then pumped from the evaporator by the compressor, which increases the pressure and temperature of the refrigerant vapor. Notice that during the compression process (B to C), the heat content (enthalpy) of the vapor is increased. The mechanical energy used by the compressor to increase the pressure of the refrigerant vapor is converted to heat energy, called the heat of compression. This causes the temperature of the refrigerant to also rise as the pressure is increased.
The resulting hot, high-pressure refrigerant vapor (C) enters the condenser where heat is transferred to ambient air or water at a lower temperature. Inside the condenser, the refrigerant desuperheats (C to D), condenses into a liquid (D to E), and, in some cases, subcools (E to F). The refrigerant pressure inside the condenser is determined by the temperature of the air or water that is available as the condensing media.
This liquid refrigerant (F) then flows from the condenser to the expansion device. The expansion device creates a pressure drop that reduces the pressure of the refrigerant to that of the evaporator. At this low pressure, a small portion of the refrigerant boils (or flashes), cooling the remaining liquid refrigerant to the desired evaporator temperature (A). The cool mixture of liquid and vapor refrigerant travels to the evaporator to repeat the cycle.