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This process is usually modeled as a throttling process for which enthalpy remains constant. Isenthalpic process (expansion in an expansion valve) – The refrigerant at state 3 enters the expansion valve and expands to the evaporator pressure.

As the refrigerant leaves the condenser, it is still under pressure but is now only slightly above room temperature. The net heat rejected is given by Q re = H 3 – H 2. In this phase the refrigerant passes through the condenser, where the refrigerant condenses and there is heat transfer from the refrigerant to the cooler surroundings. Isobaric heat rejection (in a condenser) – The superheated vapor travels under pressure through coils or tubes that make up the condenser.The work required for the compressor is given by W C = H 2 – H 1. On the other hand the entropy remains unchanged. The surroundings do work on the gas, increasing its internal energy (temperature) and compressing it (increasing its pressure). The gaseous medium is compressed adiabatically from state 1 to state 2 by piston compressor (or by centrifugal pumps) to a relatively high pressure and temperature. Isentropic compression (compression in the piston compressor) – A circulating refrigerant such as R134a enters a compressor as low-pressure vapor at or slightly below the temperature of the refrigerator interior.In an ideal vapor-compression cycle, the system executing the cycle undergoes a series of four processes: one isentropic (reversible adiabatic) process, one throttling process alternated with two isobaric processes: Expansion valve (also called a throttle valve).The typical vapor-compression system consist of four components: The figure depicts a typical, single-stage vapor-compression system. The vapor-compression uses a circulating liquid refrigerant as the medium (usually R134a) which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Vapor-compression Cycle – Refrigeration Cycle Vapor-compression cycle – Thermodynamic cycle of heat pumps.