The most common cooling source in HVAC is mechanical two-phase vapor compression refrigeration. In this cycle (Fig. below), a compressor is used to raise the pressure of a refrigerant gas.

Work energy (QW) is required, usually provided by an electric motor or steam turbine or fuel-fired engine. The compression process raises the temperature of the gas.

The high-pressure gas flows through piping to a condenser where heat is removed by transfer to a heat sink, usually water or air.

The refrigerant is selected with properties which allow it to condense (liquefy) at the temperature and pressure in the condenser. The high pressure liquid is passed through a pressure-reducing device to the evaporator. At the lower pressure, the liquid tends to evaporate, removing the heat of vaporization (QC) from its surroundings (the evaporator—technically, the heat source).

The cold, low-pressure vapor is then returned to the compressor to be recycled. Note that the heat removed in the condenser is equal to the sum QC QW. One index of refrigeration cycle effectiveness is its coefficient of performance (COP)

COP =  QC/QW (9.1)

The refrigeration cycle can also be shown on a graph of the properties of a specific refrigerant. The graph in Fig. 9.2 is a pressure enthalpy or p-h diagram with the basic coordinates of pressure and enthalpy.

The four stages of the cycle include compression (with a rise in temperature and enthalpy due to work done), condensing (cooling and liquefying at constant pressure), expansion (at constant enthalpy) and vaporization at constant pressure.

The use of the p-h diagram allows the selection of the most effective refrigerant for the pressures and temperatures appropriate to the process. To minimize the work energy required, the temperature difference between the heat source and heat sink should be minimized.

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