A. The single stage vapour compression cycle

The pressure - specific enthalpy (p-h) diagram is the mostly used thermodynamic diagram in refrigeration because of its practical appeal. Pressure, due to its association with the saturation temperatures of the refrigerant in the evaporator and condenser, is the property that characterizes the nature of the application of the refrigeration cycle (refrigerant type, domestic, commercial, industrial etc.). Enthalpy, on the other hand, appears in all energy balances in components and control volumes within the cycle. Figure 1(a) illustrates a Single Stage Standard Refrigeration cycle operating between the pressures p_C and p_E in the condenser and evaporator, respectively. The temperature differences between the refrigerant and the thermal reservoirs (cold refrigerated space, at T_L, and warm external environment, at T_H), are shown in Figure 1(b) to illustrate the external irrerversibilities associated with the transfer of heat across a finite temperature difference.

Figure 1 A Standard Refrigeration cycle depicted in p-h (a) and T-s (b) diagrams

The refrigerator COP of the cycle depicted in Figure 1 can be determined from steady-state energy balances in the evaporator and compressor (neglecting changes in kinetic and potential energy). Thus, it follows from Equation 398.3 that,