**2.1**

**Basic vapour compression cycle**

A liquid boils and condenses – the change between the liquid and

gaseous states – at a temperature which depends on its pressure,

within the limits of its freezing point and critical temperature. In

boiling it must obtain the latent heat of evaporation and in condensing

the latent heat must be given up again.

The basic refrigeration cycle (Figure 2.1) makes use of the boiling

and condensing of a working fluid at different temperatures and,

therefore, at different pressures.

Heat is put into the fluid at the lower temperature and pressure

and provides the latent heat to make it boil and change to a vapour.

This vapour is then mechanically compressed to a higher pressure

and a corresponding saturation temperature at which its latent heat

can be rejected so that it changes back to a liquid.

The total cooling effect will be the heat transferred to the working

fluid in the boiling or evaporating vessel, i.e. the change in enthalpies

between the fluid entering and the vapour leaving the evaporator.

For a typical circuit, using the working fluid Refrigerant 22,

evaporating at – 5°C and condensing at 35°C, the pressures and

enthalpies will be as shown in Figure 2.2.

Enthalpy of fluid entering evaporator = 91.4 kJ/kg

Enthalpy of saturated gas leaving evaporator = 249.9 kJ/kg

Cooling effect = 249.9 – 91.4 = 158.5 kJ/kg

A working system will require a connection between the condenser

and the inlet to the evaporator to complete the circuit. Since these

are at different pressures this connection will require a pressure-

reducing and metering valve. Since the reduction in pressure at

this valve must cause a corresponding drop in temperature, some

of the fluid will flash off into vapour to remove the energy for this

cooling. The volume of the working fluid therefore increases at the

valve by this amount of flash gas, and gives rise to its name, the

expansion valve. (Figure 2.3.)

**2.2 Coefficient of performance**

Since the vapour compression cycle uses energy to move energy,

the ratio of these two quantities can be used directly as a measure

of the performance of the system. This ratio, the coefficient of

performance, was first expressed by Sadi Carnot in 1824 for an

ideal reversible cycle, and based on the two temperatures of the

system, assuming that all heat is transferred at constant temperature

(see Figure 2.4). Since there are mechanical and thermal losses in

a real circuit, the coefficient of performance (COP) will always be

less than the ideal Carnot figure. For practical purposes in working

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