The Fundamentals of Vapour Compression Refrigeration
The basis of vapour compression refrigeration
If a liquid is introduced into a vessel in which there is initially a vacuum and whose walls are kept at a constant temperature it will evaporate at once. In the process the latent heat of vaporisation will be abstracted from the sides of the vessel. The resulting cooling effect is the starting point of the refrigeration cycle, which is to be examined in this chapter.
As the liquid evaporates, the pressure inside the vessel will rise until it reaches a certain maximum value for the temperature—the saturation vapour pressure (see section 2.8). After this, no more liquid will evaporate and, of course, the cooling effect will cease. Any further liquid introduced will remain in liquid state in the bottom of the vessel. If we now remove some of the vapour from the container, by connecting it to the suction of a pump, the pressure will tend to fall, and this will cause more liquid to evaporate. In this way, the cooling process can be rendered continuous. We need a suitable liquid, called the refrigerant, a container where the vaporisation and cooling take place, called the evaporator, and a pump to remove the vapour, called, for reasons which will be apparent later, the compressor.
The system as developed so far is obviously not a practical one because it involves the continuous consumption of refrigerant. To avoid this it is necessary to convert the process into a cycle. To turn the vapour back into a liquid it must be cooled with whatever medium is on hand for the purpose. This is usually water or air at a temperature substantially higher than the temperature of the medium being cooled by the evaporator. The vapour pressure corresponding to the temperature of condensation must, therefore, be a good deal higher than the pressure in the evaporator. The required step-up in pressure is provided by the pump acting as a compressor.
The liquefaction of the refrigerant is accomplished in the condenser, which is, essentially, a container cooled externally by air or water. The hot high-pressure refrigerant gas from the compressor is conveyed to the condenser and liquefies therein. Since there is a high gas pressure in the condenser, and the liquid refrigerant there is under the same pressure, it is easy to complete the cycle by providing a needle valve or other regulating device for injecting liquid into the evaporator. This essential component of a refrigerant plant is called the expansion valve.
This basic vapour compression refrigeration cycle is illustrated in Figure 9.1 where it is shown applied to a water-chilling set.
Figure 9.2 illustrates the changes in the state of the refrigerant as the simple, basic
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Fig. 9.1 Basic vapour compression cycle applied to a water chiller.
Vapour compression cycle takes place. The co-ordinates of the diagram are absolute pressure and enthalpy but, in order to explain the cycle and describe the diagram, it is first necessary to consider the thermodynamics of the subject.
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Enthalpy
Energy
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Compressor
Heat rejected at condenser
Fig. 9.2 Simple saturation refrigeration cycle on a pressure-enthalpy diagram.
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