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2.2 Basic components

The science of refrigeration is based on the fact that liquid can be vaporized at any desired temperature by changing the pressure around it. Water under normal atmospheric pressure of 1.01 bar will boil at 100˚C. The same water in a closed vessel under a pressure of 4.6 bar will not boil until its temperature has reached 149˚C. Liquids boiling at low temperatures are the most desirable media for removing heat, i.e. refrigerants. Comparatively large quantities of heat are absorbed when liquids are evaporated. Many of the liquids used as refrigerants in refrigeration systems have boiling points below –18˚C under ordinary atmospheric pressure. An example is ammonia, which boils at -33˚C.

Refrigeration can be achieved with ammonia without any equipment whatsoever. If liquid ammonia is poured into an open container (an evaporator) surrounded by ordinary air at ordinary atmospheric pressure, it will immediately begin to boil at -33˚C. There will be a continuous flow of heat from the surrounding air, through the walls of the container to the boiling ammonia (see Figure 2.3). Moisture from the air will condense and freeze on the exterior of the container. Such a system would work satisfactorily as far as cooling alone is concerned, but the cost of replacing the lost ammonia would be high. The ammonia, or any other refrigerant, is therefore used repeatedly. Additional equipment is needed for this purpose. The refrigerant must be delivered to the evaporator as a liquid because it absorbs heat best by vaporizing. Because the refrigerant leaves the evaporator as a vapor, it needs to be condensed to a liquid before it can be used again. To condense a refrigerant, the latent heat given off by the vapor must be transferred to another medium, such as water or air, in a condenser (see Figure 2.4). The surrounding medium must be at a lower temperature than the condensing temperature of the refrigerant.

As the temperature of the available water or air is always higher than that of the boiling refrigerant in the evaporator, the refrigerant cannot be condensed as it leaves the evaporator. To condense the vapor, its pressure must be increased to a point where its condensing temperature is above the temperature of the water available for the condensing process. For this purpose, a compressor is needed (see example in Figure 2.5).

The compressor and condenser are needed to enable the same refrigerant to be used again and again. The cost of compressing and condensing the vaporized refrigerant is far less than the cost of continuously buying new refrigerant to replace that used.

To maintain the difference in pressure between the condenser and the evaporator caused by the compressor, an expansion valve is needed in the cycle. The expansion valve separates the high-pressure part of the system from the low-pressure part. Only a small trickle of refrigerant liquid flows through the valve. In fact, the valve is always adjusted so that the rate at which liquid passes through it is the same as the evaporation rate. An expansion valve is shown in Figure 2.6. The simplest refrigeration system therefore consists of an evaporator, a compressor, a condenser and an expansion valve (see Figure 2.7).

The refrigerant boils in the evaporator at a constant low pressure and temperature. Heat is removed from the fluid being cooled. After leaving the evaporator, the vaporized refrigerant flows through the compressor. In the compressor, the pressure of the vaporized refrigerant is raised to a point at which it can be condensed by some relatively warm fluid, e.g. water. The compressor removes the refrigerant vapor. This creates such a low pressure in the evaporator that the evaporation temperature is kept below the surrounding temperature. The work input for this process is represented by W in Figure 2.7.

After being compressed, the vapor enters the condenser and is condensed at constant pressure and temperature. Latent heat is transferred from the condensing vapor through the walls of the condenser. The expansion valve has two functions: maintaining the pressure difference between the condenser and the evaporator, together with the compressor, and regulating the volume of refrigerant going to the evaporator.​​​

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