In a refrigeration cycle, the compressor has two main functions within the refrigeration cycle. One function is to pump the refrigerant vapor from the evaporator so that the desired temperature and pressure can be maintained in the evaporator. The second function is to increase the pressure of the refrigerant vapor through the process of compression, and simultaneously increase the temperature of the refrigerant vapor. By this change in pressure the superheated refrigerant flows through the system.
Refrigerant compressors, which are known as the heart of the vapor-compression refrigeration systems, can be divided into two main categories:
• displacement compressors, and
• dynamic compressors.
Note that both displacement and dynamic compressors can be hermetic, semihermetic, or open types.
The compressor both pumps refrigerant round the circuit and produces the required substantial increase in the pressure of the refrigerant. The refrigerant chosen and the operating temperature range needed for heat pumping generally lead to a need for a compressor to provide a high pressure difference for moderate flow rates, and this is most often met by a positive displacement compressor using a reciprocating piston. Other types of positive displacement compressor use rotating vanes or cylinders or intermeshing screws to move the refrigerant. In some larger applications, centrifugal or turbine compressors are used, which are not positive displacement machines but accelerate the refrigerant vapour as it passes through the compressor housing. These various compressor types are illustrated in Figure 3.2.
In the market, there are many different types of compressor available, in terms of both enclosure type and compression system. Here are some options for evaluating the most common types (DETR, 1999):
• Reciprocating compressors are positive displacement machines, available for every application. The efficiency of the valve systems has been improved significantly on many larger models. Capacity control is usually by cylinder unloading (a method which reduces the power consumption almost in line with the capacity).
• Scroll compressors are rotary positive displacement machines with a constant volume ratio. They have good efficiencies for air conditioning and high temperature refrigeration applications. They are only available for commercial applications and do not usually have in-built capacity control.
• Screw compressors are available in large commercial and industrial sizes and are generally fixed volume ratio machines. Selection of a compressor with the incorrect volume ratio can result in a significant reduction in efficiency. Part load operation is achieved by a slide valve or lift valve unloading. Both types give a greater reduction in efficiency on part load than the reciprocating capacity control systems.