In practical applications, vapor-compression refrigeration systems are the most commonly used refrigeration systems, and each system employs a compressor. In a basic vapor compression refrigeration cycle as shown in Figure 3.28, four major thermal processes take place as follows:
• evaporation,
• compression,
• condensation, and
• expansion.
Evaporation
Unlike freezing and melting, evaporation and condensation occur at almost any temperature and pressure combination. Evaporation is gaseous escape of molecules from the surface of a liquid and is accomplished by the absorption of a considerable quantity of heat without any change in temperature. Liquids (e.g. refrigerants) evaporate at all temperatures with increased rates of evaporation occurring at higher temperatures. The evaporated gases exert a pressure called the vapor pressure. As the temperature of the liquid rises, there is a greater loss of the liquid from the surface, which increases the vapor pressure. In the evaporator of a refrigeration system, a low-pressure cool refrigerant vapor is brought into contact with the medium or matter to be cooled (i.e. heat sink), absorbs heat, and hence boils, producing a low-pressure saturated vapor.
Compression
Using shaft work of a compressor raises the pressure of the refrigerant vapor obtained from the evaporator. The addition of heat may play a role in raising the pressure. Increasing the gas pressure raises the boiling and condensing temperature of the refrigerant. When the gaseous refrigerant is sufficiently compressed its boiling point temperature is higher than the heat sink’s temperature.
Condensation
This is a process of changing a vapor into a liquid by extracting heat. The high-pressure gaseous refrigerant, which carries the heat energy absorbed in the evaporator and the work energy from the compressor, is brought into the condenser. The condensing temperature of the refrigerant is higher than that of the heat sink and therefore heat transfer condenses the high-pressure refrigerant vapor to the high-pressure saturated liquid. The heat source has been cooled by heat pumping to the heat sink. Instead of using a condenser to reject heat, the refrigerant vapor can be discharged to the atmosphere, but this technique is impractical. Condensing the refrigerant gas allows reuse at the beginning of the next cycle. In some practical applications, it is desired that the condenser cools the refrigerant further, below the condensation temperature. This is called subcooling, which is usually observed in the condenser to reduce flashing when the refrigerant pressure is reduced in the throttling device. This method provides a reduction in the amount of gas entering the evaporator and hence an improvement in the system performance.
Expansion
The condensed refrigerant liquid is returned to the beginning of the next cycle. A throttling device such as a valve, orifice plate, or capillary tube for the expansion process is used to reduce the pressure of the refrigerant liquid to the low-pressure level and the boiling temperature of the refrigerant to below the temperature of the heat source. Energy losses through this pressure reduction must be offset by additional energy input at the pressurization stage.
Figure 3.28a shows a schematic diagram of a basic vapor-compression refrigeration system. For better understanding, this refrigeration cycle is shown by temperature-entropy (7-5) and pressure-enthalpy (log P-h) diagrams as given in Figures 3.28b and 3.28c. Under the steps given above, the operation of this system is:
• (1-2) Reversible adiabatic compression. From the evaporator, low-pressure saturated refrigerant vapor comes to the compressor and is compressed into the condenser by volume reduction and increased pressure and temperature.
• (2-3) Reversible heat rejection at constant pressure. From the compressor, highpressure refrigerant vapor enters the condenser and is liquefied by employing water or air.
• (3-4) Irreversible expansion at constant enthalpy. From the condenser, high-pressure saturated refrigerant liquid passes through an expansion valve and its pressure and temperature are reduced.
• (4-1) Reversible heat addition at constant pressure. From the expansion valve, lowpressure refrigerant liquid arrives in the evaporator. It boils here and in the process absorbs heat from the surrounding medium, thereby providing a cooling effect.
As shown in Figure 3.28, the essential components of a simple vapor-compression refrigeration system, as explained earlier, are:
• Evaporator. This is the device where there is heat exchange for providing refrigeration, and therefore it boils the liquid refrigerant at a low temperature, which causes the refrigerant to absorb heat.
• Suction line. This is the tube between the evaporator and the compressor. After the liquid has absorbed the heat, the suction line carries the refrigerant to the compressor. In this line, the refrigerant is a superheated gas.
• Compressor. This device separates the low-pressure side of the system from the highpressure side and has two main goals: (i) to remove vapor from the evaporator to keep the evaporator’s boiling point low, and (ii) to compress the low-temperature refrigerant vapor into a small volume, creating a high-temperature, high-pressure superheated vapor.
• Hot gas discharge line. This tube connects the compressor with the condenser. After the compressor has discharged the high-pressure, high-temperature superheated refrigerant vapor, the hot gas discharge line carries it to the condenser.
• Condenser. This device is used for heat exchange, similar to the evaporator, except that its job is to expel heat, not absorb it. The condenser changes the state of the superheated refrigerant vapor back into a liquid. This is done by creating a high pressure that raises the boiling point of the refrigerant and removes enough heat to cause the refrigerant to condense back into a liquid.
• Liquid line. This line connects the condenser with the refrigerant control device. including the expansion valve. Only liquid refrigerant should be in this line. Also, the line will be somewhat warm because the refrigerant is still under high pressure.
• Refrigerant control. This last control works as a metering device. It monitors the liquid refrigerant that enters the evaporator and makes sure all the liquid is boiled off before the refrigerant goes to the suction line. If liquid refrigerant enters the suction line. it will enter the compressor and cause it to fail.
In addition to the above listed components, there are some additional features, e.g. liquid receiver, service valves, suction service valve, discharge service valve, and liquid receiver service valve, which can enhance the refrigeration system’s operation.
