Functions of a Refrigeration Compressor

A refrigerator compressor is the center of the refrigeration cycle. It works as a pump to control the circulation of the refrigerant, and it adds pressure to the refrigerant, heating it up. The compressor also draws vapor away from the evaporator to maintain a lower pressure and lower temperature before sending it to the condenser.

The Refrigeration Cycle

A thorough understanding of the role of a refrigeration compressor cannot exist without a discussion of the refrigeration cycle, which essentially consists of the transformation of a liquid to a gas and back again. (If you're not interested in the details, just skip this step.) There are five main steps to a refrigeration circuit: evaporation, compression, condensing, receiving and expansion. 1) Evaporation: Liquid refrigerant enters the evaporator. It absorbs heat when it evaporates, which produces cooling. The refrigerant from the evaporator is fed to a tank as a weak or saturated superheated gas. The pressure in the tank rises until it equals the pressure in the evaporator. Refrigerant flow stops and the temperature in both tank and evaporator both rise to ambient. 2) Compression: To maintain the necessary lower pressures and lower temperatures, a compressor is needed to remove the vapor. Because the refrigeration circuit is closed, equilibrium is maintained. That means that if the compressor removes vapor faster than it can be formed, the pressure will fall and with it the temperature in the evaporator. Alternately, if the load on the evaporator rises and the refrigerant evaporates quicker, the temperature and pressure in the evaporator will rise. The energy that a compressor requires is called compression input and is transferred to the refrigeration vapor. 3) Condensing: After leaving the compressor, the refrigerant moves to the condenser, which gives off heat that is transferred to either air or water having a lower temperature. The amount of heat given off is the heat absorbed by the refrigerant in the evaporator plus the heat created by compression input. The byproduct of this is that the vapor changes to a liquid, which is then sent to the receiver. 4) Receiving: The pressure in the receiver is higher than the pressure in the evaporator because of compression, and thus must be lowered to match the evaporative pressure. This is achieved through the use of an expansion valve. 5) Expansion: Before the liquid enters the expansion valve, the temperature will be just under the boiling point. Suddenly reducing the pressure in the expansion valve causes the liquid to boil and evaporate. This evaporation takes place in the evaporator and the circuit is complete. There are many different temperatures involved in the operation of a refrigeration plant, but in principle there are only two pressures: evaporating pressure and condensing pressure.

Types

The main types of refrigeration compressors are reciprocating, screw, scroll and centrifugal. They are used in refrigeration, heat pumps, and air-conditioning applications, such as food processing, ice rinks and arenas, and pharmaceutical manufacturing.

Rotary Screw Compressors

Rotary screw compressors have screw spindles that compress the gas as it enters from the evaporator. The screw compressor features smooth operation and minimal maintenance requirements; typically these compressors only require changes in oil, the oil filter and the air/oil separator. Microprocessor-based controllers are also available for standard rotary compressors, which allow the rotary to remain loaded 100 percent of the time. There are two types of rotary screw compressors: single and twin.

Reciprocating Compressors

A reciprocating compressor uses a piston-actuated unloading mechanism with spring-loaded pins to raise the suction valve plate from its seat, allowing the unit to be used at any pressure ratio. This action is similar to an internal combustion engine in a car. This type of compressor is efficient at both full- and part-load operation. Further advantages include simple controls and the ability to control the speed through the use of belt drives. The reciprocating compressor is used in low-horsepower applications.

Scroll Compressors

Scroll compressors work by moving one spiral element inside another stationary spiral to produce gas pockets that, as they become smaller, increase the pressure of the gas. During compression, several pockets are compressed at once. By maintaining an even number of balanced gas pockets on opposite sides, the compression forces inside the scroll balance and reduce vibration inside the compressor. This type of compressor uses the scroll design instead of a fixed cylinder or a piston or single-sided compression mechanism, eliminating wasted space in the compression chamber and eliminating the need to compress gas again and again during the cycle (recompression). This reduces energy use.

Centrifugal Compressors

Centrifugal compressors compress refrigerant gas through the centrifugal force created by rotors that spin at high speed. This energy is then sent to a diffuser, which converts a portion of it into increased pressure. It does this by expanding the region of the flow volume to slow the flow velocity of the working fluid. Diffusers may use airfoils, also known as vanes, to improve this. Centrifugal compressors are suited for compressing large volumes of gas to moderate pressures.