What Is a Compressor and How Does it Work?
An HVACR compressor is a refrigerant gas pump in which the evaporator supplies gaseous refrigerant at a low pressure and increases it to a greater pressure. Upon being compressed, the temperature and pressure of the vapor are increased. The gaseous refrigerant is delivered to the condenser at a pressure at which condensation occurs at an appropriate temperature.
A compressor consists of two components: the power source and the compressing mechanism (piston, vane, etc.). In the case of an air compressor, the compressing mechanism is in fact compressing the atmospheric air. The air compressor works in the following way:
Air enters the piston or vane, and it is compressed by increasing its pressure and at the same time decreasing its volume. As soon as the pressure reaches a maximum set by the operator or the manufacturer, a switch mechanism prevents any further air intake in the compressor. The compressed air is used and pressure levels decrease. As soon as the pressure reaches a minimum, also set by the operator or the manufacturer, the switch allows air to enter the compressor. This procedure is repeated as long as the compressor is being used.
Performance Factors for Compressors
Factors which affect the performance of compressors are:
- speed of rotation
- pressure at suction
- pressure at discharge and
- type of refrigerant being used
Similar compressors can operate at different capacities by varying their refrigerants and compressor horsepower input. When purchasing any type of compressor, the buyer should check certain characteristics that include the machine configuration, the operation type, the price, and the operating cost. In any case, he should check the performance of the compressor and consult with the manufacturer about the most suitable and safest compressor for his budget and requirements.
Types of Compressors
The image above shows the available types of compressors. The most common ones used in refrigeration are described below:
Rotary: Compressors of the rotary type are generally low capacity equipment, used normally in home refrigerators and freezers, and not used for air conditioning. These compressors can consist of one vane, which is placed in the body, and sealed against the rotor, or multivane rotary, with vanes located in the rotor.
Centrifugal Compressors: These compressors revolve at high speed, and refrigerant is compressed by the application of centrifugal force. These compressors are normally used with refrigerants possessing higher specific volumes, which need lower compression ratios. Multi-stage units can be used to attain greater discharge pressures, and the number of stages is determined by the discharge temperature of the gas as it exits from the rotor. These compressors are utilized for water chilling in air conditioning and for low temperature freezing purposes.
Reciprocating Compressor: These compressors have pistons, and move in cylinders. Types of reciprocating compressors are:
- Open Compressors: One extremity of the crankshaft is drawn out of the crankcase, due to which multiple drives can be used with the compressor. A mechanical seal is used to check external seepage of refrigerant and oil, and escape of air towards the inside. These compressors are driven by electric motors or internal combustion engines. With belt drive, changes in speed are achieved by altering the dimensions of the pulleys, while with direct drive units the compressor is planned to operate at the speed of motor.
Hermetic Compressors: These compressors are serviceable hermetic, in which motor and compressor are enclosed in the same housing, while the welded hermetic type has the compressor and motor sealed in a welded steel shell.