Compressors – definition, classification, and types
What is a compressor and how does it work?
A compressor is a mechanical flowing device designed to increase the pressure of a gas by reducing its volume. The compressor is the main component of basic refrigeration systems and is often considered as the “heart of the cooling system”. It works as a pump to control the circulation of the refrigerant. The basic principle of the compressor’s operation is to draw in a refrigerant vapor at low pressure and temperature from the evaporator and then compress it to high pressure and temperature. Therefore, the characteristic theoretical parameter for compressors is the compression ratio, which is the gas pressure ratio at the compressor outlet (discharge side, p2) to the pressure at the inlet (suction side, p1).
The compression process differs depending on the compressor’s technology, type, and design. Compressors generate a certain amount of heat during operation, which should be dissipated to avoid overheating and to increase performance efficiency. Refrigeration compressors are designed to compress refrigerant vapors. Operation with the refrigerant in the liquid phase can cause damage of the compressor.
Classification of compressors according to the principle of operation
Positive displacement compressors
The two main types of positive displacement compressors are reciprocating and rotational. In these compressors, the refrigerant vapor is compressed in a closed space by changing its specific volume mechanically. The compression process takes place in an enclosed working space that decreases during the cycle. Compressors of this group may use pistons, spirals, screws, and rolling pistons to convey a set volume of gas with each stroke. One of their advantages is that both light and heavy gases are compressed equally well.
The principle of operation of dynamic compressors is to increase the refrigerant vapor’s kinetic energy in the rotor, which is then converted to increase its pressure. The increase in gas pressure occurs continuously when appropriately shaped moving (impeller) and stationary (vane) parts interact. The classification of dynamic compressors is based on the shapes of the rotor and the different flow directions. During the compressor cycle, the refrigerant converts its kinetic energy into potential energy. The refrigerant vapor is accelerated in the inter vane channels of the rotor which causes an increase in kinetic energy. It is converted into incremental pressure energy in the diffuser. These types of compressors are characterized by low compression with large volume flows of compressed refrigerant. We distinguish between single-stage or multistage types.
Classification of compressors by housing construction
To avoid the loss of the refrigerant during long-term operation, the compressors should be equipped with very effective seals, or even be free of all seals and openings. To describe how the compressor is enclosed and how the motor drive is situated relative to the compressed gas or vapor, we can distinguish between three types of compressors:
Fig. Types of compressors.
Open compressors are equipped with an external motor that drives the shaft, which passes through the body of the compressor. In this way, the motor drive and compressor are two separate units, connected by a shaft through a clutch or belt transmission. Internal pressure is maintained by rotary seals around the shaft.
The main advantage of an open compressor is that it can be driven by any motive power source, allowing the most appropriate motor to be selected for the application, or even non-electric power sources such as an internal combustion engines or steam turbines. The second advantage is that the motor of an open compressor can be serviced without opening any part of the refrigerant system.
In semi-hermetic compressors, the compressor and drive motor are usually housed in a single bolted housing. It is a large cast metal shell with sealed covers and screws. The detachable screw-type connection enables easy disassembly of the casing and allows for replacement of worn components in the motor and compressor.
In hermetic compressors (hermetically sealed), the compressor and motor are always integrated. The motor is designed to operate inside the housing and is cooled by the refrigerant gas being compressed. A hermetic compressor uses a one-piece welded steel casing that cannot be opened to repair or maintain the motor drive, but it also ensures that there is no route for the gas to leak out of the system.
The primary advantage of a hermetic compressor is a low ratio of g/W, thanks to its design that allows it to turn at higher speeds than open and semi-hermetic compressors. Hermetic compressors have got much more simple construction than semi-hermetic and open ones. It results in a lower failure rate, less noise and vibration, less space required to install, and easier maintenance. Hermetic compressors are also perfectly suited for inverter technology, especially with the BLDC motors. In comparison with inverter semi-hermetic compressors, the hermetic ones are featured with higher efficiency, lower energy consumption, and greater capacity modulation range.