The compressor is the element that represents the heart of the refrigerant circuit. Its purpose is to control the circulation of refrigerant inside the circuit, drawing in gas refrigerant at low pressure and low temperature, and delivering it at a higher pressure and temperature. Thanks to this process, the compressor allows the refrigerant to dissipate its own heat to the air flowing through the condenser, after which it absorbs heat again inside the evaporator.
Not all air-conditioning applications have the same capacity requirements, and for this reason there are different types of compressors available:
reciprocating compressors (divided into hermetic, semi-hermetic and open-type) are mostly used for applications with higher capacity requirements;
rotary compressors, including scroll, vane and screw compressors (suitable for high capacity applications, up to 1200 kW) and centrifugal compressors (used for applications that require higher capacities).
In this type of compressor, the pistons are designed in similar fashion to those used in a car engine: they slide inside a cylinder, drawing in and compressing the gas refrigerant. Each cylinder features a suction valve for the gas refrigerant and a delivery valve through which the gas refrigerant is sent to the condenser after having been compressed.
Reciprocating compressors are defined as open-type when one end of the crankshaft protrudes outside of the crankcase (the casing that contains the pistons and the mechanisms inside the compressor). In semi-hermetic compressors, on the other hand, both the compressor itself and the drive motor are housed inside the casing, which is designed so as to be opened for inspection and maintenance. In this case, the drive shaft and the crankshaft are one single piece. Semi-hermetic compressors are made so as to prevent air or dust from entering the mechanisms.
Reciprocating compressors are defined as hermetic when the casing is welded closed and sealed, and the heads of the cylinders cannot be accessed for inspection or maintenance. These are used in household refrigerators and freezers and in medium-capacity air-conditioning units.
The category of rotary compressors includes all compressors that perform the compression function using mechanisms that involve impellers, involute scrolls or screws. Scroll compressors (or orbiting scroll compressors) are based on a mechanism patented in 1905. This features two involute scrolls: one stationary, and one orbiting (but not rotating) around the first. Thanks to this motion, the gas contained between the two elements reaches a very high pressure and discharged through a hole in the centre. Stationary or rotary vane compressors work due to the effect of the vanes located inside the cylinders. The vanes may rotate on a cam in the centre of the cylinder (rotary), or be fixed to the walls of the cylinder (stationary). In both cases, the vanes are responsible for the movement of the gas, contributing fundamentally to the suction and compression phases. Screw compressors are based on a mechanism made up of two threaded rotors (screws) that are coupled together.
The gas is compressed due to the progressive overlapping of the lobes, causing a reduction in the volume occupied by the gas.
There are also single-screw compressors that operate by the rotation of just one cylindrical screw with a helical thread, onto which two identical rotors are coupled. Centrifugal compressors are made up of a rotor located inside a special chamber. The rotor is rotated at high speed, imparting high kinetic energy to the gas, which is forced through the narrow outlet opening, thus increasing its pressure.
This type of compressor is used for high and very high cooling capacities.