Choosing the Right Refrigeration Compressor

In an open refrigeration compressor, the motor and the compressor are separated. The compressor drive shaft is connected to the motor by a coupling sleeve or by a belt and pulleys. As such you can use different types of motors (electric, diesel, gas, etc.) according to your needs.

These refrigeration compressors are therefore not known for being compact, they are mainly used for high power. The power can be adjusted in several ways:
– by stopping some cylinders on multi-piston compressors
– by changing the speed of the drive motor
– by changing the size of any pulleys

Another advatange is that all parts of the compressor are accessible for repair, unlike hermetic refrigeration compressors.

The main disadvantage of this type of refrigeration compressor is the presence of a rotating seal on the compressor shaft which can become a source of refrigerant leakage with wear.

In a hermetic refrigeration compressor, the electric motor and compressor part are enclosed in a hermetically welded envelope.

The main advantage to a hermetic refrigeration compressor is its sealing system which is ensured by the enclosed envelope and does not depend on the state of one or several joints (i.e. the rotating shaft seal for example).

But this system does not only have advantages: the cooling of the motor is ensured by the refrigerant fluid bathing the inside of the casing which heats the fluid and is detrimental to the refrigeration cycle. In principle, the cooling capacity cannot be adjusted, except by varying the frequency of the supply current.

Another disadvantage is that it is impossible to open the enclosed envelope to make any repairs and as such if something stops working the whole refrigeration compressor must be replaced. Luckily, this system is fairly affordable.

Hermetic refrigeration compressors are reserved for low and medium power requirements.

Semi-hermetic refrigeration compressors are a compromise between open and hermetic refrigeration compressors.

As in a hermetic refrigeration compressor, the motor and compressor part are enclosed in a hermetic envelope,  but this envelope is not welded and all parts are accessible for repair.

The motor can be cooled by the refrigerant or, in some cases, by a liquid cooling system integrated into the casing.

The sealing system is thus better than on an open refrigeration compressor as there is no rotating seal on the transmission shaft. There are still static seals on the removable parts however and the sealing is therefore not as complete as on a hermetic refrigeration compressor.

Semi-hermetic refrigeration compressors are used for medium power requirements, and while they offer the economic advantage of being repairable, they cost significantly more than hermetic refrigeration compressors.

Piston refrigeration compressors are the most common on the market, they can be found in all configurations (open, hermetic and semi-hermetic) and for all power levels, from the lowest to the highest.

A piston refrigeration compressor needs to be constantly lubricated and is very sensitive to the presence of liquid at the inlet, which can cause the destruction of the valves.

Compared to other technology, piston refrigeration compressors are more compact and affordable, but they also have the least powerful technology.

Spiral refrigeration compressors, also called scroll refrigeration compressors, are made up of two spiral-shaped rollers. One of the spirals is fixed while the other traces an eccentric and orbital movement without rotation that moves the refrigerant to the center of the spiral while reducing its volume.

The main advantage of spiral refrigeration compressors is the reduced number of parts compared to piston refrigeration compressors, which means better output. This type of compressor imposes less torque variations on its engine, which increases its reliability and it is also less sensitive to the presence of liquid at the inlet.

It is limited in power (40-50kW) but several compressors can be combined in parallel to obtain a  higher level of power, reaching up to 300-400kW.

Spiral refrigeration compressors are quieter than piston ones and they are used in many air conditioning systems and modern refrigerators.

In a screw refrigeration compressor, the refrigerant is compressed by a helical screw rotating at high speed. There are two configurations: single-screw compressors and twin-screw compressors.

These refrigeration compressors give excellent output  and their power range is between 20 and 1,200kW. They also have excellent product life and are extremely reliable but they must be lubricated correctly to ensure the sealing between the moving parts, to reduce noise and cool down the refrigerant.

Among their disadvantages, it can be noted that screw refrigeration compressors are more expensive and take up more space than piston refrigeration compressors.