Refrigerator compressors: everything you need to know
Refrigeration compressors are the heart of any refrigeration or heat pump system. They impress the circulation of the refrigerant in the circuit, taking the refrigerant vapor at low pressure and at low temperature to bring it to higher values, both in terms of pressure and temperature.
The main types of compressors for industrial refrigeration and air conditioning can be summarized according to the mechanical volumetric principle and the structural design:
- Volumetric mechanical principle: reciprocating pistons and rotary blades, vane, scroll, screw (single screw or bi-rotor) and centrifugal.
- Structural design: hermetic, semi-hermetic and open.
In reciprocating compressors, one or more pistons slide in the respective cylinders, similar to car engines, carrying out the suction and compression of the refrigerant gas. Each cylinder has inlet and outlet valves.
The reciprocating compressors are available in three different solutions:
- hermetic not accessible , with built-in electric motor and cooling capacities between 1kW and 10kW;
- accessible semi-hermetic , with built-in electric motor and cooling capacities between 160kW and 500kW;
- open accessible , with one end of the elbow shaft protruding outside the casing to be connected to the separate electric motor. Cooling capacities up to 1200kW.
The variation of the cooling load of single-cylinder compressors can be carried out by starting / stopping, or, within strict limits defined by the manufacturer, by varying the rotation speed given by the electric motor.
The cooling load of the multi-cylinder compressors can be carried out by progressive unloading of the cylinders, or, within strict limits defined by the manufacturer, by varying the rotation speed given by the electric motor.
Reciprocating compressors are the most commonly used in commercial and industrial refrigeration. The semi-hermetic and open versions facilitate repairs in the event of a breakdown. According to the commercial types, there are compressors that can work with different refrigerants: R410A, R744 (CO 2 ) subcritical and transcritical and R717 in the open version.
The rotary compressors continuously compress the refrigerant gas by means of impellers, involute spirals and screws. There are, therefore, different types of rotary compressors:
- rotary compressors with blades fixed, mobile or vane;
- orbiting spiral scroll compressors;
- single screw compressors ;
- twin-rotor screw compressors ;
- centrifugal compressors.
All rotary compressors benefit from the reduction of the cooling load by varying the rotation speed of the electric motors.
Rotary blade compressors
They are all airtight. They can have a fixed blade or rotating blades on an eccentric rotor. They are mainly used in domestic refrigerators, split-system air conditioners and in medium air conditioning systems, with powers ranging from 1.7kW to 7.0kW. These compressors, according to the different types, can work with refrigerants: R134a, R404a, R507 and, in special versions, also with R290 (propane, highly flammable).
They consist of two involute spirals, one of which is fixed and the other is rotating eccentric. They are all hermetic and can have cooling capacities between 2.5kW and 70kW. Suitable for applications of medium refrigeration units and air or water cooled heat pumps. The refrigerants used in the latest generations of these refrigeration compressors are, mainly, R410A, R452B, R454B and R32.
Single screw compressors
They are made with a perfectly balanced central screw that moves one or two compression star rotors. They are available in the semi-hermetic version with built-in electric motor, or in the open version with protruding screw rotation shaft and stuffing box.
The cooling capacities are between 100kW and 1500kW. These compressors, particularly resistant also to possible backflows of liquid, are used in medium and large industrial refrigeration systems and for air conditioning in applications on air and water condensed machines.
The single screw compressors can work with different refrigerants, according to the applications: R134a, R1234ze and R513A. Open versions can compress other refrigerants such as R717 (ammonia), R290 (propane).
Two screw compressors
They consist of two screws where the male rotor is moved directly by the electric motor and, in turn, drives the female rotor. They are available in semi-hermetic or open version with a cooling capacity range between 20kW and 1200kW.
These compressors are suitable for compressing almost all commercial refrigerants: R1234ze and R513A , including R717 (ammonia), R290 (propane) and R1270 in the open versions .
Their application is mainly industrial for medium and large refrigeration capacity systems
They consist of one or more impellers in series, coupled directly to the motor shaft, or driven by the motor through a gear transmission device. The impellers rotate at very high speeds, even up to 48,000 rpm. in magnetic levitation compressors. In centrifugal compressors, unlike all those previously mentioned, the compression is not of the volumetric type, but of kinetic energy, where the blades of the impellers push the refrigerant gas through the narrow delivery opening with reduced compression ratios. The cooling capacities are generally between 240kW and 9500kW. These compressors are conveniently used in large systems, mainly with medium temperature water condensation. Centrifugal compressors adopt, according to the types, different refrigerants, such as: R513A, R1234ze, R134a.
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