A compressor is the most important and often the costliest component (typically 30 to 40 percent of total cost) of any vapour compression refrigeration system (VCRS). The function of a compressor in a VCRS is to continuously draw the refrigerant vapour from the evaporator, so that a low pressure and low temperature can be maintained in the evaporator at which the refrigerant can boil extracting heat from the refrigerated space. The compressor then has to raise the pressure of the refrigerant to a level at which it can condense by rejecting heat to the cooling medium in the condenser.

Types of Compressor

There are different types of compressors that generally used in industry are,

(a) Reciprocating compressor

(b) Centrifugal compressor

(c) Rotary compressor

(d) Screw compressor

(e) Scroll compressor

The reciprocating and screw compressors are best suited for use with refrigerants which require a relatively small displacement and condense at relatively high pressure, such as R-12, R-22, Ammonia, etc.

The centrifugal compressors are suitable for handling refrigerants that require large displacement and operate at low condensing pressure, such as R-11, R-113, etc.

The rotary compressor is most suited for pumping refrigerants having moderate or low condensing pressures, such as R-21 and R-114; this is mainly used in domestic refrigerators.

1. Reciprocating Compressor

The compressors in which the vapour refrigerant is compressed by the reciprocating (i.e. back and forth) motion of the piston, called reciprocating compressors. These compressors are used for refrigerants which have comparatively low volume per kg and a large differential pressure, such as ammonia, R-12, R-22, etc.

1. Centrifugal Compressor

A single-stage centrifugal compressor mainly consists of the following four components:

(a) An inlet casing to accelerate the fluid to the impeller inlet.

(b) An impeller to transfer energy to the fluid in the form of increase in static pressure and kinetic energy.

(c) A diffuser to convert the kinetic energy at the impeller outlet into pressure energy (static enthalpy).

(d) A volute casing to collect the fluid and to further convert the kinetic energy into pressure energy (static enthalpy).

Besides these, there are intercoolers, generally integrated with the casing, in a multistage compressor. The casing is usually made of cast iron and the impeller, of alloy (chrome-nickel) steels. The maximum stress is developed at the root of the blades.

The diffuser is normally vaneless type as it permits more efficient part load operation which is quite usual in any air-conditioning plant. A vaned diffuser will certainly cause shock losses if the compressor is run at reduced capacity and flow.

1. Rotary Compressor

Rotary compressors are positive displacement, direct-drive machines. There are essentially two designs of this compressor: (a) Rolling piston type (b) Rotating vane type

In the rolling piston type, the roller is mounted on an eccentric shaft with a single blade, which is always in contact with the roller by means of a spring.

In the rotating vane type, with four vanes, the rotor is concentric with the shaft. The vanes slide within the rotor but keep contact with the cylinder. The assembly of rotor and the vanes is off-centre with respect to the cylinder.

In both designs, the whole assembly is enclosed in a housing, filled with oil and remains submerged in oil. An oil film forms the seal between the high-pressure and the low-pressure sides. When the compressor stops, this seal is lost and the pressure equalizes. Rotary compressors have high volumetric efficiencies due to negligible clearance. They are normally used in a single stage up to a capacity of 5 TR with R-114. Large rotary compressors are used in low-temperature fields, such as in chemical and industrial processing, cold storages and freezing, as high displacement. low-stage or booster compressors at -90 to -l00°C evaporator temperature with R-12, R-22 and ammonia. They are available in l0 to 600 hp sizes with 2 to 120 cubic metres per minute displacement in one unit.

1. Screw Compressor

Rotary screw compressors also belong to the category of positive displacement compressors machine a rotary compressor essentially consists of two helically-grooved rotors which rotate in a housing.

The male rotor consists of lobes and is normally the driving rotor. The female rotor has gullies and is normally the driven rotor. A four-lobe male rotor will drive a six-gully female rotor at two-thirds of its speed. At 3600 rpm the number of compressed gas discharges of a four-lobe rotor will he 4 X 3600 = 14,400 per minute.

As in the case of other positive displacement machines, there are three basic continuous phases of the working cycle, viz., suction, compression and discharge. When the male rotor turns clockwise, an interlobe space between a pair and housing nearest to the suction end opens and is filled with the gas. There are four such pairs to be filled during one revolution in a four-lobe rotor and the suction periods overlap one another.

When remeshing starts, the volume decreases and the pressure rises. The charge is moved helically and compressed until the trapped volume reaches the discharge end. The compression ratio is thus fixed.

Further rotation simply empties the rotors of the high pressure gas until the last traces of the gas are squeezed out, irrespective of the pressure in the condenser.

On completion of the discharge phase, there is no residual gas remaining in the rotors. As a result, there is no expansion of clearance gases. The compressor has no suction and discharge valves.

There are leakage paths in a screw compressor mainly across the line of mesh between the rotors and across the clearance between the rotors and the housing. To eliminate leakage, oil is injected in a number of small jets directed towards the mesh. Oil injection also serves the purpose of cooling and lubricating along with that of sealing the leakage paths.

A slide valve, closely following the shape of the rotors is used for capacity control. At full load the valve is closed. At part load, the valve opens enabling a return flow passage to be formed so that a part of the gas drawn into the interlobe spaces can flow back to the suction side.

The screw compressor combines many advantageous features of both centrifugal and compressors, along with some of its own. As it is a positive displacement machine, high pressure refrigerants, such as R-22 and ammonia are used in it. As it is a high speed rotary machine, a large volume can be handled by it. It is, therefore, found extremely suit able for large capacity low temperature applications such as in food refrigeration.

Like reciprocating compressors, it has no surging problems. It has small pipe dimensions and positive pressures due to the use of high pressure refrigerants. Like centrifugal compressors, it has high compression efficiency, continuous capacity control, unloaded starting and no balancing problems. Also, the compressor is suitable for large capacity installations.