Refrigeration & Air-Conditioning

Module 3. Refrigeration plant components

Lesson 11
ROTARY, CENTRIFUGAL COMPRESSORS AND SCREW COMPRESSOR

11.1 Introduction

The different types of compressors used in refrigeration plant includes rotary, centrifugal compressors and screw compressor

11.2 Screw Compressor

It consists of two helical rotors which are installed in a common housing. The refrigerant vapour is drawn into the gaps between the teeth on rotation of the rotors. The gas is gradually compressed along the length of the rotors. These compressors belong to category of positive displacement compressor.

Screw compressors are used in large installations. One of the greatest advantages of the screw compressor is that the capacity can be varied down to 10 % of full load without excessive loss of efficiency. The working of screw compressor is indicated in Fig. 11.1. It is also reported that screw compressor consumes little less energy as compared to reciprocating compressor. It also requires less maintenance as compared to reciprocating compressor. These compressors are compact in design and require less maintenance. PLC based compressor control facilitates automation for higher efficiency.

Fig. 11.1 Working of screw compressor

11.3 Centrifugal Compressor

The centrifugal compressor is generally used for refrigerants that require large vapour displacement and low condensing pressure. Single stage centrifugal compressor can develop compression ratio to about 4.5. For higher compression ratio, multi stage centrifugal compressors with inter cooling are employed.

The impeller draws low pressure vapour from evaporator. The high speed of impeller leaves the vapor refrigerant at high velocity at the vane tips of the impeller. The volute casing collects the refrigerant vapour from the diffuser and it further converts kinetic energy into pressure energy.

Centrifugal compressor requires less maintenance due to less number of moving parts.

11.4 Rotary Compressor

These compressors are positive displacement type and the refrigerant is compressed due to the movement of blades. There are two types of rotary compressors. The whole assembly of both the types of rotary compressor is enclosed in a housing which is filled with oil.

11.4.1 Single stationary blade type rotary compressor

This type of compressor consists of a stationary cylinder, a roller and a shaft having eccentric. A blade (sealing blade) is set into the slot of the cylinder in such a manner that it maintains contact with the roller by means of a spring. When roller rotates, the vapour refrigerant ahead of the roller is being compressed and the new intake of gas takes place.

11.4.2 Rotating blade type rotary compressor

This consists of a cylinder and a slotted rotor containing a number of blades. The centre of the rotor is eccentric with the centre of the cylinder. The blades are forced against the cylinder wall by the centrifugal action during the rotation of the motor. As the rotor turns, the suction vapour entrapped between the two adjacent blades is compressed and is discharged through the discharge port.