Construction and Working Principle of Reciprocating Compressors

Construction and Working Principle of Reciprocating Compressors Points : construction and working principle of reciprocating compressors Reciprocating Compressor (Positive Displacement Compressors) Reciprocating compressors are available in sizes as small as 1/12 kW which are used in small domestic refrigerators up to about 150 kW for large capacity installations the reciprocating compressors are classified as:
(i) Open type
(ii) Sealed or hermetic type; and
(iii) Semi-sealed or semi-hermetic type.

Open type compressor is flexible in the sense that by varying the speed of the compressor, different capacities can be obtained. It can be operated by any prime-mover such as electric motor, petrol-diesel engine, steam engines etc. In the field, the motor can be easily changed in case of motor burn out. The refrigerant system is not affected by motor burn out. Disadvantage however is that the shaft seal is most vulnerable point for leakage of the refrigerant.

Sealed or hermetic type, however has no need for the shaft seal. These are usually made in capacities from 0.4 kW to 110 kW. Sealed compressor for R-12 and for R-22 are commonly available. Though 5 kW sealed compressors are quite in use. At present hermetic type compressor units particularly in centrifugals are being manufactured in very large sizes. The compressor and motor have a common shaft and are assembled in a single body ‘and the whole assembly is fixed in a steel shell, joints of which are welded. The losses due to a drive package and shaft seal friction are also eliminated, giving power per ton of refrigeration less as compared to open type, but the operation can be only by A.C. electric supply. In the event of the burnt out, highly corrosive hydro-chloric and hydro-floric acids are formed due to the disintegration of the fluorocarbon refrigerants like R-12, R-22 and R-1 I etc. at the high temperature associated with burn out. The system, therefore, gets contaminated and before a repaired or new motor-compressor assembly is fixed, the system has to be thoroughly flushed.

In hermetic compressors, as the compressor motor assembly is suspended inside a steel shell, the winding and the rotor cannot get natural cooling. The cooling of the winding and the rotor is accomplished by cold refrigerant vapour coming from evaporator. The suction vapour is made to pass through passages round the motor stator and around the rotor before entering the compressor suction and thus cool the motor. The lower the temperature of suction vapour, easier it is for the heat flow from winding and the rotor to the vapour. Therefore, the evaporator and suction line superheat should be minimum to obtain maximum cooling of motor. The refrigerant leakage is of utmost importance. Since the refrigerant from evaporator is responsible for cooling the motor, there are chances of motor burnout when refrigerant has leaked away and that cannot be found out as the motor will show no signs of over-leading at all.

Semi-bolted or semi-hermetic compressors differ from those of sealed type since motor is accessible for repair just by removing the bolted plate. So motor can be repaired without any difficulty. In the sealed or hermetic compressors the seal has to be cut for repairs of motor or compressor. Thereafter the seal is again welded to bring back to original condition. This is quite cumbersome and costly affair as compared to the repair done in semi-bolted type.

In order to reject heat during compression process, the compressors are provided with fins for cooling and water jackets for water cooling system. Hermetically sealed compressors are cooled by incoming refrigerant vapour. The compression temperature of R- 12 system is much less than R-7 17 and R-22, the compressors using R- 12 are usually air cooled by forced air over fins. In case of R-7 17 compressors water cooling is most common R-22 compressors are either air or water cooled.