Wednesday, 28 December 2016

Hermetic Compressor & Semi-hermetic compressors

A hermetic or sealed compressor is one in which both compressor and motor are confined in a single outer welded steel shell. The motor and compressor are directly coupled on the same shaft, with the motor inside the refrigeration circuit. Thus the need for a shaft seal with the consequent refrigerant leakage problem was eliminated. All the refrigerant pipeline connections to the outer steel shell are by welding or brazing. The electrical conductors to the motor are taken out of the steel shell by sealed terminals made of fused glass. The figure below shows the cut-away view of a hermetic compressor. One can see the cooper windings inside the outer shell and also the refrigerant conections (copper pipes). Hermetic compressors are ideal for small refrigeration systems, where continuous maintenance (replenishing refrigerant and oil charge etc) cannot be ensured. Hence they are widely used in domestic refrigerators, room air conditioners etc. Since, the motor is in the refrigerant circuit, the efficiency of hermetic compressor based systems is lower as the heat dissipated by the motor and compressor becomes a part of the system load. Also material compatibility between the electrical windings, refrigerant and oil must be ensured. Since the complete system is kept in a welded steel shell, the hermetic compressors are not meant for servicing. A variation of hermetic compressor is a semi-hermetic compressor, in which the bolted construction offers limited serviceability.

Semi-hermetic compressors are identical sealed type, but the motor and compressor built in manufactured housing with screw sections or access panels for ease of maintenance. These compressors are manufactured in small and medium capacities and their engine power may be up to 300 kW. For this reason, they are cheap, and another advantage is that they are compact. In addition, they have no problems with leaking. On Fig. 3.5 shows a new type of semi-hermetic reciprocating compressors for medium-and low-temperature commercial refrigeration equipment. They are issued to alternative refrigerants (e.g.. R-134a, R-404A and R-507). Fig. 3.5a shows a cutaway view of a single-stage octagon series semi-hermetic reciprocating compressors with a nominal engines with a capacity of 60 and 70 HP With integrated ripple mufflers and performance management (100-75-50%), smooth, efficient and compact piston semihermetics now available for this category of potential. They can work with refrigerants R-134a, R-407C, R-404A, R-507A, R-22. Fig. 3.5b shows used a two-stage semi-hermetic reciprocating compressors for extremely low temperatures and its main feature is the two-stage compression in a single package. A two-stage compression, the compression ratio of the share, thus avoiding extreme temperatures and achieve very reliable operation. In particular, for commercial refrigeration systems with high load variations, energy-efficient operation at full and partial load (capacity up to four stages) to all common refrigerants can be at a reasonable price. In addition, it is recognized features of the octagon, compressors, which even pay with a double in tandem configuration.

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