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Reciprocating Compressors

Most cooling systems in use today rely on reciprocating piston-type compressors. Reciprocating compressors are manufactured in three types:

1. Hermetic - compressor-motor assembly contained in a welded steel case, typically used in household refrigerators, residential air conditioners, smaller commercial air conditioning and refrigeration units.
2. Semi-hermetic - compressor-motor assembly contained in a casting with no penetration by a rotating shaft and with gasketed cover plates for access to key parts such as valves and connecting rods.
3. Open - compressor only with shaft seal and external shaft for coupling connection to belt - or direct-drive using as electric motor or natural gas engine. These are largely used for ammonia refrigeration applications as hermetic designs cannot be used with ammonia refrigerant, and for engine-driven units.

As the piston nears the bottom of its stroke within the cylinder, the intake valve opens and the refrigerant vapor enters. As the piston rises, the increased pressure closes the intake valve. Then as the piston nears the top of its stroke, the exhaust valve opens permitting the vapor at the higher pressure to exit. Reciprocating compressor capacity is a function of the bore and stroke of the piston-cylinder configuration as well as the speed of the machine, and the clearance tolerances. Compressor capacity is also related to the compression ratio.

The mechanical design is rugged and reliable but has one significant limitation. Reciprocating compressors are designed to handle vapors, not liquids. When liquid enters the cylinder on the intake stroke, it tends to damage the valves on the compression stroke and possibly the compressor itself. This is why chillers incorporate liquid-to-suction heat exchangers, which assure some level of vapor superheat at the compressor suction. Capacity is controlled by multiple staging of smaller compressors or in large multiple cylinder reciprocating compressors by unloading banks of cylinders on the compressor. This tends to make the machine most efficient at full load. Therefore, for maximum efficiency recips should generally be operated at full load. This is the reason small compressors are cycled on and off in most residential and small commercial applications.

The mechanical design is rugged and reliable but has one significant limitation. Reciprocating compressors are designed to handle vapors, not liquids. When liquid enters the cylinder on the intake stroke, it tends to damage the valves on the compression stroke and possibly the compressor itself. This is why chillers incorporate liquid-to-suction heat exchangers, which assure some level of vapor superheat at the compressor suction.

Technology Types (Resource)

Reciprocating compressors are manufactured in three types:

1. Hermetic- compressor-motor assembly contained in a welded steel case, typically used in household refrigerators, residential air conditioners, smaller commercial air conditioning and refrigeration units.
2. Semi-hermetic - compressor-motor assembly contained in a casting with no penetration by a rotating shaft and with gasketed cover plates for access to key parts such as valves and connecting rods.
3. Open - compressor only with shaft seal and external shaft for coupling connection to belt - or direct-drive using as electric motor or natural gas engine. These are largely used for ammonia refrigeration applications as hermetic designs cannot be used with ammonia refrigerant, and for engine-driven units.

Manufacturers

 

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