Compressors having external drive require a gland, or seal, where the shaft passes out of the crankcase, are termed open compressors. The drive comes from an external air-cooled motor or other prime mover. Open-drive compres­sors are a requirement when using a refrigerant such as ammonia, which is not compatible with copper used in electric motors. Open compressors may be belt driven or directly coupled to the shaft of the electric motor. Belt drives offer the opportunity to match the speed to the capacity requirement, but they must be adequately guarded and since a lateral load is imposed on the compressor bearings, not all compressor models are suitable. This type of drive is widely used in transport applications where compressors are driven from the vehicle engine or from a separate diesel engine.

The shaft seal for open-drive compressors usually comprises a rotating car­bon ring in contact with a highly polished metal facing ring, the assembly being well lubricated. To maintain contact under all working crankcase pressures, the carbon ring is spring loaded and to allow for slight movement of the shaft.

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