Incoming gas may contain particles of dirt from within the circuit, especially on a new system. Suction strainers or traps are provided to catch such dirt and will be readily accessible for cleaning on the larger machines.

All but the smallest compressors will have a strainer or a filter in the lubri­cating oil circuit. Strainers within the sump are commonly of the self-cleaning slot disc type. Larger machines may also have a filter of the fabric throwaway type, as in automobile practice. Reciprocating compressors have splash lubri­cation in the small sizes and forced oil feed with gear or crescent pumps on all others. A sight glass is normally fitted to semi-hermetic and open compressors to indicate oil level.

When the compressor is idle, the lubricating oil will contain a certain amount of dissolved refrigerant, depending on the pressure, temperature and the refrigerant itself. At the moment of starting, the oil will be diluted by this refrigerant and, as the suction pressure falls, gas will boil out of the oil, causing it to foam.

To restrict the refrigerant quantity in the oil to an acceptable amount, heat­ing devices are commonly fitted to crankcases and connected so as to remain in operation whenever the compressor is idle.

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