REFRIGERATION
MONITORING THE LUBRICATION OF RECIPROCATING COMPRESSORS
R
eciprocating compressors utilize moving parts, which need to be protected by a dynamic oil film. Since the oil in the compressor is continuously exposed to refrigerant, it follows that the oil will also contain a certain amount of refrigerant. The amount of refrigerant contained within the oil depends on the type of refrigerant and oil used, the oil temperature and the refrigerant pressure. The lower the oil temperature and/or the higher the refrigerant pressure, the higher the amount of refrigerant solution in the oil. A Daniel plot is typically used to indicate the percentage of refrigerant in the oil, for a given pressure and temperature. Oil heaters are recommended for most refrigerating compressors, regardless of whether the compressor is installed in an indoor or outdoor unit. Of course, the viscosity of the oil must be high enough to support long-term, reliable operation. This is true for all modes of operation, including refrigeration and defrost. The only time it is possible to omit a crankcase heater is when the crankcase will not be exposed to temperatures below the minimum as stated in the compressor manufacturer’s operating instructions, and when the refrigerant charge does not exceed 1 1/2 to two times the oil charge. In systems that use a pump-down approach, the oil heater could be omitted if the crankcase temperature can’t drop
below 10°C or 50°F. Omitting the crankcase heater is risky and it is good practice to install one in all cases. As the piston moves towards the valve plate the oil film between the piston and wrist pin, along with the oil film between each crankshaft journal and connecting rod, are exposed to very high forces. It is extremely important this layer of oil is not squeezed out prior to the piston reaching the end of its compression stroke. Refrigerant always reduces the viscosity of oil. The refrigerant type, oil type and refrigerant pressure determine the viscosity of the oil. For example, in Figure1 we can see BSE32 oil at 50°C (122°F) will contain 30 per cent refrigerant R407C when the pressure of the refrigerant is 15 bar or 218 psia.
Figure 1 Refrigerant content
The return gas superheat, and ultimately the discharge temperature, have a large influence on the oil viscosity in the crankcase. Therefore, compressor operating instructions include minimum discharge temperatures that must be present during all modes of operation. There are two main lubrication methods used in reciprocating compressors. These methods are: positivedisplacement (using an oil pump) or dynamic, using a rotating oil disk (centrifugal lubrication). Some smaller
Phil Boudreau
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M e c h a n i c a l
Phil is the Ontario sales manager for Bitzer Canada Inc. and provides training and technical support for Bitzer’s clientele. He can be contacted at: pboudreau@bitzer.ca.
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