Page 125 - Reciprocating Compressors Operation Maintenance

112 Reciprocating Compressors: Operation and Maintenance

The oil should be applied in such a way that only a minimum amount
will remain in the path of the moving parts. Notwithstanding differences
between plain and antifriction bearings, the moderate conditions in com-
pressor crankcases are such that one weight of oil provides satisfactory
service for both types of bearings.

Oxidation

Much of the circulating oil in compressor crankcases is broken up into
fine spray or mist by splash or oil thrown from rotating parts. Thus, a
large surface of oil is exposed to the oxidizing influence of warm air, and
oxidation will occur at a rate that depends on the operating temperature
and the ability of the oil to resist this chemical change.
Oil oxidation is accompanied by a gradual increase in viscosity and,
eventually, by the deposition of insoluble products in the form of gum or
sludge. These deposits may accumulate in oil passages and restrict the
flow of oil to bearings.
Conditions that promote oxidation in crankcases are mild, however,
compared to oxidizing conditions in compressor cylinders.

Water

Although water may enter compressor crankcases by condensation
from the atmosphere during idle periods or possibly from leaking jackets,
there is generally little water present because of the continuous venting
of water vapor at crankcase temperatures.
Normally, therefore, there is little opportunity for the formation of trou-
blesome emulsions, which could combine with dust and other contaminants
to form sludges that would restrict the flow of oil to lubricated surfaces.
A good compressor crankcase oil will, nevertheless, need adequate
water-separating ability to resist the formation of harmful emulsions and
to permit water to collect at low points where it may be drained off.

Oils for Crankcase/Bearing Lubrication

The type of oil used for the lubrication of bearings and running gear
components must comply with the compressor manufacturers* recom-
mendations, but generally a good quality, non-detergent mineral oil
should be used. These lubricants should contain rust and oxidation