Page 124 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 111

covers, and shafts are effectively sealed. Vents are provided to permit
crankcase breathing, and when these are equipped with simple air filters,
the entrance of contaminants is minimized,

FACTORS AFFECTING BEARING LUBRICATION

Fluid-Film Lubrication

In general, the factors such as load, speed, temperature, and the pres-
ence of water and other contaminants have a moderate effect on com-
pressor bearing lubrication. All lubricated surfaces in compressor
crankcases are provided with an excess of lubricating oil.
During operation, surfaces are separated by relatively thick oil films
that normally prevent metal-to-metal contact and wear and reduce fric-
tion to the low value of fluid films.
Under these conditions, the most important property of an oil is its vis-
cosity at the operating temperature. Oils of suitable viscosity will distrib-
ute rapidly and evenly to form a film that will adequately resist being
squeezed from loaded areas.

Thin-Film Lubrication

During operation, fluid oil films exist between bearing surfaces. How-
ever, while stopped, adequate fluid films can no longer be maintained,
and some metal-to-metal contacts may occur.
Under these conditions, and regardless of bearing type, the film
strength property of oil is of increasing importance. An oil having ade-
quate film strength will resist rupture and, thus, minimize metal-to-metal
contact, friction, and wear. Diester and polyalpha-olefin (PAO)-based
synthesized hydrocarbon oils are often compounded for these require-
ments and should be considered for modem compressor installations.
It should be emphasized that only thin films of oil exist at points of
frictional contact in antifriction bearings. Hence, to minimize metal-to-
metal contact and to reduce friction, a lubricating oil having adequate
film strength is required.
Cooling, cleaning, and protection against rusting are also important
functions of the lubricating oil, and the oil viscosity should be in a range
that provides ready distribution.