Page 84 - Reciprocating Compressors Operation Maintenance

Design and Materials for Reciprocating Compressor Components 7 1

NON-LUBRICATED OR OIL-FREE CYLINDER CONSTRUCTION

There are many applications for industrial compressors in which oil in
the gas stream cannot be tolerated. Oil-free compressed air is essential in
industries such as the food industry, the brewing industry, and the packaging
(Pharmaceuticals) industry, as well as in some industrial air control systems.
But even in general industry or manufacturing, there may be reasons to
consider reducing the amount of lubricating oil used in the compressor
cylinders. Excess oil can build up in the discharge valve port areas, and
even the best premium grades of compressor oil will oxidize when sub-
jected to high temperatures. These oils may eventually form gummy or
sludge-like deposits, which reduce the performance of a compressor and
can, in some cases, lead to fires in the air system if they are allowed to
build up. For these and other reasons, non-lubricated operation has
become increasingly popular.

HISTORY

Before looking at the design details of the oil-free reciprocating com-
pressor, it may be of interest to review what has been produced in the
past and to examine the present "state of the art."

Oil-free cylinder designs were created in the early 1930s. These cylinder
designs used water for lubrication and saw service in brewery applications.
Soap and water lubrication was used for compressors pumping oxygen.
In about the mid 1930s, the first high-pressure, 2000 psi non-lubricat-
ed air compressor was made using carbon rings. In subsequent years,
many single- and multi-stage compressors were made using carbon as the
wearing material for the piston and rider bands. This carbon piston ring
construction is shown in Figure 2-33.

This was a "non-floating" type piston, which meant that the carbon
rings transferred the weight and load of the iron piston onto the cylinder
bore. Piston rings with expanders were used to seal the gas.
Another type of construction was a "floating" piston, in which a tail
rod was used with a small auxiliary crosshead. The tail rod supported the
piston and prevented it from touching the bore. Carbon-rider rings were
not used.