Page 41 - Reciprocating Compressors Operation Maintenance

28 Reciprocating Compressors: Operation and Maintenance

ings vary from fractions to more than 20,000 HP per unit. Pressures
range from low vacuum (at intake) to special process compressors for
65,000 psig or higher. For industrial plant air service, they are used from
fractional HP to 2000-3000 HP and in pressure ranges from low vacuum
to perhaps 500 psi for soot blowing.
In common with all positive displacement compressors, the reciprocating
compressor is classified as a "constant-volume variable pressure" machine.
For most applications, they are the most efficient built today. They can
be fitted with capacity control devices to closely maintain their efficiency
at partial loads (reduced capacity output They can be built to handle
almost any commercial gas, provided corrosion problems in some
extreme cases can be solved. Gas cylinders are generally lubricated,
although a non-lubricated design is available when warranted.
Because of the reciprocating pistons and other parts, as well as some
unbalanced rotating parts, inertia forces are set up that tend to shake the
unit. It is necessary to provide a mounting that will stabilize the installa-
tion. The extent of this requirement depends on the type and size of com-
pressor involved. These machines are normally designed to be installed
in a building, but can be fitted for outdoor installation.
Reciprocating compressors should be supplied with clean gas. Inlet fil-
ters are recommended on air compressors. These compressors cannot sat-
isfactorily handle liquids that may be entrained in the gas, although
vapors are no problem if condensation within the cylinder does not take
place. Liquids tend to destroy lubrication and cause excessive wear.
Reciprocating compressors deliver a pulsating flow of gas. This is
sometimes a disadvantage, but pulsation dampeners can usually elimi-
nate the problem.
Reciprocating compressors, like the rotary sliding vane and helical
lobe screw machines, are positive displacement compressors. This means
that gas is compressed by trapping a charge of gas and then reducing the
confining space, causing a build-up in pressure.
The reciprocating compressors, more commonly called "piston com-
pressors," compress gas by use of a piston, cylinder, and valve arrange-
ment. Figure 1-17 shows volume reduction and subsequent increase in
pressure, as a piston moves in a cylinder,

COMPRESSOR CLASSIFICATIONS

Manufacturers design compressors to fill definite user needs. These
compressors fall into two general groups, moderate duty machines and