Page 43 - Reciprocating Compressors Operation Maintenance

3O Reciprocating Compressors: Operation and Maintenance

which contain the discharge valves. This water is circulated through
a radiator/fan arrangement identical to that in an automobile engine.
3. Water-cooled compressors have jackets cast as an integral part of
the cylinder and heads to remove the heat of compression. These
compressors do not depend on radiation to dissipate the heat and
can therefore operate at higher ratios of compression.

Trunk Piston Design

The moderate duty compressor, whether air-cooled or water-cooled, is
built similarly to the automotive engine and uses the length of the piston
to guide it in the cylinder. This is called a "trunk piston" design.
In the trunk piston design, the sides of the piston thus act as the guid-
ing surface in the cylinder bore. For this reason the piston must be fairly
long in relation to its diameter. With the trunk piston design, compression
is possible on top of the piston only; it is called single-acting.
Trunk piston design compressors allow for higher rotation speeds than
those designed with a crosshead.
Generally, the bearings, wrist pin and piston rings are lubricated by oil
thrown from the crank, a process known as "splash" lubrication. In some
cases an oil pump is added, and the wrist pin, crank pins, and bearings
are pressure lubricated.

Heavy Duty Compressors

It is generally agreed that for a reciprocating compressor to be consid-
ered heavy duty or "continuous duty," it must be both water-cooled and
double-acting.

Water-Cooled

We have seen that whenever gas is compressed, heat is generated. Prop-
er cooling of the internal parts of the compressor in order to maintain the
coolest possible temperatures at critical points is a basic part of the design.
In the case of the water-cooled reciprocating compressor, the cylinders
and cylinder heads are surrounded by water jackets, and the heat transfers
through the metal to the water much more efficiently than heat transfer
through metal to air. Water-cooled reciprocating units handle cooling more
efficiently than comparable air-cooled units. This allows for continuous
duty (operation at 100% load and 24-hour days) with low maintenance.