Page 53 - Reciprocating Compressors Operation Maintenance

4O Reciprocating Compressors: Operation and Maintenance

the gas between stages. It is important that such interstage pressure
restrictions be considered, including the pressure drop involved.

Driver

Generally speaking, if choice of driver is dictated by the available
source of power, the heat balance, waste gas use, or any other factors in
the plant or process, the compressor that best fits the driver should
receive first consideration. In other words, the centrifugal compressor
always receives first consideration if the driver must be a turbine. The
reciprocating compressor should probably always receive first considera-
tion if the driver is to be an electric motor. This is because the majority of
motor-driven centrifugal compressors require a speed-increasing gear.
The converse is true if turbines are used.

Oil Contamination

Oil contamination can have a bearing on compressor type selected.
The degree of importance attributed to oil contamination will also influ-
ence decisions. Partially lubricated or non-lubricated reciprocating com-
pressor designs may be more suitable than the small centrifugal (or com-
mon helical lobe units with inexpensive seal systems).

Soil and foundation conditions must be considered in compressor
selection. In a few instances, this dictates the type of machine. A cen-
trifugal compressor operates without producing unbalanced forces. The
foundation need only support the dead-weight load with adequate stiff-
ness to maintain alignment.
The type of reciprocating compressor most widely applied today in
chemical and process plants is designed specifically for a minimum of
unbalanced forces. Generally, a foundation that will support the dead-
weight load of the compressor and maintain alignment of compressor and
driver will have adequate mass to absorb the small unbalanced forces
that may be present.
The centrifugal machine will usually be favored from both the founda-
tion and floor space standpoint if this is the only consideration.