Page 51 - Reciprocating Compressors Operation Maintenance

38 Reciprocating Compressors: Operation and Maintenance

unless one can utilize the smaller moderate duty unit. The latter require
(generally) limited foundations.

Capacity Control

Variation in demand for gas must be considered; it usually ranges from
full compressor capacity to zero. The different types have widely varying
abilities to handle this range, some being much more economical at part
load than others. The reciprocating compressor is particularly favored in
this area,

Variety of Choice

Within the four general divisions of compressors, there is a certain
freedom of choice between arrangements. Similarly, many selection
problems can be solved by considering more than one of the available
reciprocating compressor designs.

Oil Free Air

Any requirement for air completely free of oil will rule out the rotary
vane and most of the helical-lobe (screw) type compressors. There are,
however, some screw compressor models which are offered as oil free
and thus do not operate with oil in the rotor chamber.

Cost of Air

Compressed air is never "free." It costs a considerable sum, and the
selection of a compressor should always consider this factor, particularly
for heavy duty service. Two facts should be mentioned:

1. Power cost is by far the largest item in over-all air cost.
2, Power cost over the service life may be many times the first cost of
any compressor.
SELECTION OF PROCESS COMPRESSORS

The process gas compressor is called upon to handle many diverse
types of gas. Its capacity control requirements have a lower range (50%