Page 54 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressors and Their Applications 4 1

Multiple-Service Units

For sound economic reasons, process plants have been installing fewer
compressors to do a given job, using larger sizes and no spares. When a
process requires handling of many streams, a common occurrence in
some types of plants, machines can be so configured that several streams
may be handled by a single driver.
It is rare that more than two services are handled in a combination of
centrifugal casings having a common driver. There are problems of
capacity and pressure control, as well as other factors.
With reciprocating compressors, greater flexibility is possible in the
number and size of streams that can be handled on a single driver.
Desired capacity control is usually more easily attained as well. As many
as six separate streams have been handled on large existing compressors.

First Cost

There is no formula for establishing the relative cost of the centrifugal
versus the reciprocating compressor. If volume, pressure, k factor, and all
other factors are the same, then gas specific gravity can influence the
cost of the centrifugal, but it will have little effect on the reciprocating
compressor.

Power Cost

As has been mentioned earlier, power cost throughout the service life
of a compressor is many times the first cost. While it is possible that the
more efficient machine may be higher in original installed cost, the
power savings over a period of years will usually quickly pay off the dif-
ferential and return a profit for the remaining life.
Except at very low ratios of compression, the centrifugal compressor
is inherently less efficient than the reciprocating compressor.
Very large volumes, low ratios of compression, and low final pressures
favor the centrifugal compressor. No one today would think of using the
reciprocating compressor for blast furnace blowers. Conversely, higher
ratios of compression and higher terminal pressures favor the reciprocat-
ing compressor in other services.