Page 52 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressors and Their Applications 30

being a quite frequent minimum even with reciprocating units where
control possibilities are the greatest), and they are practically never
"spared." Twenty-four hours a day, seven days per week operation is the
general rule for months at a time. On-the-line continuity, or "availability"
of operation, is highly important.

Gas Characteristics

Gas composition and characteristics can have a decided influence on
compressor type. A low gas inlet density, for example, will usually affect
the centrifugal compressor to a greater degree than it will affect the posi-
tive displacement machine. A centrifugal machine handling low density
gas will require many more stages (a larger unit) than when handling a
high molecular weight or specific volume inlet gas. Reciprocating and
other positive displacement compressors are not seriously affected by the
gas, molecular weight, specific volume, or inlet density.

Process Conditions

In any compression problem, for a given compression ratio, the flow
rate to be handled establishes the physical size of the equipment under
consideration.
If 80,000 CFM of gas must be compressed at near atmospheric suction
conditions, a centrifugal compressor would almost certainly be used for
the lower stages of compression.
In centrifugal compressors, if suction pressure is raised, the discharge
pressure will exceed the design point, the horsepower will increase, and
excess pressure may have to be throttled down. If the suction pressure is
lowered, the centrifugal will not compress to the desired discharge pressure.
Therefore, it is most important that the suction and discharge pres-
sures, their variation, and resulting influences be accurately evaluated.
Low compression ratios with reasonable capacities favor the centrifu-
gal compressor. High compression ratios and higher pressures favor the
reciprocating machine. One could not attempt to define the borderline in
pressure between reciprocating and centrifugal compressors because
there are too many other factors to be considered.
In some multistage process compressors, the interstage pressure is set
by the process. This may be for washing out undesirable elements,
adding gas, or carrying out chemical reactions that change the nature of