Page 226 - Chemical process engineering design and economics

Compressors, Pumps, and Turbines 209

would become vertical at the choke limit, indicating that the flow rate has
reached a maximum. Controls to prevent operation too near the choke
limit usually are not required. The design point is selected to allow for an in-
crease or decrease in the flow rate if the process conditions vary.
In Figure 5.13, the operating range of the various compressors are shown
for comparison, except for the rotary compressors which are expected to occupy
a region between the reciprocating and centrifugal compressors. Figure 5.13 can
help to guide the process engineer in selecting a compressor design.

COMPRESSOR POWER

To size a compressor requires calculating the power needed for compression.
This can be done by assuming an isentropic compression and then correcting the
result by dividing by an isentropic efficiency. The power can also be calculated
by assuming a polytropic compression, and then correcting the result by dividing
by a polytropic efficiency. Both methods will be considered. The isentropic
method is also used for blowers and vacuum pumps, but the polytropic method
could also be used if data were available. First, we need to derive relationships
to calculate the compressor power.

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