Page 150 - Compression Machinery for Oil and Gas

138 SECTION II Types of Equipment

Intercooling
In each stage of compression, the increase in fluid density caused by the
increase in pressure is partially offset by an increase in temperature. Thus,
the temperature rise decreases the average fluid density compared to the isother-
mal compression case, and it is well known that this increases compression
work. Naturally, this problem is magnified in a multistage compression process.
Intercooling refers to the installation of a heat exchanger between the exit of one
stage and the inlet of the subsequent stage in order to remove the heat of com-
pression from the previous stage or stages, thereby increasing the average fluid
density over the compression process and reducing compressor power
consumption. For inline compressors, intercooling is done with a back-to-back
configuration between the last stage of the first section and the first stage of the
second section. This is practical because back-to-back machines have case pen-
etrations and utilize external piping between these two points. However,
straight-through inline compressors do not have a means to incorporate a
traditional heat exchanger for intercooling because the only case penetrations
are for the inlet to the first stage and exit of the last stage. By contrast, IGCs
have the ability to be intercooled between every stage since piping exists
between the exit/inlet of every stage, which can offer the best thermodynamic
efficiency for the total compression process.
As an example, Fig. 4.3 shows the compression process for a 13:1 pressure
ratio (PR) air compressor. The minimal compression work is represented by the
theoretical line of isothermal compression. It can be seen that the actual com-
pression work is minimized by increasing the number of compression and inter-
cooling stages.
In general, IGCs can accomplish the same overall compression with fewer
stages than an inline compressor due to the cooler inlet temperature and with
improved efficiency since the additional intercooling brings the total compres-
sion process closer to isothermal. Of course, there is no “free lunch”: heat
exchangers introduce pressure losses and require energy to flow cooling fluid
through the nonprocess side. Therefore, the thermodynamic benefits of inter-
cooling must be optimized with these penalties in mind.

FIG. 4.3 Different compression processes with intercooling.