Intercooling
The benefit of multistage compression in itself is mainly that of raised flow capacity or volumetric effectiveness for a given pressure ratio. The multistage compression also permits appreciable saving of work when cooling of the gas among stages is resorted to. The cooling is generally completed by a water cooled tubular heat exchanger that also serves as a receiver among the phases.
The work kept by inter-cooling in the idealized two phase reciprocating compressor is explained on the indicator diagram which is as shown in figure below.
Figure: Flow Diagram
Figure: Two-stage Compression with Inter-cooling
Cooling by the cylinder water jackets is never very effectual. The compression curve is always nearer to the adiabatic than to isothermal. And hence, the gas discharged from the initial phase at state x is at a higher temperature than inlet temperature T1; when the gas is cooled to state y at temperature T1 the volume coming into the second phase will be less than the volume leaving the initial phase. The compression in the second phase will proceed all along a new polytropic curve at low volume. The cross hatched region among the two polytropic curves in figure shown above symbolizes the work saved by inter phase cooling to the preliminary temperature. Real cooling might be to some other temperature, though it is conventional to talk about cooling to T1.
The saving of work by two phase compression with inter cooling will based upon the inter phase pressure px selected. Of course, as px approaches either p1 or p2, the procedure approaches single phase compression. Any saving of work should rise from zero to a maximum and return to zero as differs from p1 to p2.