Discuss Isentropic, Isothermal, Polytropic Process 2

[NMLS]

I have read that by multi-staging in air compressors or turbochargers,(allowing the air to pass through intercoolers between stages) returns the air to its original temp. and brings it compression (P-V) curve closer to that for an Isothermal operation. I assume that without intercooling the operation would be Polytropic. Is it correct to say that in an Polytropic process some cooling applies, therefore, somewhere in between isentropic and isothermal, real compression procession processes are generally polytropic in nature. The cooling would be the heat given of to the surrounding area and carried away with the compressed air. In an Isothermal process maximum cooling occurs and in an Isentropic process cooling occurs during. I have read that in an Isothermal process the temp. remains constant and in an Isentropic process the entropy of the working fluid remains constant. Can you please clear this up for me? THX.

 

[amorrison]

Recommend you post this question to

forum391 HT and Thermodyanamics

or

forum404 ME-other topics

 

[CBraver]

You are correct that an isothermal process implies constant temperature and that isentropic implies constant entropy. Adiabatic implies that were was no heat transfer. You are correct in that very few processes are completely isothermal, etc .... but oftentimes the heat loss is negligible in relation to the system that therefore is ignored to make calculations simpler. An adiabatic process allows you to use the relationship P1V1^k = P2V2^k (k=1.4 for air).

Multi-stagged air compressor systems can do what you say (when designed properly), or at least close to it. For a simple example, a 2 stage air compressor might intake air at 70F, compress it (piston compressor, centrifugal/turbo, whatever), cool it through an intercooler, compress it again and then cool it again through an aftercooler back to 70F. There will be two profiles on a Pv diagram showing that. A single stage would have one large one. The two stage will require less work (although there is still work required to cool the air for the second stage in the intercooler). The downside is cost.

Does that answer your questions?

 

[Truckdriver2]

The area under the curves is equivalent to the work done. In the cooled machines there is less area under the curves - hence less work for the same effect.