Refrigeration theory: Why does the compression follow an isentrope?

[EVriderDK]

When you have a perfect compressor the compression will follow an isentropic line and an h,log(p)-diragram (enthalpy,pressure diagram) but why? What is entropy exactly and what does it mean in such a system?

In real life the compressors have an isentropic efficiency of maybe 0.8, which will make the compression on the diagram deviate from the isentropic path. But why? Is it just because the friction etc. in the compressor heats up the refrigerant?

 

[njoysci]

The following explains it nicely:
http://www.learnthermo.com/T1-tutorial/ch10/lesson-B/pg04.php

 

[EVriderDK]

Hey I do not think that it does.

 

[engnr_arsalan]

Compression process is isentropic(ideally),isentropic means that process is entirely reversible and adiabetic.
adiabetic means no heat transfer,this can be achieved via proper insulation etc.now coming to the irreversibility,fluid friction and friction between the cylinder wall and Piston are the causes of irreversibility that causes a compressor to deviate from isentropic path..
why compressor should follow isentropic path? ans: "ideally" compressor is not generating any heat,no losses at all,all the energy given to compressor is transferred to fluid.therefore,entropy change is zero(which violates the 2nd law of thermodynamics)
compressor heats up the refrigerant? and: yes,in comprsr with pressure,temp of ref also increases.