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?