Refrigeration theory: Why does the compression follow an isentrope?

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Discussion Overview

The discussion centers on the isentropic nature of compression in refrigeration systems, particularly focusing on why ideal compressors are expected to follow an isentropic path in an h,log(p)-diagram. Participants explore the implications of entropy in this context and the factors that lead to deviations from ideal behavior in real compressors.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the definition of entropy and its significance in the context of compression, suggesting a need for clarification on these concepts.
  • Another participant asserts that the ideal compression process is isentropic, defined as entirely reversible and adiabatic, and explains that irreversibilities such as fluid friction cause deviations from this ideal path.
  • Concerns are raised about the real-world isentropic efficiency of compressors, which is noted to be around 0.8, leading to questions about the role of friction and heat generation in this deviation.
  • It is mentioned that in an ideal compressor, no heat is generated, and all energy is transferred to the fluid, resulting in zero entropy change, which is said to conflict with the second law of thermodynamics.
  • One participant agrees that the temperature of the refrigerant increases with pressure in the compressor, indicating a relationship between these variables.

Areas of Agreement / Disagreement

Participants express differing views on the nature of isentropic processes and the factors affecting real compressors, indicating that multiple competing perspectives exist without a clear consensus.

Contextual Notes

Some assumptions regarding the ideal behavior of compressors and the definitions of entropy and isentropic processes remain unresolved, as do the implications of irreversibility in practical applications.

EVriderDK
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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?
 
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Hey I do not think that it does.
 
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.
 

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