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Old Mar 9th 2010, 02:23 AM   #1
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Refrigerants, Rankine COP, Enthalpy and Entropy

I am trying to understand the characteristics of refrigerants which determine their limits of performance, say for a heat pump. I understand the importance of characteristics such as critical temperature, condensation and evaporation temperatures, also the compression ratio. However, a crash course in Thermodynamics has left me confused about a number of other terms and relationships.

I am looking at a pressure enthalpy diagram for R12 with its ideal Rankine heat pump cycle plotted. I am trying to understand the Rankine coefficient of performance:

COP Rankine = ( Hd1 – Hd3) / (Hd1 – Hs2)

Where
Hs2 = Enthalpy at saturated vapour, evaporator side of compressor
Hd1 = Enthalpy at superheated vapour, condenser side of compressor (after undergoing isentropic compression from Hs2)
Hd3 = Enthalpy at saturated liquid, condenser side of compressor (after undergoing isothermal condensation from Hd1 )

My understanding of Enthalpy , H, is that it is = U + PV, where U is the internal energy( heat ) and PV is the energy associated with the pressure or motion of the vapour pressure.

Entropy still leaves me confused both in its qualitative and quantitative definitions: I have heard terms such as “how organized or disorganized a system is” or “the availability of energy to do useful work”

So my questions are this:-
1) What quality does the Rankine COP tell you about the refrigerant?
2) Why does the Enthalpy change between S1 and D1: Surely the distribution of energy between U and PV will change but not the sum (U +PV )?

Hope you can help

Rimmer
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Old Mar 9th 2010, 03:54 AM   #2
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What I'm trying to understand is what is it about the refrigerant which enables us to give it a quality, COP, without reference to a real design. I.e. I'm used to the definition

cop = useful energy delivered / energy required to deliver useful energy


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