Physics Help Forum Refrigerants, Rankine COP, Enthalpy and Entropy

 Thermodynamics and Fluid Mechanics Thermodynamics and Fluid Mechanics Physics Help Forum

 Mar 9th 2010, 02:23 AM #1 Junior Member   Join Date: Mar 2010 Posts: 2 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
 Mar 9th 2010, 03:54 AM #2 Junior Member   Join Date: Mar 2010 Posts: 2 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 rimmer

 Tags cop, enthalpy, entropy, rankine, refrigerants

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