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Overall Heat Transfer Coefficient (Exhaust Gas to Refrigerant)

  1. Mar 4, 2013 #1
    Hi there,

    I am doing a project modelling and simulating a gas engine waste heat recovery system using an organic rankine cycle.
    As part of this project I would like to size the surface area of a heat exchanger (if only roughly). I have found the appropriate formula using Q = U.A.LMTD and all the variables are fine and I believe I am calculating correctly but I'm assuming an overall heat transfer coefficient ("U") of ~400 W/m^2.K. I have no idea if this is accurate
    The two fluids exchanging heat are an exhaust gas (heat source) and various refrigerants (R134a, R245fa), I was hoping someone might be able to suggest an accurate "U" for these fluids and if possible something to reference.
    I don't have a fantastic knowledge of heat exchangers by any means, do I assume the heat exchange surface to be made from metal and is it relevant whether i specify plate or shell and tube etc?
    Any help would be greatly appreciated, thanks advance for your time.

    Ronan
     
  2. jcsd
  3. Mar 4, 2013 #2

    SteamKing

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    The U in heat transfer is a property of the material used to fabricate the heat exchanger and the geometry of the unit. It is not a physical property of the fluids passing thru the unit.
     
  4. Mar 4, 2013 #3
    oh ok thank you, I was confused because any information I have searched for contained tables with both the hot and the cold liquid for example: gas to water and a corresponding "U" value...
    Would you know how to obtain a relatively accurate value for my predicament or an educated guess perhaps?
    Thanks again for your post
     
  5. Mar 4, 2013 #4

    SteamKing

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    It depends on the material used to construct the heat exchanger and the physical layout of the heat exchanger itself.
     
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