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Do Refrigerants have more heat capacity than water?

  1. May 27, 2015 #1

    rollingstein

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    I was reading this report prepared by PNNL which says (page 16):

    "Using refrigerant to deliver heating and cooling requires less energy because of the larger heat capacity of the refrigerant relative to air and even water. Less mass flow is needed to deliver the same amount of heating or cooling."

    http://www.gsa.gov/portal/mediaId/169771/fileName/GPG_VRF_Report_-_FINAL_DRAFT_4-16-13

    Is this really true? I see for Chlorodifluoromethane (R22) a latent heat of vaporization of 233 kJ/kg.

    How is that more heat capacity than water?
     
  2. jcsd
  3. May 27, 2015 #2

    Simon Bridge

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    Latent heat of vaporization is not the same as heat capacity.
     
  4. May 27, 2015 #3

    rollingstein

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    What is the usable heat capacity of R22 in a typical refrigerant application? I assumed that was the latent heat.
     
  5. May 27, 2015 #4

    Simon Bridge

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    Is "useable heat capacity" a term of the art somewhere? Please provide a reference.
    Check your assumptions against what the author is talking about - i.e. ability to transport heat compared with air and water.
    Remember to compare like with like - what is the latent heat of vaporization for air? Does it make more sense, in that case, to compare the specific heat capacity (at constant pressure say)?
     
  6. May 27, 2015 #5
    Something important that you forget is the temperature range you can use when transporting heat from the room unit, wich will be at room temperature and the chiller on the roof where the temperature must be above freezing if you use water. The temperature range will be less than 20 degrees K if you use water. Other refrigerants can have a much larger temperature range that can include the boiling point, so you have to include the heat of vaporization as well.
     
  7. May 27, 2015 #6

    rollingstein

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    Understood, thanks!

    So essentially we are comparing H_vap_R22 versus Cp_H2O * delta_T_H2O

    With a delta_T_H2O of 20 C the refrigerent gives a higher heat capacity i.e. lower mass flows.
     
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