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Natural convection from hot pipe in water

  1. Jun 20, 2011 #1

    I was having this very silly problem in heat transfer. Please could you see whats going wrong !

    I am working on defrost water tray modelling. In that, the water collected during defrost is heated by the compressor o/p coil so as to aid evaporation. I was trying to find out the heat transfer rate, in watts between the coil and water . It is given that the coil is at 60 deg Celsius.

    When I try to calculate the h value and the Q value using natural convection correlations, I am getting a convection coeff (h) as large as 1500 W/m^2.K. Whereas it should be in the range of 20-100 for natural convection in water.

    Its basically a copper tube in water. Its OD is 5mm. Length of coil is 75cm. For natural convection calculations I have taken charateristic length as its OD ie 5mm. Is that correct. ? The correlations that I apply are for horizontal pipe in fluid.

    The value of Nusselt no. I land up with is 15. This give me h value of 1500 W/m^2 K (Q~600 Watts), which is very huge.. ! (using h=(nu*k)/d). k(thermal conduct. ~.5 W/m.K)

    Which of these assumptions do you think could have made this go so off.. ?

    Could anyone point out where I am going wrong with this !

    Thanks !
  2. jcsd
  3. Jun 20, 2011 #2
    First, a Nusselt no of 15 for natural circulation is not unreasonable. So for a 5mm tube, it is what it is. On the other hand, the smaller surface area should affect your heat transfer rate.

    This was for the outside surface of the tube correct? What are the conditions inside the tube? How many tubes? How long are the tubes? What is the temperature difference driving heat flow?
  4. Jun 21, 2011 #3
    I agree with edgepflow. Copper tubes in water will transfer heat at pretty good rates, so 1500 W/m^2*K is reasonable.
  5. Jun 21, 2011 #4

    Thank you for your response..

    Yes, I too verified, that a copper coil 5mm od and 75cm length at 60 deg cels. when placed in water at 40 deg cels. can provide heat rates as high as 1500 watts.

    In my case, my mistake was in assuming the coil to be at 60 deg cels. This coil is actually the condenser in the refrigerator. The ref. fluid that flows through it is at 60 deg cels. and I wrongly assumed that to be the temperature of the coil.

    On accounting for the 2 convection resistances (1/hA) at the ref. and water surface, I found that the coil (whose resistance is negligible) is at a temperature of 42deg, only 2deg greater than the temperature of water.

    This gives me heat rates of 20-30 watts which is realistic for the case of the condenser coil in the defrost water tray.

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