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Convection over horizontal pipe in air

  1. Feb 24, 2010 #1
    I am working on a problem involving heat transfer from a horizontal pipe in air. I am having trouble with it and not getting the solution I expect, and I think it is because of the convection component of the heat transfer. The general formula I am following for convection from a horizontal pipe is:

    Q = h*A*(Tsurface-Tambient)


    h = convection heat transfer coefficient
    A = pipe surface area
    Tsurface = surface temperature of pipe
    Tambient = air temperature around pipe

    I am using the following formulas for calculating the heat transfer coefficient:

    Natural Convection:

    h = 1.32*((Tsurface-Tambient)/D)^0.25

    Forced Convection:

    h = C*Re^n*Pr^(1/3)*k/D


    Re = Reynolds number
    Pr = Prandtl number
    k = thermal conductivity of air
    D = outside pipe diameter
    C,n = constants dependent on Re (can't remember their name off-hand, see chart)

    Reynolds Number C n
    0.4-4 0.989 0.330
    4-40 0.911 0.385
    40-4,000 0.683 0.466
    4,000-40,000 0.193 0.618
    40,000-400,000 0.0266 0.805

    Are these formulas appropriate (natural convection in particular as this was given to me by a colleague and I have no clue where he got it)? The forced convection is from a text book (which I don't have at the moment), however the book did not specify the orientation of the pipe. If anyone could chime in, or maybe provide a link to a 'how-to' on calculating convection around pipes that would be great, thanks!
    Last edited: Feb 24, 2010
  2. jcsd
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