Convection over horizontal pipe in air

[hemi_fan]

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)

where:

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

where:

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 textbook (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!

 

[Chestermiller]

I would evaluate the heat transfer coefficients from both equations and see how the values compare.