3 replies to this topic

[jonlin]

Hi all,

I am trying to establish if a cooling leg is long enough. I am trying to calculate the overall heat transfer coefficient of the pipe. I can easily get the convective heat transfer coefficient inside the pipe, the conductive heat transfer through the pipe wall, however I am having some difficulty getting the external convective heat transfer coefficient.

The only item I am missing to complete my calculations is the Reynolds Number. How do I calculate the Reynolds number of air outside of a pipe? I suspect I need to use the hydrualic diameter, however all the literature sources only seem to show me how to calculated hydraulic diameter for internal pipe flow. I need to calculate it for external flow over the pipe.

Thanks all!

[breizh]

Hi ,

For natural convection , you can use the correlation from Lorenz :
Nu =0.55 *(Gr*Pr)^.25

Hope this helps
Breizh

[jonlin]

Thanks Breizh,

I am actually factoring in wind passing over the pipes at a known velocity. Hence I think forced convection would apply in this case.

[srfish]

There is a section in Process Heat Transfer by Serth that uses the pipe diameter for calculating a Re number. The section is " Forced Convection in External Flow". It starts on page 2/62.