Thanks Daniel for the input - if we make the problem a bit complicated - say fins are attached at the outer periphery of inner tube. Now ??

Hi Daniel, Hakeem,

For your application daniel the use of the hydraulic diameter is correct for overall heat transfer in the annulus (even if you may have to switch for inner diameter for let's say local heat transfer consideration).

Hakeem if fins are attached at the outer periphery, it's a little bit trickier : you would want to go for the formulea bellow :

dHe = [(df^2- do^2)/2p + df*t/p + do(1 - t/p)] ÷ [1 + (df - do)/p]

with

df = external diameter of fin

do = external diameter of bare pipe

p = face-to-face distance between adjacent fins

t = fin thickness

Hope it helped a bit :-)

Dear Fahd Boukrouche - Its really helpful to me - Could you please give me the reference/source of the formula

Here is the source :

http://www.eng-tips.com/viewthread.cfm?qid=248736

You may want to have a look at that paper also (formula 7) for an other expression of Dh (maybe more accurate)

Article • Saad A. El-Sayed et al, “Experimental Study of Heat Transf...

Dear Fahd Boukrouche - I have more one question which is beyond the scope of the base question - What will be the characteristic length of the shell region of a shell-and-tube configuration ? We can make the problem simple - no fins, say 2 or 3 tubes.

You can use the more general form of hydraulic diameter, which equals four times the net flow volume divided by the wetted area (take care: volume and area not area and perimeter)

The net flow volume is volume of shell - volume of tubes - volume of fins...etc

The flow area can be all contact area, outside surface of tubes, fins and inside surface of shell

Regards