Heat Exchanger - Air to Air

[interface222]

Does anyone have experience with Air to Air heat exchangers and some nominal values for design. I am attempting to design a low volumetric flow heat exchanger. The diameters are fairly small. Does anyone have experience in this? I am getting large numbers for the required length. Is this true for Air to Air recuperative exchangers with small diameters and low volumetric flow?

Many thanks

 

[speco]

Interface222,

The overall U value obviously depends on a number of factors, especially the Reynolds number. I have worked with a number of them, mostly as recuperators operating at about atmospheric pressure. I would expect to see an overall U value in the range of 6 BTU/(hr-ft^2-Deg F)with decent velocities, and lower if the velocities are low.

This would be based on a round tube surface in a exchanger with one stream flowing transverse to the tubes.

Regards,

Speco (

http://www.stoneprocess.com)

 

[25362]

Speco is right. One thing to remember is that the overall heat transfer coefficient OHTC is never greater than the lowest tube-side or shell-side individual HTC.

From my sources the OHTC for industrial air preheaters are in the range of 14 to 20 kcal/(h.m².^oC)

 

[interface222]

Thank you. I appreciate your responses.
I'm using a recuperative heat annulus heat exchanger with low velocities.

I think finding the correct Nusselt number is of course my largest source of uncertainty. But my results are believeable.