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Heat Exchanger - Air to Air

Heat Exchanger - Air to Air

Heat Exchanger - Air to Air

(OP)
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

RE: Heat Exchanger - Air to Air

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 (www.stoneprocess.com)

RE: Heat Exchanger - Air to Air


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)

RE: Heat Exchanger - Air to Air

(OP)
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.

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