Flue gas as external or internal flow? 2

[crissmyrn]

Hello everyone,
I need your opinion on sth. I need to heat air with combustion flue gas. All other things been the same, what would be best: have the air in pipes and the flue gas outside the pipes or vica versa? Assume a theoretical geometry - I am just asking what's the best practice with regards to heat transfer effectiveness.
Thank you all. Warm regards,
Christos

 

[kfoh]

Hey Christos,

if I understood you right, you want to know whether the flue gas or the air should flow in the pipes or the shell respectively?
From my point of view it is dependent on many factors what the best configuration should be, however in pure terms of heat transfer the gas with the smaller heat transfer abilities should flow in the shell, since there are more options to compensate by using additional surfaces, like fins.

You can evaluate that by first assessing the Prandtl and Reynolds numbers of each fluid in the pipe. Then you can use the darcy friciton factor to determine the nusselt number. With this, and the thermal conductivity you should be able to determine the heat transfer coefficients to make a decision.

kind regards

 

[crissmyrn]

Hi Khof,
Thank you - your answer is very helpful. I will calculate the heat transfer coefficients.
Regards,
Christos

 

[EdStainless]

The other thing to consider is the pressure drop that can be tolerated by each flow, since the shell side will usually have lower pressure drops than the tube side.

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P.E. Metallurgy, Plymouth Tube

 

[gruntguru]

Perhaps also consider the insulation requirements. If the average temperature of the air is significantly less than the flue gas it may make sense to run the cooler fluid (air) in the shell.

je suis charlie

 

[racookpe1978]

1. Dirt, scum, water vapor, soot, and "crud" All maximum in the flue gas, right? So, you,d prefer to have the flue gas (the dirtier side) to be in the smoother (easier to blow clean) side of the air-air HX. Cooler, cleaner, no dirt, no corrosion all on the air side, right? So put the fins on the air side.

2. Insulation. Flue gas = hot, air side cooler, so put the flue gas "inside" the air side to maximum the heat exchange,and to minimize losses from the HX to the room. You'll need less insulaiton, and your HX walls will be cooler (less stress and less corrosion.)

3. Fans. You got to move both air flows, but your air-side fan will be easier to service and to replace than the corroded and degraded flue side fan. So, put a "clean air" blower on the furnace on the inlet side, burn the pressuized (somewhat) air and fuel, and let the hot exhaust gases naturally flow out and to the HX after the burners and up the stack.

Put the air-side fan on the air side and you have two cooler, cleaner, cheaper-to-run fans.

4. Put the fins on the air side in the clean air.