×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Flue gas as external or internal flow?
2

Flue gas as external or internal flow?

Flue gas as external or internal flow?

(OP)
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

RE: Flue gas as external or internal flow?

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

RE: Flue gas as external or internal flow?

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

RE: Flue gas as external or internal flow?

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Flue gas as external or internal flow?

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

RE: Flue gas as external or internal flow?

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources