Firetube Heat Flux Calculations

[engpes]

I have what seems to be a simple question, but I cannot seem to verify.

I am trying to calculate the fire tube heat flux in accordance with API 12L. My question is:

1.) When calculating the flux area on a "U-type" fire tube, can you account for the entire length of tube on both passes?

For example if I have a U-type fire tube that has a total length of 20 ft (10 ft one way and 10 ft back), can I use the entire 20 ft for the area calculation?

Thank you in advance.

 

[EnergyMix]

Generally you don't count the U bend area.

 

[engpes]

I am not really after the U-bend, I am really looking at the return straight run of pipe 10' long.

Thanks,

 

[EnergyMix]

You count the straight length of tubing, but I've not normally done it as "one length of 20 feet." I normally treated each pass on its own, i.e., 2 10 foot lengths. That's why I made the comment about the U-bend. I know some people try to add the U-bend area and claim the heat transfer that occurs in that area (and there are formulas to calculate that). But it isn't the same as the heat transfer in the straight part of the heat exchanger (and, depending on the design, you might get limited to no heat transfer in the U-bend region.)

So it ends up being what is your effective heat transfer area: Is it really 10 feet of tubing times two or does that 20 feet also include the U-bend area?

 

[engpes]

It is 10 feet of tubing x 2 (there and back). Is is 20 feet of straght run (10 there and 10 back). I am not counting the area in the turn.

Thanks!

 

[EnergyMix]

Yes you need to count both straight sections. Not trying to confuse you (sorry if I did.)

 

[engpes]

Not at all. Thank you very much!!!

 

[chicopee]

I would be very careful about having U bends in fire tubes. Tube erosion from the flue gases and cracking from thermal expansion can be problematic.