Finned tube thermal design 1

[AndreChE]

Dear all

I want to reuse a heat exchanger to solve a plant problem by cooling a heavy hydricarbon with CW but I might have a problem with it.

Its design temperature is 149ºC (300ºF) and operating temperature of the hot fluid is 155ºC. With the pipe layout and length I was able to estimate a 5ºC temperature loss from the source to the HE inlet.

So, we are at the temperature operating limit but the design is not good. Since I don't have another suitable HE and to built a new one it will take some time... I had the idea of install a portion of finned tube from the source to the tie-in point of the line.

However I'm not able to calculate the heat/temperature loss of such a finned tube.

Where or how can I solve this equation. I'm not so familiar with finned tube thermal design. Is there any manual,good book o internet software that might help me?

Regards,
AndreChE

 

[sshep]

Wouldn't it be easier to just go for a rerate of the exchanger? 300F seems so low as to make a successful rerate highly likely. -sshep

 

[AndreChE]

Sorry...what do you mean with rerate the exchanger?
Submit new design conditions to local authorities with an independent organization qualification report?

I already had that in mind...

 

[sshep]

Hello Andre,
As a chemical engineer myself, I am usually in a situation like you- I know what the minimum rating needs to be, but am not clued up on pressure vessel codes or qualified to do code calculations myself. This is must be done by a professional mechanical engineer certified in your state (assuming you work in the USA). If the exchanger records exist and inspection/repair/alteration history is available, then he will be able to take care of everything.

This exchanger seems to me like an easy candidate for rerate as the original design temp is seems quite low, and the new requirements are within 50F. A simple internet search give the following rerating process overview:

http://www.carmagen.com/newsletter/news17.htm

http://www.carmagen.com/newsletter/news4.htm

I am sure you can easily find even better information- these forums must be loaded with mechanical engineers who can do this sort of thing in their sleep so advice is readily at hand. You will also be able to sleep much better if you don't have to trust to a makeshift finned pipe solution.

best wishes always,
sshep

p.s. Some advice: With these type fast process modifications (reuse of old equipment in new service) any needed relief system requirements often get overlooked until the very last- don't get caught at the last minute by something like that.

 

[GL431]

I would use the same approach as you used to estimate the temperature drop in the bare pipe, substituting the fin surface for the bare tube/pipe surface and taking an overall transfer coefficient of 40-60 W/m2/K say.

As a busy engineer it normally does not pay to go into too much detail into these kind of calculations (if you do not have the right calculational tools). If you require any level of detail or accuracy you should use the HTRI Air Cooler Rating programme (or a programme like it) or ask an engineering contractor to do it. That is if HTRI has the natural draft option (cannot remember whether it does that with any accuracy).

If you are going the finned tube route, what does your technical surveillance authority say? After all, you would be modifying a pressure bearing part.

Uprating your heat exchanger would be a good try. Is 155°C against CW a good solution? Normally I would limit CW use to process streams of max 60°C for fear of calcium deposits, water boiling in the exchanger, organisms growth, etc. So there are some aspects to look at there. Also, if it is a fixed tubesheet design, is the heat exchanger designed to take the thermal expansion loads (temperature differential limitations)?