Natural convection outside helical coil

[mfqd13]

Hi everyone,

I'm ending some calculations of a helical coil immersed in a liquid solution.
Inside the tube runs superheated steam that condensates along it. Here i don't have problems, even though we're talking about condensation.
My problem is to calculate the heat transfer coef. outside the Helical coil inserted in a vertical tank with 80m3 and aprox 8m height. To determine the heat transfer coef. outside the helical coil (inside the tank) i've used natural convection outside an horizontal tube. This isn't in fact what i have, but is an approximation, and as we all know, the experience in this kind of calculations matters, so the overall coef. seems ok and between normal values for this kind of applications.
BUT, i would be much pleased if my calculations were 100% correct, so i've searched a lot in every reference book like Incropera, Holman, VDI Atlas, Steam, Kern, etc and also here in ENG TIPS in other forums, but when appears a reference to a helical coil, it's often related with heat exchangers in counter-flow (witch isn't whats going on here because i have natural convection) or the approach is often related with inside coil heat transfer calculations.

Is it possible for someone to give me some references to look in to determine the natural convection coef. outside a helical coil?

Thanks!

 

[MisterE2]

upright helical coil or vertical?

Also, it doesn't sound like a hand calculation will work all that well there, you have a lot of different parts to the convection process in the coil, in a section of the coil, water will heat up and rise to another section of the same coil which will be heated up even more affecting the solution to that new section of the coil. If the coil is copper, then you can make some assumptions like, the coil can be treated as constant temperature.

 

[mfqd13]

I don't know what you mean with "upright", but i think it is vertical.

What tou say about the convection phenomena is correct and thats why i asked for some help with specific formulas for this application.
The tube material is SS 316.

Thanks

 

[MisterE2]

vertical makes it a bit more defined, but it now has a entirely new set of problems, now the material isn't a uniform temperature, steel has a thermal conductivity of about 25, whereas copper is 400. To answer the question of why there isn't a formula, you have a lot of different sections of helical coil which all have different temperatures dependent on the surrounding fluid and rate of flow through the tubing, and you still have the problems above. You won't find a formula because there are too many variables, which require lots of solutions for each wrap of the helical coil, then the solution of the next wrap of the helical coil is dependent on the lower wrap, the lower section's heat transfer will continuously insulate the upper sections heat transfer via thermal insulation. I would say just simulate it, I think ansys has something for it.

Though, if you want a basic overestimate of the heat transfer, you could do a metal hollow tube in a tank of water with natural convection.

 

[mfqd13]

Dear MisterE2,

Thanks again for the reply!

I agree 100% with you. This is a specific phenomena that varies with a ton of things like dimension of the coil, outside fluid properties, etc... So i've searched a lot and in several books, articles and foruns the conclusion was the same that we are facing now.
I also agree with you that in this specific situations, if we really want to obtain exact results we need to simulate this in a software.

For pratical situations in industry, of course i will continue to use my calculations that have been working until now, but in the future i will give a try a simulation in a software like ansys or autodesk cfd to compare the results.

Some day, when i try to put this in a software, i will share some results here. For now i think this post is finished.

Thank you once more for the replies!