effectiveness of a "penetrating fin"

[yazguli]

Hi
If you make a cut on the inner pipe of a double pipe heat exchanger or make a hole on it and attach a rectangular or a pin shaped fin on (in) it,(and of course, weld it) so that it is extended from inside of the pipe to it's outside,how would the fin effectiveness be changed?
I wonder if in this case the meaning of "base surface of a fin" is somewhat changed and that the surface of the inner part of the fin(or even a portion of it) could be considered as a part of pipe's surface,not being covered with something that would decrease the conduction heat transfer through the pipe's surface.
regards

https://files.engineering.com/getfi...35-af3e-39a7459aac15&file=20221201_055742.jpg

 

[IRstuff]

A picture might help make sense of your post.

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[yazguli]

IRstuff
I edited my post and sent a picture.thank you for your attention.

 

[IRstuff]

I think you are asking if the fin penetration of pipe wall changes interior fin's heat transfer properties; if so, then no. The entirety of the system's properties changes and would be modeled with fluid-fin; pipe wall, and external fin, since the heat has to travel through the pipe wall boundaries to get outside, regardless.

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[EdStainless]

It all depends on what part of the system is limiting heat transfer.
You have to model the resistance to heat transfer at each step in the system.
Those fins look more like they are for mixing and keeping the fluid temp uniform.
And they would be a bear to try to clean.

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[georgeverghese]

If you intend to increase the inside htc, look also for vendors who supply and design turbulators to increase tubeside Nre, but be wary of the results they may indicate. Many heat exchangers with these installed have not met the intended thermal duty in actual operation. In some OpCos, internal turbulators are not permitted. Tubeside pressure drop will also increase considerably. If at all used, it must obviously be in clean service on tubeside.

 

[yazguli]

Thank you, every one.
Let me explain what I intend to know and to do, a bit more.
In fact I’m going to make a double pipe heat exchanger to recover heat from the exhaust of a diesel generator as a project for my M.Sc. experimental thesis. (Of course I’m too old to be a student, but for some reasons, I am!)
I need to have a clear idea of what would be the optimum design .That’s why I’m asking these questions.
It is said that by attaching fins on a pipe you gain a larger surface for convective heat transfer, losing a smaller but hotter surface (fin base surface) which could be used for conductive (as well convective) heat transfer.
I wonder if this kind of fin would help “save” this surface.
The picture I’ve attached is of a rough prototype of the inner pipe. Fin length inside the pipe must be shorter.
First I intended to move the flue through the inner pipe, and the water to be heated through the annulus, but due to the references it is advised to move the gas through the annulus .In this case the inner pipe will be of smaller diameter (2 or 2 ½ inches, 50 or 65 mm) and the outer one about 6 inches to comply with back pressure requirements of diesels.
On the other hand, I expect this kind of fins could act as turbulators and baffles (as they move the fluid around) simultaneously.
I am aware of the pressure loss which will be imposed on both fluids, but I’m somewhat sure it will not be a serious matter of concern.
I am going to put a water tank above the diesel and make the best of natural convection of water. All of this is aimed to have some warm water to be used in construction sites in which diesel generators are the only sources of power.Inner tube will be removable to clean the soot.
Thank you for reading such a lengthy post.

 

[EdStainless]

If you are going for transfer from hot air (exhaust) to water then the fins need to be attached to the water containing tube and sticking into the gas containing one.
And with exhaust you have to worry about acid dew point corrosion so making that part of it out of 409SS might be a very good idea.

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[IRstuff]

How much of an improvement are you expecting? You seem to be putting the cart before the horse, analysis-wise. Typically the gas-side transfer is the weakest by far.

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[georgeverghese]

Brown Fintube offers off the shelf fintube double pipe heat exchangers - fins on the outside. Run the engine hot exhaust through the shell - it makes thermal design sense, since hot gas exhaust side would, on a plain surface, have a lower film htc than water on the tubeside. So with fins, the controlling htc may indeed shift to the water on the tubeside. If memory serves me right, they offer either longitudinal fin or transverse fin options.

 

[pierreick]

Hi,
This ppt presentation may give to you some hints.
Pierre

 

[yazguli]

Edstainless
Thank you for your reply
As you say, acid formation is a concern and the materials must be suitable. Of course I’m not sure the leaving gas temperature is going to be less than dew point temperature. Speaking about the fins, as the h of the gas is at least ten times less than that of water, the fins must be attached on gas side. My main question is “what if they go through the pipe’s wall to some extent and come in contact with water either?”
IRstuff
Thank you for your answer
If you mean heat recovery by ”improvement” it is a lot. A 265 kw (355bhp) Volvo diesel rejects 216 kw (12284 BTU/Min) of heat to its exhaust. If you mean “more enhancement by this kind of fins” compared with customary ones, it might be some or none. This is what I want to know. It is obvious that the fins must be in gas side. If you see longer fins inside the pipe, it is because at first I intended to move the gas through it.
Speaking about the cart, in some experiments it may happen, especially when you have no opportunity to know how the horse is standing!
georgeverghese
Thank you for your attention and information.
I have some experience in HVAC, so I am going to make the heat exchanger myself. Of course I am not obliged to.
pierreick
Thank you for the file. Despite some differences in specifications and applications (Max 300º F exhaust temperature in boilers vs 910 in diesels), it is useful.