Plain Tubing Vs. Fin Tubing

[millsjj]

Hello,

I am currently investigating the redesign of a heat exchange unit that uses SS helical coil for heat transfer purposes. One avenue of investigation I am reviewing is to use fin coils instead of the standard 1.25" tubing to reduce the overall length of the required coil whilst giving similar sq.ft of heat transfer area. I have had an extensive search online but cannot find anything to give a comparison between fin and non-fin pipe. Does anyone have any info on this subject or could point me in the right direction please?

 

[TD2K]

http://www.wlv.com/wp-content/uploads/2014/06/databook3/DataBookIII.pdf

Start here, it's more complicated that just thinking "5 ft of finned coil = the surface area of 20 ft of plain coil so my heat transfer will be the same". How successful fins will be will depend on the fluids and the anticipated heat transfer coefficients on each side. Process air coolers for example use fins on the outside because the heat transfer you get with air is relatively poor so the additional area of the fins compensates for the low heat transfer coefficient.

Now if you had steam condensing on the outside of tubes to heat say molasses, fins on the outside likely aren't going to help you since the steam side is likely going to have a far greater heat transfer coefficient than the molasses side.

Fouling is another factor to consider. If the outside isn't a clear service, fins may not be a good idea.

 

[georgeverghese]

Also, fins need to be corrosion resistant. See also Process Heat Transfer by DQ Kern for finned heat transfer. Also in Perry Chem Engg Handbook. Also beware of vapor blanketing limits on heat transfer rate if these fins are in vaporising service.

 

[racookpe1978]

IF two fluids (not gas-to-gas) the much greater flow resistance of the added fins on the fluid side needs to be considered.

It may require a pump change if fluid. Probably not a fan change if gas, but that's a possibility.

 

[millsjj]

Hi Guys,

Thanks for the responses. To shed a bit more light the heat transfer is for hot water to cold water inside of a tank with hot water flowing through the pipe in question. I understand that it is not just a case of matching sq. footage, as the fins cannot be treated as direct heat exchange surface, but I had thought there would be some measure of conversion rate to standard pipe of the same dimensions.

As I am dealing with the same medium with decent heat transfer and also there being no issue with the added flow resistance of the fin tube I would consider the design of merit. It is more a case of how the two compare to each other in terms of performance.

Thanks

 

[dbill74]

With water on both sides of the heat exchanger I don't see the benefit of the fins, unless you are going to put fins inside and outside the tube.
If you are looking to replace an existing heat exchanger with a smaller foot print, consider using a "flat-plate" heat exchanger.

 

[millsjj]

Hi dbill,

Any chance you could elaborate further on your opinion as to why you don't see a benefit? I have been looking around and seen that fin tubes are used in water heat exchangers commercially but they dont seem to be the norm. This may be what you're eluding to in that it is not an overall cost effective method but still cant find anything definitive.

Currently reading through the suggest texts and from what I can see it would be a method of reducing the footprint. Unfortunately the plate HE wouldn't really be in line with the avenues I have been asked to explore at the moment so wouldn't really be an option.

 

[IRstuff]

How is the cold water circulating, if at all? That will make a large difference in the heating capacity if the water isn't flowing with any velocity.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[millsjj]

The flow of cold water will be intermittent and will for extended periods have no forced circulation.

 

[georgeverghese]

Would guess this finned design would only be of use if the tubes are lying on a horizontal plane, and fins are transverse fins - ie natural convection currents get to flow through the space between the fins - else you'd have a large fraction of the fins with no thermal currents going past it.
With SS coils in aerated hot water service, check with a materials selection engineer if chlorides will influence materials.

 

[Dougt115]

Unless you are making the finned tube yourself the manufacturer should have the information you want. It is a selling point to convince you to buy his product after all.