https://www.sciencedirect.com/search?qs=double%20helical%20heat%20Exchanger&show=25&sortBy=relevance

You design it as any other counterflow heat exchanger. Since both flows will typically be turbulent, there is only a moderate increase in the heat transfer and friction due to the secondary flow effects (curvature effects), hence it is ok to dirsregard this effect. [For the internal flow there is a multituce of correlations for Nu and f for helical flow; for the annular flow there is a general lack of correlations. However, my experiments in the 1980s showed negligible impact of curvature on Nu and f for turbulent flow.) Regarding manufacturing, be aware that the tubes tend to become oval if the curvature radius is too small; also note that keeping the inner tube in position may need some additional considerations.

Dear Muddada G.

Your desired design is simple and you can depens on heat transfer book by Holman.

Good morning, all.

I note that some parties have not found a relationship between f factor (and hence I assume pressure drop) with curvature. Some books, e.g. Spiers, suggest otherwise (i'll try and get the reference) More critically, you will find that at 400C, radiation transfer between the walls (hence heat pick-up) is important. There are papers published on this in the years (¬ 1970 - e.g. Nature, 257, 367 (1975); Lloyd & Weinberg , also Proc. Roy. Soc. A (1978), Jones et.al.). The "pipes" were of rectangular cross section. More recently, better calculating techniques were published (¬ years 1990, ¬2000). Again I'll advise some references.

Regards Stephen.

Dear Muddada Ganapathi

I think this article can be useful to you.

Article Shell-side thermal-hydraulic performances of multilayer spir...

There are many researches on this type of heat exchangers. (analytical, experimental and numerical). You can find with a simple search on "tube in tube helically coiled heat exchanger".

Good morning Muddada Ganapathi,

You have above several techniques and sources to help you in your analysis of coil-in-coil heat exchangers. In your case, your flow rates are significant (approx. 41 kg/h for steam and 122 kg/h for flue gas) and your temperatures are high (exit temperatures of approx. 720 K for steam and 1470 K for flue gas). At these temperatures radiation heat transfer will be significant. The heat exchanger wall temperatures will be sufficiently high that materials such as copper may not be appropriate. Do you want to maintain heat exchanger integrity if the steam rate drops dramatically without a corresponding fall in flue gas parameters? If yes you will need to use a fairly sophisticated material for construction and many of them have high coefficients of thermal expansion. You do not state any fluid pressures.

With a coil in coil unit the model is a little simpler than the double spiral ("Swiss Roll") heat exchanger unless there is heat exchange between the individual loops of the coil and not simply between the inner and outer walls. You have not mentioned external (or any) insulation that you wish to use. Is heat loss significant?

I look forward to seeing how this develops (and I have not forgotten the references that I promised).

Regards Stephen.

Spiers seems to have been re-issued in 1977 as J.W. Rose, H.M. Spiers and J.R. Cooper, under the title "Technical Data on Fuel", publisher Wiley. Like my earlier copy it is out of print. My own copy gives the correction factor to be applied to the pressure drop through helical coils. There are subsequent proposed factors and a search will identify these - there is information in the literature. For the annular flow, as a first approximation, evaluate the pressure drop for a straight pipe and use the same correction factor for a helical coil but modified by using the hydraulic mean diameter appropriate to annular flow.

Have you considered how to fabricate these helical coils? A double pipe exchanger, with walls of reasonable thickness, will be difficult to bend! How do you achieve constant, accurate spacing? What are the design and operating pressures?

Regards Stephen