large diameter corrugated HX vs lcoil ength

[pybyr]

I know I already raised it in my most recent post in a different thread, but I'd really actively invite the input of the excellent minds on this group (who may already think that concept got settled as to the basic suitability of corrugated stinless) regarding the question of whether a shorter length of large diameter corrugated tubing (one and three quarter to 2+ inches) would work as well or better as an in-tank HX than the "usual" long spiral of relatively small diameter (1/2 or 3/4) smooth copper.

https://www.hearth.com/talk/threads/21362/

If this "short and fat instead of skinny and long" HX coil would work on a "physics" level (and I invite input on how to figure the optimal or needed length, diameter, and ratios between for a 150 kBTU boiler into a 1350 gallon tank) then the shorted length might offset the higher cost of the corrugated stainless

Thanks

 

[DaveBP]

I walk past my propane tank out back several times a day and never thought of CSST for heat exchanger tube. One of the great things about this forum. I'm tossing around DHW schemes myself. HX's in the pressurized storage tank? Sidearm indirect? Superstore Indirect? Never thought of corrugated gas tube.
First impression was that the corrugations are pretty narrow and that the water flowing through might not eddy out into those 'grooves' to pick up the heat and bring it back into the flow. You can stretch the tube to open up those corrugations so water could get out there and back in more easily (and get some free extra length with every purchase!). It will stretch quite a bit if you're not coiling it up into a small diameter helix to fit into a flange opening.
As to larger diameters, yes you get more surface area with larger diameters but you get even more volume to exchange the heat from. For equal lengths, double the diameter and you do get double the surface area but you get four times the volume to extract heat from. Plus, with larger diameters the water is more inclined to run up the middle and never get out to the OD to do any exchanging. Smaller diameters are more effiecient in terms of surface-to-volume ratios but if you go too far in that direction the friction and velocity (higher velocity means less time in the exchanger to swap heat) bite you. I don't know enough fluid dynamics to know where the golden compromise is. You just end up using a bit of SWAG either way, I think.
I still want to think about it some more.