# Make a 22mm copper pipe into a coil??



## Old Beaver (23 Nov 2021)

My son is making a hot tub. He needs to make a 3 metre x22mm copper pipe into a coil for heating the water. I needs to be about 350mm diameter. 

Anyone know how to make it without collapsing the copper pipe??


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## Cabinetman (24 Nov 2021)

Not an expert and never done it but I think you fill it full of dry sand, bung up the ends and it won’t collapse as you bend it, probably best to anneal the pipe first, that will make it easier to bend. He could do several lengths of micro bore jointed from the same starting point, the heat transference would be far better, and can be bent without too many problems. Ian


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## Just4Fun (24 Nov 2021)

Even with a pipe bender 22mm copper takes significant effort to bend. Without one I would not be confident of producing a decent coil. At the very least I would want a cylindrical former to bend the pipe around.

You can get springs to go inside the pipe to prevent kinking. If you fill the pipe instead, use salt or sugar instead of sand so any remnants you don't get out will dissolve rather than circulating around to damage a pump or whatever.


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## Phil Pascoe (24 Nov 2021)

You can or at least could buy copper pipe in coils that was much more flexible, but even that I doubt would go to 350mm diameter.


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## Fitzroy (24 Nov 2021)

As Cabinetman alludes to, 22mm copper pipe will have pretty poor heat transfer properties, even industrially we don’t go above 19mm and 14mm is more common. The effectiveness is very non linear and starts to drop steeply beyond 19mm.


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## Fergie 307 (24 Nov 2021)

Flatten one end and solder it up. Then fill the whole length with kiln dried fine sand. Then solder a cap on the end. Now you need a former, wood will do, drill a hole in the flattened end and screw or bolt it to the former. Then use a blow torch to heat it up as you bend it around the former. Afterwards you just cut the ends off and pour out the sand. You can clean the inside using a suitable sized bottle brush, just attach a long enough bit of string and pull it through. A tip I was shown years ago for getting a pull string through pipe conduit is to tie a piece of foam to the end of the string, shove it in one end of the pipe, then use a vacuum cleaner on the other end of the pipe to suck the foam through with strinng attached. Have done this with 15mm and it works well, dont see why it wouldnt work with 22.


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## chris.s (24 Nov 2021)

Soft coil copper pipe from refrigeration suppliers is what is required to make coils except it isn't 22mm as still in inch sizes 1/4 to 7/8 and connections made with flared nuts so tool required.
Best to look for a refrigeration company thats not booked up installing domestic heating systems or cut a coil out of an old hot water tank as it heads to the scrap yard replaced by a new heat pump system.

ps. or look at solar heating suppliers for ready made coils eg. Solarcoil.co.uk Manufactures of solar retrofit coil heat exchangers


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## paulrbarnard (24 Nov 2021)

You can but 10mm pipe in a coil rather than straight. Wednesbury Wednesbury Microbore Copper Pipe Coil 10mm x 10m
Inter leave three lengths of that and join the ends together with standard fittings.


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## Sachakins (24 Nov 2021)

Dry sand method best, but I would go with 15mm max.


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## glenfield2 (24 Nov 2021)

You can buy soft copper certainly up to 1/2in - used it on our canal boats. Regular 22mm copper is hard to bend even with a bender - and you couldn’t do coils that way anyway. Quality seems to vary too - we got some from S***wf** a while back and it wouldn’t do 90deg bends without creasing whereas tubes from another source bent fine.


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## Fitzroy (24 Nov 2021)

Love some early morning calcs. Overall heat transfer coefficient with pipesize in this application is estimated at:
22mm : 1105 W/m2/°C
15mm : 1492 W/m2/°C
10mm : 1726 W/m2/°C

However as you can see these are per unit of surface area and smaller pipes have less area per meter of pipe length. Assuming a 30 degree average temperature difference from hot side to cold side (ie coil at 70°C and hot tub at 40°C) the heat input per meter of pipe works out at:
22mm : 2.2kw
15mm : 2.1kw
10mm : 1.6kw

The original 3m of 22mm would equate to 6.6kw of heat input, to achieve this same heat input you would need 3.3m of 15mm pipe and 4.3m of 10mm pipe. The two caveats for 10mm pipe are:
- At 4.3m of length you could struggle with flowrate through the pipe, and you would be better with to 2.15mm lengths in parallel rather than one long lenth
- 10mm pipe has little structural strength and you would need to protect this from someone leaning on it etc!

A coil of 22mm pipe will also look pretty cool, IMHO, so there is the aesthetics to consider.


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## Old Beaver (24 Nov 2021)

Fitzroy said:


> Love some early morning calcs. Overall heat transfer coefficient with pipesize in this application is estimated at:
> 22mm : 1105 W/m2/°C
> 15mm : 1492 W/m2/°C
> 10mm : 1726 W/m2/°C
> ...



Wow! that was something I could never work out. I think a dual or parallel system, two feeding pipes would be good too. Thanks,


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## xy mosian (24 Nov 2021)

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.
I have fitted a pre-made coil similar to this to a hot water tank for heat transfer from a central heating system. I cannot for the life of me remember what it was 'politely' called. There may be a variation somewhere which would be useful.
geoff


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## Fitzroy (24 Nov 2021)

Old Beaver said:


> Wow! that was something I could never work out. I think a dual or parallel system, two feeding pipes would be good too. Thanks,


It's my profession, or was before I became a middle manager, but I can still vaguely remember how to use the software for simpler tasks, and it's good to stretch the grey matter occasionally for technical tasks.


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## Jones (24 Nov 2021)

Soft copper pipe as used for air conditioning units comes in a handy coil that probably wouldn't take much effort to coil tighter, or simply get an old hot water cylinder and use the heat exchanger coil from that, you might be able to buy them new for fitting when solar panels are added.


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## guineafowl21 (24 Nov 2021)

350mm diameter means a circumference of about 1.1m so you won’t even get 3 full turns in the coil. You’ll need a stout, secure form to haul the pipe around, even if you anneal it, which will require a reasonably powerful heat source.

I made this wort chiller for cooling beer ready for the fermenter, from 10m of 10mm copper pipe. I didn’t need to brace the pipe internally; just wrapped it round a strainer post. It worked well, but I now use a counterflow exchanger which is far more efficient. Relying on convection for heat transfer is slow - ideally both fluids should be pumped.


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## Fitzroy (24 Nov 2021)

My analysis was based on 1m3/hr, so roughly equivalent to a central heating circulation pump.


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## Henniep (25 Nov 2021)

This may have been said using other terminology but there is bendable and non-bendable copper tubing (this may be the 'soft' type that some have referred to) The bendable type generally has a thicker wall and is more mallible.


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## TRITON (25 Nov 2021)

Old Beaver said:


> My son is making a hot tub.


Would this be a hot tub that looks very similar to an alcohol still


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## hanser (25 Nov 2021)

Some thoughts from me:
I think annealed soft copper pipe is what you should use. Some here to 15mm Copper Tube - Metric - Annealed Soft BSEN12449CW024A-H050 - 10m Coil.

You could google youtube along the lines of 'copper tube coils' how to

You find the sort of coil you want to make in hot water cylinders - do you know a plumber/scrappie that could let you have a half decent cylinder - per chance you could repurpose the coil?


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## Spectric (25 Nov 2021)

Go down a scrapyard and look at the old copper hotwater cylinders and they have a copper coil inside that may be what you want. Failing that why not use the plastic plumbing pipe which comes in coils anyway, and will take water at 70 degree's C.

Failing all this then why not use a cast iron hot tub and have an open fire underneath, just be careful or you could cook.


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## Ozi (25 Nov 2021)

Just a thought, it doesn't have to be a round coil. What ever diameter of pipe you chose buy a standard pipe bender and make a square or hexagonal coil - you may curse me later.


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## Dave Moore (26 Nov 2021)

Hi,
I’ve sand loaded and bent standard lengths of copper pipe years ago at work but I wouldn’t recommend trying this way to make a coil. I think it would be better as suggested to use a soft copper coil being careful when opening it out to the diameter you want as copper work hardens. We use silver solder for our work as it takes a higher pressure and we are working on Oxygen lines.
Regards,
Dave


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## Myfordman (26 Nov 2021)

Has the effect of hot tub chemicals on copper pipe been considered before effort is put into bending?


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## jonn (26 Nov 2021)

Old Beaver said:


> My son is making a hot tub. He needs to make a 3 metre x22mm copper pipe into a coil for heating the water. I needs to be about 350mm diameter.
> 
> Anyone know how to make it without collapsing the copper pipe??


1. Anneal the pipe - properly.
2. Make a jig like shown in the picture of some hardwood. See to that the flanges are close to the *diameter* of the pipe.
3. Put the cooled annealed pipe in the tool, and start bending. The flanges will prevent the pipe collapsing.
4. Take your time, don't force the bending.
And yes, it works very well. Used similar ones to bend brass piping for fuel lines on RC model airplanes.
PS: Don''t put sand in the pipe as that will have the opposite of the desired effect ☹


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