Camping hot tub. Engineering problem?

I'm trying to build a camping hot tub. I've done it roughly before, just by a coil of gal pipe in a hot fire and water pumped though with a little electric pump.



It's great for the kids to have a nice hot soaking in before bed, and then the adults can get in. All good fun. Takes a few hours to heat up 600 litres of water to 40 degrees. '



But I want to do it properly. So I bought some 3 mill steel and some 1 inch gal pipe and fittings. A bit of flu from bunnings and welded up a heater. Fitting the gal pipe and fittings inside the steel box,...





Finished off the heater. Painted with high temp paint,.....





Gave it a test run yesterday,....





It did work, but all the pipe conections are leaking too much. I stuffed up by not putting pipe sealant on all the joints. With all the heat of the fire, I thought normal pipe sealant wouldn't do the job and would just melt. I actually suspected it would be a problem, but I wasn't worried, because I just thought I'd weld every joint up if there was leaks. In a test run there were leaks, so I did weld up every joint. However I forgot how bad galvanised steel is to weld. It doesn't weld clean, it has bubbles and pops and spits and as I'm not a great welder I couldn't get the joins to be water tight.

I'm also hoping that it will work using the principal of thermosiphon (the pumping action created by rising hot water), so therefore not needing to be pumped. But that also calls for an airtight, water tight seal.

So now I hear there is high temp pipe sealant. Can anyone recommend a pipe sealant that would withstand being inside a fire? Mostly the joints wouldn't get real hot as in theory there is water running though them, however there might be a time when the system is dry and temps could be above 1000 degrees?

The old pipes are no good now, as they've been welded. I've pulled them out and ready to replace with new gal pipe, as soon as I find a pipe sealant that would do the job.


See ya's.
 
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Coiled copper pipe? No joins then.

BTW top idea!


Copper would be great if you could guarantee the system will always have water flowing through it. However I don't think that will be the case. What about when the firebox is still half full of wood and raging hot, and the tub temperature is getting close to 40 degrees [And your on your sixth beer] and you need to shut it down? Easiest thing to do is shut off a few taps and let the water out of the heater system. Copper pipes would, could possibly melt. Gal steel pipe, no worries. I will have taps in place to let the water out.

It is a bit of a fine art in getting the water to the right temp, and being able to deal with the fire that's still going.

And of course you can't shut off taps without letting the water out or there would be an explosion and scalding hot water going everywhere.


See ya's.
 
What a great thing to take camping.
I would grind off the gal - using a wire brush on the exposed thread - and weld them properly. The water won't be under pressure. Can you do a test on one joint and see it that works?
 
Nice effort!

Gal tube won't work. The heat of the fire would burn the galvanising off, creating a heap of noxious gases in the process (could you smell it? Maybe not if you kept running cold water through it and the fire wasn't directly on it) and then you're just left with some dodgy pipe. Go for a mild steel of decent thickness instead. And bring a spray pack of cheap veggie oil or similar so you can coat it between uses to stop it rusting - like seasoning a cast iron camping pan. Put stoppers in the ends of the pipe so air can't get in when you store it. Maybe after pouring some oil in for good measure. FYI boiler tube in power stations is all steel for this reason but then the water is treated.

Thermo-siphon won't work either - or it will take ages - or you would need a massive heat exchanger at lower temperatures in the flue - pick any of those. It works for hot water systems on domestic house fires during winter because they don't mind a slow heat up time and you don't have as much water to heat. Also creates the potential to boil the water in the pipes which creates pressure (ie a boiler) and can melt the pipe, depending on how close the fire is. A charcoal fire in an enclosed contraption like that can certainly get proper hot. So I'd definitely go with the small water pump.

Best way to shut down would be to shut off the flue and air intake to the fire, if it's airtight?
 



Not sure what the heat range or flow rate would be in that set-up ,stainless steel marine grade 316 welded would give you a better heat transfer,that's a good set-up..
 
Great setup!!!

What if the hot box had a floor so that the pipes could be immersed in a water bath. Providing pipes remain immersed, the temp of the pipes could never get above the boiling temp of water (100 degrees).

Would also make it easier to regulate the flow of water thru the pipes to a uniform temp.
 
Great setup!!!

What if the hot box had a floor so that the pipes could be immersed in a water bath. Providing pipes remain immersed, the temp of the pipes could never get above the boiling temp of water (100 degrees).

Would also make it easier to regulate the flow of water thru the pipes to a uniform temp.

And no problems with leaks.
;)
 
And no problems with leaks.
;)

Your heat transfer efficency wouldn't be high enough. You would struggle to get the bath water up to 20degs.

Though we have done a similar thing with a hot pot. Basically recycled water through a large boiling pot. Again it's hard to get the temp right, and is no where near as efficient as this set up.

Your set up looks neat but personally there is no way I would ever take something so bulky camping.

I also think stainless or copper would be the way to go. How did they used to do it 'in the old day' I'm too young to remember. :eek:
 
Thermo-siphon won't work either - or it will take ages - or you would need a massive heat exchanger at lower temperatures in the flue - pick any of those.


There's plenty of thermo-siphon water heaters available on the market. Cost a good few thousand dollars though.


This is a dutch tub,....



The problem I see with this design is that smoke and ash would be going everywhere. A bit primitive I reckon.



This is a Chofu Wood-Fired Water Heater,....



All stainless steel. Good setup. Better pic here, from the other side,





Thermo-siphon definitely works. I'm just trying to build my own setup.



Best way to shut down would be to shut off the flue and air intake to the fire, if it's airtight?


Yep, I can shut off the air intake on my heater. However after going for a few hours the bottom of the heater and the bottom level of pipes will always be covered in a few inches of red hot coals. It will heat the water for a long long time after shutting off the air.

The ideal situation is to time the fire to perfection and have it nearly died down as the water in the tub hits 40 degrees. Then it will keep it at that for a few hours. But that will be hard to do. What if a drunken mate decides to pile a heap of wood into the fire? You have to have a contingency plan for that? You need to be able to let the water out of the heater sometimes when things don't pan out. And that means everything will be dry and get molten hot.


See ya's.
 
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Great setup!!!

What if the hot box had a floor so that the pipes could be immersed in a water bath. Providing pipes remain immersed, the temp of the pipes could never get above the boiling temp of water (100 degrees).

Would also make it easier to regulate the flow of water thru the pipes to a uniform temp.


The pipes have water in them. I don't see what you are talking about. The fire is going in the bottom of the steel box. There is no floor in my box, but pipes running across the bottom. The beauty of this is that in the morning you lift it up and the ash falls out.


See ya's.
 
or you would need a massive heat exchanger at lower temperatures in the flue - ?


I've looked at getting energy out of the flue. Problem with the flue is it's too high. Thermo-siphon will only work if the plumbing is below the water level in the tub.

You could get heat out of the flue, but it would have to be pumped. I want to do away with the pump if possible. One less thing to go wrong.


See ya's.
 
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From what I understood he was saying have the pipes running through a tank full of boiling water. Rather than having the pipes heated directly by the fire. However as I said the heat transfer would be too inefficient. You would hardly heat the bath water.

Blacky
 
I was assuming you wanted hot water quicker than 4-5 hours, which is what a thermo siphon would take on a hot tub the size of your first photo. I also suggested mild steel because it's a lot cheaper and easier to weld than stainless.
 
I was assuming you wanted hot water quicker than 4-5 hours, which is what a thermo siphon would take on a hot tub the size of your first photo. I also suggested mild steel because it's a lot cheaper and easier to weld than stainless.


OK. I know it will take just a few hours when pumped through? Perhaps it will take longer by thermo-siphon?

Yep mild steel would be fine. I just bought the gal pipe as that's what I saw in the local rural store. I've been trying to work out how to get it all bended into the right shape to fit in a metal box. When I saw the gal pipe a light went off and realised I didn't need to get anything bent. But now I gotta stop it from leaking when it's dry inside a box if it get to a thousand degrees?


See ya's.
 
From what I understood he was saying have the pipes running through a tank full of boiling water. Rather than having the pipes heated directly by the fire. However as I said the heat transfer would be too inefficient. You would hardly heat the bath water.

Blacky


Yep. Gotcha. We already have a beer keg that we use for hot water. Tip cold water in the top, it goes down a tube to the bottom, and hot comes out the spout at the top. Works great for small amounts of water. As you said though, very inefficient and hopeless for what I'm talking about.

Gives enough hot water to give toddlers a bath.




See ya's.
 
The pipes have water in them. I don't see what you are talking about. The fire is going in the bottom of the steel box. There is no floor in my box, but pipes running across the bottom. The beauty of this is that in the morning you lift it up and the ash falls out.


See ya's.

Have the pipes in a boiler. Like putting a radiator in a kettle.

Pump the water through the radiator (your pipes) they can now be sealed so that there is no direct water exchange between the boiling water on the fire and the hot tub water being circulated by a pump.

Water in circulation to the pool should never reach boiling point (but could).


Your mates stoking the fire will only boil the water in the tank at the fire and in likely put out the fire when the water boils over.

Problem will be replenishing the boiling tank on the fire.

Cheers

PS seems I am slow at typing you understood the drift.
 
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