Camping hot tub. Engineering problem?

[HiEquity]

Not sure about the cities, but around here at least most people 50 years ago or more had a water heater, heated from their slow combustion stoves and using thermo-siphon. They were called 'donkey' heaters for some reason. I vaguely remember them.


See ya's.

They still use theirs on the farm. And what's wrong with burning wood? It's a fully renewable resource and turns back into CO2 regardless of whether you burn it or let it rot on the ground. Much better than using gas or coal (electricity) if you don't have an air shed to worry about.

And quick to make when you have PTO powered bench saws available on the farm...

 

[The Fence]

And what's wrong with burning wood?

Critters habitat. sssssssshhhhhhh......greens will hear you.

Actually one must have a lic to cut firewood these days. No firewood allowed to be taken from roadsides as far as I know due to the above issue, critters habitat.

 

[topcropper]

braze pipe joints
brass brazing rod will flux in with zinc in gal coating, flow along pipe threads
leave watertight joints to about 1200 degrees (when brass will melt out of joint)

I think I will try a high temp sealant first. If that's not perfect I'll try that Bob. Haven't done much brazing for a while though.

Ta.


See ya's.

 

[Dylan33]

Take the box to an engineering / welding shop. Ask them to make up a stainless steel coil to fit in the box with as much tube as they can fit in with the thickest wall. The more pipe you can stuff into the box the more efficient it will be.
It will last forever without the fear of corrosion etc.

Good luck.

 

[DEC]

Take the box to an engineering / welding shop. Ask them to make up a stainless steel coil to fit in the box with as much tube as they can fit in with the thickest wall. The more pipe you can stuff into the box the more efficient it will be.
It will last forever without the fear of corrosion etc.

Good luck.

And how much will that cost in Australia? It'd probably be cheaper to fly to Thailand to get it done and have a weeks holiday while your at it.

 

[depreciator]

I've done that sort of job with my trusty stick welder. A mate of mine ran some gal pipes in the back of a pizza oven to heat water. We just took a bit more time with the welds - especially getting the gal off first. Can't remember if we used that thread tape before screwing the joints together. I'd say we would have.

 

[DEC]

I think I will try a high temp sealant first.

something like permatex 59214 is supposedly good for 200 deg C

 

[topcropper]

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 don't think the amount of water flow will make much difference to heating speed. I already know I can heat 600 litres of water to 40 degrees in two hours by pumping the water though a fire. A big flow of water adds just a few degrees. A small flow adds a lot more temperature. It ends up the same. I experimented with this years ago. If thermo-siphon means the water is moving very very slowly, it might be taking it from 15 degrees to 60 degrees in one lap though the heater, in which case it will still heat the 600 litres up at the same rate at 20 times the water flow.

See ya's.

 

[topcropper]

I installed the new gal pipe, and coated the thread with some 'loctite' high temp exhaust sealant, for temps up to 300 degrees. It works perfectly. No leaks, so it's now heating efficiently. It is also heating by thermo-siphon, so no pump needed.

I get the fire going, and then fill the tub from a tap. It goes though the heater while filling, so the water is warm by the time the tub is full. I then switch off the tap from the hose and turn on the tap from the cold outlet and it starts heating by thermo-siphon.

Took 90 minutes to heat 600 litres from 15 degrees to 40. I managed to time the fire perfectly, so at 40 degrees I shut off the air and it continued to hold the tub at 40 degrees for the next few hours.

The kids have been in it for hours now. Can't wait to take camping. Will get some funny looks from people I reckon.


See ya's.

 

[topcropper]

Took the hot tub camping over the school holidays to West Kunderang on the Macleay River in the middle of the Oxley Wild Rivers national park. The idea was for it to be going in the late arvo so the kids could have a hot bath before bed, but after a day or two I ended up having it go all day. The river water was still a bit cool despite the hot days, and so all the kids would swim and kayak for an hour or two and then jump into the hot tub to warm up again. I usually had to empty the water out in the arvo and heat it up again that night as the water got so dirty from the kids getting in and out all day. Otherwise the mums wouldn't get in it at night. Dirty water, Yuk!




It must heat relatively efficiently as it doesn't take much wood. I was also worried about the water getting too hot, but it just wasn't an issue. You soon learn how much wood it takes to heat the water, and that's how much you put in the box. It was easy. Then, to keep the water hot it takes hardly any wood. Just throw in a small piece every hour or more so the fire doesn't go out. The sealant works 100% and I've probably had 40 or more fires in it now.

The Loctite 596 high temp sealant has sealed the pipes perfectly and as there is always water running though them the temps would never be greater than 100 degrees anyway.



I've now bought an electric heated spa from Gumtree for $150. Picked it up and bought it home.

http://www.gumtree.com.au/s-ad/old-...reestanding-american-style-hot-tub/1028809784

A bargain I thought. Owned by a retired couple. They reckon everything still worked on it? Apparently it had a leak somewhere? And would have been all too much trouble and expense to get fixed? These cheap spas are always for sale on gumtree or Ebay as people can't fix a leak or they get too expensive to run using gas or electricity. I was basically being paid to cart it away for them, and I was happy to.

I'm going to heat this spa using wood. I will build a bigger version of the camping heater, with bigger box and pipes and it should heat the 1500 litres in no time for no cost. Will let the water out after a few days so won't need any chemicals or chlorine.





The second hand spa at home. It's up on 44 gal drums while I work on it.



See ya's.

 

[Coconutwheels]

Love it! Placing my order for a heat box

 

[topcropper]

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.

HE, looks to me that after about 40 fires in the box that the galvanising is still fine. The pipes are black with soot, but when you scape it all away the silver galvanising all seems to be there on the pipes? I'd say it's because the heater has never been run dry, and I will strive to never let it run dry. The pipes have probably never got hotter than 100 degrees even with flames directly on them as there is always water in them?

I'm going to again use gal pipe, just bigger, on the newer, bigger, non portable water heater.


See ya's.

 

[HiEquity]

HE, looks to me that after about 40 fires in the box that the galvanising is still fine. The pipes are black with soot, but when you scape it all away the silver galvanising all seems to be there on the pipes? I'd say it's because the heater has never been run dry, and I will strive to never let it run dry. The pipes have probably never got hotter than 100 degrees even with flames directly on them as there is always water in them?

I'm going to again use gal pipe, just bigger, on the newer, bigger, non portable water heater.


See ya's.

Fair enough TC - I can't argue with that experience. Clearly the pipe just isn't getting that hot because the effect of the water overcomes the effect of the flames. Makes sense I guess given the much better heat conduction of water over air.

I was just going from the BBQ industry where it's a huge no-no to use any galvanised components as the nasties do burn off and end up on your food. Obviously doesn't apply if the pipe is kept cool with water!

Thanks for the update... nice to know the thermo-siphon works too.