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Rocket Hot Tub (burn tunnel to large?)  RSS feed

 
Bob Carmellio
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I setup the start of a rocket stove today. I made it out of brick with a large j-tube shape. I was unable to get any of that super rocketry effect I see on you-tube and my fire was a bit smokey. It does draw in the right direction though, but at the temps I'm getting at the end of the barrel are lower then I thought. Maybe I had unrealistic expectations of flames moving through the burn barrel. The reason I made the burn barrel so wide is that large is that I have a heat ex-changer made out coiled copper tubing 1/2inch wide, 8" diameter coil, so I was hoping to utilize a feed tube that was slightly larger than 8". My thinking now is that either my burn tunnel is to large or the wood i'm using to test (stick found on the ground) have too much moisture, and are creating smoke. I would appreciate any guidance on how to make this work better.

Observations:
*I barely see flames jumping into the smoke stack when I look down the smoke stack (is this normal?), maybe I have to much length in the bottom of my J-Tube?

*Lots of Smoke (I get a fair amount of smoke, i'm going to try for some dryer wood) any other tips?

*My Burn Tunnel too large?

My Plan:
To install the my copper coil at the end of a system. see sketch in next post.



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Size of Burn Tunnel
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my heat Exchanger
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Rocket Hot Tub Site.
 
Bob Carmellio
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Stove Burning in Action.
IMG_2584.JPG
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Stove loaded to the max.
rocketHot_tub.jpg
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The plan
 
Peter van den Berg
gardener
Posts: 519
Location: +52° 1' 47.40", +4° 22' 57.80"
61
forest garden trees wofati woodworking
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Bob,
You've got three aspects which are wrong.
#1. Too much mass, try to stack the bricks on its side. That way you'll end up with about half the mass.
#2. The riser is too short, absolute minimum is twice the length of the tunnel and/or three times the depth of the feed tube.
#3. All these bricks are loosely stacked, the crevices are everywhere which will draw in air at the wrong spots. Use some clay/sand mix to seal those.

Dry fuel and insulation all around the thing will help.
Please don't try to extract heat from inside the feed, tunnel or riser. You'll end up with a back puffing and temperamental stove which is unable to come up to proper (about 1000 C.) working temperature. Ordinairy bricks are unable to withstand such levels.

 
Bob Carmellio
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Thanks for the Tips Peter.

I would love to see this stove get up to 1000c. That would make my day. Given your suggestions I see I can build the stove in two ways

1.) 3-4ft insulated circular heat riser at 6" diameter inside of 8-9" brick chimney. at top of heat riser run heat over heat ex-changer in the wider 8" chimney. Allow upper chimney with heat ex-changer to breathe with ventilation spaces if I get reverse flow issues.

2.) same as above with but use an 8"diameter system, and build my chimney larger. (how much insulation space do I need between, heat barrel and bricks on average? 2 inches enough?) Do larger systems need longer/shoter heat risers?

I would like to hear your or any one else's thoughts on 8" vs 6" j-tube designs and if for any reason 8" might not work as well as a 6" design I see many people use. I figured 8" would mean more heat faster for trying to get all that hot-tub water up to temp quickly. I'll put a pot of water top of the chimney if I have to harvest every little bit of heat faster. I basically want to get a few hundred gallons of water from 50 to 100F+ as fast as I can.

Wow after saying all that I almost think partially exposing the barrel of the rocket mass heater inside the hot-tub would work well if I could stop water from leaking around the outside of the barrel. (the hot tub I'm making is a stone & cement job). i'll try that idea, next if my heat ex changer is a fail & I find a barrel.

 
Adam Stjohn
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Yeah, it seems to me that placing the copper tubing directly inside the burn tunnel, heat-riser, or heat-exchange-barrel ... placing it directly inside any of that is way too extreme.

I don't know, you might even melt the copper? Yeah, actually you might: the melting point of copper is just under 2000 degrees Farenheit, and in the book ,rocket mass heaters, by Ianto Evans and Leslie Jackson, the interior of the combustion unit can get up to 2000 degrees Farenheit. (see page 59)

Also, partially exposing the heat-exchange barrel to the water sounds extreme. An interesting EXPERIMENT, but also EXTREME and potentially blindingly steamy hot-death-to-your-eyeballs when the hot tub leaks or splashes and hits that extremely hot steel. Another thing is that the heat-exchange barrel is intended to exchange heat with the air, submerging it in water may affect the temperatures and the internal burn-processes: colder burn, less clean, less efficient.

You may want to check out this podcast: http://www.richsoil.com/permaculture/613-pocast-098-hot-water/

Also, definitely, this (not just about floor heating, but also water heating-- check out the 4th picture down): http://www.permies.com/forums/posts/list/40/5937

 
Bob Carmellio
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MMM Hot eyeball death....I'd like to stay clear of that. If I can find a drum I'd still like to still try the submerged barrel approach. Seems like you cant beat that heat exchange on a submerged barrel, id just have to wear protective eye wear or more practically protect the barrel from people . Thanks for the posts. lots of nice things to think about. I just ordered my Vermiculite! To give the insulated metal heat riser a try.

I am so AMPED to get the Heat riser cooking at 2,000F! i'll be pumping water through my copper with a bilge pump from the river right next to the tub to fill it. I don't think I'll risk melting any copper unless my pump fails, or my battery dies. ill place the heat exchange just above the heat riser maybe at 3.5-4 feet and let it soak up as much heat at is can for about 2ft. If it all works well, i'll get to hang out with friends in the winter and jump from icy stream to hot tub...That is my fantasy anyway if I can manage to bring that much water up to temp.
 
Adam Stjohn
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Bob Carmellio wrote: I don't think I'll risk melting any copper unless my pump fails, or my battery dies.


If you have a water in-take from the bottom of the tub (where cold water will be), and then your out-put is up at the top of the tub, you may be able to get a convective draft (inside the copper tubing), so that you wouldn't need a pump in the first place. (it would be fail-safe too: SO LONG AS THE COPPER HAS WATER IN IT TO BEGIN WITH (so you'd need a way of ensuring that) and so long as the stove is hot, then the water would be flowing-- no chance for a pump failure.

Eh? Make sense?
 
Bob Carmellio
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Great idea, ill design the heat exchanger inputs and outputs as you suggest and see if I can get a convective flow. I'll use the pump to fill the tub through the heat exchanger on the first go, then driconnect the pump house and hope for the best....i'll let you know how it works out.
 
Consider Paul's rocket stove mass heater.
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