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Cooling feed tube with aluminum fins --- Plate aluminum under burn floor diffuses heat to hearth  RSS feed

 
Dale Hodgins
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I only became aware of the need to cool the burn tube this morning while reading this thread --- http://www.permies.com/t/32099/rocket-stoves/doubling-power-RMH-cooling-feed --- The author has built a metal contraption to draw heat from the tube. He heats water with it. A few seconds later, I thought of this.

My metal working skills are thus --- I can make really nice straight cuts and I can do simple curved cuts. I can sand sharp edges. That's it, my full arsenal. Most of us are familiar with how the aluminum fins on an air cooled engine work. Aluminum is great at moving heat around. It transfers heat many times faster than steel does. The large surface area of the fins allow heat that is drawn from the engine to be given off to the cooler air. The structure of a firebrick feed tube lends itself perfectly to the insertion of plate aluminum fins. It's just a matter of cutting the plate to size, so that a little sticks into the burn tube and several inches protrude beyond the brick, into the cool air near the floor. The plate could be bent at a 90 degree angle so that one surface could rest tight against the inside of the burn tube. Not only will this reduce heat build up in the burn tube, it warms the air near the floor where it could be used as the supply for a secondary burn.
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Plate aluminum under the insulated burn floor will diffuse heat to the much larger hearth. There would still be a little insulation between the aluminum and the wooden floor. Since aluminum allows heat to travel along it's length very easily, any heat that makes it through the insulated burn floor, will be directed laterally. Say we use a 30 inch square plate under a 6 inch system. the whole thing is 900 sq. inches. The part beneath the fire is 36 sq. inches. This large radiant area will easily keep the plate under the fire relatively cool. Pans full of water could be warmed while sitting on the plate.

Plate aluminum is worth less than $1 per pound as scrap metal. I could see using 20 lbs. of it between the fins and the hearth plate.
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thomas rubino
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Dale; Interesting idea about aluminum fins hanging out. Are you making one ? What thickness are you thinking ? In my current location i could see myself wacking my ankle as i go by . However my next rmh goes in the shop where i have much more room. I hope you are building this, i would like to see what your finished product looks like.
 
Dale Hodgins
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I'm definitely building several RMH units. The configuration of the burn tube is still under debate. A simpler design would involve a scrap steel square tube of the right diameter. A big flange could be welded onto the top end. This flange becomes a heating surface. Another chunk of plate steel with a loop handle on the top side and a stove gasket on the bottom, could seal the opening when the burn is over.

We've all seen big steel, hollow tube posts in industrial buildings. They typically have a bottom plate that has holes to receive the bolts that stick up from the concrete floor. The bottom foot of one of these would be perfect. Turned upside down with a hole cut to match the tubular steel, we'd have a ready made cook top and feed tube. Being a part time demolition guy, I've had these. Most are too big, but some 6 and 8 inch posts are used. I may scour a scrap yard and then there's my friend who own a foundry. I've given him lots of scrap, so he'll let me scour his pile for the right chunk.

EDIT
--- My brother is currently bidding an industrial demolition that has tons of tubular steel. He wants a couple of RMH units as well, so hopefully the right sizes are present. He wants to see a few at my place first. This means I get dibs on the first few posts. He has paid for welding before and this gives a good incentive to scrounge the right shape.
 
Dale Hodgins
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This is a poorly welded table base, but the shape is right. A heavy, structural steel example will usually have reddish primer paint and well done welds.

Turned upside-down, we get a feed tube liner and a cook top with 4 holes to hang implements from.
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Edit --- later that day --- It turns out that I may be an idiot. --- Smoke back is generally caused by design flaws in the system and is not inherent in well designed units. I will do quite a bit of further investigation, in consultation with others before continuing here. I think I've been designing fixes for things that don't need to be broke. This is a good lesson in doing searches. Rather than searching out reasons why a stove will smoke, I sought a means of stopping it. Kind of like designing an better way to run water out of the livingroom when the roof needs fixing.
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Peter Ellis
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Interesting ideas. Is anyone else amused by the apparent contradictions in wanting to cool the feed tube and, at the same time, pre-heat the air for the feed tube? I do understand the purpose, I just find amusement in what can appear to be two contradictory goals.

I am particularly intrigued by the thought of a large aluminum base plate under the feed tube and extending out from under. With the rate at which aluminum can transfer heat, it could be quite the effective heat exchanger, pumping more heat directly into the room. With appropriate care, it could make a great spot for drying boots.
 
Dale Hodgins
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The whole concept is new to me. Now that I know about it, I'm looking at it as an opportunity to have some instant heat, a place to hang stuff and a pot or skillet that also serves as an adjustable closure.
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Edit --- Later that day --- It turns out that I may be an idiot. --- Smoke back is generally caused by design flaws in the system and is not inherent in well designed units. I will do quite a bit of further investigation, in consultation with others before continuing here. I think I've been designing fixes for things that don't need to be broke. This is a good lesson in doing searches. Rather than searching out reasons why a stove will smoke, I sought a means of stopping it. Kind of like designing an better way to run water out of the livingroom when the roof needs fixing.
 
Dale Hodgins
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It turns out that I may be an idiot. --- Smoke back is generally caused by design flaws in the system and is not inherent in well designed units. I will do quite a bit of further investigation, in consultation with others before continuing here. I think I've been designing fixes for things that don't need to be broke. This is a good lesson in doing searches. Rather than searching out reasons why a stove will smoke, I sought a means of stopping it. Kind of like designing an better way to run water out of the livingroom when the roof needs fixing.
 
Peter Ellis
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While I understand that properly designed and executed rocket stoves do not smoke back, I think that there are still some potential benefits to some of the ideas that have been raised here.
Sometimes an idea is a good one despite being off its intended target.
 
Dale Hodgins
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Peter Ellis wrote:While I understand that properly designed and executed rocket stoves do not smoke back, I think that there are still some potential benefits to some of the ideas that have been raised here.
Sometimes an idea is a good one despite being off its intended target.


Yes, I often go off on mental tangents. I seldom write stuff down until I'm dead sure. You can rest assured that most things I post will continue to be infallible.

The plate under the burn floor idea is something I came up with while in my teens. A relative had a wood stove on an asbestos concrete mat, which still got hot by the wood floor. I suggested a bed of sand with the aluminum to spread the heat over a wide area. All of the aluminum fin and tube stuff was spur of the moment without too much put into asking why.

I've been toying with the idea of using scrap aluminum shavings, and aluminum extrusion in the cob, to quickly move heat from the flue area. This would allow for more even heating of a RMH Trombe wall that has a flue running throughout its length. Some cob would be 4 feet from the heat and not readily able to absorb during a burn. With a powerful heat wick like aluminum, a more even distribution would be possible.
 
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