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Post by spedley on Apr 21, 2015 12:11:46 GMT -8
I'm currently looking at ways to pre-heat the fuel and I want some feedback about using heat from the chimney to pre-heat air. In the following diagram the stove is essentially two right angled pipes, one inside the other. The air is drawn in through the outer chamber which pre-heats the air (good) but slightly cools the chimney (bad). It does however prevent a lot of the heat escaping from the system (like insulation) and recirculates it back into the stove. The heated air is then redirected by the hinged door through the fuel and up the chimney. The main point I'm looking to answer is... "If for example I lose 100 degrees from the chimney to pre-heat the air does the increase in air temperature raise the chimney temperature by 100 degrees?" I know there are various pressure and heat capacitance equations involved but do they even slightly cancel each other out?
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Post by DCish on Apr 22, 2015 3:28:00 GMT -8
Greetings Spedley. Here are my thoughts.
Any wood fire has two parts to the burn -- pyrolysis (heating the fuel to produce smoke), and combustion of pyrolysis products (smoke). A rocket stove focuses on insulating the second (combustion) zone to maximize temperature to encourage more complete combustion. The scenario you describe violates this principle. By cooling the combustion zone you decrease the heat available to support complete combustion. At the same time as you rob the combustion zone of heat, you are transporting that heat toward the fuel, encouraging it to pyrolyze faster, producing more smoke. My guess is that by pyrolyzing faster while simultaneously cooling and robbing the flame front of heat, you would get a very smoky burn. Not to say you shouldn't perform the experiment, but that is the result I would expect.
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Post by spedley on Apr 22, 2015 11:42:32 GMT -8
That is 50/50 what I'm expecting. I'm still not certain that the opposite might happen - by increasing the intake air temperature and the amount of smoke there may be extra combustion which more than compensates for the losses. I've cut the internals and I'm ready to weld tomorrow (60mm x 2mm) tubing. I haven't sourced the outer tubing yet (90mm min) but have to get a decent flame from the inner chamber first to compare against.
My back-up plans are: A) start with an outer pipe almost as high as the chimney with the aim of taking most heat away from the top of the chimney where combustion should already have occurred or heat output is greatest. B) failing that, put an additional air intake pipe vertically up through the bottom of the stove to introduce fresh air directly into the chimney C) and also I will try hanging a 10mm steel core down the centre of the chimney with the hope it stores enough heat to negate the losses from the chimney wall.
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Post by peterberg on Apr 22, 2015 12:06:21 GMT -8
Spedley, Are you aware that a correct working rocket heater will eat steel for breakfast? When it doesn't, it isn't up to standard.
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Post by spedley on Apr 22, 2015 13:27:56 GMT -8
At present I am just experimenting.
The [very] little I think I know is that a large traditional campfire is roughly capable of 800C. Achieving this in a simple stove is good and anything above this very good. Steel at this temperature would have a nice red glow. Steel should be ok up to 1100C and starts to melt at 1300C-ish
In theory the maximum temperature of burning wood is well over 1800C but at this temperature combustion is as good as it gets. If I manage anywhere near this with a few hours work I want a medal!
Perhaps cooling my chimney may be beneficial after all :-)
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Post by satamax on Apr 22, 2015 13:54:10 GMT -8
Spedley, search for spalling, and see what you can find. A J is perfectly capable of 1000C°, and a batch can reach 1200C°. Metal melts above that. But over 600C°, it starts to loose it physical strengh, then about 800/900C° it's chemical stability. You've never wondered whay it's around thoses temps that tempering is done? Below 900C° there is unburned gases, which is no good. Well, i gonna save you some work www.youtube.com/watch?v=7RyvsZD1_CUAnd don't even think that thicker is better, as it delaminates and becomes like puff pastry after a while!
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Post by spedley on Apr 23, 2015 10:16:13 GMT -8
That was a bit of a disaster. Started with a J tube and barely managed 400C, mostly around the 240C mark. Thought it may be the extra long feed tube but shortening this made it worse. I'm pretty sure that the problem is the small diameter tubing (56mm - 2 and a bit inches). Getting sufficient fuel in the feed tube and sufficient air flow seems impossible (they are mutually exclusive).
I'll re-evaluate my design to address this issue.
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Post by spedley on Apr 24, 2015 2:33:34 GMT -8
After a realisation I have found the idea to be floored. Whilst I was hoping for positive feedback from the heated air I totally forgot the more significant air volume drop. A 100C rise in air temp equates to a 25% drop in air volune which would mean a higher flow and more heat loss.
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Post by spedley on Apr 25, 2015 14:00:18 GMT -8
Sorry Satamax, I didn't get around to replying. Thanks, that is a line of videos I have not come across before. I knew steel wouldn't be great but didn't realise it would be that unsuitable! I have now addressed the issues with my very first test ... Obviously it is very basic but 50% of my project is to learn how to weld. You can see where I cut off the feed to to attempt to improve things. I have now re-evaluated and re-designed the stove which I'll start a new thread here
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