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Post by dmachado on Jan 26, 2011 7:03:49 GMT -8
Hello everyone, in my search for a way out of oil fired boiler central heating, I have decided to go with firewood + wood pellets mixed system. The pellets are for a continuous burn that can be variable, but the firewood would be very important to get about 80% of the heating needs, depending of the day... This would mean maybe about 80kwh on a cold January night, outside temp -2 or -3ºC. I guess this would then mean burning 30-35kg of firewood at 80% efficiency. I know we can get above that. I recently found this forum and already I have learned a lot about the RMH and some of the variations in test. The concept I am working on is a RMH, but aimed to heat water, and capable of taking a load of some 30-40kg, 65-90lbs of wood. For now, never mind about where the flame goes, I'll explain it later. My best concept would be a mix between the barrel pocket rocket and the J shaped brick rocket, built from firebricks. With a cone shaped feed area all around the burn area at the bottom, that could take the 90 lbs of wood (say 10-12 split logs, 10x10x40cm roughly)... It could be half a cone, or a wedge shaped feeding area, with a lid. The air feed would be a tube paralell to the raiser, ending just above the bottom of the burn zone, in an angle to be determined, to avoid backdraft and smoke etc, or parallel to ground, for example. What do you think of this?  |
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Post by grizbach on Jan 26, 2011 8:44:50 GMT -8
Hello dmachado, I feel that as this unit would heat up, the stack of wood would get too hot and start to pyrolize. Causing too much burnable gasses to get a clean burn.
Maybe you want the wood to index down one at a time? Looks like it would jam.
A more horisontial wood feed could bring the air in around the wood to cool it and limit the "Thermal runaway effect".
grizbach
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Post by Donkey on Jan 26, 2011 17:37:30 GMT -8
Assuming that the fuel column feeds properly, with the air inlet in that position, you will have a permanent smoky area at the top of the feed tube. Every time you opened it (for feeding or unsticking the wood or whatnot) you would get a stream of smoke in your face. The top of the feed and the lid would very quickly get gummed with creosote.
Otherwise, it would probably work just fine..
Oh.. Also, you would want to build in an ash and/or hot coal drop. Depending on the type of wood used, I've noticed that the hot coals can build up fast and choke the horizontal part of the feed (the burn tunnel). If you're feeding by hand, it's not a big deal, you just use a stick or fire-poker and rake a big enough opening so it can draft well, then you wait for the coals to burn down a bit before stuffing in more wood.
An automatic feed won't be that smart and likely as not will choke up early and often if not given the space.
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Post by dmachado on Jan 27, 2011 4:51:30 GMT -8
I understand, thank you for both opinions.
So the feed tube should also be where the air goes in, the ideal angle will be when I feel the wood weight pushing down on the other logs.
What are your thoughts about the fire burning backwards into the feed tube? maybe a simple weight sensor and a small fan could ensure that draft is kept of the right direction?...
Also a grate under the burning area and a bit farther from it is needed, to get coals out of the way, but allowing them to burn completely. Of course, daily cleaning may be in order.
Thanks!
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Post by Donkey on Jan 27, 2011 9:02:49 GMT -8
The best angle for the wood is directly vertical, falling straight down into the burn area. Air should flow down around the wood and in, insuring that smoke (fuel) flows directly into the fire with the air and everything goes in the right direction. 'Course, it won't work as well with the feed tube as you've drawn it (if air came in the top). The heat riser should be taller than the top of the feed. Let me re-phrase that.. It will work for a while, as long as air flowing in keeps the feed tube cool enough. If a little flame backs up the wrong direction and the feed heats up, the stove could reverse direction pretty fast. A little fan could help, but one of the nice things about rocket stoves is that you don't NEED fans and stuff.. I always try to design away from moving parts, they are always the first to fail and if your design REQUIRES them, you tend to get a big surprise at exactly the wrong time.. You want to pile in a bunch of wood and forget it... There are other ways to go about that.. There's peterbergs bell stove, which has a rather standard type of firebox. Also, I've kinda got that going on in my stove at home link here. People keep asking me for an internal diagram of that stove and I keep putting it off... Sorry folks, I'll do it soon. |
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Post by dmachado on Feb 1, 2011 5:55:19 GMT -8
You want to pile in a bunch of wood and forget it... There are other ways to go about that.. There's peterbergs bell stove, which has a rather standard type of firebox. Also, I've kinda got that going on in my stove at home link here. People keep asking me for an internal diagram of that stove and I keep putting it off... Sorry folks, I'll do it soon. You are right, I want to get some 6-8-10 logs burning and forget them till the next day, as I am not coming out of the house to poke them or anything like that. I guess a firebox is what I need. Thanks. |
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Post by dmachado on Feb 2, 2011 2:39:55 GMT -8
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Post by dmachado on Feb 18, 2011 9:13:32 GMT -8
...so I started laying out a few bricks, fun enough, and I got this one idea: Firebox size approx. 8'' wide, 16'' long, 16'' high, mostly closed, with ventilation from the back at aprox. 1/2 height (natural draft or fan, don't know yet, maybe a small fan on the safe side). Small brick "ramp" inside the firebox to keep the firewood over the burn area which leads to a burn tube about 8'' wide and 4'' tall (forced draft may be required). Length of tube maybe 8'' also. For now, my riser's height is determined by my downer's height, so it will be close to 6' tall (!!), with 180 degree downturn into the downer - a 40 gallon ex-wood fired water jacket with a 4'' flue in the middle. So, all the way up with heavy insulation, all the way down with pretty good heat transfer/cooling. I may install a spiral on that side to make the flue gasses go a longer way and extract more heat if necessary. Stainless steel tube, corrugated, 3/4'' surrounds in a spiral the riser bricks under its insulation (fireclay+expanded clay balls) and enters the riser from the top down to about half height (hot hot zone), exiting to the 210 gallon hot water tank. So the water at say 100ºF exits (pumped) the bottom of the hot water tank, goes inside the downer and gets pre-heated, then around the riser on the outside, then inside the top half of the riser then to the top of the big tank, hopefully and at least at 160º-175º. Some 60 or 70 feet of tubing will be used. Pre-heated secondary air at 1/3 of the riser will be tested, using the grate tubes as hot air carriers for perfect combustion inside the riser. Just got to get me a couple dozen more bricks! I'm hoping the flue gases will be pretty cool with this setup, and I can get it vented to the outside almost at ground level.  |
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