jga2z
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Post by jga2z on Dec 4, 2013 9:53:31 GMT -8
I have a rather long thread over at permies on my build for further details if needed. RMH redesignThis post concerns the problems I'm having with draw, and giving the parameters of my system for analysis and comment. I have about 36 hours before heat is essential with lows in the single digits. I had on previous iterations needed a booster fan on the exhaust , but things were running pretty good the last several burns without it but the test run tonight to dry up the new cob didn't go so well. I was having a hard time getting even a moderate draw, and with some questionable wood I was burning after I had it going for a bit had a smoke back disaster. Aside from some added cob to my inground exhaust duct (which shouldn't have any affect on draw/gas flow) the only change is I cut off the small straight part of the transition area hole in the the lid I'm using for the bottom of the barrel. This added maybe 8sq in to the area of that hole which I keep thinking was too large anyway. I did it to make the opening smoother into the area below. I'm evaluating and recalculating things and think that hole should perhaps be somewhat smaller. It's around 90+sq in now, and I think it should be closer to 70sq in for my 6" J-tube. I think I read about sizing this exit area here, but I forgot. The large distance between the riser insulating duct pipe (14") and the 55gal barrel (22") of 4" all around might also be too large. Although I think the size of that gap will just let more or less heat radiate off the barrel as long as it isn't to small (less than 2") Going by the book if an 8" system has a CSA of 50sq in, and the 2" barrel gap gives an area of 125sq in (a factor of 2.5) this seem like the right size to start with when creating a tapered transition area (venturi) to the exhaust duct. Given those assumptions then a 6" system might be 70sq in or also a gap of maybe 2" in a 30 gal barrel (I don't have one to know it's size) also a starting point size for the transition area. Here's how my flow goes, ~6" J-tube (fire brick) CSA of ~28sq in, coming out of riser with 2" gap to barrel, 125sq in area between riser and inside of barrel, ~90sq in exit hole into venturi transition area and out a 6" exhaust duct. 32ft effective length of duct and a 10ft vertical stack. If I'm using a 55gal barrel (since I don't have a 30gal one) with a 6" system should I decrease the gap between the barrel and riser insulating cover to less than the current 4"? In doing so if I lose radiant heat off the barrel and it goes into the exhaust duct that's fine, more heat for the mass. Given the formulas that might mean a gap of only just over an inch to get the ~70sq in area. This would be rather hard to transition in to a manifold out of the bottom of the barrel. Or can I just reduce the size of the transition area manifold hole from ~90sq in to 70sq in? Hopefully this is enough info for some input on the flow issue. I need to do something quick so any help would be appreciated.
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Post by matthewwalker on Dec 4, 2013 10:49:12 GMT -8
It's definitely not that any of your dimensions are too large. If anything, you could up that top gap a bit, 2" is just a bit tight in my experience, and there's no such thing as too large, again, in my experience. Almost for sure you just have a "cold plug" from the new cob. Get that fan back on and run the whole thing hot as you can for as long as you can to dry out the new cob. Once you get some heat in there it will run fine on it's own. If you are not insulated from the surrounding earth though, you will have the same problem with a cold start every time. You might need to accept that you will need the fan on start up, or shorten the system, or create a bypass to get things going before you try to get flow through the whole mass.
Basically, if you cool the gases too much on the way to the vertical exhaust chimney, they will plug up the system with heavy, dense gas that won't rise up the chimney. Once you get heat at that end, the whole thing will start to flow. Your wet cob is extracting a ton of heat, and it's exacerbated by evaporation. Good luck.
Edited to add: That's a fairly long run for a warm bench indoors on a 6" system. Once it's all dry and warm you should be able to run it, but it's right on the edge in my opinion, and being buried in the earth is going to be a handicap for sure. You have one of those systems that may be better off just spilling the cold exhaust out at ground level rather than trying to get it to go up a chimney, but then you will have wind issues to deal with. If you use a vertical chimney, you need to save a bit of heat to drive it.
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Post by satamax on Dec 4, 2013 12:52:46 GMT -8
I'm with Matt on this cold plug isue, and on the "downspout exhaust"
Just an idea. Why don't you try to do the two?
Leave your pipe in, the ground, as it is, and extend it so gases go out at floor level, outside obviously. And make a vertical chimney at 2/3rd of the lengh, with a T, which can be closed off. May be with something like a butterfly vane. May be up high, so the actual vertical chimney can act as a bell (ish) when closed.
Just a few ideas. I have a cold plug with my system. I pour a glass of 90 proof alcohol or meths in the chimney to prime the system. And light the cold stove with 90 proof too. Cleaner, faster, and creates a good draw/push at first.
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jga2z
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Post by jga2z on Dec 4, 2013 16:45:35 GMT -8
Satamax- what do you mean by a downspout exhaust? So nobody thinks the large opening in the bottom is slowing things down? I've had the barrel top up to over 700deg, mid 600's easily. The duct in the ground has gravel under bottom 1/3, and cob on sides and top, then 2x8x16 cap blocks over the top (set in fresh cob. I've had some permies comment on the ground sucking up all the heat but I think the gravel insulates it from the surrounding earth enough.
Are you guys saying that I could run the heater with no vertical stack and it would work? I originally thought that was the case, but a lot of people on the permies forum think the vertical stack is essential. I've tried it all ways. When I use the fan it's right on top of the elbow coming out of the ground. I've tried a 5'stack, but it was working pretty good with 10'. No top cap yet which is next on the list. Can I use 2 elbows and have a 180 at the top? or would that cause problems. 62 this afternoon, 28 tonight, 20's and snow tomorrow. Gotta start the fire now.
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Post by matthewwalker on Dec 4, 2013 17:01:24 GMT -8
There's no hard and fast rule, but think of the physics. You need heat for the gasses to rise out the vertical chimney. New, wet cob, cold earth, long flue run...by the time the gasses are getting to the vertical they are stone cold and plug up the system. So, use a fan or shorten the run to get hot gasses there. Once they are there, they will create the stack effect and pull the gasses through faster, so you will have warm gas there and the vertical stack will help. If the gasses are cold at the end, they can't rise up a vertical chimney, so right at that moment, a vertical stack is hurting you.
Overall, I far prefer a system with a vertical chimney at the end, but you have to save a little heat to make it work. I think your system will probably work once it's dry, but you will have to work a bit to get it dry and warm.
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jga2z
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Post by jga2z on Dec 4, 2013 20:01:54 GMT -8
I'm in for a minute while running the RMH. I tried pulling the stack off and it won't hardly draw at all. The temp at the bottom of the stack is maybe 60-70, but it's 35 out now. Top of barrel temps are around 600. With the stack it's drawing OK. No smoke back, but the sticks are tending to wick up with flame. Thanks for the input. Will post more results later.
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Post by satamax on Dec 4, 2013 23:27:46 GMT -8
John, well, it would be good to have the option of being able to chose from the two. Mind you, if you remove the vertical chimney, and it doesn't draw at all, your cold plug happens before. How flat is the pipe in the ground? Wouldn't it go down a bit. May be only a few inches, but that could impede with your draw.
Youy're already going down a bit with your barrel on top of that mound. I like your transition. Seems good. I don't know how you have insulated the burn tunel, and heat riser, but i suspect it might not be enough. I don't know about cob, there's near to none where i live. But it seems to me that it's more of a heat sing than anything. The ground into which your pipe is laid is a heatsink too. And the moisture is not easily driven out.
I know it's not time for the big modifications, but what i would advise you do do, dig all that again, a bit deeper this time, lay a path of extruded styrofoam or polyurethane pannels, two feet wide about four to six inches under the tube. And insulate verticaly as well, on the edge of the bottom pannels, up to ground level, then fill that with cob and your pavers on top. Then you will be sure to have a mass which will dry. Otherwise, moisture is pushed away from the cob, into the ground, then sucked back by the cob. And you are radiating all of your heat into the ground, endlessly. If you don't like chemical products, lay a thick film of waterproof tarp in your trench, then fill with sand. I have the same kind of problem, and only 10ft underground. I had to insulate 6ft of vertical stack, and about 14 of horizontal run. And even then i have to prime the chimney stack.
Mind you, there's a daft test to see if it can be made to work without digging again.
Put a T at the bottom of your ten feet stack. Buy a plug for the side hole. Open that when it's fitted, pour 90 proof, meths, or anything which can burn without being too risky, in the chimney. Don't use petol, unless you absolutely have to. Leave the cap/plug open, then, set light to your combustible. (i don't like doing this with newspaper, too slow and not easy enough) and prime your chimney stack, till it's hot enough that you can't touch it for long. Then, light your rocket. I don't know if you should close the cap or not. On mine, i leave it open till smoke comes to it. Mind you, my system is fiddly and weird.
Remember, if you don't insulate the ground around the pipe, you will have to do this every time as long as there's moisture in the ground.
Damp ground acts as a nice heat exchanger, but you have more radiating surface and temp differential than you can power with a rocket.
Hope this helps.
Max.
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jga2z
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Post by jga2z on Dec 5, 2013 23:53:18 GMT -8
John, well, it would be good to have the option of being able to chose from the two. Mind you, if you remove the vertical chimney, and it doesn't draw at all, your cold plug happens before. How flat is the pipe in the ground? Wouldn't it go down a bit. May be only a few inches, but that could impede with your draw. Duct in the ground is fine, no flattening at all. Working pretty good now after drying up a lot
Youy're already going down a bit with your barrel on top of that mound. I like your transition. Seems good. I don't know how you have insulated the burn tunel, and heat riser, but i suspect it might not be enough. I don't know about cob, there's near to none where i live. But it seems to me that it's more of a heat sing than anything. The ground into which your pipe is laid is a heatsink too. And the moisture is not easily driven out. Burn tunnel is cobbed thick around the sides,2 layers of firebrick and a 4" concrete cap blocks underneath the whole J-tube, but the top has only maybe 2" of cob over it. I wasn't thinking ahead when I did that. It was mainly because that's where the lid for the bottom is and positioned there so the top gap on the barrel is 2" above riser. This might explain why contrary to standard observations the side of the barrel with the smaller gap (feed tube side) is hotter. It could also be because of my freak barrel that is deformed at the top (swelled like 3" up from lip) from having frozen water in it for several winters. It didn't rust out because the barrel had dried paint in the bottom. I'm still hoping that the duct is insulated enough from the surrounding ground by the gravel. I don't measure any temp increase near duct compared to other areas of the floor. I know it's not time for the big modifications, but what i would advise you do do, dig all that again, a bit deeper this time, lay a path of extruded styrofoam or polyurethane pannels, two feet wide about four to six inches under the tube. And insulate verticaly as well, on the edge of the bottom pannels, up to ground level, then fill that with cob and your pavers on top. Then you will be sure to have a mass which will dry. Otherwise, moisture is pushed away from the cob, into the ground, then sucked back by the cob. And you are radiating all of your heat into the ground, endlessly. If you don't like chemical products, lay a thick film of waterproof tarp in your trench, then fill with sand. I have the same kind of problem, and only 10ft underground. I had to insulate 6ft of vertical stack, and about 14 of horizontal run. And even then i have to prime the chimney stack. Way too much work for a limited return at this point. I have a drain trench surrounding the dome for drainage. Only rarely during a wet spring is the ground there saturated.
On the current iteration of the system I have yet to need to prime when starting up beyond a small paper fire in the burn tube, and covering part of the top of feed tube (which still has the air inlet at the bottom rear) to get it to start up and draw OK.Max, I do appreciate your input and take the advice to heart.
I'm actually kind of giving up on thinking that I can ever get the cobbed duct given it's limited mass (whether isolated or not) to retain and radiate enough heat to keep my dome warm. So I'm going to use water tanks as my thermal mass. Not my fish tanks as I had previously contemplated since the fish won't tolerate major temp changes like would be needed to store and then radiate heat. Although they will have to be kept at a stable temp by some means. I'm going to use a few 55gal plastic barrels that will be heated by the RMH.
John
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Post by satamax on Dec 6, 2013 0:55:56 GMT -8
John, when i say flat, i mean horizontal, not squashed.
When i talk about moisture, it's a small percentage in the ground, but that still transfers heat between the grains of whatever you have in the ground. Cob is nice and cheap, but keeps moisture in it forever. Not soaked ground. Just 5 or 10% moisture would transform your ground into a heat sink.
It's simple, with any mass, you have gas temps, mass temps, mass heat exchange surface, mass specific heat storage capacity, radiating surface, and heat loss through your walls and roof.
Here you have a small exchange surface, the surface of your flue tube, sending heat into an "infinite" heat sink, which radiates the heat gathered, straight away into the surounding air and whatever is on the ground. You have to make all thoses parameters work together. But my opinion from the start is that in a greenhouse, where the walls and roof are not insulated, there's very little chance to heat anything with a rocket.
Because there's a huge difference between heat input from a tiny rocket, even if you recover most of the energy, and heat loss through the surface in contact with the outside. Even if you have a damper in between, like mass.
Iirc, one who has succeded at keeping his fish warm (ish), did raised beds of mass with fishtanks above.
You could look into the half barrel system, and insulating thoses beds on the bottom and sides.
But i'm under the impression you were expecting some kind of miracle. Well, i'd rather rely on the laws of physics.
Hth.
Max.
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Post by patamos on Dec 6, 2013 9:23:50 GMT -8
Hi gang,
My name is Pat. I'm getting ready to design and implement a few systems here on the north wet coast, so have been researching backwards as the weather draws me inwards. I want to start by thanking you all for the amazing contributions you have made to making our world a saner place. And special thanks to Donkey, Lasse, Max at Firespeaking and Peter at Aprovecho for taking the time to help me nut out a big outdoor rocket fired kitchen project last summer. As i get my cyber act together i will send pics and vids
I was following this post and thought i'd chime in about Annualized Geo Solar heating - a method i have implemented with success on SOuth Vancouver Island. AGS is a technique of intentionally dumping heat into the ground, usually under the house and partially insulating the floor in a way that slows the rate of return to the floor surface (via conduction then radiation) long enough to be useful in the winter. Generally speaking 10ft of loam = 6 months of travel time. For more info about this approach google Don Steven's AGS Tokyo Paper.
Anyway, John, seeing that you are likely dumping more heat into the ground than desired, and given that ground moisture has a tendency to wick upwards if there is a dry surface through which to evaporate (matric force i think)... there is the likelihood that wet ground will is wick the heat away. The wetter the ground the more effectively it will wick. Nothing like water for thermal conductivity... That said, there are a few things you can do to slow this mode of heat loss and thereby take advantage of a longer flywheel heat sink of warmed earth beneath your dwelling.
1 - dig a curtain drain around the building's perimeter. The deeper the better. This will help to keep the ground under the building dry - especially during the wet season when much of that stored heat can wick away in a hurry. 2 - reduce or eliminate any evaporative qualities of your floor surface. If cob, get as much oil on/into it as you can. This will impede the matric force.
When designing an AGS system there are two approaches. One is to insulate the sub floor earthen mass bottom and sides and partially insulate the top. A mostly closed battery so to speak with a large radiating (uninsulated perimeter floor) surface. This is usually a very impractical retrofit.
The other approach is to create a perimeter cape of waterproof insulation extending up to 20ft in every direction from the central heating element. The idea is that by the time the heat travels that far it will have dissipated and radiated into the areas of the building you wanted it to. Insulating can be as simple as used forestry seedling trays with a couple of layers of 6mil poly over top, maybe some old carpet for puncture resistance and general mechanical protection, then 6"+ of soil and ground cover. Landscraping anyone?
I know this is all a bit off topic, but the general idea is if you can keep your surrounding ground relatively dry, the heat you are dumping into your sub floor does not have to be entirely lost.
Hope this is of some value
well being
pat
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jga2z
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Post by jga2z on Dec 6, 2013 9:29:25 GMT -8
Max- I had physics classes in high school and college, but that was 20-30 years ago. I am still really in to theoretical physics and string theory. I just didn't think thru all the parameters when designing my system. Just made the mistake of going on some apparently bad examples. Pat-Yes I have a perimeter drainage to keep water out from under the dome. The ground is pretty dry and the entire floor has 3-4" of limestone gravel covering the dirt. My aquaponics system is a 275 IBC, top cut off and inverted for floating grow bed over bottom of IBC fish tank and 4 creek gravel filled half barrels, times 2. 2400lbs of gravel total. 1 system just has the fish tank running. I have insulated the fish tanks and will do the half barrels today. It boils down to this, for my situation I will need depending on temps in the 0-20 deg range to get the inside up to 65 I need 34-49K BTU's /hr or some where in the range of 100-144#'s of wood / day. Or 6-8gal of WMO which I have over 150gal. More to ponder John
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Post by patamos on Dec 10, 2013 23:07:40 GMT -8
Hi John, Given the floor lay up you have described - with open gravel over dirt... i'd say some moisture will be prone to wick its way up to evaporate. The denser your subfloor soils, the more this will happen. Most clays being the ultimate wickers due to ionic binding of the hydrogen molecules...
That said, much (perhaps only some) of that heat should still remain within the building's perimeter envelope, and therein reduce the overall heat load/loss. If we think of teepee and pit house cultures, part of how those dwellings remain liveable through intensely cold winters is from the central fire conducting heat downwards into the earthen subfloor mass, some of which re-conducts back upward when the floor surface temperature is cooler than the subfloor
well being
pat
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jga2z
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Post by jga2z on Dec 11, 2013 10:53:47 GMT -8
Pat, My dome is over the former garden spot. VERY black dirt zero clay at least a couple feet down down as I have dug. It was somewhat loose soil prior to putting several tons of gravel on top of it. The floor has not seen direct precipitation(except for condensation drips)for about 4 months.
Been having fair luck with last burn getting top barrel temps up over 600 for extended periods. Interesting observation last night. I was trying to move some warm air off the barrel around and put a fan on low blowing on the bottom a few feet away. Within a few minutes the top temps went close to 700, but the bottom temps went below 200. Unfortunetly I think this means that the exhaust temps are also lower and reducing the heat sent into the mass. The reverse of what I need.
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jga2z
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Post by jga2z on Dec 16, 2013 23:28:35 GMT -8
Ok more updates for this thread. I am now having a bit of draw problem. Satamax has commented over in my permies forum thread. I suspect that my in ground (surrounded by cob and or gravel)duct is suffering from a cold plug, likely at the end of the run where a foot or so is outside in the cold and snow. I have kept the area close to the vertical stack clear of snow. I tried a waste oil setup for a day or so and had only fair results (another thread perhaps)Subsequently I cannot get much of a draw. Even though the top of barrel temps are 600+ and quicker to achieve than before I have almost no rocket sound and weak draw (no smoke back). Base of flue stack is ~90-100. Not sure if it should be higher unless barrel temps are higher. I will try to mitigate the possible cold plug at the vertical stack base by isolating and insulating the last few feet of the duct from the surrounding ground. Beyond that I am considering mods to the rocket heater itself to get a better flow and heat output. Rebuild to a 8" system. How would that work with a 6" ~30ft exhaust? Just add to feed, burn and riser tube lengths, or just riser? Riser height is currently ~42" very bottom (floor of burn tube) to top. Here is a pic of the core minus riser. I added another layer of full bricks under the base with an ash pit space at bottom of feed tube. Barrel lid as base is still used sans duct exit manifold.
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Cramer
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Post by Cramer on Dec 22, 2013 7:47:42 GMT -8
I almost dread posting this and possibly showing my ignorance but if I am off base I am sure that one who knows much more than I will be quick to chime in and correct me so here goes... It seems to me that if you have a cold plug now with a six inch all the way through your system (more or less) then you will likely really smoke yourself out if you try to put an 8 in combustion system on a six inch system/chimney stack. In the picture you posted of the heat riser section, I wonder... how much does one of those bricks weigh? They look to me like dense firebrick which may be absorbing the heat from secondary combustion. They will well withstand the heat in the riser but are not the best thing to use as a riser stack. Maybe it is just me but I think once everything is dried out (which may well take a very long time but over time I think it will happen) I think you may well be able to store a fair amount of heat in your ground. I don't think though that without some above ground storage and dissipation that you will be able to maintain the temperatures you seek. From one who lives here (see picture)... I very much like your greenhouse!
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