Satamax Antone wrote:Hot gases tend to rise so in the downdraft leg you would be pushing hot gases down. Not sure the riser can push this. It already strugles with a barrel without chimney.
But isn't the same thing happening with a bell system?
Hot gases tend to stagnate up top, and as these are cooling, they go down, and exit through the chimney. But because the volume of the bell is far bigger than heat riser and flue, the bell acts as a faster cooler than a downdraft channel, Reducing the hot plug effect. A updraft/downdraft channel acts as a siphon, more or less, where the bell having more surface doesn't do this effect as much. Gases, in the updraft/downdraft channel, have far less contact surface to cool down, hence stay hotter for a longer time, forming that hot plug, which doesn't happen in a bell. Theoretically this could happen in a bell, when all gases, and surrounding mass reach a steady state, being all at the same temp. But this can't happen as easily, as the bell's temperature exchange surface is bigger, and it's outer radiating surface is even bigger.
I don't know for a batch yet. But for a J, i don't think more than 15/18 inches could be done easily. Because the draft might become uneven between all the pieces of wood. And reverse at some spots. Plus, there is dilution of the heat with excess O² and the walls of the burn tunnel and heat riser being soo far that they don't concentrate the heat anymore.
Gerry Parent wrote:
Re-looking at your photos, hooking up your pipe directly to the manifold barrel IMHO is not a very good way to go. I would almost guarantee that this is the bottleneck area that is killing your draw.
Gerry, Jim has a fair bit of space. And i don't think this is much of a problem.
This could be better, but not by any means a problem.
Graham Chiu wrote:A bell I believe just refers to the container of the gases. So, if you had a double wall with the gap holding the gases, then that's still a bell.
A bell, to act as such, has to be of a certain volume, to let the convection happen, and avoid bypassing. The absolute minimal floor surface area has to be over 5 times the CSA to avoid bypassing.
In case of a wall, is is easily attained. If not too thin.
Jim Higgins wrote:Thanks Graham. I understand the concept of the bell being just a chamber holding the gases. The problem for my intent is that all the heat is kept at the top of the bell while the cool air is at the bottom. I need the whole wall to be warm. This is why I believe I will go with ducted space whether that is via metal duct work or simply building duct space with the brick or other material I end up using. With the duct work, at least as I understand now, all heat travels the entire duct work, thus heating the mass all along the way. If the ducted spaces progress upward instead of around and back (however many times) in a low bench, the heat will build up from the bottom to the top of the wall where the exhaust will exit and go through the roof. If my understanding so far of the premise behind the ducting principle is wrong, I would appreciate some explanation. Since I have not yet read the work books available I realize I may be blowing smoke ;-}
Well, the exhaust of a bell is not at the top, but at the bottom. It's like an inverted siphon. Gases enter the bell at the bottom, rise to to their buoyancy, since they are hotter and less dense. cool down at the contact of the surrounding mass, sink back down towards exhaust at the bottom.
Heat, in the mass travels in all directions. So your whole wall will be hot after a while.
If you want the wall to be all heated more evenly, you make it a double bell. Top and bottom one.
Peter van den Berg wrote:Think of 15 mm thick and possibly a cross welded underneath to make it more rigid.
would it make sense to paint the underneath of the 15mm plate with heat resistant paint and/or some kind of rust protective paint?
Also would you protect the underneath of the steel plate with some ceramic fibre blanket (if so how thick) to minimize the heat shock? At least at the place directly above the heat riser? The plate is about 380mm above the heat riser.
Nono, i don't think the ppaint would resist for long at the top of the bell, inside!
At what height will be the top of your bell?
Because, besides cooking, a steel plate doesn't serve much purpose, except if you want to heat a room above. Two over dimensioned metal I beams, supporting firebrick slabs is easier to do. Or a poured single slab.
OK. I think your mass is too long. Reduce your heat riser, for sure, you will extract less heat at first, so you will have more for the mass. Explaination, less température differential means less heat exchange. So, more heat going to the mass. And, i don't know what is aluminium insulation. Just get batt insulation. Aluminium is usually a heat conductor. Rockwool is good stuff.
Additionally before placing the drum back on, I put 2x4s over the riser and measured the gap between the two drum sections. I got 1.25 inches. Since the 2x4s are 1.5x3.5 that would give me 2.25 inches of space between the riser and the top of the drum.
This time during the burn I had to close off 50% of the feed tube to get any draft and when the drum hit 400 degrees it flamed out and I got smoke blow back and fire out the feed tube.
I sealed off the feed tube with bricks and let it die out.
About this time, it occurred to me that maybe my mass duct really didn't have enough rise, so I tore that apart and reinstalled it with a 3 inch rise in each 10 foot section with a 1 inch rise between the end elbows for a total of 7 inches of rise before it hits the chimney. And I had the same issue as before: at 350-400 smoke blow back made its appearance.
In the meantime I met a young gentleman at one of the local fireplace shops who had attended one of Paul's RMH Seminars, and explained to him the problems I was having and showed him my photos. He offered his opinion that the CSA of the exit duct going to the Mass was probably not enough. So, back to the books and sure enough there is a 3 inch CSA requirement there I had totally overlooked (much to my chagrin).
I tore out the back of the support bricks and again cut them to the size I needed and also considered since the surfaces of the ceramic board, the tiles and the firebricks that support the riser were not perfectly smooth, I would put a thin (1/2 inch) layer of cob between the riser and the core to make sure everything was sealed up tight.
This having been about the fourth time I had torn things apart, another thing became apparent. While the silicone is wonderfully easy to apply and does seal things up, its a real pain in the rear to get off and clean up. So, this time since I was cobbing things up, I decided to use cob for the resealing of the drum.
see 15-sealed by cob
16-exit duct csa
With the new duct gap in place and everything rebuilt, the final burn was more dismal than any of the others such that even a small fire to start came back through the feed tube.
So this is where I currently sit. There has to be a choke point somewhere, and having just read the post on how to make your RMH rockier, the only other think I can think of is to lower the riser, or move the whole thing back the the three inches I moved it forward.
While I'm not freezing, as the propane back up is in place, my pocket book is hemorrhaging to keep the propane flowing. So the sooner I can fix this the sooner my pocketbook will love me again lol.
Thanks in advance for your posts and constructive comments.
This sounds like a typical case of condensation plug.
I have been fighting with this before.
First of all. I would be happy if you could increase a bit your top gap. 3 or 4 inches is better, imho. Even if this is a square riser. Re checking the numbers, you have a theoretical gap surface of 45, where the absolute minimum is 37 sqin. I see you have a red spot, and that's no good. I mean, the metal expands, and buckles down, chocking your gap, and hence the burn.
the chimney which consists of 3, 4 foot runs , 1, 90 degree elbow and 2, 45 degree elbows to get outside.
If your chimney going through the roof? Outside part insulated? It is not going through a wall? I think you need to insulate that chimney outside. And may be inside too. Down to the barrel.
When you do a burn, can you hold your hand on the chimney for any length of time, say 4 feet above the barrel? If yes, this is no good, this means your exhaust gases are too cold. Best time to check would be when the stove fires back.
To me it all seems sound. Except that the burn seems bad, by the look of the soot inside the barrel. But that's normal if it is stalling.
What did you use for mass? Pebble or gravel is mildly insulating due to the air gaps, so it shouldn't be a problem. But the first layer where all the air gaps touch the pipe, let it radiate faster than a solid mass. So this could be a potential problem.
We have heard here, of several semi failures, where people burned, with bare pipes. And the heat extraction was too much, so the stove stalled. I think you are in the same kind of case.
When i say condensation plug. The steam in the exhaust (usually in a vertical part) condensate into droplets, this releases heat above, re heating whatever moisture is above, so the system still functions for a while. It works as an ensemble. Since the gases, and the steam or fog are all gases, they tend to move in the same direction. But the more water condensation in that column of gases, the heavier the column of gases becomes. Until a complete stall. And a reversal. Shooting flames inside, then putting the fire out, due to the lack of oxygen in those gases.
Do i make sense there? But with your black pipe stuck to your barrel, this shouldn't happen.
You say you are buried in snow. What kind of outside temps, since this can effect your build too. If it is really really cold, this could effect your system, if it is really on the verge of condensation. Also, do you have a sort of outside air "intake"
Anyway i stop babbling away.
The solution i see. Insulate the vertical chimney above the barrel. If that doesn't work. Reduce the length of your tubes in your bench, say a foot at first. If that doesn't work, cob the pipe, may be on two or three inches. If that doesn't work, insulate the whole pipe in the bench. And if this doesn't work, burn the whole house down, and move south!
thomas rubino wrote:Hi max;
It is a diamond wheel, I just think of it as masonry blade. It still is on a hand grinder and the safe thing to do, would be to get a wet saw... Luckily I'm almost done cutting, so no need...
Thomas, since you need to soak the bricks before using them. A good trick to cut with a diamond wheel without water, is to soak your bricks which need cutting, overnight in a bucket. If not perfect, it helps.
Another trick, mind you, i have never used it with a 5 inch grinder. But on a 9. 1/4.
Is to ude a hand pump sprayer to wet your cut. I use something like this:
Fox, the gap is the tightest space between top and heat riser. Exterior, interior, mid way, whatever! Just the tightest gap. If you do a trumpet bell end, you can reduce the gap, since the diameter is larger, and so is the circumference.
David, i think theoretically, in a perfect burn, you could reach 900 celcius at the top of the heat riser. So, a third of that on the cooking area would be amazing. On my batch, the walls of the "barrel" get around 285/295C°But that's approximate. And cooking plate was 191° last night. But the cooking plate is on the firebox. Not on top of the riser.
Well, i like a little mass in the heat riser. But i will try a five minute riser or two in the future;
To explain about my idea of adding mass outside the riser. On inside the barrel.
I have my lovely workshop heater, you've seen pics now. The heat riser is made of ceramic flue elements from tona. Rockwool, perlite/vermiculite mix. And all of that is dry stacked, held in place by dry stacked pozzolan flue elements. Let say, i burn for four hours, or five. The heat riser outside layer's heat is still feelable the next morning. 8 hours after the end of the burn. You believe me, you don't. Up to you!
I would say, in your case, it's well worth a try, or/as well as lining the inside of the barrel too.