Ed Carter wrote:So, the question is whether the top, front and end dimensions of the bench can be increased by installing heat reflector along the back wall of the bench bell?
Yes, that has been done before although not in the shape of reflection but insulation instead. Personally I implemented this in what is now called the Brussels build, the back wall is a neighboring building and insulated so the volume of the bell is much larger. It worked right out of the box so to speak.
Ed Carter wrote:Along the same lines, I suppose the bell could be shaped as a right triangle with its hypotenuse running from the floor to the back of the benchtop, again using heat reflector.
Bells could be virtually any shape, as long as the volume is large enough to slow the gas velocity down sufficiently thereby allowing the forces of gravity do its thing. See bell theory.
Orin Raichart wrote:Has anyone put the p channel under the box instead of on top? (seems to me the air intake would be inline with the flow of flame and gas instead of being forced downwards which disrupts the horizontal flow....yes, I know I would have to blow/clean the p channel out if it was placed on the bottom) ?
A p-channel on the bottom of the firebox is called a floor channel, its workings are different and it's a bit more complicated to make. As such it's similar to the secondary air channel developed by Matt Walker.
Orin Raichart wrote:In Option 1, how long does my 6" riser have to be? Kirk Mobert's doesn't seem to be as tall as described by the spreadsheets or formula for these types of stoves. The answer seems to be 43" for a 6".... can it be 32 to 36" and work correctly?
Donkey's cyclone batch rocket is a smaller system, it's a 5" system if I remember correctly. So the riser is shorter as compared to a 6" system.
I second Thomas, take down your J-tube and use the materials to create from scratch. Trying to adapt your existing heater will inevitably lead to cramped compromises.
John Goodbody wrote:Last of the 3 was so pronounced that sent the house vibrating before I maneged to grab phone and make this clip.
Sent the house vibrating? That isn't a slow pulse but a full-blown pulse jet sound, probably at 40 times a second! You doesn't say anything about fuel consumption while it is doing this but in my experience the combustion is accelerated greatly so the fuel is racing through. Below is an example of what my 4" development model was capable of 7 years ago. I'm still in the dark what is causing this, sorry.
Your RMH is acting as a valveless pulse jet, see Pulsejet by Wikipedia.
John Goodbody wrote:In another thread I also mentioned that during last two firings, the draw was breaking up, sounding almost like steam train, rather than rocket.
Does this hint anything to anyone?
Anyone had same issue?
A slow pulse, like a steam engine, 5 times a second or so points to a restriction somewhere in the system. The expanding gases would slow down, accelerate again, slow down, et cetera.
Most causes has been mentioned here I'd think, but I'll post those again, it might be helpful in some way.
Construction faults: Top gap between riser and barrel, manifold not enough space, too many bends, cramped transition to chimney, cold outside chimney.
Places where the hot gases need to change direction there should be more space. A 90º bend 150% of system csa, 180º bend 200%, so the space above the riser should be twice as spacious as the riser's csa suggests, in my own opinion. The same goes for manifold and bends further down the system. Sometimes there's a build mistake that causes a large piece of cob fall down later on and block the system partially.
All these spots can act as a restriction although it doesn't look like that.
Restrictions that occur through time: ashes buildup in the tunnel/riser transition, on top of riser, in manifold, in elbows in a piped bench system. Straight single bell systems rarely clog up, floor area of most designs is large enough to house vast amounts of ashes. One of my designs showed signs of restriction down the line after ten years of use, cleaning the bell's floor fixed that issue.
1) It will take a skilled mason to build a brick bell which is gas tight. I would start with plastering the bricks in order to seal all the small leaks.
2) You say the chimney isn't above the roof line: at which level is it? Some people think a single-wall chimney pipe would do as the outside chimney, please believe me, it won't. Requirements for a good chimney: 3 feet above roof line, straight, smooth inside and insulated.
3) And yes, the vermiculite layer could still be wet since it tends to stock moisture in a very efficient way. A dozen firings won't remove that in my experience, keep firing that rocket ferociously.
Solving point 1 and 2 is absolutely necessary in my opinion.
Graham Chiu wrote:With 130 mm pipe I would have wrapped the blanket around the pipe and then burnt the steel pipe out over time. I presume that's acceptable
Over time, the steel pipe will deform badly, pinching the flow and disturbing the burn process until it finally collapses. Which means there's at least one burn season the heater won't work properly at all. Been there, done that, learned from it.
Ron Curtis wrote:I am thinking of refractory bricks for the feed and chamber and refractory tubes 4 x 33cm for the riser
Ron, those refractory tubes aren't suitable for use as a riser. They will crack, quite badly due to uneven heating. This could be alleviated to some degree by making one slit lengthwise. No guarantees it won't crack this way but they're stand a better chance to stay undamaged.
Mart Hale wrote:
I want to make a mold for making rocket oven cores with air crete for Peter's Van design.
I am curious where I can find more information about the molds made that have been tested.
All my casted experiments were done in a coated plywood or white melamine MDF box. All the sculpting was done with extruded polystyrene foam, made to size and shape with a table saw and smoothed by hand.
The foam parts were glued into place using double-sided adhesive tape, the kind that is used for fixing carpet edges to the floor. Any crack was sealed using acrylate sealant so no cement water leaked behind the foam parts. In fact, the pictured core is done like that.
Looks nice, I'll give you that. The top of the firebox is restricted/lined with firebricks but the sides of the firebox are still steel as I see it. Lining this also with firebrick, full or split ones, would help a lot in burning temperatures and through that, cleaner burning. I fully expect insulating the riser as well would raise efficiency again, gaining more heat out of the same fuel.
All that done, you would be able to add a second barrel to extract more heat. And burn the paint off, no doubt, I sincerely do hope you have doors in front and rear of your garage to vent the muck outside.
My original 6" development model started with one 55 gallon barrel and ended with three of those, while the first one already impressed me no end.
The construction as in the Mallorca build is used a dozen of times now and works well. Done in a number of different materials for the outer skin, raw stabilized bricks, mud bricks, normal building bricks, solid concrete bricks and hollow bricks filled with cob.
Anything above the riser in double skin with an expansion joint between the skins is what I would recommend. That said, a bit more won't hurt anybody.
The same construction is used in the Brussels build, see http://batchrocket.eu/en/applications#opensystems and scroll down to the second item.
Keep in mind that the top half of the bell will be slower to heat up and cool down this way as compared to the lower half.
The phenomenon of opening cracks on the outside during heating up is due to the inside of the bricks heating up too fast so the outside is lagging behind. Ultimately, those bricks could "walk apart" in time so the cracks won't close again when cooled off. Mark however, it's still quite normal to see a few hairline cracks in some places.
Gayle Reynolds wrote:But with this bell system, am I wrong in understanding that it's a big empty chamber, and you might have 34-4 inches of stone, masonry, etc? Because if so, I would constantly be worried about a leak between the bricks, or even the fact that masonry is much more porous than a flue pipe would be? Also, even without an actual leak, due to the porous nature of masonry, what's to keep the toxic fumes from soaking into the masonry and building up?
A masonry heater in operation works in underpressure because of the pulling draft of the chimney. So CO won't leak into the house, unless you employ a chimney damper that closes 100%. I experimented with masonry heaters for years, using a gas analizer. This device also measures CO inside the room where it is situated and it didn't even flinch once in ten years while the diverse heaters were in normal operation. It did measure CO when the workshop was full of thick smoke though, but that seems only logical. One wouldn't be able to smell, feel or see CO but smoke is an entirely other matter.
Having said that all, building rules in US asks for a double walled heater with a thickness of at least 8", if I understand correctly.
Reading your first post over I come across the idea to place a steel plate of a quarter inch on top of the U-turn. Such a plate will inevitably warp spectacularly because of the temperature difference.
William LeMieux wrote:So, all downdrafts are not created equal? Please explain the differences between the downdraft in a bell design and the downdraft in a tube design. They both have them.
In my minds eye the main function of the heat riser is to thoroughly burn the gasses locked inside the wood. By the time the gasses reach the top of the riser, that function has already taken place. The bell or downdraft tube is simply a means by which the gasses would exit the heat riser, cool, then sink. By having the downdraft tube isolated from the heat riser, the downdraft tube would naturally be cooler since it's not absorbing any heat from the riser. It should function better than a bell design.
That's the way I see it anyway. Please correct me if I'm wrong and point out any flaws in my thinking.
Satamax is right, not all downdrafts are created equal. I think you mean a kind of U tube, placed upside down. One leg being the riser, the other one being the downdraft channel. The difference with a bell is that all the hot gases need to pass through that U while a bell is spreading the heat load. Moreover, hot gases tend to stay in the highest region so the heat stress will be enormous in that spot. I've seen some examples build out of firebrick, within weeks the top of the construction cracked badly. I can't say anything about it being working correctly or not, although I'm inclined to think one would run the risk of equalizing the temperature in both legs so the draw would be hampered badly.
In other words: where heat stress in a barrel or bell is spread out, this isn't the case in a U-turn. So heat stress will be concentrated in the bend of the U.
But if you want to see for yourself, please build it and post the results.
This morning a new milestone has been passed: the batchrocket site is viewed by 100,012 unique visitors since the site went online in April 2016. In September I estimated this would take place around Christmas but it happened two weeks in advance.
It looks like the number of 300,000 views will be exceeded in the second half of Februari, it's now 277,126.
Graham Chiu wrote:What temperatures are you recording at the top of your riser, and at the point where the top of the bell is?
During development of the batch box in 2012: highest recording directly above the riser 900 ºC. Top of the bell I don't know, at the time I was interested only what happened with the combustion core. All bells (and whistles) were there to extract heat otherwise I couldn't use the gas analizer.
Nono Junang wrote:What do you think about using 25mm of ceramic fibre board instead of the steel plate and cross weld? I have used some for the bottom of the heat riser. They are rated 1260degC, come in 1000mmx500mm size and cost NZD30 and are also rigid enough to close the top of the bell. So since my opening is 1070mmx700mm, I would need max. 3 pieces glued together. I still have some leftover of ceramic board glue.
I wouldn't get much immediate heat release, but that would be ok. And that would be a lot cheaper and lighter than a steel plate.
Yes, temperature-wise this would do the job. I don't have any experience with the ceramic board glue you mention. Of course you could try it and see whether it leaks and how to solve that. Is it possible to build your bell a little smaller so it can be done with just two boards?
Hmmm... Actually, it should be bigger instead because the top plate doesn't extract heat in this case.
Nono Junang wrote:Hi Peter,
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.
I've come to understand the steel top plate of the bell isn't meant for cooking in your case. My recommendation of making it more rigid isn't because of corrosion but warping instead. So paint is out of the question, it would burn off quite quickly as Satamax mentioned.
The second thought is doable and works, just one inch (25 mm) of superwool would be enough to take off the high heat. I am not sure this will prevent warping but since the 5 minutes riser is a lasting solution, shielding the top of the bell on the inside with superwool should be working. The only problem you are facing then is the need for a heat resistant adhesive to glue the superwool to the steel.
Nono Junang wrote:The second box of bricks is not a bell but rather a channel flowing like a snake (not straight) with opening going right / left / right.
I have made this to increase the path of the exhaust so that it gives even more energy to the bricks. The height of that channel is 1.7m.
What you did is making the gas path longer, this is what the Austrians do. Just to let you know batch boxes are particular picky about friction in the smoke path, that's why I always advocate bell constructions cause they are virtually frictionless.
Nono Junang wrote:I plan to cap the top of the first bell with a plate of stainless steel 1.5mm thick. I would also like to make a 200mmx200mm opening on that plate just on top of the heat riser and place a glass there. In addition I would like to have a mirror on the ceiling right on top of that glass so that I can have a view of what is happening inside the heat riser from the top.
A steel plate of 1.5 mm thick will warp badly at that location. Think of 15 mm thick and possibly a cross welded underneath to make it more rigid.
That glass will stain with soot during the start of the burn, and will turn to white when the temperature goes up. But in my experience this isn't worth the effort, you won't see anything after a few burns.
During the Innovator's gathering in Montana Erica Wisner tried a 2" batch box core entirely built of insulating firebricks. The thing needed a longer riser in order to get it going. And it could only be run with nothing more than chop sticks size fuel. Conclusions: there should be a real fire in there, not a smoldering pair of small logs. The fire can't be scaled down that far and insulation is an issue in the sense it shouldn't be scaled down as well. Smaller burn chambers are disadvanteous due to the unfavourable volume / wall surface area ratio.
I've built a number of 4" batchbox rockets myself and in my opinion it could be done. But you need to keep feeding it in order to achieve a clean burn longer than some minutes.
Graham Chiu wrote:Anyone have a clearer idea of why these pyrex doors shatter? Are they being exposed to more than 500 deg C? Have they shattered just sitting there, or is it thermal shock?
When the glass shatters in little crumbles it isn't pyrex but tempered glass instead. When it cracks in a non-straight line it is pyrex which happened to be borosilicate glass, maximum heat resistance is up to 500 ºC. The real stuff is special ceramic glass, heat resistant up to 900 ºC and insensitive up to a certain level to heat shock. This ceramic stuff is sold under the names of Robax or Neoceram, only two producers in the world as far as I am aware of.
Edit: I've never seen a piece of Robax to shatter or even crack. Not even in very extreme circumstances, provided it is able to expand.
Taking an old-fashioned woodstove and glueing a riser to it doesn't mean it becomes suddenly a clean burning efficient wood combustor. The proportions of the firebox need to be right, the port size need to be very precise and the riser insulated and high heat resistant.
There are some projects been done like this, but the wood stove needed quite a rebuild.
Dan Hatfield Ii wrote:Do you have any resources (sizing or maths) in bell bench building (stratification chambers).
I’m considering going that route to save money and time on flue fittings.
A bell bench isn't any different as compared to an upright bell. Please use the tables given on the website. In case the bench is built as a second bell deduct 15% of the total cross section area of the system.
By the look of it in the drawing, the woodstove seems to be a Turbomasse stove. That type of stove does need a lot of draft to work properly so cooling down the exhaust gases won't work, the chimney need to be quite hot in order to keep the thing going. To be frank, just adding a bench or other heat extractor to a random stove is bound to fail. In case it does work, it'll come together with a lot of pollution and tar dripping out the chimney pipe.
Satamax Antone wrote:Hey Peter, no worries.
I must not have been awake yet! Have you heard this one? I'm pretty sure i know how it can be replicated. Well, kindof.
OK, the first one is what I meant with a rapid pulse. Hard to count but it seems to be something between 35 and 40 pulses a second. I didn't know about this one, although it's published nearly 3 years ago. I am all ears how it could be replicated, imagine one could run such a small thing with pellets and get this type of performance!
The second one is the slow pulse, the Testo won't be happy with this for sure.
Satamax Antone wrote:Is your's pulsating like this,
If yes, that's bad.
As always, I hate to contradict you Max. A slow pulse like twice or 4 times a second is not good, I do agree with that. But in order to know what speed the pulse in the video was going I slowed it down at the time and calculated it as something like 40 times a second, much like a pulse jet engine. The Testo gas analiser confirmed it was a very clean burn while the fuel was racing through as nothing else. Together with absolutely top temperatures of the small drums as the result. As far as I know of, nobody has mimicked this behaviour until now so it might has been the result of a number of specific circumstances.
So I wouldn't regard this as a good example of a pulse like a steam engine, and I might stress again such a slow pulse is considered wrong behaviour of the heater.
Geoff Laughton presented this rocket stove water heater in a video published May 12, 2013, that's true.
However, the guy who built it is someone else, his name is Tim Barker. He built this appliance and wrote an article published November 23, 2012 by Permaculture Research Institute.
Here's the article, complete with diagrams of its workings.
Tim built other water heaters based on rocket stoves, notably in Missoula, Montana during the Innovator's Gathering 2014, to name one event.
See how it goes the coming couple of months, there might be some changes in behaviour while you learn to operate the heater.
Just send me a purple moosage (personal message) when it's ready and I'll send you the mail address to deliver it to.
Daniel Ray wrote:Thanks again to Peter for such an excellent design and all of those here that gave their advice along the way. Just awesome work!!!
Glad to be of help, always nice to see people half a globe away having build it and are happy with it.
On another note: this might be an opportunity to place it as an example on the batchrocket site. Since it's the first full batch box with barrels and a piped bench that I know of. Could you write a comprehensive article about your build, proportions, where you aquired the door and fixed it to the firebox, pictures, that last video?
The end temperatures could be measured with a turkey pin thermometer. I won't expect you to buy a Testo gas analiser but most of the builds in the applications chapter never saw one either.
Let's say another month or two and then evaluate and write the article?
You are right Max, although for an 8" J-tube system that would probably be 4 barrels on top of each other. That would require a high barn or something like that to reach that upper limit.
Sounds interesting to me, anyone who like to have a tower batchrocket like that?
Eight inch system, stacked barrels only. Calculating 1.5 m² for each barrel totals up to 6 barrels, 0.89 cm high each. A tower of 5.34 meter (17.5 ft) high, twice as much as my former workshop heater!
Would be really exiting to run such an appliance, don't you think?
In case you want to cook on the top of the barrel, 3" would be fine. In many cases the top of the barrel buckles in, making the top gap suddenly cramped. When you don't want to cook on it, the top gap could be anything. Up to a yard is perfectly possible, 4" is a safe bet. This will provide room for the fast streaming gases to go through the 180 degrees transition without much friction.
My rule of thumb: minimum space for any 90 degree direction change 1.5 times minimum. For any 180 degree direction change 2 times minimum. The same goes for a manifold, by the way, people tend to make that too cramped.
Absolute minimum for an 8" system is 2", accompanied with the risk of buckling in and severely restriction of the system flow.
Personally, I once tried a top gap of 4' and the thing kept working like a charm. Couldn't get any higher since the ceiling wasn't any higher, a practical top limit probably doesn't exist.
Graham, just a few points:
There's way too much mass in your setup, the bricks are taking up lots of heat. Also, since it's a small system, the ratio between volume and wall area is very unfavourable. Scaling down of the original test bed means the wall thickness should be scaled down as well.
Because it's dry stacked, there will be a lot of small crevices, this could together easily be more than the whole of the inlet.
The air inlet is way too big and it's closed at the bottom.
The exhaust opening is smaller than the csa of the riser.
All these factors contribute to the sluggish running and low temperatures.
Use split firebricks if possible, insulating firebricks of whatever thickness would be close to ideal.
Mix a clay slip and use that between the bricks when you stack it. A running bond is always better to get the darn thing airtight.
Make sure that the air inlet is at the bottom of the firebox opening. About 25% of the riser's csa should be plenty.
Enlarge the exhaust opening to 150% of the riser's csa, the gases aren't willing to go through sharp corners so they need room to flow freely.
When you've done all that it should go much, much better. Even the steel plate on top of the firebox would get much hotter, especially when the air inlet is low in the firebox and the top end is closed.
edit: the riser seems to be quite short. For such a small system, it should be at least the recommended length. Let's see for a 4"system that would be around 29", measured from the floor of the riser/firebox (which should be the same figure).
edit2: Listening carefully to what you are telling in the video, I'd get the impression the port is up to the top of the firebox and lifted from the bottom. This is utterly wrong, the thing need to be lower in order to work properly. This design is the result of lots of work experimenting. It's highly optimized, as such it is a very, very tight design. Any deviation from the recommendations means you are almost certainly doing something that I've tried already and ticked it off my looong list(s) because it didn't add to proper running, rather the opposite.
Please follow the design as close as you can and it will do what it says on the tin!
Looks very expensive to me, dimensions are double width and just 5' long. That's not a full 1" thick roll I'd think.
Edit: Just checked, a full roll of 1" thickness and 2' wide is 24' long, delivered in a carton box. Weight depending on the density, #64, #96 and #128 are common densities. In the Netherlands it can be obtained for just under € 70.-, something like $ 82.- excluding shipping.
Edit2: The above price is for a cheap material that contains carcinogetic components. The real Superwool is not bio-persistent and cost €142.- or $167.- excluding shipping.