Critique my Design Please

I have a design that I have not seen as yet ... similar to a Russian stove but using a batch box RMH ... Please watch my Short Video explaining my Idea and feel free to enter your suggestions before I start my build
https://youtu.be/NTIMIxev9CA

First comment: use perlite-clay instead of perlite-Portland cement for the insulating base. The clay will eventually fire hard; Portland cement will break down at high temperatures.

I have some concern about the hard red brick for the firebox surround; it is more brittle than old soft red brick and cannot tolerate heat-cool cycles as well. I think I would start the firebrick lower in this situation. The roof slab or whatever above the heat riser needs to be high-temp refractory, as you will get blasts of 1500-2000 F heat there. A corbeled firebrick top can work for spaces up to about 26" wide (mine is 26" square inside), or you can make brick arches if the wall tops are supported well enough, or refractory slabs.

Small holes need to be much larger in total than a single opening, as you get edge friction effects for each hole.

You don't need a special thimble to go from the first firebox surround to the rest of the bell, just a good sized opening or two at the bottom of the dividing wall.

You are making the bell considerably more involved than it needs to be. I understand supporting the roof slabs, but making effectively three secondary bells plus a collection chamber will mostly ensure that the part nearest the firebox will be significantly hotter than the other end. I would make one big open space inside the outer walls, with a porous cross wall for strength leaving large holes at top and bottom for free circulation. If you want more roof slab support, making pillars of brick where needed will add mass without increasing the overall size. The exhaust pipe running from the roof of the bell down to near the floor (known as a "plunger tube") is a good, standard idea. It may be helpful to make the bottom foot sliding so it can be raised or lowered for best performance. You want an access panel or hatch in the side so you can reach the exhaust for adjustment, and for inspection and cleanout generally.  Even a short piece of 8" stovepipe with a cap will be good.

I think the perlite-cement top slabs will work fine, as the heat there will not get high enough to degrade the Portland cement.

  I agree with everything Glenn posted. Especially the small hole effect.  An orifice only flows 62% what it's cross section implies you need 2x the cross section (40") you do not want a restriction to flow.

  Your design is a double bell and you have restricted the flow path to the second bell.  If you make that 3rd wall solid and only have a passage at the bottom, adjusting the Chimney temp by raising the bottom of the Chimney will be less efective.  This double bell could very difficult to start 1st fire in Oct every year.  

I suggest you make that 3rd divider the same as the other 2 and just have an effective single bell.

I do not see clean outs, you need 1 before the first divider and in the far wall.

Counting bricks it looks like you have the size correct for a single bell (32x32x32) 4.5 bricks x 5 x 13 courses.  However there is a 200°f burn hazard at both ends of the bench and it is tall for a chair.  

options

Brick in the chimney so the surface temperature is touchable.

Make it a single bell and move the Chimney over next to the fire box. Now both burn hazards are next to each other.  

Consider making it longer 4.5 bricks x 7 bricks x 9 courses, Is about 24" tall. (I don't think there is room.)

Do you need the bench? Turn the bell tall and skinny.  4.5 bricks x 4 bricks x 16 courses.  You lose the bench but moves the whole foot print left away from the door.  This is the thermal mass heater / Russian stove look.  Only needs 1 clean out.     Tom

Glenn Herbert wrote::
Small holes need to be much larger in total than a single opening, as you get edge friction effects for each hole.

You are making the bell considerably more involved than it needs to be.

I think the perlite-cement top slabs will work fine, as the heat there will not get high enough to degrade the Portland cement.

Hi Carl, I agree with Glenn and Thomas for most of their comments. The batchrocket is developed with a real bell in mind, not switchbacks like the old-fashioned Russian stoves used to be made. This was done in such a way that the top could be closed with the same large bricks they used for everything else.

But the Russians from 2 centuries and more ago didn't build a batchrocket as the combustion core. The main difference: the batchrocket design is a very powerful combustor, it'll produce an enormous amount of heat in a relatively short time frame. And because of this rocket-type behaviour, there's lots of gas expansion in the bell's cavity. Every change of direction of the gas stream is friction, let alone pinched areas from one cavity to another like your mock-up shows. If there is one characteristic which every batchrocket is showing: they are very, very picky about friction in the smoke path. Even when it don't look like that, it will play up nevertheless when the thing is running full bore.
Besides that, your construction means starting it in fall every year will be a bitter fight before it will play ball.

Please consider a large single bell, no switchbacks of any kind. In case you want to build a smaller bell and a bench beside, treat these as one single bell. To this end, the bench should be completely open to the bell, in fact the opening between bell and bench should be as large as the entire inside of the bench, i.e. the height and depth of it. Even a 2" high lintel construction that is lower than the bench' seat inside, will hamper the gas stream big time. I've experienced this phenomenon twice, in two different heaters, before it dawned on me what caused it to malfunction.

As a note: it isn't enough to use firebricks on flat for the top end of the bell. The gases will get very hot in there with a fat chance that the expansion/contracting cycle will drive the bricks apart. Best construction to date for a single walled bell is the one done like the Mallorca build. See https://batchrocket.eu/en/designs#squarecore
All wall higher than the riser, bricks on edge, a layer of 1/2" superwool and split firebricks as the inside liner. This way, the firebricks are allowed to expand without cracking the red bricks outer layer apart.

In case you want to have more mass, make the whole bell double walled and you will be surprised at the amount of bricks that will go in there!

Having said all that, it's your house, your heater. If you want to try untrodden territory instead of tried and proven methods, please go ahead.

Thank You Glenn, Thomas and Peter for your Input ... I will be changing my design using your suggestions ... Peter is it possible that the link for the build drawings for the Mallorca build are available in another format ... I am unable to open these and my computer is not compatible with the SKP format to download the program ... but I do get your Build plan here ... Thank You All, Carl

Carl Boehm wrote:Peter is it possible that the link for the build drawings for the Mallorca build are available in another format ... I am unable to open these and my computer is not compatible with the SKP format to download the program ... but I do get your Build plan here ...

No other format, sorry.
There is a free version of the SketchUp application that runs in a browser. https://www.sketchup.com/plans-and-pricing/sketchup-free
If and when all this fails, there's the video on youtube which is pretty well done, most of the details are there.
https://youtu.be/Yo5lBIcx_8U



Still not clear? Ask questions please, though be specific. A long list of questions won't work because I don't have time being busy with my computer for several hours to assist just one build.

I've Decided on a single Bell system ... One Last question ... How Much clearance should there be between the Riser Top and the top of the bell ... Minimum - is there a maximum ?
Thanks to all who have helped

That's an easy one. At least the same dimension as the system size.  I would prefer more, twice would be close to ideal. In practise, there isn't such a thing as a maximum. I tried a "top gap" of about 40" in a 6" system, 10 years ago, without any adverse effects. It just ran, without as much as a hiccup with very good results according to my trusty Testo.