Seeking advice for a stacked brick rocket-stove sap evaporator/summer outdoor stove.

I have searched and read every thread I could find on rocket sap evaporators, beginner rocket stoves, brick types, materials, refractory/non-refractory, and so on.  I am not looking to build something to make a serious amount of syrup, like this (https://permies.com/t/54381/Rocket-Stove-Maple-Syrup-Evaporator), as I am only running 3 taps, more as a spiritual practice of reciprocity with the forest I care for.  I thought my airtight wood stove would handle the mere 3 gallons or so of sap a day but it can't even keep it at a boil.  My other motivation is to get some experience working with brick and mud for my future build of a small rocket mass heater for the tiny dome home I will be building and, hopefully, have an outdoor cooking stove for the Summer.

This is what I have to work with.  A lot of old brick, salvaged from an old building I assume, hand formed clay and solid.  Some odds and ends of some newer brick, some look like they are made of concrete and are very heavy. An unlimited supply of nearly pure clay, mixed with rocks, underlaying the topsoil (and yes, I know I have to screen out the gravel), and a lesser amount of both aged and dry as well as fresh and wet donkey manure. I can buy sand, it doesn't grow here.

I need some advice on dimensions for the feed tube (? where the wood goes in and burns),  as well as how chambers are laid out/ how they run.  I cannot just buy "plans" as I don't think anything exists that suits my needs, plus this is really a learning exercise as well.   I want a "cooking" surface that can accommodate 2 standard steam-table pans, 16 1/4" x 9 3/4", and a canning pot 12" in diameter at the same time. And preferably, no sheet metal parts.  

Below I'll list some useful threads I have read already, just so you know what I have already searched through and maybe understand.  

Any and all advice gratefully accepted. I promise to follow up with photos and diagrams of the build!

https://permies.com/t/137193/bricks-simple-rocket-   Basic dry stack with good thread, but too small (cooking surface area) for boiling sap.

https://permies.com/t/30551/Fake-fire-brick#238559  THE BEST AND MOST THOROUGH treatment on different brick types and their uses, including understanding why/how to insulate various parts of stoves. Posted 9 years ago by Erica Wisner; I hope she is still on staff!

https://permies.com/t/61789/Designing-combination-shop-heater-maple  Good post and thread but the sketchy drawings didn't give me any real information I could work with.  It also isn't really what I need since it is also a mass heater build, too big.

https://permies.com/t/63359/Maple-Syrup-Stove-Evaporator  This gives an idea of the cooking surface I'm looking for but the build is unwieldy and of concrete block, with nothing showing how it is constructed.  The thread of answers don't apply to me as I don't need to produce so much and can't get into manufacturing metal doo-dads.

Hi Freyda! I am just east of Binghamton.

The two evaporators I have built have used the shells of a kitchen stove and freezers to house the firebox and support pans and chimney connections. If you have access to a derelict appliance and a sawzall, angle grinder, or tinsnips, that will make it easier to build a durable small evaporator. You will get much better performance with a rocket fired unit if you add a chimney, probably 6 to 10 feet of 6" galvanized duct for your size. I would advise an L-tube design, with a horizontal feed/burn tunnel around 7" wide x 8" high x at least 24" long, and a riser of similar dimensions and at least 24" high from the floor. If you have a really large number of bricks and don't wish to make a permanent or moveable setup, you could just stack bricks for the whole thing (plus metal chimney).

I would make your feed and riser as described, then at the top of the riser, make a trough shape leading from the riser top with around 4" clearance to the bottoms of the pans and heading toward the chimney collection/mounting point. A flat floor with brick walls wide enough to hold the rims of your pans, transitioning to wide enough to hold your canning pot, would be what you want. You will really want some sort of closure around the 12" canning pot so that you can keep draft through the fire to the chimney. Big tin cans cut and flattened and bent as required would do okay in your small setup. Use one or more of these at the end to connect the chimney stovepipe. I am not at a location where I can easily make sketches to post right now.

If your clay is anything like mine, between the clay and gravel there is quite a bit of silt and sand, and you may not need to add anything to make good cob. For mass cob, I don't even bother to remove anything smaller than around marble to golf ball size, depending on the intended use. Only for finish layers or thin bedding layers do you need to screen the clay. Adding chopped straw will give good wet and dry strength.

A good way to make the feed/burn tunnel with brick is to make walls of bricks on end, flat faces facing the fire, and spaced so that they can support bricks laid across the tops of the walls. You will need to stabilize this by containing the wall bricks. Anything functional is fine, from more bricks as mass backup, to metal sheets or bands, to a thick layer of cob. You will want to insulate the top and sides of the tunnel and riser as much as you can. Scraps of fiberglass will work for a few years. Obviously you need noncombustible insulation Cob mixed with a large amount of perlite would be a good long-term insulating stiffener.

Glenn Herbert wrote:Hi Freyda! I am just east of Binghamton.

The two evaporators I have built have used the shells of a kitchen stove and freezers to house the firebox and support pans and chimney connections. If you have access to a derelict appliance and a sawzall, angle grinder, or tinsnips, that will make it easier to build a durable small evaporator. You will get much better performance with a rocket fired unit if you add a chimney, probably 6 to 10 feet of 6" galvanized duct for your size. I would advise an L-tube design, with a horizontal feed/burn tunnel around 7" wide x 8" high x at least 24" long, and a riser of similar dimensions and at least 24" high from the floor. If you have a really large number of bricks and don't wish to make a permanent or moveable setup, you could just stack bricks for the whole thing (plus metal chimney).

I would make your feed and riser as described, then at the top of the riser, make a trough shape leading from the riser top with around 4" clearance to the bottoms of the pans and heading toward the chimney collection/mounting point. A flat floor with brick walls wide enough to hold the rims of your pans, transitioning to wide enough to hold your canning pot, would be what you want. You will really want some sort of closure around the 12" canning pot so that you can keep draft through the fire to the chimney. Big tin cans cut and flattened and bent as required would do okay in your small setup. Use one or more of these at the end to connect the chimney stovepipe. I am not at a location where I can easily make sketches to post right now.

If your clay is anything like mine, between the clay and gravel there is quite a bit of silt and sand, and you may not need to add anything to make good cob. For mass cob, I don't even bother to remove anything smaller than around marble to golf ball size, depending on the intended use. Only for finish layers or thin bedding layers do you need to screen the clay. Adding chopped straw will give good wet and dry strength.

Dear Glenn,

I am so excited that there is a Rocket Scientist so close to where I live!  This raises my hopes that my plans will go beyond the dreaming and planning stages.
Thank you so much for such a thorough answer to my query.

I actually had nearly an entire lengthy response written yesterday and then they disappeared when my hand hit something on the keyboard.
Not the first time, so now I’m writing in Word and then cut/pasting to Permies!

I will attempt to do a plan drawing tomorrow but first I want to be sure I understand your suggestions clearly.

Taking the build from front to back:
Feed tube/fire box: tunnel of brick, 2’ long
Riser: 90° turn upward, brick 2’
Cooking surface: 90° turn to horizontal continuing in line, open top trough of brick. If pans lengthwise, 4’ length
Base for chimney: 1’ more.
So, if everything continues in a straight line, I would need 9’ for the stove, plus some more room for a flat base and clearance for loading wood and around the chimney area.

This brings up a few questions in my mind.

1.Does everything have to proceed in a line? After the riser, would a right angle turn to square the footprint interfere with the draft?

2. The trough will have to flare out only 1” to accommodate the narrow width of the pans. I am assuming you mean to set the pans down into the opening? Since they have a lip all around, setting them with the bottoms resting on the brick would leave gaps of at least an inch between the pans where air would escape.

3. Would widening the trough to 15 ½” accommodate the length of pans crossways (to shorten the overall length) kill the draft?

4. I’ve read in some other threads discussions of expansion/contraction stresses on these stoves.  Is that a concern with dry-stacked bricks and clay soil/cob? (I’m pretty sure we share the same clay soil type. Where we’ve pulled it out of planting holes for fruit trees, just piled up next to the hole and baking in the summer sun, it solidifies into near concrete that withstands fall rains and winter snows!)

That’s enough questions for now.  Based on your answers, I’ll draw up the rough plan tomorrow and post it.

Thanks again for your help!
Freyda

Glenn Herbert wrote:A good way to make the feed/burn tunnel with brick is to make walls of bricks on end, flat faces facing the fire, and spaced so that they can support bricks laid across the tops of the walls. You will need to stabilize this by containing the wall bricks. Anything functional is fine, from more bricks as mass backup, to metal sheets or bands, to a thick layer of cob. You will want to insulate the top and sides of the tunnel and riser as much as you can. Scraps of fiberglass will work for a few years. Obviously you need noncombustible insulation Cob mixed with a large amount of perlite would be a good long-term insulating stiffener.

I'm wondering if, at least for the first trial runs of working out this design, if I could use wood ash as an insulation material.  I could maybe mix it with enough water to make it like a mortar and press it around the burn tunnel and riser and then contain it with concrete blocks.  Also thinking of bedding the floor bricks in it.  Would that work?  I have loads of the stuff that's accumulated from using an airtight stove to heat the house.   I don't seem to be successful searching for threads on insulation materials for rocket stoves here.  

Also, if you could look at my previous questions in the thread I'd really appreciate it.

Thanks again.

The riser would not add anything to the horizontal dimensions. The cooking surface would overlap about a foot over the firebox, and I would orient the pans crosswise to the flow for more compactness. So you could fit the whole thing in less than 6'. One of the evaporators I built has the firebox at right angles to the pan. The fire doesn't care about that.

I did mean to sink the pans down into the trough supported on their rims, both for better sealing and better heat transfer.

I think the wood ash combined with a bit of clay would work fine for sealing, stabilizing and insulating the firebox and riser. If there is any charcoal or unburned material in it, that would be consumed as you heat the firebox leaving some voids.

Glenn Herbert wrote:The riser would not add anything to the horizontal dimensions. The cooking surface would overlap about a foot over the firebox, and I would orient the pans crosswise to the flow for more compactness. So you could fit the whole thing in less than 6'. One of the evaporators I built has the firebox at right angles to the pan. The fire doesn't care about that.

I did mean to sink the pans down into the trough supported on their rims, both for better sealing and better heat transfer.

I think the wood ash combined with a bit of clay would work fine for sealing, stabilizing and insulating the firebox and riser. If there is any charcoal or unburned material in it, that would be consumed as you heat the firebox leaving some voids.

Thanks so much for the clarifications and answers to my questions.  If it's not snowing tomorrow,  I'm going to get started on my build.  Probably won't get any farther than carrying all the bricks and other materials to where I want to build and trying to get it all level. Oh, and there's getting the new tire on the wheelbarrow to get the clay with. I'm glad we left some piles of it next to the fruit trees we planted last year, but the orchard is quite a hike up the hill!