Stratification chambers (Bells) explained

A stratification chamber is more commonly known as a bell.
A bell for a Rocket Mass Heater is nothing more than a box of a specified size with walls and a roof.
It can be any shape that fits best in the space you have.
The hottest air rises to the top and slowly sinks lower as it is replaced by hotter air (Stratifying)
At floor level, the gases locate the exit chimney and leave the bell. Ideally, exit temperatures are over 140F and below 200F  having shared the majority of heat to be stored in the bell.
Tall and skinny, low as a bench, a large cube, it can have a flat top or an arched top, and it can even go under a set of stairs as long as there is easy gas (hot air) flow.
Similar to a European masonry stove, bells can also be built as a "double skin" with a brick box inside a brick box, creating twice the mass, holding the heat twice as long, and almost zero chance of exhaust gasses entering the living area.
Unlike a European design with its serpentine channels, the stratifying bell is simple to construct.
It can be made from any nonflammable material available, sheet metal, brick, stone, or even cement board or block.
They can be designed to shed heat quickly (metal) or built with mass outside or inside the enclosure to hold the heat for 24 hours or more.
They can be built as a work of art or as utilitarian as needed to keep warm.
Depending on what design RMH you build, firebricks or insulation are used for the roof of the bell, particularly over the riser.

The most important part of bell construction is to remain at or under the specified maximum ISA (internal surface area)
Peter has determined the maximum size of a bell for each size of the first-generation Batchbox design.
5" Batch Box can have an ISA of 39.8 sq ft
6" BB can have 57 sq ft
7" BB can have 77.5 sq ft
8" BB can have 101 sq ft of internal surface area
If you build the bell to match the BB size and have a proper chimney then you should have no problems with a poor draft.
Note) those numbers are for the original first generation of Batcbboxes, Shorty core uses a reduced ISA, and I do not know the numbers for the Double shoebox variants.
Surface area is simply width x height. Each wall is measured, and if uninsulated, the roof is measured; the floor does not count, nor does the Batchbox core if inside the bell.
Those numbers are converted to sq ft and added together to determine the total bell ISA.

BYPASS)
A bypass is a shortcut to vent hot air from the top of the bell into the chimney, creating a strong draft.
Commonly, a 4"  pipe with a blast gate is used to control flow.
A properly sized bell mated to the matching  BatchBox core size, with a proper chimney, should not need a bypass.
A bypass will allow you to use a larger ISA than the published numbers. (But is not recommended by Peter)
A bypass can be a wonderful tool to ease cold startups, even with an appropriately sized bell.
A bypass can be confusing to a novice stove operator, often leaving them open for an entire burn.

Integrating a BatchBox core into a bell)
The most time-consuming and confusing part of a bell build is mating the core into the face of the bell.
Each build is different.
A core can be at floor level but is more commonly raised on a metal stand or sitting on a brick plinth.
Offset angle brackets and all-thread rods (tension frame) can be used to secure the core to the door backing plate or the door airframe.
Superwool insulation is used around your core to protect the metal tension frame pieces from warping.
Tapcon bolts can also be used to further secure the backing plate.
A door is hinged to the backing plate or the airframe.

The majority of this information comes from Peter Berg's website https://batchrocket.eu/en/building














 

Thank you.
I'm trying to figure out whether I can build a small rocket mass heater to warm two rooms , total maybe 10x25x7' high. I can imagine a location in the middle of the outside wall, but wonder how much space I need and whether the stratification chamber would help it be smaller. There's lots of information on Paul's site to help build, but it doesn't help me answer the first question. What's the minimum size?

Hi Shodo;
For J-Tube design cores 5" can be built, and 6" is the recommended minimum
For Batchbox designs 4" can be built, smaller yet has been done but again it is not recomended.

For stratification chamber size,
I listed the Max ISA for each core size from 5" up.
If you want to go smaller than max your exit chimney temperatures will start rising beyond the range where we would like them  140F-200F
It is ok to do that just not very efficient.

Thanks. I'll look into it more.

Hello Shodo,

If  you are looking to build a batch box, Peter van den Berg has given a good write-up about question you asked on his website:

https://batchrocket.eu/en/building#size

Question on ISA.

Is the internal surface area related to the size of the fuel load in the batch box? As in one load of fuel will heat this much area?

Or.......if you ran a 6 inch J tube.......and just kept it going.........if you had an ISA say 150% the maximum for the batch box......would it eventually get it up to temp?

Hi Eugene;
The Isa numbers are how large your bell can be with no bypass for your core size.
The core size change has a larger firebox but also has different dimensions.
The size of the fuel load itself is immaterial to the volume of heat output. (A batchbox does not have to be fully loaded each time)

You can make a larger bell, but it will need a bypass, which will need to stay partially open for quite a while.
If you close it too soon your dragon will stall and your house will fill with smoke.
So now (if you leave it partially open) you are sending high heat out your chimney rather than the 150F -200F that RMHs are famous for.
If you want more heat for a longer time, consider building a double-skin bell.

thomas rubino wrote:Hi Eugene;
The size of the fuel load itself is immaterial to the volume of heat output. (A batchbox does not have to be fully loaded each time)

I read  that in a way that is doubtful, ie fuel quantity is not relevant  to total output of heat.

Hi Ian;
I guess I worded that wrong.
The temperature and the efficiency of the burn are the same with a whole batch or a  half batch.
Total heat output varies by the type of wood, its dryness, and how much weight you place in each load.

A J-tube the same system size as a batch box will eventually heat up the same bell fully. How long it would take depends on the specifics of the situation.

Glenn Herbert wrote:A J-tube the same system size as a batch box will eventually heat up the same bell fully. How long it would take depends on the specifics of the situation.

Not quite correct, sorry about that. Having a J-tube of the same system size as a batchrocket doesn't mean they have the same power output. Which means the batchrocket bell is grossly oversized when combined with the J-tube. Without a bypass of some kind, the J-tube won't come up to clean burning temperature at all.

Been there, done that, learned from it.

I think it depends on the particular installation. If you are depending on a heated chimney to get enough draft, and you had a way oversized bell that shed heat quickly, you might never get really hot or a really good burn. If you have a massive bell sized just right for a 6" BB and good natural draft, a 6" J-tube would burn fine and would probably take 3 or 4 times as long to heat up the bell.

My 8" J-tube burns fiercely as soon as the kindling is lit even if the bell (about 50 ft2 ISA, 9" thick brick + cob) is stone cold, no assist from a warm chimney.

Hmmm... In October 2015, I started out with a 6" batchrocket and a bell of 64.6 sq.ft (6m²). The chimney being 6" (15cm) diameter, insulated and straight up. It was a complete disaster, the darn thing refused to play ball for two weeks straight. I brought it down to 53.8 sq ft (5m²) and it was managable, it was willing to work. Just before this 11th heating season I brought the ISA up to 57 sq ft (5.3m²), not accidentally the accepted maximum ISA, and it still worked. Although I had to be very careful not to rush a cold heater into full burn. This particular heater doesn't sport a bypass, by the way.

So, you are saying that your 8" J-tube is coupled to a 50 sq ft bell and works well. I think you are right here, this could be a little bit larger and still running well, in my opinion. But I highly doubt this same J-tube core would run as well with a bell with an ISA that's double what you have, 101 sq ft (9.4m²) to be precise. That's the maximum ISA size an 8" 1st generation batchrocket could serve without running into problems.

What I meant to say is this: in about 13 years, time and again the J-tube proved to be about half as powerful as the 1st gen. batchrocket for the same size. This isn't speculative, or an opinion, just hard numbers and the result of literally hundreds of experiments.

I agree that a J-tube depending on a heated chimney to work well needs to be sized for a batch box 2" smaller, as mine coincidentally is. (I didn't know the batch sizing parameters when I built mine.)

A J-tube with natural draft not needing the chimney warmed to burn well, I believe, could work fine in a larger bell while taking longer to store as much heat. The ISA of my bell is irrelevant to the early functioning, when any size bell would still be cold.

Peter van den Berg wrote: in about 13 years, time and again the J-tube proved to be about half as powerful as the 1st gen. batchrocket for the same size

Thank you Peter. Very valuable information.

I was thinking that I read that somewhere here a while ago.
So for J-tubes with bells we would simply take the 1st gen bbr ISA and divide it by 2.

And if the core isn’t within a bell the core surface counts as well? And a barrel obviously counts too.

Glenn Herbert wrote:A J-tube with natural draft not needing the chimney warmed to burn well, I believe, could work fine in a larger bell while taking longer to store as much heat. The ISA of my bell is irrelevant to the early functioning, when any size bell would still be cold.

Forgive me Glenn, for not mentioning why the above statement won't hold, so here we go.
Of course you are entitled to believe whatever you like.
But... there is a certain effect that is firmly based on physics, the kind that won't be influenced by faith. That effect is mostly referred to as "chimney stall". About +/- 20 minutes into the burn, the exhaust gases into the chimney need to be warmer than 60 ºC (140 ºF), otherwise the chimney draw will cease to exist and all smoke will stream into the house. What I mean with temperature measurement, is done in the very center of the chimney pipe, where the stream has its highest temperature and velocity.

I stumbled upon this phenomenon many years ago and it took a lot of time to understand what the hell was happening. As you may know, combustion of woody material will produce heat (obviously), CO² and water vapor. Quite a lot of the latter, about half a liter of liquid water for every kilogram of bone dry fuel. Translated in imperial measuments: 30.5 qubic inches of water for every 2.2 lbs of dry fuel. As such, it is a by-product of the combustion process, much like natural gas. When the fuel wasn't as dry to begin with, this water content will be added to what is going into the chimney.

For now, we concentrate on the water vapor. This will be sent into the chimney and when the temperature is low enough, something between 40 and 50 ºC (104 and 122 ºF), the vapor will condensate on the chimney wall into liquid water and runs down. Lower in the chimney it's warmer, so the water evaporates again and is added to the vapor that's already there. So it rises into the chimney, but since the gasses are more saturated with water vapor now, it will condensate in an earlier state and lower in the chimney so it runs down again. This process will be repeated over and over again, consuming more and more heat, until there's no more heat to carry the vapor to the outdoors and the chimney will reach the state what we call "stall". No more draw, all smoke and water vapor is streaming into the house.
Sometimes, the stall can be deminish by itself and the draw seems to be restored. But in almost all cases, within minutes the chimney stall shows up again.

If you like to check the above explanation, extend your bell by 100%, start the thing up stone cold and watch what happens.

Benjamin Dinkel wrote:And if the core isn’t within a bell the core surface counts as well. And a barrel obviously counts too.

No, the core surface won't count as part of the ISA. Internal walls and ceiling of the bell is what we are talking about, not the outside of whatever part is sticking out of it. A barrel is also part of the ISA, that's true.

What I use as the simplest calculating method: count all the walls and ceiling as if the core wasn't there at all.
The core, when in the bell will extract heat at first but radiate out again later. When the core is sticking out there need to be something around it, otherwise it will be much too hot. Most of the time there will be a second wall around the core to dampen the heat, in that situation.

Peter van den Berg wrote:
But... there is a certain effect that is firmly based on physics, the kind that won't be influenced by faith. That effect is mostly referred to as "chimney stall". About +/- 20 minutes into the burn, the exhaust gases into the chimney need to be warmer than 60 ºC (140 ºF), otherwise the chimney draw will cease to exist and all smoke will stream into the house. What I mean with temperature measurement, is done in the very center of the chimney pipe, where the stream has its highest temperature and velocity.

I would like to praise Peter for his explanation here,  as most of you know, I am blessed  to have tremendous draft 97% of the time, It literally can suck a newspaper crumpled  up right out of my hand and do so even without a fire lit, but a warm stove. I attribute this to Peters well designed batch box that I built a couple years back.

Anyway, if I get in a hurry and rush a cold start, exactly as Peter described can happen. But oh so- I dare say- fixable, with gentle adjustments on the bypass, and in my case, you can hear when it is going well. I never experience the need or requirement when my stove is warm

Well done Peter

Scott

Peter van den Berg wrote:

Glenn Herbert wrote:A J-tube with natural draft not needing the chimney warmed to burn well, I believe, could work fine in a larger bell while taking longer to store as much heat. The ISA of my bell is irrelevant to the early functioning, when any size bell would still be cold.

Forgive me Glenn, for not mentioning why the above statement won't hold, so here we go.
Of course you are entitled to believe whatever you like.
But... there is a certain effect that is firmly based on physics, the kind that won't be influenced by faith. That effect is mostly referred to as "chimney stall". About +/- 20 minutes into the burn, the exhaust gases into the chimney need to be warmer than 60 ºC (140 ºF), otherwise the chimney draw will cease to exist and all smoke will stream into the house. What I mean with temperature measurement, is done in the very center of the chimney pipe, where the stream has its highest temperature and velocity.

I stumbled upon this phenomenon many years ago and it took a lot of time to understand what the hell was happening. As you may know, combustion of woody material will produce heat (obviously), CO² and water vapor. Quite a lot of the latter, about half a liter of liquid water for every kilogram of bone dry fuel. Translated in imperial measuments: 30.5 qubic inches of water for every 2.2 lbs of dry fuel. As such, it is a by-product of the combustion process, much like natural gas. When the fuel wasn't as dry to begin with, this water content will be added to what is going into the chimney.

For now, we concentrate on the water vapor. This will be sent into the chimney and when the temperature is low enough, something between 40 and 50 ºC (104 and 122 ºF), the vapor will condensate on the chimney wall into liquid water and runs down. Lower in the chimney it's warmer, so the water evaporates again and is added to the vapor that's already there. So it rises into the chimney, but since the gasses are more saturated with water vapor now, it will condensate in an earlier state and lower in the chimney so it runs down again. This process will be repeated over and over again, consuming more and more heat, until there's no more heat to carry the vapor to the outdoors and the chimney will reach the state what we call "stall". No more draw, all smoke and water vapor is streaming into the house.
Sometimes, the stall can be deminish by itself and the draw seems to be restored. But in almost all cases, within minutes the chimney stall shows up again.

If you like to check the above explanation, extend your bell by 100%, start the thing up stone cold and watch what happens.

Thank you Peter for this explanation, which gave me a real light bulb moment. I have seen this happen several times with the little 4" J-tube in my glasshouse, when a perfect fire that has been burning for a while (usually less than an hour) suddenly goes into reverse.

A tee at the bottom of the vertical chimney would provide a way for the condensate to drain off and alleviate this problem.

As I'm about to attach a bench to our Matt Walker stove, this thread contains invaluable information.

On my stove, I built in a bypass at the back of the core direct into the chimney for easy lighting, and also for the possibility that some day I may want to cook on the stove without heating the whole thing and the bench.  Yes, it does require thought and discipline not to leave the bypass open and waste heat up the chimney, but it's well worth that in my view for the ease of lighting.  We live in a fairly warm climate with only 2-3 months of not super-cold weather.

So a few observations and perhaps clarifications......

Perhaps what makes the batch box more powerful is there is a larger stack of wood all burning at same time.......vs the j-tube which has one or a few sticks burning at a single point of flame........the tip of the spear?  Like twin 4 barrel carbs vs. a 2 barrel on a gas engine? Perhaps the analogy does hold up if you think of the heating of the mass as the load on the engine?

Peter's explanation of the draft temp condensation issue is very helpful. Had never seen it put that way, but does make sense. So taking that forward........some mitigation remedies...........

1. Hotter stack temps.......don't stall the engine with too much load?
2. Less stack height? Can recall issues when folks put rockets in basements and ran flues up 2 or 3 stories and they didn't work? So short stacks to chimney top vs tall? Then there would be issues with inside chimney stacks vs. outside?
3. Does it matter if chimney flue pipe is single or double wall?

Seems like there are a lot of engineering variables to consider.......and perhaps some guildelines set forth as something to consider and follow?

Hi Eugene;

An 8" J-Tube has more than just a few sticks burning. With a 7.5" throat, wide open all the time, they roar.
After many modifications, they have been "maxed out" in terms of output.

A 6" Batchbox derives its power, not from the size of the load of wood, but from the size and shape of the box and the port size in the back, and receives a supercharge of cool oxygen through the secondary air tube at the throat of the port, creating a cyclone or double rams horn of fire reaching greater burn temperatures than a J-Tube can create.

Eugene Howard wrote:So a few observations and perhaps clarifications......



1. Hotter stack temps.......don't stall the engine with too much load?
2. Less stack height? Can recall issues when folks put rockets in basements and ran flues up 2 or 3 stories and they didn't work? So short stacks to chimney top vs tall? Then there would be issues with inside chimney stacks vs. outside?
3. Does it matter if chimney flue pipe is single or double wall?

Seems like there are a lot of engineering variables to consider.......and perhaps some guildelines set forth as something to consider and follow?

I probably sound like a broken engine to those that have followed my comments in the past, but will repeat a few here.

1)  Never have a seen a fire getting to big for the design, basically meaning every design has the "proven dimensions"   for the size of the flue. With that in mind, I would answer, that NO, sized right, you will not stall the engine with to much heat,

2)  Are you calling the flue a "stack" or are  you referring to something else, my flue/chimmeny or maybe what you call stack is 35' plus tall.  So not sure what is meant by something  not working because stack height.  I will put forth that restrictions, will be first and formost the problem with flues.

3) would seem to best study what you need to know about keeping warm air rising.

Again proven ways have been just that, proven!

Many apologies..........yes I do tend to get the terms flue/chimney and stack height cross threaded........stack height sometimes used with BBQ smokers, which I'm also interested in. I'll try to use flue from now on to refer to......well......the flue. BTW, with BBQ smokers, taller stack heights are used to increase the draw, power and heat generated inside smokers. Stack size, both in diameter and height matter. Those also include dampers which are used to slow down rate of burn (smokers can get "rockety" too if allowed to take off and go.........), so dampers are used to slow down rate of burn and to force heat and smoke to back up into the smoke chamber. Offset smoker designs going thru an upheavel as designers are trying to direct the heat thru the smoker chamber to spread it out evenly for better use and results. Top down cookers gaining in popularity over traditional bottom up cookers. One should think it would be settled science by now, but it's not.

Also, thought of yet another factor that might affect chimney / flue slow downs or reversals.......and that is a source of make up burn air. Way back when, our houses and buildings were drafty, so source of make up air was easy.......there was no shortage of cold air coming in........but as homes become better built and tighter........they run out of burn air. Initially this was couched in terms of not wanting to burn heated interior air thru our wood heating appliances, but past that.......a tight house runs out of air.......basically the stove pulls a vacuum and stalls when it runs out of make up burn air. May not apply to everyone, but I suspect it may apply to some.........and some of those not aware of what is going on.

So yet another question.........if I were to use lump charcoal as my source of fuel (vs wood), would these rocket heaters.......batch or j-tube......work as well, or even at all? My guess is no.........or at least not nearly as well.

Some observations..........youtube videos from well known sources.........

First up.......one of my favorites.......but what I note is the size of the fuel load........and how it burns.......and how vortex waxes and wanes thru stages of the burn So what happens downstream of batch box when the flames go out? Where does the radiant heat from still burning charcoal go?



Next up.......will use a Liberator as example of a j-tube...........this is NOT a batch system.........a continuous burn that does not vary......meaning all components of the fuel......charcoal and gases........all burn at same time.......and as soon as fuel runs out, all heat generation stops. Unless a thermal mass is installed downstream, the heating stops. In practice.....as folks are using these.....it gets to be a problem.



And lastly what in my mind may represent best of both worlds.............with a twist..........replace the batch box of the masonry heater with a j-tube running thru appropriately sized bell........which BTW.......takes us back to the original rocket mass heater.......with j-tube, barrel riser and cob mass. But all components of the fuel burn at same rate and can burn continuous for as long as it takes to heat the load of the mass to see you thru?



OR........combine the lovely flame and vortex of the FOX batch system, as core for a masonry heater. The FOX batch system could be a commercial made outfit no different than one of the truly horrible heatilator type pre-fab fireplace cores. Do that and maybe we can take these mainstream into homes all over the place?

Charcoal burning would require a different design, similar to coal fired masonry heaters. They utilize grates to provide air to the solid fuel that will not produce any light gases that would form a vortex.

I realize this thread is about bells and such, but perhaps related..........as I think of original rocket mass heater design.........j-tube, barrel riser and cob style bench as thermal mass..........you light up the fire and keep it going for a short while until thermal mass heats up......then let fire go out and ride the radiant heat of the thermal mass for as long as it lasts.  Rinse and repeat.

What happens if you just keep the fire in the j-tube going........continuous? Does the whole thing ever reach an equilibrium point or does it just keep getting hotter in the room to the point it eventually drives you out? Once the thermal mass heats up, what happens to flue temps? Do they start rising also to point where you are no longer storing heat but losing it up the flue/chimney?

My 8" J-Tube in an uninsulated greenhouse in northern Montana was run nonstop from apx 7 AM to 10 PM all winter long.
I call it extreme burning. With an exposed barrel and 45' of buried 8" piping, we could easily top 80F while it was in the low teens outside.
My chimney exhaust gas temperatures were well over 340°F; they should be 180°F- 220°F.
By 7 AM the next morning, it would be in the low forties or upper thirties and dropping steadily.

My new all-brick double bell first-generation BatchBox is in the exact location. And easily brings the room to the lower sixties with three fires a day.
The next morning, it will be in the upper fifties and creeping downward at a tenth of a degree every half hour or so.