Question to double skin bell of BBR

[firerob]

Hi everybody,

first of all: THANK YOU ALL VERY MUCH for all of these wonderful information here!!
Especially Donkey and Peter vdB for all of the work you put into this and the sharing of it!!

Right now we are putting the finishing touches on the cob house that we were building during the last
3 summers and now it's time to install the heating system before it starts to get cold here in Hungary.

I've been researching and reading about BBR's (mostly here) for a few years and now I feel confident
to build a Batch Box Rocket Stove Mass Heater. (Years ago I built a J-tube barrel Rocket Stove).

We won't be able to build the system first outside to test it because we are pressed for time and dry space,
so we will have to build it up directly inside and hope for the best

That's why we would really like to bounce our plans off of you guys here, just to make sure that we
didn't miss or overlook anything important or that we are having any major or minor errors in our design.

And also we have one big question regarding the design of the upper part of the bell (see below).

All of your input will be very much appreciated - I admit, I'm kinda nervous to build this, because we
will fully rely on this BBR in the upcoming winter and if it fails half way through the winter,
we'll have a serious problem!


Here are the specs:

It's a 180mm system with a single bell. The core (firebox + heat riser) will be inside the bell.
The exit flue will be outside of the bell. The exit flue is made of aluminium, the coldish gases
will be collected 6cm above ground level and then from there they go straight up for 4 m
(through the roof). No bends.

The core will be raised 40cm off the ground level (so the firebox sits higher than the exit flue).

To build the core we have already made (moulded) insulated bricks resp. heat riser elements

out of perlite and (soil-)clay.

The perlite bricks with which we'll build the firebox (bottom, sides, top and backwall) are 15cm x 30cm x 5cm.

The heat riser elements are round: inside diameter 18cm, outside diameter 30cm, they are 13cm high
(we have 10 pieces = 130cm).

The perlite bricks will be mortared together with a mix of waterglass and grog powder and they will be
supported from the outside by a welded rebar frame. The rebar frame won't be insulated with ceramic
fibre blanket and I hope that this far down in the bell spalding won't be a problem.
Any concerns regarding this?

The inside of the firebox will be plastered with a 2-3mm layer of waterglass and grog powder to make it
more durable against abrasive forces. (It's very hard, I made some tests).

All of our design and numbers are based on the datasheets and info on batchrocket.eu, which means:

- Firebox measurements: 52 x 39 x 26 cm

- ISA of bell: 7,37m² (at this point in the planning) (excluding the floor, including the core support)
- Port size, P-channel, etc - everything according to batchrocket.eu/en/building#dimension


The core support (40cm high) will be a closed box (NOT coloums!) out of regular used (fired) bricks
(not firebricks!). Will this box design effect the flow of gases toward the exit in a detrimental way?

Between the core support box and the walls of the bell there is a space of 11cm. Will this be enough for
the coolest gases at the bottom to flow comfortably towards the exit flue in the backwall of the bell?


The bell will be build with single skin up to almost heat riser top and then double skin with ceramic fibre
blanket in between. The ceiling (also double skin with ceramic fibre blanket in between) will be supported by
L-Profile iron which will be wrapped in ceramic fibre blanket.

Pretty much like the built of the "Free University, Brussels" open system. Pictures you can find here (scroll down):
batchrocket.eu/en/applications#opensystems

Looks like they used regular bricks laying flat up until almost heat riser top and then they switched to
double skin with the inside skin of - what looks like - firebricks and the outside skin from regular bricks
upright and ceramic fibre blanket in between.


Now, the big question is this: could we build the entire bell from regular (fired) brick. Which means
the single skin part and both skins of the double skinned part, including the ceiling?
Would the regular bricks withstand the heat? Is this safe to do? If the ceiling collapses it's a disaster.

Again, we really appreciate every input! Thanks a lot in advance.

Rob & Zsofi

[peterberg]

Sept 15, 2023 10:04:12 GMT -8 firerob said:

It's a 180mm system with a single bell. The core (firebox + heat riser) will be inside the bell.
The exit flue will be outside of the bell. The exit flue is made of aluminium, the coldish gases
will be collected 6cm above ground level and then from there they go straight up for 4 m
(through the roof). No bends.

Ahhh, the first blooper. Please use a steel stove pipe, no aluminum. The exhaust temperature cannot be guaranteed, in the midst of winter when you fire the heater maybe three times a day the alu pipe could give way.

Sept 15, 2023 10:04:12 GMT -8 firerob said:

The core will be raised 40cm off the ground level (so the firebox sits higher than the exit flue).

Sensible.

Sept 15, 2023 10:04:12 GMT -8 firerob said:

To build the core we have already made (moulded) insulated bricks resp. heat riser elements
out of perlite and (soil-)clay.
The perlite bricks with which we'll build the firebox (bottom, sides, top and backwall) are 15cm x 30cm x 5cm.

The heat riser elements are round: inside diameter 18cm, outside diameter 30cm, they are 13cm high
(we have 10 pieces = 130cm).

The perlite bricks will be mortared together with a mix of waterglass and grog powder and they will be
supported from the outside by a welded rebar frame. The rebar frame won't be insulated with ceramic
fibre blanket and I hope that this far down in the bell spalding won't be a problem.
Any concerns regarding this?

The frame isn't a problem, rusting would usually occur in summer, when moist is able to collect in the very dry heater. And in the first month of running the thing, a fine layer of tar will form on all surfaces, so steel is nicely preserved.
Sad to say, the perlite bricks might be not up to the job of holding up for many years. The achieved temperatures are too high for perlite, which is softened when heated to more than 850 ºC. Top temperatures in the firebox 950 ºC, in the riser 1200 ºC. You might get away with it, but I would use hard firebricks for the entire core.

Sept 15, 2023 10:04:12 GMT -8 firerob said:

The inside of the firebox will be plastered with a 2-3mm layer of waterglass and grog powder to make it
more durable against abrasive forces. (It's very hard, I made some tests).

Be careful with waterglas, it's irritating for skin, lungs and eyes.

Sept 15, 2023 10:04:12 GMT -8 firerob said:

All of our design and numbers are based on the datasheets and info on batchrocket.eu, which means:

- Firebox measurements: 52 x 39 x 26 cm - ISA of bell: 7,37m² (at this point in the planning) (excluding the floor, including the core support)
- Port size, P-channel, etc - everything according to batchrocket.eu/en/building#dimension

The core support (40cm high) will be a closed box (NOT coloums!) out of regular used (fired) bricks
(not firebricks!). Will this box design effect the flow of gases toward the exit in a detrimental way?

Why using a closed box? Why not a steel stool for it? Or even some half-brick columns. Completely open under the firebox is close to ideal, exhaust gases are friction free to enter the exhaust opening and all fine dust will settle on the floor. A couple of years ago I did the core support this way: one L-shape steel profile at the back, resting in the left and right bell wall. Left and right of the firebox another profile, resting on the front wall and at the back on the other profile. Welded together in such a way that the firebox floor slab was enclosed on three sides by the steel.

Sept 15, 2023 10:04:12 GMT -8 firerob said:

Between the core support box and the walls of the bell there is a space of 11cm. Will this be enough for
the coolest gases at the bottom to flow comfortably towards the exit flue in the backwall of the bell?

As you mentioned, the exhaust opening is 6 cm above the floor. This is not enough, make that at least 10 cm. See the stream profile as an imaginary ring: the circumference of the opening with a height of 11 cm. The surface area of this ring should be at least 2 times the cross section area of the exhaust. In your case, it's 2.44 times so it's possible but another construction has other benefits.

Sept 15, 2023 10:04:12 GMT -8 firerob said:

The bell will be build with single skin up to almost heat riser top and then double skin with ceramic fibre
blanket in between. The ceiling (also double skin with ceramic fibre blanket in between) will be supported by
L-Profile iron which will be wrapped in ceramic fibre blanket.

Why single skin? Remember, the Mallorca build was done this way in order to create a reasonably responsive heater, it's an Mediterranean island. In a colder climate, a double skin heater has a lot more mass and will evening out the delivered heat much more. So, it all depends on the place were you are.
By the way, the ceiling was done with steel T-profiles upside down. In case these aren't to be found in a good size: use two L-profiles and weld these together back to back. Easy enough for a person with welding equipment and it is a sturdier construction. Also, superwool at the underside of the steel isn't really necessary, provided there isn't one directly above the riser. Further away from the riser is also advisable, so a larger distance from riser to ceiling.

Sept 15, 2023 10:04:12 GMT -8 firerob said:

Looks like they used regular bricks laying flat up until almost heat riser top and then they switched to
double skin with the inside skin of - what looks like - firebricks and the outside skin from regular bricks
upright and ceramic fibre blanket in between.

The switch to bricks on its side wouldn't leave enough room for the superwool and split firebrick liner. So in this heater, we used an intermediate layer of bricks on flat, turned 90 degrees in the horizontal plane. Have a look at the pictures, or at the sketchup drawing for that matter.

Sept 15, 2023 10:04:12 GMT -8 firerob said:

Now, the big question is this: could we build the entire bell from regular (fired) brick. Which means
the single skin part and both skins of the double skinned part, including the ceiling?
Would the regular bricks withstand the heat? Is this safe to do? If the ceiling collapses it's a disaster.

I honestly don't know whether or not the regular bricks would stand the heat, it depends on how wide the bell is and how high the ceiling above the riser. The further apart, the more the heat is spread over a larger area. Normal bricks are fired at a temperature between 900 and 1100 ºC, depending on the clay composition. As a rule of thumb, the resulting bricks are heat resistant for about half of the baking temperature. Not guaranteed, but this is the ballpark you need to think about.
Gas temperature spewing out of the heat riser: about 900 ºC, so a ceiling that's sufficiently away from the riser and large enough would absolutely necessary. No numbers, sorry for that, as far as I know this isn't done before. As a note: the ceiling won't collapse due to cracking of a couple of bricks. There are more than one layer of bricks in a ceiling, with superwool in between.
In case you want to go this route: keep an eye on the ceiling during winter. I think it won't cave in just like that but start sagging first which should be visable from outside.

[fiedia]

Hello firerob,

Regarding temperatures inside the bell, I did some measurements inside my 200mm BBR. Here is a typical run :

You can see that the brick ceiling above the HR will be too hot for normal bricks. You would better use refractory material. In contrary, the temperature decreases very rapidly underneath the ceiling. One row of refractory bricks under the bell ceiling should be enough.

Regarding bell exit, the chart tells also how fast the temperature decreases inside the bell. The bottom of your bell will stay cold. Unless you have a bench after the bell, you can put the whole bell and exit 40cm above the ground, it will not affect much the heating performances. My core sits 70cm above the ground.It is much much easier to load. The core window radiates more efficiently through the room at this height. 

Fumes will condense inside your exit pipe. Either use an insulated pipe or make sure that the brown stinky juice can not escape from your bell. If it stays inside the bell, it will dry eventually.

Keep us informed. Drawings are often better than descriptions. They may show some important details.

[firerob]

Sept 16, 2023 2:31:49 GMT -8 peterberg said:

Sept 15, 2023 10:04:12 GMT -8 firerob said:

It's a 180mm system with a single bell. The core (firebox + heat riser) will be inside the bell.
The exit flue will be outside of the bell. The exit flue is made of aluminium, the coldish gases
will be collected 6cm above ground level and then from there they go straight up for 4 m
(through the roof). No bends.

Ahhh, the first blooper. Please use a steel stove pipe, no aluminum. The exhaust temperature cannot be guaranteed, in the midst of winter when you fire the heater maybe three times a day the alu pipe could give way.

Steel would be my first choice too, but it was much easier to find 180mm tube from aluminium here. Steel pipe with this diameter is hard to find and then very expensive.

Your point is good though! The exit flue temperature of around 100°C is not guaranteed. Hmmm...that's true, I guess.

Aluminium melts around 700°C. I can imagine that the exit temp might go up to 200°C, maybe 300°C or maybe even 400°C if the mass in the bell is really fully heat saturated, 

but I cannot imagine it going up to alu melting temp!?

With all of your experience from all of the BBR's that you built: did you ever see the exit flue temp going up so high that it could melt aluminium?

What would have to happen to get the exit flue temp to over 600°C?

Sept 16, 2023 2:31:49 GMT -8 peterberg said:

Sad to say, the perlite bricks might be not up to the job of holding up for many years. The achieved temperatures are too high for perlite, which is softened when heated to more than 850 ºC. Top temperatures in the firebox 950 ºC, in the riser 1200 ºC. You might get away with it, but I would use hard firebricks for the entire core.

Ah, that's sad news indeed. I never heard this info before. I just read that others are building the core out of perlite/clay mixes and since nobody reported failures, I opted for this.

I guess at this point we'll have to take our chances. I was planning anyway to take the bell partly apart next summer to check on the insides. Hope it'll last at least one winter.

Sept 16, 2023 2:31:49 GMT -8 peterberg said:

A couple of years ago I did the core support this way: one L-shape steel profile at the back, resting in the left and right bell wall. Left and right of the firebox another profile, resting on the front wall and at the back on the other profile. Welded together in such a way that the firebox floor slab was enclosed on three sides by the steel.

Beautiful idea!!! This way it's much easier to build, it's better for the gas flow and L-Profile we need anyway for the top! That's the way we'll do it. Thank you.

We'll just have to keep in mind that without the core support box we are loosing some 0.2m² of ISA. Which means we will have to make the bell a little wider and/or deeper 

(higher we cannot go, we are already pushing 2.2m).

Sept 16, 2023 2:31:49 GMT -8 peterberg said:

Sept 15, 2023 10:04:12 GMT -8 firerob said:

Between the core support box and the walls of the bell there is a space of 11cm. Will this be enough for
the coolest gases at the bottom to flow comfortably towards the exit flue in the backwall of the bell?

As you mentioned, the exhaust opening is 6 cm above the floor. This is not enough, make that at least 10 cm. See the stream profile as an imaginary ring: the circumference of the opening with a height of 11 cm. The surface area of this ring should be at least 2 times the cross section area of the exhaust. In your case, it's 2.44 times so it's possible but another construction has other benefits.

I think there is a misunderstanding. The exhaust hole will be 20cm x 20cm big and it will be starting at 6cm off the ground (the height of 1 brick). 

This are 400cm² and CSA of riser is 255cm². I could make it bigger, say like 20cm high and 30cm wide (=600cm²). 

Should I go bigger than 400cm²?

Sept 16, 2023 2:31:49 GMT -8 peterberg said:

Why single skin? Remember, the Mallorca build was done this way in order to create a reasonably responsive heater, it's an Mediterranean island. In a colder climate, a double skin heater has a lot more mass and will evening out the delivered heat much more. So, it all depends on the place were you are.

That's interesting. I never thought about a full double skin bell. 

I never heard it or saw videos or pictures neither. 

It would take around 8h until the outer brick surface starts radiating heat, I would assume!?

That would need much more CFB than we have at the moment. I need to think about this.

Sept 16, 2023 2:31:49 GMT -8 peterberg said:

Sept 15, 2023 10:04:12 GMT -8 firerob said:

Looks like they used regular bricks laying flat up until almost heat riser top and then they switched to
double skin with the inside skin of - what looks like - firebricks and the outside skin from regular bricks
upright and ceramic fibre blanket in between.

The switch to bricks on its side wouldn't leave enough room for the superwool and split firebrick liner. So in this heater, we used an intermediate layer of bricks on flat, turned 90 degrees in the horizontal plane. Have a look at the pictures, or at the sketchup drawing for that matter.

Yeah, I saw the pictures and love the design! It would have been tricky to build otherwise and it even gives a little "mantle" around the bell. 

Plus it breaks up the monotony of a plain 2,2m high wall!

The question is though: this intermediate layer will stick a few cm into the bell. Will this not be detrimental for the gas flow downwards?

I could also cut the bricks a little shorter, so this layer would be flush on the inside. (I just did some dry stacking). Do you think this would be worth the effort?

Thank you, cheers

Rob

[firerob]

Hi fiedia,

intersting chart, very cool. Thanx. I'm just not sure if I understand it correctly.

HR bottom 800°C surprises me! (I assume this means inside of the heat riser behind the port).
I would have expected the temp to be much higher (pushing 1000°C or more).

But anyway, why is there only a peak to 800°C? Shouldn't the temp stay on a plateau at least for as long
as the fire is going full blast?

Because if there would be a plateau (temp would stay on 800°C for, say 40min), then
wouldn't the temperature 22cm below the ceiling go up eventually?
Namely in the same rate as the top part of the bell gets heat saturated?

Where is the mistake in my thought process?

Cheers,

Rob

[peterberg]

Sept 17, 2023 7:14:11 GMT -8 firerob said:

Yeah, I saw the pictures and love the design! It would have been tricky to build otherwise and it even gives a little "mantle" around the bell. Plus it breaks up the monotony of a plain 2,2m high wall!
The question is though: this intermediate layer will stick a few cm into the bell. Will this not be detrimental for the gas flow downwards?
I could also cut the bricks a little shorter, so this layer would be flush on the inside. (I just did some dry stacking). Do you think this would be worth the effort?

No, not at all, cutting the bricks isn't wortwhile. Besides, laying the bricks like that is best done with the same weight at both sides. Moreover, the outer shell has to be built first and flush with the rest of the bell, this is the easiest. Been there, done that, tried it.

For gas flow, see the example of the two barrels on top of each other. Both have the bottom cut out, leaving a ring of approx. 25 mm wide. This looks like breaking the downstream along the wall and it does, but this is marginal. A few cm above this ridge the temperature decends some 50 degrees C, and comes back to the higher temp again 10 cm below the ridge. What happens is the stream bends away from the wall to avoid the ridge and returns to the wall below it. Logical, energy streams from high to low, so it tends to stay close to the wall because that's the coldest.
Sorry, can't find the drawing that illustrates the effect.

[peterberg]

Rob, this exhaust opening of 20x20 cm is large enough, especially as you are planning to lift the core above the floor with nothing underneath. This ensures a low gas velocity low in the bell so the gases will change direction much easier.

[fiedia]

Sept 17, 2023 7:53:35 GMT -8 firerob said:

Hi fiedia,

intersting chart, very cool. Thanx. I'm just not sure if I understand it correctly.

HR bottom 800°C surprises me! (I assume this means inside of the heat riser behind the port).
I would have expected the temp to be much higher (pushing 1000°C or more).

Yes it is behind the port, 20cm above the HR bottom. It can reach 1000°C but you do not want this. Best is to stay between 700°C and 900°C. Reaching 1000°C  often means that you get a dirty burn.

Sept 17, 2023 7:53:35 GMT -8 firerob said:

But anyway, why is there only a peak to 800°C? Shouldn't the temp stay on a plateau at least for as long
as the fire is going full blast?

Because if there would be a plateau (temp would stay on 800°C for, say 40min), then
wouldn't the temperature 22cm below the ceiling go up eventually?
Namely in the same rate as the top part of the bell gets heat saturated?

Where is the mistake in my thought process?

Cheers,

Rob

The peak seems narrow because the chart shows a 12 hours run. Have a look at the chart below with a 2 hour range. 

Here is a new chart with 4 runs a day. I see no plateau and the bell is not getting much warmer..

If you want higher temperatures at the bottom of your bell, You have to insulate the upper part. You may get a more uniform temperature but the bell skin temperature will raise slower and remain lower. Up to now my problem was to get a bell warm enough to heat my room. Therefore, I did not insulate bell and firebox walls.

[foxtatic]

Sept 17, 2023 6:13:24 GMT -8 fiedia said:

Hello firerob,

Regarding temperatures inside the bell, I did some measurements inside my 200mm BBR. Here is a typical run :

You can see that the brick ceiling above the HR will be too hot for normal bricks. You would better use refractory material. In contrary, the temperature decreases very rapidly underneath the ceiling. One row of refractory bricks under the bell ceiling should be enough.

fiedia Just to be real clear on your chart, does "-22cm" and "-44cm" mean these temps were taken on the inner bell wall at 22cm and 44cm below the ceiling? 
Someone is using concrete as their bell wall material in another post and I always read that was a no-no. But if the temp drops that low just 22cm below the bell ceiling... I'm tempted to use concrete at least at the base of the bell. Maybe even halfway up before transitioning to red brick (then fire brick above the riser of course.)  

[firerob]

Sept 20, 2023 9:57:33 GMT -8 fiedia said:

Ok, thanks for the scaled down chart, but I am still confused about the missing plateau: 

According to batchrocket.eu/en/building#size the wood load in your 200mm system is around 12kg.

The chart shows that after 12 minutes the temperature in the HR reaches 900°C, this still makes sense but then 24 minutes into the burn the temp drops to 700. 

Shouldn't 12kg of wood give a strong burn for more than 24 minutes, which means that the HR temp should stay up instead of dropping down 200°?

I am not trying to be difficult, I just have the feeling that there's something I have not understood yet about the dynamics.

Furthermore, you have a 3 bell system, right? 

If the temp in bell 1 never goes beyond 250°C measured 44cm below the bell ceiling, then how warm can the gases be that enter your bell 2?

Do you have numbers for bell 1 exit temperature?

-rob

[firerob]

Ok, so we decided NOT to use the perlite/clay bricks and build the
core from firebricks with cfb around.

I'll document our progress here and I hope it'll help someone down the
road as much as it helped me to watch hof's documentation
"6.3" brick rocket stove in Russia"

These are the perlite bricks we made in vain 😪



This is how far we are right now





With the heavier core (firebricks instead of perlite)
we need to re-inforce the L-profile support structure
today.

[firerob]

Why are the pictures just showing up as links?

What do i need to change to see the pix directly?

Thx

[Vortex]

idrive seem to be blocking the ability to embed pictures. The links you posted are to the pages your pictures are on, not the pictures themselves, but there seems to be no way in the webpage to extract the actual picture links as the html is blocking right click function.

[firerob]

Sept 29, 2023 0:55:44 GMT -8 Vortex said:

idrive seem to be blocking the ability to embed pictures. The links you posted are to the pages your pictures are on, not the pictures themselves, but there seems to be no way in the webpage to extract the actual picture links as the html is blocking right click function.

Thanx Vortex.

Does it work with Dropbox to embed the pictures?

[Vortex]

I've never used Dropbox but it seems it's possible: www.dropboxforum.com/t5/Create-upload-and-share/How-do-I-embed-images-with-a-direct-link-from-Dropbox/td-p/245432