Batch Rocket - Build Question

I have a question for you rocket scientists out there. I have my brick bell started, the combustion core will be on a stand fully enclosed in the bell.

In the pictures below you'll see the chimney exhaust box on the side of the bell. I believe the opening is now tall enough to bridge across and I'll complete the stove pipe connection details once the bell is completed. The exhaust opening is 13" wide x 8" high, I can go one more course of brick to make the opening taller but I'm thinking this is sufficient. First question is whether my opening is large enough as is?

The second question has to do with the bridge across the opening. I'm planning to use angle iron to support the brick. I can place some 16 gauge sheet metal on the angle iron for full support. Wondering if I should place superwool gasket around the steel to isolate it from the brick or will it be cool enough this low down to not worry about it? Any other suggestions for the construction detail of the bridge will be appreciated. My hope is to get it right and never have to revisit this section as it will be a whole lot of brick to tear out if it needs to be rebuilt.

Thanks in advance for any input.

Glenn,

It would be helpful to know what size cross section your batch box is going to be.  Are you designing for a 8 inch batch box, or another size.   This will determine whether or not your exhaust port is large enough.  I have seen angle iron of mild steel used in many fireplaces and RMH iterations.  I see no problem with what you've done here.  I think you have a nice start and you were wise to ask for guidance at this juncture in your build.

BTW.  I'm not really a rocket scientist.  I'm just a knucklehead.  But the real Rocket Scientists are not far away and I'm sure they'll be chiming in soon to help you further.

Thanks for your comments Thomas... and a good question. I should have mentioned that this is a 6" system. My understanding is that the exhaust box opening as I am building it is not a critical dimension, other than not wanting it too small. I would just like some further validation since I am right at the point that I can easily make the opening one more course of brick taller.

Perhaps worth mentioning that the ceiling of the box will immediately connect to my existing 6" stove pipe that is 24' high from an old cook stove that I removed. I will also be installing a bypass in the upper part of the bell to get a good draft when starting.

Oh, and BTW... I'm a knucklehead too aspiring to be a rocket knucklehead.

From the image it looks that the interior circumference of the bell will be around 3 m (15 modular bricks). If the system is 150 mm, the required ISA 5.3 m2, so the height of the bell would be 1.77 m.
So if this is a 150 mm (6") system, your opening to the chimney is sufficient.
You can make it smaller: 6x6", so it will be easier to span it with bricks without any metal. You can chamfer the corners of the bricks to the entrance to the chimney for smoother exhaust flow.

Also, I would place the bricks on the bottom of the bell - it's a good practice.
Are you planning to make a bell with two wythes of bricks? It's too thick. One is enough for medium storage capacity heater, or for higher capacity - the outside skin bricks laid on shiners and inside laid on stretchers.
I also suggest a cleanout opening with a metal door, or at least with removable bricks. It should be placed on the bottom of the bell in such a way that every part of the bell can be accessed for cleaning. It's best to position it one course of brick above the bottom floor, so the ashes/dust during cleanout will be blocked by the first course threshold.  If needed, more cleanouts should be incorporated.

Thank you for your review Cristobal.

I received feedback from Peter vdBerg confirming that my exhaust box is well sized. I took his recommendation on bridging the main wall across the opening and laid 3/8" sq. bar stock across and dry stacked those 6 bricks. They will be mortared solid on the next course.

Thanks to Cristobal's review he brought to my attention that my ADD had kicked in and I was so hyper-focused on the brickwork I forgot to install the clean-out door frame. It was a good experience to deconstruct the back wall and see how tight the fireclay/sand mortar actually functions and then the ability to rehydrate and reuse the mortar. Note to self... no more cutting brick with the angle grinder indoors.

Tomorrow I'll complete the 4th course and then switch to building the core.

With the 4th course of brick now complete it’s time to switch gears and fabricate the core stand then build the core. Focus today was to do a final dry build of the lower part of the core to confirm dimensions then weld up the stand. One little detail to point out that is minimal but ensures the core is level is shown in the 2nd picture. The angle iron that runs front to back is notched out before welding so that it sits flat with the rear angle iron that sits on the brick. With the two legs in the center the structure is super solid and perfectly level.

Glenn,

May I ask why bricks on steel? Coefficient of expansion of regular steel is around twice that of a clay brick. The picture looks like the bricks are snugly fitting between the angle steel.
Metals in stoves, kilns, furnaces are being used, but I would only use it where really necessary: air channel in batch-box, ceiling suspension, outside frame for reinforcing, etc.

Hi Cristobal... the angle of the picture does not show that there is perhaps a bit under 1 mm gap between each side of the brick, accounting for the radius of the inside corner of the angle iron. The steel will not expand appreciably in that direction anyway, it will mostly move lengthwise. The brick in the stand will be dry fit against the steel and is the foundation for split bricks that make up the core that will rest on top. Also, I don't expect the temperature to be very high this low in the bell so I don't see it as an issue. The clay brick on the inside of the next course will be isolated from the steel with superwool gasket and generous space between them.

It would be best to nudge out the vertical flange of the angle iron at the back of the firebox, where it is placed in the seam between bricks.

I would recommend to extend the exhaust box a couple of layers, forming a bond with the rest of the heater's wall. On top of that, half the width of a brick smaller around than the outer perimeter of the box, A steel plate with the hole for the stove pipe already cut out. Cut the hole with a diameter that is about 20 mm smaller, bend the circumference all around the hole and hammer it further in such a way there's a flange formed.

The steel doesn't need to be large gauge, what I did a couple of times was using the bottom of a barrel. Cutting and bending/hammering the hole and flange first and once that was done cut the plate out of the barrel. By doing it this way, the plate won't deform badly. Place the plate flange down and lay another two layers of bricks on top of the plate, together with the outer skin of the bell. The plate and bricks will be a very sturdy combination this way. The size of the finished hole and flange should be the same size as the crimped end of the stove pipe.

Hope this will make sense.

Thank you Peter... yes, it all does make sense.

The one question I have is your first recommendation of notching out the vertical section of angle iron for the brick of the inner bell skin. I initially thought about doing that but then thought the angle iron will loose horizontal structural rigidity. Since this element just affects the inner skin of the bell and will not be visible I can simply cut those bricks in half to allow the vertical section to be maintained. Do you think the center support leg on that piece of angle iron will be sufficient to eliminate any concern in this area? I'm curious if there is some additional reason that I am not considering?

I understand your recommendation on the exhaust box and will continue a few additional rows to lock it in. I also understand your recommendation for creating the connection between the exhaust port and the stove pipe. I need to buy a few barrels for the next build in another building and my source only has closed barrels, so I'll use one of them to fabricate the connection plate. Definitely a great idea using the barrel so the plate maintains a flat profile while making the flange.

Glenn Littman wrote:Do you think the center support leg on that piece of angle iron will be sufficient to eliminate any concern in this area? I'm curious if there is some additional reason that I am not considering?

Together with the horizontal ends between the bricks the support leg will be sufficient to keep the whole of the firebox in place. But you are right, due to the double skin construction you could cut up the bricks where the steel is meeting the wall. Don't forget to use a dot of superwool at the ends that are surrounded by bricks.

Just curious, what is the size (L,W,T) of your bricks? My own 150 mm (6") red bell system is double-skinned, made from castable refractory sporting an inner skin of 120 mm (4.72") and an outer skin of 50 mm (1.97"). Total weight a bit over 2000 kg (2.2 US tons).
Since you are building a same sized double-skinned system out of bricks, it would be interesting to know how it would behave as a higher mass construction.

Peter, my bricks are 8" x 3.75" x 2.25". My very rough initial estimate before beginning construction is a mass of perhaps 3 tons. I was going to wait to have the core completed with a riser height of 8X or more (I'll see what the actual height is without cutting the top course of IFB). Then I can calculate total brick and weight.

I was very fortunate to have a good refractory supplier a few hour drive from me that I have built a great relationship with. They happened to have a previous manager that for some reason left 3 pallets of brick in their yard. When I told them what I was doing with the refractory they said if I wanted the brick it was mine for the taking. They just wanted to space back in their yard. A big win for both of us.

Further interesting is that custom casting is a big part of their business and they are really interested to track my build. Once I complete this one I plan to talk with them about cost and details for casting cores. But, one thing at a time, lots to do to get this one done.

Back to work today on the batch rocket... the core is now complete except for the superwool wrap of the firebox and riser. Tomorrow I'll begin playing with some ideas for securely mounting the door. Since the core will be fully enclosed by the bell I'm pretty sure this needs to be fully designed and fabricated before I get too much further. Then back to laying the red brick, fabricate and install the stove pipe tie-in to the exhaust box, then more brick until I bridge the top of the firebox, then fabricate and install the door.

I'm pretty sure I have the proper next steps identified. I'll be grateful for any comments if I'm overlooking anything.

Glenn,

You built it so quickly that my comment will be too late.
The firebox should be contained in your bell, this way:
-you would store more heat in the skin
-there would be better thermal separation between the firebox and the door
-you could install the door(s) using stainless steel brackets:

Heater oven door installation

That's the best method:
-entire weight of the door is evenly distributed on the outside wall, which matters a lot especially when a weak clay mortar is being used
-the brackets are separated with refractory wool so different expansion rates will not break the wall

To break it, entire wall would have to be destroyed. In case of bolts in bricks, after several years the bricks may start moving independently.
The brackets are attached with three 4-5mm (3/16") bolts or rivets per side. Brackets are attached to the left, right and top.
Of course what you did will work, but now you have to figure out something that was already tried in hundreds of heaters.

Thanks for sharing the video Cristobal.

My plan is to totally isolate the door structurally from the bell in the following manner. With the core complete I'll weld 1 1/2" angle iron to the core stand. This will consist of a vertical piece inset about 1" back from the inner skin that is the height of the firebox. I'll weld a 2nd piece on angle iron on a 45 deg angle for rigidity front to back. Each vertical angle will have 1/2" threaded rod captured tight with a stover nut on each side of the angle and protruding out the front to hold the door frame. The door frame will have a rectangular opening corresponding to the secondary air tube so the tube protrudes through slightly and acts as a placement guide. I'll offset the angle iron to allow room for the Superwool blanket to cover the core and use Superwool gasket between the door frame and outer bell skin.

I think this is a practical design. It isolates the door frame and mounting structure from the bell skins and will allow for any unforeseen needs to remove the door frame should any modifications need to be made in the future. Thoughts?

By the way... for anyone wondering about the color difference of the inside of the firebox from the external firebrick. I had read a very in depth thread on Donkey's forum about the degradation of refractory brick over time due in large part to the effects of wood combustion and mainly the alkaline content of the resulting ash. One of the proposed ways to slow down this process is through the use of a ceramic coating and the author cites numerous products available globally for this purpose. I sourced the one called ITC 100HT on Amazon. Here's the link to that thread https://donkey32.proboards.com/thread/3909/firebricks-refractories-fail-silica-flux.

The coating is not inexpensive but considering the level of effort required for me to teardown my heater in the future for a firebox rebuild I felt it to be a small investment if I can get additional life from the system. Time will tell...

Cristobal Cristo wrote:You built it so quickly that my comment will be too late.
The firebox should be contained in your bell, this way:
-you would store more heat in the skin
-there would be better thermal separation between the firebox and the door
-you could install the door(s) using stainless steel brackets:

In general, what you describe is probably right for other mass heaters. I have done this several times and came to the conclusion a batchrocket core is quite a bit different. The inside of the firebox will get much hotter than expected, due to the hot gases that have to bent down to the port. As a result, the firebox' ceiling will get mighty hot, even directly behind the door.

Every normal brick that is exposed (even indirectly) to the inside of the firebox will start wandering due to thermal expansion, causing the outer skin to crack. Because of this phenomenon the inside of the core should be entirely separated from the bell's outer skin. Having the door mounted as is shown in the video will create muchos problemos as well regarding the door lintel in a batchrocket.

Been there, done that, won't do it again.

Glenn Littman wrote:It isolates the door frame and mounting structure from the bell skins and will allow for any unforeseen needs to remove the door frame should any modifications need to be made in the future. Thoughts?

Sounds like the right method. Assuming you are going to use a steel lintel: it would be best *not* to weld the lintel to the side frame. This lintel will expand quite a bit lengthwise, keep it separated from the bricks and firebricks with superwool. And also, a dot of superwool at the ends of it.

I Just Posted My Build Today which is similar to yours ... I would suggest that you put a
clean-out above the door to enable the top of the firebox to be cleaned of any ashes that may not fall to the floor.

I think I was misunderstood here. I did not recommend using stainless L-brackets for the presented structure and only for a heater with a firebox fully contained within the skin envelope.
Regarding the skin lintel - if it's a jack arch or sectioned arch or any other type, it has to be built with a space above the top bracket (and also spaces on the left and right). Bricks can not rest on the frame/bracket.
This solution is used both by Kuznetsov and batch box builders (with kachel or brick skin) in central Europe, so they can give warranty that the doors will stay where they should during the life of the heater.

Peter van den Berg wrote:Assuming you are going to use a steel lintel: it would be best *not* to weld the lintel to the side frame. This lintel will expand quite a bit lengthwise, keep it separated from the bricks and firebricks with superwool. And also, a dot of superwool at the ends of it.

Thank you Peter. Would you recommend the same lintel approach as I used (and you recommended) over the exhaust box entrance header? In that location I used 2 pieces of 3/16" square stock for each brick header.

An unrelated question for you Peter. I love to have data on my systems and intend to embed 2 or 3 thermocouples into the build. Where in the riser would you expect to see the highest temperatures? This is where I'm thinking of placing the 1st thermocouple. I'm also planning to embed a thermocouple in the mortar between the first course of refractory brick in the walls adjacent to the top of the riser.  Also, maybe a 3rd thermocouple in another location further down. My pyrometer only handles one thermocouple at a time so this is really just for spot checks when curious.

Cristobal Cristo wrote:Regarding the skin lintel - if it's a jack arch or sectioned arch or any other type, it has to be built with a space above the top bracket.

No arches in my design. That only adds complexity and other than a potential esthetic I don't see that an arch provides any functional benefit from a heating perspective.

The door frame mount is basically complete. I welded a piece of 1" x 2" tubing to the side of the core stand as a spacer to mount the vertical angle iron and added a 45 deg support welded to the cross-braces on core stand. Unfortunately, I ran out of argon after adding the spacers so please excuse the lousy looking flux core welds. Regardless, the vertical angle iron is solid in all directions. There is plenty of room to add the superwool blanket around the core. I decided that since the door frame will be around 20" high it would be best to have 3 bolts on each side to ensure the frame doesn't warp over time. I had a short piece of 1/2" all-thread laying around so I placed it on the bracket to illustrate the idea. Stover bolts will keep the rod secure over time.

Glenn Littman wrote:Thank you Peter. Would you recommend the same lintel approach as I used (and you recommended) over the exhaust box entrance header? In that location I used 2 pieces of 3/16" square stock for each brick header.

You could do the same with the door lintel, yes. Please keep in mind whatever you do: between the steel and the top of the firebox there should be a generous gap, to be filled with superwool.

Glenn Littman wrote:An unrelated question for you Peter. I love to have data on my systems and intend to embed 2 or 3 thermocouples into the build. Where in the riser would you expect to see the highest temperatures? This is where I'm thinking of placing the 1st thermocouple. I'm also planning to embed a thermocouple in the mortar between the first course of refractory brick in the walls adjacent to the top of the riser.  Also, maybe a 3rd thermocouple in another location further down. My pyrometer only handles one thermocouple at a time so this is really just for spot checks when curious.

Highest temperatures in the riser would be halfway the heigth of the port, about one inch in front of the rear wall. Beware, this could be as high as close to 1200 ºC (2190 ºF), so make sure your thermocouple is able to withstand that level of temperatures. May I suggest another, possibly more relevant spot? Right over the riser, dead center. Highest I have measured in that spot: just over 900 ºC (1650 ºF). Top of the bell would be a good spot as well, and maybe another one two foot down also.

Peter van den Berg wrote:You could do the same with the door lintel, yes. Please keep in mind whatever you do: between the steel and the top of the firebox there should be a generous gap, to be filled with superwool.

My plan was to wrap the entire core with superwool blanket, which would include right up to the face of the firebox opening. The steel bar would then rest on the superwool and the clay brick on the steel bar. I'm thinking that I may have to cut the clay header bricks a little thinner to account for the thickness of the superwool + steel, but I'll see what that gap looks like when I get there. I also plan to place a layer of superwool gasket material between the door frame and the brick.

Peter van den Berg wrote:Highest temperatures in the riser would be halfway the heigth of the port, about one inch in front of the rear wall. Beware, this could be as high as close to 1200 ºC (2190 ºF), so make sure your thermocouple is able to withstand that level of temperatures. May I suggest another, possibly more relevant spot? Right over the riser, dead center. Highest I have measured in that spot: just over 900 ºC (1650 ºF). Top of the bell would be a good spot as well, and maybe another one two foot down also.

When you say: "Right over the riser, dead center"; do you mean right at the top of the riser? Not in the ceiling directly over the riser, correct? Below is a picture of this thermocouple, it's rated for 2,500 ºF. My thought was to drill a hole in the IFB and use a little Sairset between the ceramic beads of the thermocouple and the IFB to secure it. The probe is 8" long so I can easily center it in the riser. Do you have a better recommendation for securing the thermocouple rather than embedding it into the top course of brick?

Making progress...

Fabricated the top plate for the exhaust box out of the top of a barrel and attached a stovepipe tee to provide a clean-out in that area. Attached the plate with tapcon's, placed Superwool gasket underneath and wrapped over top. Still need to slice a brick in half to fit under the tee.

Wrapped the firebox and lower riser with Superwool blanket. Still need to wrap the riser with blanket. Made a template for the door frame. Attached 4 of the 6 threaded rods to the door frame bracket.

Continuing to lay brick. About 4 more courses to go before bridging the firebox. Once this is complete I'm hoping to be able to lay 2 courses per day since there will be no other details to attend to until I reach the top of the riser.

I think Glen's been playing us.  Pretending he doesn't know what he's doing.  Lol.

Spectacular build Glen.  Looking fantastic!

Thomas Tipton wrote:I think Glen's been playing us.  Pretending he doesn't know what he's doing.  Lol.

Spectacular build Glen.  Looking fantastic!

Thanks for the compliment Thomas. The credit goes to the fantastic people that contribute on Permies and Donkey's forums. It was lots and lots of hours of reading and researching before I began. Couple that with direct guidance from Tom Rubino and Peter van den Berg and a touch of my OCD and I'm really happy with the build so far. Can't wait to fire it up and bring life to the dragon.

Making progress since last post. I'm now past working around the firebox so I can get an inner and outer skin course complete in 4 hours. The bridge across the top of the firebox went well and the gap is filled with Superwool. I'm 5 courses of brick away from the transition on the inner skin to refractory brick.

Looks fantastic, nice and neat, well done.

Fox James wrote:Looks fantastic, nice and neat, well done.

Thanks Fox. It is a really fun project. I'm just getting inpatient anticipating firing it up, especially today, the temperature outside right now is 7 deg F and 46F in the shop.

I'm only laying one course of brick per day and finishing each day setting the corner bricks for the next course so they are well seated to provide the reference points for staying level and plumb. I still need to fabricate my door which will be based on the https://dragontechrmh.com/how-to-build-a-batchbox-door/  design from Thomas Rubino. So I'm guessing that it will be another 3 weeks or so before the first fire is lit.

Progress is moving much faster now sporting my new Dragon Technology t-shirt. It really does make a difference!

Wow, Glenn;
You and your Batchbox are styling!
I'm sure I can see an improvement in your brick-and-mortar work since acquiring your DT staff shirt!
Looking forward to seeing your Dragontech door build.
Keep up the good work and keep those photos coming!

Question for you Rocket Scientists as I am about a week away from beginning to build my bell roof. I'm wondering what thickness angle iron is appropriate to support the weight of the refractory brick + clay brick and not deform (sag) over time due to the extreme heating localized directly over the riser? The open span that the steel will bridge is 22". I have 3/16" thick angle but can easily pick up some 1/4" if that would be advisable.

Also, just confirming that a Superwool layer should be installed between the refractory brick and the clay brick, correct? I do intend to wrap the steel with Superwool gasket and a dab at the ends for steel expansion space.

Glenn Littman wrote:Question for you Rocket Scientists as I am about a week away from beginning to build my bell roof. I'm wondering what thickness angle iron is appropriate to support the weight of the refractory brick + clay brick and not deform (sag) over time due to the extreme heating localized directly over the riser? The open span that the steel will bridge is 22". I have 3/16" thick angle but can easily pick up some 1/4" if that would be advisable.

Also, just confirming that a Superwool layer should be installed between the refractory brick and the clay brick, correct? I do intend to wrap the steel with Superwool gasket and a dab at the ends for steel expansion space.

Love it, I am ever so slightly ahead of you with my postings..   Answers there.

Glenn Littman wrote:I'm wondering what thickness angle iron is appropriate to support the weight of the refractory brick + clay brick and not deform (sag) over time due to the extreme heating localized directly over the riser? The open span that the steel will bridge is 22". I have 3/16" thick angle but can easily pick up some 1/4" if that would be advisable.

The 3/16" thickness is plenty strong enough. You'll need to have a T-bar, mounted upside down. Best is to have two pieces of angle iron, welded back-to-back. The vertical flange, which is the part that is hidden between the bricks is doubled this way so this is a stronger as compared to a bought T-profile.

Superwool gasket on the horizontal flange where the bricks are resting is enough, around the underside feels more secure but bears the risk of rust in it. Make sure the bricks are nowhere touching the steel and you are fine.

Thank you Peter for the quick reply. As per your instruction, I'll duplicate the welded T profile like I did with the core stand using my 3/16" angle iron. Note that in the US, T profile steel is pretty much a thing of the past. I spoke with steel suppliers in 3 different cities and they all told me that they haven't seen this available in the market in many years.

Peter van den Berg wrote:Superwool gasket on the horizontal flange where the bricks are resting is enough...
Make sure the bricks are nowhere touching the steel and you are fine.

Peter, can you clarify and detail a little deeper based on my questions below. The roof refractory bricks will sit in the T-bars with Superwool gasket on the horizontal flange, no problem. Then you say that the bricks should not touch the steel anywhere.

Should there also be SW on the vertical part of the T since there will probably be contact?

Should the point where the steel is resting on the clay brick below be isolated with SW?

Glenn Littman wrote:

Should there also be SW on the vertical part of the T since there will probably be contact?

As long as there's some space between the vertical steel flange at both ends of the firebricks, those are allowed to move freely. Of course you could use superwool in that spot although it isn't necessary.

Glenn Littman wrote:

Should the point where the steel is resting on the clay brick below be isolated with SW?

Yes, definitely. All around the inner skin one layer, and a second between the steel t-profiles and where the top bricks are resting on the front and rear of the heater's inner skin. All this to ensure there will be a good seal so there won't be any leaks.

On top of the firebricks one wool layer of preferably one inch thickness. On top of that a layer of ordinary bricks, in order to keep the wool in place. The seams could be filled later on, see that the mortar of this layer won't connect to the outer skin.

Hope this is clear.