Will this work? (Temperatures in different parts of bell)

Hi smart people,
We're planning for a batch-box rocket in our future cabin, and ran into a problem. The thing is, we'd very much like a cooktop on the rocket, but any system size but the very tiniest ones would put the cooktop at an uncomfortable height, especially since I'd like to try for a char-gathering system like I described here, which adds to the height. So my question is, would it be possible to have the ceiling of the bell at two different levels, and the cooktop on the lower one (see drawing)? Or is the temperature in the bell insufficient for heating a cooktop everywhere except directly above the heat riser? Is there any data on temperatures in different parts of the bell? (Note: the drawing is just a five-minute thing I did in Paint, and is definitely not to scale. Just to show what I was thinking.)

Hi Eino;
Reverse your drawing and put the shorter roof over the riser.
To cook, you will want the higher temps that are available over the riser.
There is no doubt that it will also fill the taller section of the bell with hot air.
Or, as you are building a batchbox, you could leave a portion of the core exposed out from the bell at the bottom and cook on it.

Thanks Thomas! The reason I thought to put the taller roof over the riser was to have enough space between the top of the riser and the roof, while getting the cooktop low enough to be comfortable. But, well, it was a long shot.

I did consider a partially exposed firebox top before, but wasn't sure it'd work. How much of the firebox top can be exposed without losing so much heat that it compromises the cleanness of the burn? And will a metal stove top work, or will the temperature in the firebox degrade the metal? I would have guessed this would go under the "no metal in core" rule? Or is there another material that would work better?

Hi Eino;
You would leave from the front door till just shy of the riser out from the bell
There are options for batchbox roofs.
A flat large heavy firebrick
A home cast, flat large "firebrick"
A thick cast iron griddle
A kiln shelf.
Standard firebricks can be angle cut to form a peaked roof.

In my shop, I use an uninsulated cast iron griddle topped with plate steel and plain clay bricks.
It gets plenty hot to cook on the bricks.
The sides and bottom of my batch are insulated with #8 1" thick Morgan Super wool.
In our studio, we have a full-size large firebrick as a roof.
It has 1" superwool over the top and a steel plate covered by clay bricks.
It does not get as hot as fast as the shop stove does with the uninsulated cast iron but you certainly could cook on it.

Ooh, that sounds better! I don't know why I got stuck on the idea of a standard stove top being the only option for cooking on. Thermodynamics is thermodynamics, after all, and the heat will spread. I really like the idea of a brick cooktop. Thank you!

Heino, do yourself a favour and lift the whole of the core from the floor, 50 cm to the top of the firebox floor. This will save your back while loading the heater. Then have it stick out like Thomas suggested. The top of the firebox will get plenty hot due to the fact that the hot gases need to go down to the port (or gate, whatever one prefers). So there will be lots of heat delivered to the top of the firebox. It would be best to use a small cast iron cooktop for this.

The combustion quality won't be compromised, after all behind the firebox itself there's the afterburner which will take care of all unburned gases and tar. This has been done quite a number of times so it's safe to say the thing will work nevertheless. Normally this top-of-the-firebox heat will end up in the bell but the fire won't know and don't care about where this heat goes.

So a cast iron cooktop is enough as a roof on the firebox in its own right? Or should there be some other material (firebrick, or insulation?) under the cooktop? Won't the cast iron undergo spalling at the temperatures in the firebox? I had got the impression that any metal in any part of the core (other than secondary air inlet) was a bad idea? Thank you for taking the time to answer my stupid questions.

Hi Eino;
As my dad used to say, the only "dumb" questions are the ones you do not ask...
Most Batchboxes are only burnt 2 times a day, not enough to really damage the cast iron.
My shop stove with the uninsulated cast iron griddle gets burned back to back fires all day when I am working out there.(so does our studio RMH)
I call that extreme, batchbox burning...  not something you would do in an insulated home.
Indeed, there are some color changes on the cast iron top. Not enough for me to worry about at this time.
I do have a spare cast iron top if it suddenly cracks.

I will say that the large firebrick (12"x24"x2' thick) on my studio RMH is completely unaffected by the extreme burning.

Eino Kenttä wrote:I had got the impression that any metal in any part of the core (other than secondary air inlet) was a bad idea?

In general, that's true. In a more precise way: no metal in the hottest parts of a rocket. In a batchrocket, the firebox isn't the hottest spot, the riser is. Main difference is the temperatures, maximum 850 ºC (1560 ºF) in the firebox and upto 1200 ºC  (2190 ºF) in the riser. It is a true afterburner, a blown fire burning pure woodgas is raging in there. Coupled to an oxygen-rich environment, bare metal in there will be destroyed in short order.

Okay, that's quite the difference. 850 ºC is still plenty hot though... Nice to know we can use a cast iron top, so we have options.

Again, thank you so much for the help, you guys are awesome!

Hello to all. I am building a 180mm batchbox based on Peter's plans, with a floor channel. and was thinking of using metal for the firebox. And i just saw  Peter's answer for the temperature there reaching 850C. So, 3mm steel plate is going to survive this?

Hi Kostas;
Your metal will warp and spall quickly.
You need firebricks in the core.
There is a metal called RA330 that sometimes is used as a liner in a batchbox core.
It is extremely expensive.

Thanks for replying Thomas, and so quickly. heavy firebrick  is the option so far, but i suspect they are sucking a lot of heat at the first stage of the burn, and delay  good combustion in the riser. they weigh almost 80 kgrs (riser is made from insulating ones) . i would use these for the firebox too, but i fear abrasion due to knocking of the wood there, will damage them . So i thought i should protect their surface somehow.
i would prefer the system to be quick in reaching proper temperatures as it will be fired once a day probably, since i am in Greece.
Another option for the firebox i am considering these days is ytong bricks. they are rated for withstanding 1200C and fireproof, but haven't heard of anyone having used them. Thet are also very light, like the IFB's, they even look identical.

Kostas, you could use split fire bricks (32mm thick)  backed with insulation?

Thank you for responding Fox, and yes reducing the width of the dense firebricks is a very good option.
I was actually checking that yesterday, and found some fireboards 1,4mm thick for pottery ovens. And on the outside I could put the ytong insulating bricks.
But there is also another consideration. The insulated fire breaks I have for the riser are quite low in alumina content and are prone to quick degradation due to the ash 's corrosive attack.. I found a very interesting discussion about this topic in donkey 32 Pro boards. So I am considering buying more insulated fire bricks, with high alumina content this time, and using these for the hottest parts of the core and leaving the previous ones for the cooler ones. They'll all have to be painted with a paint based on zirconia for protection. It is more expensive but mostly I'm not sure if even the higher alumina content ones, which are harder ,can withstand the impact with wood in the firebox.

Hi Kostas;
Soft insulated bricks are very easy to damage just by loading wood.
I know this because I  tried it.  I thought I would just be careful...
I find it hard to believe that a "paint" will make them durable enough.
Insulated bricks work best in your riser.

I guess it depends on how gentle you are with loading the wood?

Zircon drys to a stone like hardness and is said to adhere very well to fresh insulating brick but I have not tried it on insulating brick myself.

I have tried it on vermiculite board, the zircon, was indeed very abrasion resistant but, it cracked and fell off after about 40-50 hours.

There are different grades of insulating brick, generally the higher the temperature, the softer the brick and the more open the structure becomes.

Coating open pore, insulating fire brick with Zircon is said to work very well and I can imagine it would form a hard  surface but nothing like as durable as hard fire brick, for heavy use, I am sure hard fire brick is the best option.