Batch mass heater bell dimensions

The bell size dimensions chart can be found at https://batchrocket.eu/en/building#metal

I want to build a batch mass heater. The bell size chart doesn't make sense to me, unless it assumes there's no heat battery bench and all heat transfer radiates through the bell.

A standard oil barrel measures 88cm tall and 56cm wide (diameter). Roughly 34.6" and 22" inches.
I understand its ISA would be approximately the surface area of the cylinder without its bottom circle (just the top one).
Hence
2πr(h+r) -  π r²
2*3.14*0.28*(0.88+0.28) - 3.14*0.28²
=1.79m² (19.26 square feet)
Adding barrels vertically would add surface area on the sides.
So for 3 barrels
3*2πrh + π r²
=5.12m²  (55.11 square feet)
So 3 oil barrels would be slightly short of the 5.3m²(57 square feet)  recommended bell for a 6"  (15 cm) riser.

And this does not account the possible surface area that would be lost in the mounting of the bell onto the combustion chamber unit (just in models that the bell encompas the burning chamber).

But often 6" riser systems come with just one barrel bell and a heat battery bench.

1. Does that chart assume there would be no heat battery bench?

Also accordind to this chart for a 9" riser I would need 11,2 of these barrels.

Number of barrels ≈ (ISA - top circle) / surface area of cilinder wall
(11.4m2 - 3.14*0.28²)/ 3.14*0.56*0.88
=11.2

Seems too many. I haven't seen anything like this.

2. What am I getting wrong?

Hi Dave;
Well, you have managed to confuse yourself and me as well!
If I understand you correctly you are wanting to build a 9" batch box with a bell.
Peter's numbers for bell ISA, are assuming your riser will be inside the brick bell.

It sounds like maybe you want to build a 9" batch box (huge), using a standard oil barrel over the riser with a seating bench beyond it?
A nine-inch batch box is a monster, using one barrel directly over the riser there will be extremely high heat radiation from that one barrel.

Tell us what your plans are.
How large an area do you want to heat?
If you want a batch box with seating,  a brick bell over the riser and an open cavity bench to sit on beyond is easy.
Why do you want a barrel at all?

Thanks for your reply Thomas.

I want to heat up a relatively large room (10x12m, 3.5m high, so 420m3, or 33x39feet, 11.4 feet high 14,672 ft3
It's an old house, thick walls (bricks and cement) non- insulated many windows. Rarely it gets to 0C, 32F, but 5C 40F is a common minimum in winter. So I estimated Q=G*V*DT=1.8*420*17≈13.3 KW mean output power

This is a multifunctional room consisting of 4 unpartitioned areas that include workshop space and a chill-out area with a projector/cinema. It sits on the ground floor of a three floor house.
There's often people working some hours in the morning/afternoon in the workshop, and people chilling in the cine section some evenings.
Considering it's a room that is regularly used at different times I thought it would be appropriate to split that heat generation in a heat battery bench and a instant heat radiation bell.

I considered the barrel because of budget and building speed.
I already have a barrel and reckon I could get some more cheaply. I will get some free refractory bricks, but building the whole bell structure with them could be quite expensive, specially if going for a large riser bell.

I am aware the barrel won't last as long as the bricks. I am unsure if I could use non-refractory bricks for the bell. That would make it significantly more affordable.  I can source mud, so I considered building the bench with ordinary bricks and cob.

This will be my first batch. I prefer to stick to a proven design, and will avoid the barrel and chose bricks instead if the chosen plan states so.

Hi Dave;
Well, to start you only need refractory bricks at the core and possibly over the riser.
Your bell can / should be plain clay bricks.

If you are building a batch box rather than a J tube I think a seven-inch will supply you with plenty of heat, a nine inch is a monster!
As far as using barrels, nothing wrong with that.  They last for many years. My first RMH with a barrel I built in 2013 ... the same barrel is still in use and in perfect shape.

Here are a few links to my shop stove build.
It started out life as an 8" J tube with a single barrel and a brick bell.
Then I converted it to a 7" batch box with double barrels and a brick bell.
You will be amazed at how well and how long that clay brick bell will hold heat!.
 https://permies.com/t/94980/Brick-Bell-Shop-Heater
https://permies.com/t/138779/Breaking-News-Shop-Dragon-admitted
https://permies.com/t/181421/Batch-box-rebuild
https://permies.com/t/190599/Finishing-touches-Shop-Dragon-rebuild

The direct answer to your question about bell ISA is that that area refers to the whole of the internal chamber(s) between heat riser and chimney exhaust. It can be one large volume, any combination of masonry and steel, or divided into two or more volumes.

Thanks for you reply Glenn,

So I understand from your reply the (internal) surface area of the bench counts towards that ISA. Please correct me if wrong.

david higos wrote:1. Does that chart assume there would be no heat battery bench?

A bell can be any shape, so a hollow bench can be part of a bell as well. It's illustrated in a couple of instances on the batchrocket website, here's one.
https://batchrocket.eu/en/applications#culdesac
A combination of a barrel and a brick bell of whatever shape is done multiple times already.

david higos wrote:Also accordind to this chart for a 9" riser I would need 11,2 of these barrels.
Number of barrels ≈ (ISA - top circle) / surface area of cilinder wall
(11.4m2 - 3.14*0.28²)/ 3.14*0.56*0.88=11.2
Seems too many. I haven't seen anything like this.
2. What am I getting wrong?

One barrel is roughly 1.8 m², including the top, you calculated this yourself. For a 9" batchrocket''s bell (which is huge!) you'll need 12.3 m², this number is in the table. This 12.3 m² divided by 1.8 will give you approximately  6.8 barrels, certainly not 11.2. Your calculation is clearly wrong, although I fail to see where.

You've sent me a mail with exactly the same questions. Either you are asking me questions personally or publicly, not both at the same time. I would prefer publicly, so more people are able to read the information that's relevant to them. Saves me answering the same questions over and over again.

Upps... Sorry to bother you Peter, I can understand. I felt I was writing to a website and not to you. Apologies. Thanks for telling me how to reach out better.

I can neither see why I got that final wrong result number. But I see now that that figure is now closer to 7 oil barrels for that hypothetical 9"heat riser.

I understand quite a few more important things better from all of your replies, and also get rid of some of my misunderstandings.

A 7" riser is enough for my needs. I can now visualise a combination of elements (bell and benches) that make a 7.2 M2 or 77.5ft2 ISA.

It's late here. Bye for now. Thanks a lot.

thomas rubino wrote:It sounds like maybe you want to build a 9" batch box (huge)

Peter van den Berg wrote:For a 9" batchrocket''s bell (which is huge!)

Hi David, just to highlight a minor detail...  If two batch box experts are hinting that a 9" batch box is "huge", I'd suspect that you might be able to get away with a smaller one and save some money.

Yes indeed Mike.

I wanted to estimate the amount of building materials (and corresponding budget) hence I tried to work out the size of the riser based on the recommended  KW from the formula I found. That accidentally  helped me to realise some of my missconceptions.

I have moved on. I go with that double recommendation indeed. Don't care about the KW formula much anymore. I can now work out an estimate for these materials. It helps to narrow things down a bit.

I need to do some more research again on materials and trade-offs now.