Question about bell sizing

Hello all! My question regarding bell sizing is based on information found at http://batchrocket.eu/en/building#bellsizing which shows a calculation of 57 ft2 on ISA for a 6" batch box, now my question is since that is the maximum nominal size as I understand, if my bell has less ISA will it still function properly? Is the only disadvantage to a smaller bell area the smaller mass amount and therefore a smaller thermal battery?  I appreciate any input!

Hi Jon; Smaller is ok , just don't go larger.  Smaller area equals smaller thermal holding. Can you put 2 smaller bells in place ?

A smaller bell will function properly, the exhaust will be hotter, that's all. The mass amount is very much depending on the thickness of the walls.

So if you build your bell with thicker walls the net effect will be that it's taking longer before it's "full" and also longer to cool down again.

It's possible to build a double bell, which is more complicated, but also about 15% more efficient as compared to the single variant. I am a firm advocate of a single bell because it is so darn simple to build, without upstream channels and whatnot.

Peter van den Berg wrote:A smaller bell will function properly, the exhaust will be hotter, that's all. The mass amount is very much depending on the thickness of the walls.

So if you build your bell with thicker walls the net effect will be that it's taking longer before it's "full" and also longer to cool down again.

It's possible to build a double bell, which is more complicated, but also about 15% more efficient as compared to the single variant. I am a firm advocate of a single bell because it is so darn simple to build, without upstream channels and whatnot.

Thank you so much for your input! This is exciting news as I have very thick bricks for my bell/bench, however also trying to limit foot print as this is going in my garage. I was stupidly blown away by Brian kopps's results and initial heat output off his barrel. From the limited firsthand experience and knowing my intentions of a garage heater it seems the barrel heat will be almost enough in itself, so a slightly smaller bench may be perfect for my needs. I used to use a borrowed propane convection heater for my garage, I'd put it on full output for 6-10 minutes and then down as low as I could without it shutting off, and my garage would be plenty hot for at least 1-2 hours this way.

Jon McLain wrote:

Peter van den Berg wrote:A smaller bell will function properly, the exhaust will be hotter, that's all. The mass amount is very much depending on the thickness of the walls.

So if you build your bell with thicker walls the net effect will be that it's taking longer before it's "full" and also longer to cool down again.

It's possible to build a double bell, which is more complicated, but also about 15% more efficient as compared to the single variant. I am a firm advocate of a single bell because it is so darn simple to build, without upstream channels and whatnot.

Thank you so much for your input! This is exciting news as I have very thick bricks for my bell/bench, however also trying to limit foot print as this is going in my garage.  

http://www.mha-net.org/visit-with-peter-van-den-berg/

I'm slowly gathering material to build my heater, slowly due to too many interests. I have picked up 8 older blocks that I'm told are fire rated. I am going to use insulated fire bricks for my box core, and a ceramic riser I got off Brian Kopp. I was thinking of using clay bricks for the base of my barrel mount and these blocks for the bell/bench. I am hoping someone can help me either with a good idea or bad idea on these due to them being concrete. My concern is obviously they may crack or break. However being only used in the bench they will receive much less heat than any type of direct fire and be shedding it on the outside ad it absorbs so I'm thinking it may work. Any thoughts?

https://m.lowes.com/pd/Cap-Concrete-Block-Common-4-in-x-8-in-x-16-in-Actual-3-625-in-x-7-625-in-x-15-625-in/3608974

Concrete blocks should work fine for the mass. I might keep them isolated from flue/duct contact for the first few feet of duct run (with cob or refractory cement), but after that the gas temperature will not be high enough to damage the block.

You do want good contact between the blocks and the flue gases encased in ducting; even if you don't have access to much clay, using cob to fill between duct and blocks is a good choice. Air gaps will kill the thermal conduction into your mass.

Glenn Herbert wrote:Concrete blocks should work fine for the mass. I might keep them isolated from flue/duct contact for the first few feet of duct run (with cob or refractory cement), but after that the gas temperature will not be high enough to damage the block.

You do want good contact between the blocks and the flue gases encased in ducting; even if you don't have access to much clay, using cob to fill between duct and blocks is a good choice. Air gaps will kill the thermal conduction into your mass.

Appreciate the input, however these are solid blocks at 16*8*4 so I was planning on sealing them into a bench right off the bottom of my barrel as sort of a large-scale bell without need of a transition pipe. So the only 2 openings will be wood feed and exhaust

To save space and increase stratification, you could make the bell/bench taller without increasing footprint.

 Also, standard clay "chimney bricks" (not firebrick) could be used to create a 6" CSA "duct" accross the bottom of the inside of the bench, exhausting at the "opposite end" of (the inside of) the bench from the barrel, to give the gasses more time to cool before they come in contact with the less heat resistant concrete blocks covering the bench.

 After the barrel and manifold, the red clay should hold up OK, I believe, but others will surely jump in and correct me if I'm wrong.