Sizes for single bell systems

[satamax]

You count 1.86m² for a full barrel. With the top, not accounting for the bottom. Two half barrels should be aproximately the same.

So that's 5.58m². Well, i think thoses measurements are very conservative.

[rk3745]

Peter,
Are the sizes of ISA calculated for a specific material?
Some materials have much more heat absorption capability and conduction than others. The faster heat absorption the smaller size of ISA?

[Deleted]

The weakest link determines the strength of a chain.
No material can absorb heat faster than it can be delivered.
The ability of gases to deliver heat is rather low.

Convective heat transfer
www.engineeringtoolbox.com/convective-heat-transfer-d_430.html

[rk3745]

Thanks karl,
It is a surprise for me!

[AlexHarpin]

Hi Peter! I have difficulty calculating the ISA of a system of 215mm / 8.5 "


Can you help me understand the formula please?

[peterberg]

It is simple, really. The riser csa of a 150 mm system is (rounded off) 177 cm2. The corresponding recommended bell Internal Surface Area is 5 m2. With a very good chimney stack that could be about 10% more.

The csa of a 215 mm system is 363 cm2. This figure divided by 177 cm2, being the csa of the 150 mm system gives a factor of 2.05. So your bell could be at least 2.05 times 5 m2 which is 10.25 m2.

Max thinks this is very conservative. And yes, it is, I am aware of that. I use a heater with the firebox elevated 50 cm from the floor of the bell and the chimney is too short, just 4 m, with a higher appartment building close by. I mounted a rotating chimney cowl and tuned the balanced ventilation to just a little bit of over pressure in the house. Since yesterday, it looks like my 5 m2 bell could be slightly larger. Time will tell whether this will prove to be true or not.

What I earmarked as the recommended values should result in a heater which will work in almost all circumstances.

[AlexHarpin]

Thank you Mr. Berg, I did not calculate the right way, I did not have the right numbers for csa ...

I prefer to build on conservative measure and avoid pre-heated the chimney or a bypass. If the trend continues I should start the project very soon. I'll post pictures!

[esbjornaneer]

Oct 23, 2015 2:06:53 GMT -8 peterberg said:

Hi all,

During the course of this year there has been some new experiments conducted regarding bell size. In my own opinion, those experiences point strongly to a smaller Internal Surface Area (ISA) than assumed before. In order to keep the gas velocity in the heater high enough to get a good burn there should be at least an end temperature of 80 degrees Celsius (176 F) in the chimney stack directly after the bell.

Recommended bell size for a 150 mm (6") system is down to 5 m2 (53.8 sq ft)now.

The 80 deg C is at what stage of the burn? And I guess that would be the same comming out of the 2nd bell if there is one. Thanks for clarification.

[peterberg]

At the height of the burn with the noise of the thing rumbling with two different "voices". When there's a 2nd bell, that 80 C should be at the start of the vertical stack, yes.

[twd000]

would it be possible to use cinder blocks rather than steel drums for a bell? The internal passages of the cinder blocks would provide a lot of surface area in a small volume. They have a lot of thermal mass for absorbing BTUs and re-radiating them..

[peterberg]

Correct, but these cinder blocks aren't able to withstand the temperatures involved. More than 520º C (968º F) and the silicon christals in the concrete are expanding big time, one of the reasons concrete buildings can burn down. Besides that, the portland cement which is used as binder isn't up to that sort of heat. The riser is able to spew out 900º C (1650º F) so the cinder blocks above the level of the riser are destroyed in no time.

[twd000]

I was afraid that would be the limitation. Any idea how far away from the riser outlet would be a safe temperature? If I could use firebrick in the ceiling above the riser to survive the 900C environment, could I use conventional masonry farther downstream in the channels? I see final chimney outlet temperatures of around 100C; I wonder how quickly it dissipates from 900 to 100C?

[peterberg]

Oct 23, 2016 5:29:20 GMT -8 twd000 said:

I was afraid that would be the limitation. Any idea how far away from the riser outlet would be a safe temperature? If I could use firebrick in the ceiling above the riser to survive the 900C environment, could I use conventional masonry farther downstream in the channels? I see final chimney outlet temperatures of around 100C; I wonder how quickly it dissipates from 900 to 100C?

Like I said, in a bell system anything above the level of the riser end should be firebrick. Don't know about a channelled system if that is what you meant.

[etownandrew]

Oct 23, 2015 2:06:53 GMT -8 peterberg said:

All the numbers assume the heater is coupled to an adequate chimney stack. Better stack, larger ISA, worse stack, smaller ISA, a bypass and/or a primer box of some kind. Also, the idea is that inside the bell system is very little drag. When the drag is increased by a not very favourable exhaust opening which is situated in a corner close to the floor for instance, the bell should be smaller in order to keep gas velocity adequate.

What makes a chimney an "adequate chimney stack"? Is a chimney required to be insulated to be considered "adequate"? I have a 30' tall interior masonry chimney that I am planning to use for an 8" batch rocket masonry heater. I'm thinking that I need to derate the ISA recommendations for my chimney as it will be taking some heat from the exhaust. Also, it has never had great draw when I have used it with a 55-gallon two barrel stove.

[peterberg]

Ideal chimney stack is straight up, round csa, smooth inside and insulated. And oh yes, for most of its entire length inside the house envelope. Your chimney is probably a masonry one, which is bound to take up some of the heat itself.