Single bell and heat distribution

[independentenergy]

hello.
I tried to think about how you distribute the heat in a single bell.
To personally I did some tests with a small masonry stove and in fact the heat builds up, of course, at the top of the bell and lantamente falls, when the wall has absorbed all the heat, the temperature of exhaust smoke starts to rise, therefore the efficiency of the system decreases.
this is obvious ..
is what I thought of using two bells in order to optimize the distribution of heat even on the low end of the bell.


single bell
hypothetical distribution of heat




double bell
hypothetical distribution of heat







What do you think?

[Daryl]

Super glad you posted this.  I am curious too about the gas flow in the double bell.  I am wondering if the hot gases going up from the bottom of bell 1 would hinder the descent of cooled down gases coming from the top of bell 2.  

Love this stuff.

[independentenergy]

is a legitimate question, I think there are no problems, but I think there is a mixing between hot and cold gas at that point, also because I believe that the second bell at the top will produce cold gases that will tend to go down ... not to I'm sure ...
expect the experts

[independentenergy]

another design, shifted to the left with only one channel to the right and exhaust fumes in the bottom of the second bell, and eventually, oven.

[independentenergy]

Bypass is used to start the fire and ignite the chimney, or to warm up before the oven

[DCish]

Your last model is very reminiscent of a kuznetsov over-under double bell (scroll to bottom) www.stove.ru/index.php?lng=1&rs=16

[independentenergy]

wow, very nice that site ....
then the theory is correct, the site says that the heat is distributed better, that's what I meant

[DCish]

Not only does it improve the vertical heat distribution in vertically stacked bells, Peterberg has calculated that for each additional bell, total ISA required goes down by a fixed percent (i forget the exact number), opening the potential for a more compact stove. It also inspires other ideas for creating multiple heat zones in a storage mass (e.g., warmer and cooler portions of a bench; wood drying area that is warm but not hot, etc).

[peterberg]

The total required ISA goes down by 15% as compared to a single bell, all else being equal. I've tried this with a triple bell as well and that went down by another 15% or thereabout. This is talk about stacked bells, when these are side by side the situation is different.

[independentenergy]

ok thanks.
So if I understand, if a batch-box 150 mm ISA should be about 6/7 square meters, this value must remove 15% if I add a bell and even 15% if you add another.
With three bells removes 30% of total ISA than a single bell, is that correct?
So suppose you have a data base of ISA 6 square meters, and want to use two bells:
6-15% = 0.9
then 5.1 square meters total ISA

if I used 3 bells:

6-30% = 1.8
then 4.2 square meters total ISA
correct?
thanks

[peterberg]

Jun 25, 2014 13:20:08 GMT -8 independentenergy said:

ok thanks.
So if I understand, if a batch-box 150 mm ISA should be about 6/7 square meters, this value must remove 15% if I add a bell and even 15% if you add another.
With three bells removes 30% of total ISA than a single bell, is that correct?
So suppose you have a data base of ISA 6 square meters, and want to use two bells:
6-15% = 0.9
then 5.1 square meters total ISA

if I used 3 bells:

6-30% = 1.8
then 4.2 square meters total ISA
correct?
thanks

Not entirely, it's 6 sq mtr minus 15% equals 5.1 sq mtr. For the third bell this will be 5.1 sq m minus 15% equals 4.335 sq m. Keep in mind, these values has only been found in one experiment and could be different in different circumstances. The shape of the bells, for instance.

[pyrophile]

In my opinion, bell's size depends on the materials used in the bell.
Metal conducts heat very well, then bell's size should be smaller.



The single metal bell in picture 1 has about 6sq m and could be greater.


In picture 2, the lengthy single brick/concrete bell has maybe 8 or 9 sq meters (there are baffles insides) and could be greater, I think.


A single bell can be empty or full of bricks like the bell/barrels of dragon heaters, I think.

Benoit

[peterberg]

Jun 30, 2014 12:47:37 GMT -8 pyrophile said:

In my opinion, bell's size depends on the materials used in the bell. Metal conducts heat very well, then bell's size should be smaller.

Yes, you are right. But surprisingly, the difference in internal surface area (ISA) isn't as large as one might think. The maximum I've found usable for a metal bell made out of three 55 gallon oil drums turned out to be about 5 m2. Maximum of a brick bell using the same 6" batch box system happened to be 6 m2, only 20% larger.

Probably, there are more factors into play then just the difference in materials. For example, in the metal bell heat is transferred from gases to metal and from metal to air mostly. Two heat transfers very short after each other from convection to conduction and then to convection again. In a brick bell the heat is transferred only once from gases to brick during the entire burn. After that, heat is transported by conduction inside the brick wall for one hour or more, depending on the thickness of the wall. As an overall effect, in the brick bell heat transfer happens only once. When the heat is finally coming out as radiation mostly, the fire is long gone.

[pyrophile]

Well... I see in both cases -metal or brick bell- different kinds of heat transfers, radiation too.

In both stoves I tried to maximise radiation (as there were no additionnal stage to heat in the building ).

In the metal bell of the photo above (or the third one below) I tried to minimize convection and maximise radiation in order to heat persons more than air. The cylinder is standing up, facing persons, at their height (less than 2m), radiating towards them. (I remember that radiation rises with temperature, rising as a square as the temperature rises, if I am not wrong). Surface temperatures are from 150 to 300° C, if I remember well, 90 or maybe 100 above (insulated).

Of course, it also heats air by convection. But I don't think it is in an important proportion (considering also that it is a very huge place, without windows, concrete made, 5-6 meters high, always very cold, even during sommer).

Convection would be much higher, I believe, if the cylinder was horizontal, offering more possibility to air to surround it , getting hotter and go up. Like this from internet :

or this one I wouldn't make again :



Let's come back to the water tank, here during the first trial :



The water tank cylinder is not too high : it does not heat the air above people's height. But the uge room is maybe 5 or 6 meters hig). The warmer part of the cylinder is at people's head's height.
A bit lower would be better but difficult to manage (height of the internal riser, necessity of insulation below the firebox, etc...).
To counterbalance a low height, I had to rise the diameter.
Generally speaking, I stopped building high stoves as russians, Chernov or americans do (and as I did). I add bells horizontally and not anymore vertically. I try now to make stove that radiate at people's height.

One of the problem is that half of the radiation of the cylinder goes towards the wall and then is "lost"...I was not allowed to put it more in the middle of the room...radia at 1tes

I add that the top of the cylinder is insulated (insides) to prevent it to radiate towards the roof.
So is insulated the rectangular rocket stove on the second photo, behind it, beetween stove and wall. It is 2 tons heavy, but could have been a bit heavier.


I would add that bell effect is, in my opinion, more important in your 3 drums bell than in my water tank.
I consider that bell effect is the most important ABOVE the internal heat riser, not around it where gases mostly go down, not being able easely to go up and down.
Then, I think that "your" 3 drums bells does "bell" (!) a lot. "Mine" has only a little part above the riser and then doesn't maximise bell effect (as I did not want it to radiate above people's head). Of course, the cylinder being rather large, there is more "bell effect" in the space around the riser as there is more space for convection. The best for me would have been to have a really large cylinder...Not easy to find!
Then, with about the same surface and about the same material (but "mine" is thicker in metal), I think that we have not the same transfer and I think that I could have much more than 6 square meters as radiating surface. That means that "my" exhaust temperature should be much higher than "yours"!
Maybe I should add a second bell.

I add that in the brick stove shown, the large bell/space at the right of the riser is another way to maximise the bell effect without having so much space above the heat riser. I think that it acts more fully as a bell than "my" water tank. Its volume is better used. (But of course, it doesn't send the same heat in the room : it is much less powerfull).

I may be not very clear in my explanations and maybe wrong also, but it is good to me to share and maybe oppose differents ways of understanding things! With cordiality, of course!

Benoit

P.S : I add a technical detail for measurements, I don't know if I am right :
On this first trial, one can see black painted spots to allow infra red thermometer measure as IR measure is not easy on métal.

The punk aesthetic goes well with the place...

[peterberg]

Benoit,
There's one fact missing I believe. When radiation temperature is higher than 100 C, the air molecules will be heated directly resulting in a large increase in convection. Not sure about the height of the temperature though. All modern infrared radiating panels won't sport high temperatures because of this phenomenon.

Actually, this single bell idea with the combustion chamber inside it isn't acting as a bell pur sang. Simply because of the constant feeding with hot gases stratifying won't occur. So I think in your situation it would be best to keep the first and hottest bell small and add another bell beside it which is fed and exhausted close to the bottom of it. Beware though, don't situate the exhaust hole against the floor. This could be acting as a restriction, about half of the exhaust diameter as distance between floor and hole will alleviate that. Of course, when you are using a wider exhaust shape, like a funnel progressing from a large rectangle to the required circle, that would be ideal.
Hope all this makes sense.