Project:RMH,horizontal feed Batchbox model 6",

[peterberg]

Mar 17, 2013 10:37:58 GMT -8 dvawolk said:

But is there any difference if i have one bell with 7 sq. meters or two bells with summed inner surface of 7 sq. meters?

Oh dear, this is complicated. I know this is hard fact but I am not able to explain it properly. I am not sure I've understand the mechanism fully myself. So I will tell you what I do know about this.

Yes, there's a difference between a double and single bell system of the same inner surface. Another way to express this: a double bell, utilizing the same inner surface area as the single variant, is about 30% better at extracting heat. To get those bell systems on a par the double version do need to be 30% smaller by volume. The total surface area will not go down as much, by 15% in reality. The floor of each bell doesn't count in this area calculation because the gases won't warm the floor, almost none at all.

So in theory, your stove should contain 5.95 m2 inner surface to achieve the same heat extraction as a single bell of 7 m2.

Here's a page from the Rocket/bell project, reply #86 of January 2011 where I've wrote down the same conclusion.

[dvawolk]

Hi! I added sketch.

There are two bells. Each has the same inner surface (minus floor) but totally different volume... So even by staying with the single bell system one can lower the volume to some degree...
A bell is usually a prism. If one would have a prism with top view of a letter L or F or E then the inner surface would become very large compared to inner volume of a bell.
My bench bell has "L" top view but this is only the first step of enlarging inner surface with not-so-much enlarging inner volume.

I think that it will be hard to get a simple bullet-proof formula for matching inner surfaces of single/double bells. It's because the temperature of gases is lowering to the bottom of a bell. And each next "brick surface" taken away from the bell will leave more and more heat unused.

For example:
The exhaust from the bell has 50°C.
Then we take away one row of bricks and exhaust temperature rises to 60°C
Then we take away the second row of bricks and exhaust temperature rises to 75°C (not 70°C).
That's all because the difference of the outside and gas temperatures is higher when we are closer to the source of hot gasses.

What comes to my mind, also, is this:
if we compare single bell and double bell system with the same burner and with the same exhaust temperature. I would say that double bell has more of a traction for gasses? Or not?

Klemen

Attachments:

[peterberg]

It's quite common is Russia to incorporate some columns inside a bell in order to support a couple of smaller top plates. This will add surface area which in turn is increasing the heat extraction capacity. So you are right, building a bell using a large E-shape will greatly enlarge the inner surface area as compared to the volume. I will call that ISA from now on, unless somebody object against it.

My calculations were done using a more or less cubic size and shape. I think it would be best to drop the volume definition altogether and communicate about this topic in ISA terms. The percentage of 15% is probably the only reasonable fact to go by, so let's stick to that for the time being.

All this is boiling down to rules of thumb, nothing more, nothing less. It surely will work unless the shape of the bell do become too complicated. The goal is to be able to build a mass heater suited to a given chimney, fire box volume and end temperature.

And yes, a double bell will impose more friction to the gases, if that is what you mean. It's still a very small force as compared to a labyrinth or contraflow construction. The separation of gases and the extraction of heat is done by means of gravity, being the main reason of the low friction properties of a bell system.

By the way, this paradigm of a double bell being more efficient than a single one was recognized by I. S. Podgorodnikov, who has done a lot of work on this during the first half of the 20th century. The "Teplushka", a very well-known stove and oven construction is one of his designs. My information about this is derived from the site of Igor Kusznetsov but that doesn't respond at the moment.

Beware, not all what Kusznetsov write is hard fact. His theories about "free gas movement" hasn't been proved, exhaust measurements hasn't been done.

[dvawolk]

Mar 18, 2013 1:44:46 GMT -8 peterberg said:

I think it would be best to drop the volume definition altogether and communicate about this topic in ISA terms.

Sorry, Peter, but now it became clear regarding volume, also. It does have a place with lowering the exhaust temperature...

Imagine that we mark (in red color:-) one liter of hot gasses coming from the heat riser into the first bell. Here we press stopwatch. If the volume i smaller it will come out to exhaust hole faster! So the larger the volume, the better the heat extraction.

The volume is directly connected to lowering of velocity of heat gases in the bell. Not so with the ISA.

So to "estimate" the exhaust temperature, the bell volume and ISA should be taken into consideration...

I won't complicate with my heater! I will wait so the weather becomes dryer (again, it is snowing with 1°C - humidity everywhere). Then i will put electric heater in my second bell for a while and i will measure humidity on the exhaust. And after that i+ll try it again!

Just a thought. It would be nice to really be able to "MARK" the gasses red. Easy velocity measuring in certain parts of a bell. But probably only wishfull thinking. I doubt of a colored smoke withstanding 1000°C

Cheers,
Klemen

[Donkey]

... But once the stove has filled with exhaust, from feed through to chimney, the gasses MUST exit at a rate high enough to be replaced from behind (burning more stuff) without backing up. The speed through the system will be directly tied to the chimney, NOT bell size.
Seems to me that once the stove is started, running and hot, volume will matter LESS than ISA.

??

[dvawolk]

I will modify slightly:
The volume is directly connected to lowering of velocity of heat gases IN the bell. It does not affect (vividly) the velocity of gases in the exit holes (which have the same CSA as system).

The bell is lowering the speed of gases because of larger CSA (4 times (or more) of System CSA - SCSA in future?!) which automatically adds to larger volume in the bell...

Klemen

[marko100]

That's a lot of reading.
I do not know where to begin to respond.
Klemen doing the calculation on the maximum temperature of the outer surface of the oven. And the total power in kW, at this temperature.

In a bell system ISA is more important.

Things that are important and are interrelated:
Speed of gases and residence time of flue gases. Bigger volume of the bell the lower speed of gases and therefore has a greater residence time. Greater the residence time the greater the heat transfer.
Temperature difference between the transmitter ( flue gases) and the receiver(inner wall of bell). The greater the temperature difference smaller area need for greater temperature drop.
Factor for heat transfer of material that we use in our case fireclay. This factor tells us how much heat is able to take the material at a given temperature difference at a given time. Fireclay is a very poor conductor of heat, so we need such a large surface area (ISA).


@ Peterberg
In my opinion, a double bell is more efficient due to two heat exchangers that work at a higher temperature difference between the flue gases and the wall bell. That why it needs less ISA.


Lep pozdrav Marko

[satamax]

Marko, another material, which is readily available for everybody, and works better than cob at transfering and storing heat, concrete. I don't know about refractory concrete thought.

[Deleted]

Mar 18, 2013 7:26:32 GMT -8 Donkey said:

The speed through the system will be directly tied to the chimney, NOT bell size.

The numbers of particles passing at any point have to be the same.
That's why volume affects the speed of fluids.
With gases especialy if there are huge temperature differences it becomes more complicated.
The volume affects speed, pressure and temperature, which affect the possible rate
of heat transfer, which in turn will affect the the pressure and thus temperature
of the gases...........
This is a snake that bites its own tail.

Even at the entrance it is not the chimney alone affecting the amount of air
entering the system.
There are entrainment effects too.
The faster the air enters the more additional air will be entrained.
Eg. the Peter channel depends on entrainment effects.
One more snake biting the own tail.

[marko100]

satamax
Fireclay is another word for chamotte in our country.
Concrete is transfering heat a lot faster then chamotte but for storing heat chamotte is better. Refractory concrete acts like chamotte.

Karl I fully agree with you.


LP marko

[marko100]

Can anyone from the forum administrators view what is wrong with the system clock? I am writing this at 20:35 system clock shows 2:37 pm. It is very disturbing to monitor the threads.

LP

[dvawolk]

Marko:
Modify time zone settings in your profile...

[dvawolk]

Hi, all!

I need some clarification regarding thermal conductivity and ISA of bells with different materials.

Peter: you did and educated guess that one needs approximately twice as much of ISA if using bricks, compared to metal drums.

But the guess of 1:2 here is not connected with thermal conductivity of materials because the thickness of these two options is very much different. Right?

If one would use 6cm thick firebricks vs. 6cm thick metal, then ISA would be more like 1:80

Thermal conductivities of materials[1W/(m.K)]:
IRON: 70
BRICK: 1
CONCRETE: 1
AAC: 0.2

Deriving proportions of ISA for abovementioned materials can be useful only if one uses them i the same thicknesess.
Is this the right conclusion?
----------------
What i am thinking is:
i want longer bench (second bell):
i am running out of ISA:

Solution?
I can make one wall with AAC. This way i can make 5 times more ISA with AAC compared to concrete. Of course i will use AAC where i don't need warm surface - like a wall of the bench that is facing the wall of my apartment. The other part of bell will be concrete or brick - usefully conducting heat to outer surface.

And if i make sure that the AAC won't come in touch with temperatures of more than 200°C all should be fine?

[Deleted]

Apr 3, 2013 2:50:39 GMT -8 dvawolk said:

Thermal conductivities of materials[1W/(m.K)]:
IRON: 70
BRICK: 1
CONCRETE: 1
AAC: 0.2

Thermal conductivity of casting mixes can be significantly improved by adding
extremely hard minerals.
Eg. alumina (corundum is the densest form of alumina) or silicon carbide.
Alumina is usualy the cheapest.
Thermal conductivity of minerals depends on purity and to a lesser amount
density.

See also:
Heat transfer coefficient
en.wikipedia.org/wiki/Heat_transfer_coefficient

For the Internal flow look at forced convection.
www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html

[peterberg]

Apr 3, 2013 2:50:39 GMT -8 dvawolk said:

If one would use 6cm thick firebricks vs. 6cm thick metal, then ISA would be more like 1:80.

So, you are implying a bell made of firebrick should be 80 times as large as one of metal? No that doesn't sound as the correct figure.

A recently built stove in the Netherlands, made for a large part of unfired bricks and an estimated total Internal Surface Area of 6.5 m2 (70sq ft.) is doing quite nice at the moment. It's early days yet, the thing isn't dried out completely and the chimney temperature didn't exceed the 35 C. (95 F.) until now. The shape of the (single) bell is much more complicated than the two stacked 55 gallon barrels. The chimney is 15 cm (6") diameter, 7 m (23') straight up and insulated.

Incidentally, I am thinking it over to go there tomorrow. The builder will report his findings anyway, so I should be able to tell you more hard fact the day after tomorrow.