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Post by pyrophile on Jan 2, 2015 4:12:39 GMT -8
Hello everybody and Happy New Year! I would like to propose another opinion on Internal Surface Area of bells. It is my opinion and I may be wrong, of course! It is generally accepted here that a bell must be limited in its ISA and that each stove size determinates a maximal ISA. For example, the max ISA for a 6" stove should be about 6 square meters. I don't share this idea and I will try to say why. Let's imagine that we build a 6" rocket in an workshop, without its bell, "naked". The workshop has two doors, open, on opposite walls. It has a high ceiling. This workshop is like a bell, without pipe. Much bigger than 6 square meters. We could even imagine that we are not in a workshop but in a cave (une grotte) with one entry and one output. Or ,even, only one entry, like in a bread stove which is a bell used since a long time. Let's have also a little candle, parafine or wax, witch, in comparison, sees the wall of the workshop or of the cave as a very huge bell. Let's light both candle and rocket stove. What does happen? The big size of our bell don't interfere with both fires. The wall of the workshop are like an infinite bell for the candle! Of course, the workshop's wall won't heat with the candle, but they will -a bit- with the rocket. We could close the doors and drill holes in each door to let fresh air come in and, in the other door, put a pipe for the exaust. The little candle would also stil burn, despite the size of its bell. Any candle sees the closed place where it is as a very huge (infinite?) bell. One can also light a candle in a mass stove, let little air comme in and open slightly tne exaust tube and everything will work, despite this big ISA for a candle!
I would like to know your opinions about that, maybe there is something that goes wrong!
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Post by shilo on Jan 2, 2015 6:07:08 GMT -8
if you need a draft created by the heat, it don't work
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Post by pinhead on Jan 2, 2015 7:16:36 GMT -8
I'm somewhat of the same mind, though for different reasons; I think ISA is an incomplete metric.
For instance, a 6" stove inside a well-insulated house can handle a much greater ISA than the same stove in an uninsulated barn.
In other words, I believe the delta T is just as important as the ISA. Recommending a "maximum ISA" for a particular stove is ignoring this important piece of data.
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Post by peterberg on Jan 2, 2015 7:19:56 GMT -8
I think in a cave like you describe the rocket is acting like it is in open air. No wonder it would work.
In a more practical way: First, building a bell system which is twice the size of the accepted maximum the exhaust temperature will be low enough to cause condensation of the produced water vapor inside the exhaust. Accumulating water in the exhaust system will cause the chimney to stall eventually.
Second, such a system do need a chimney stack high enough to reach above the uppermost top of the bells when run outside. In a building, the stack need to reach above the highest point of the roof in order to stop the building being a better chimney than the chimney.
When built large enough, the bell approach the same environment as the open air. Of course this is a sliding scale, nothing to pinpoint where it would start or stop.
But it's an interesting thought, maybe you are voluntering for the job to find out the practical boundaries?
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Post by satamax on Jan 2, 2015 11:02:12 GMT -8
Guys, Peter.
I have has the same kind of thoughts as Pyrophile.
Kind of!
A room inside a house, in the middle, rather large. With two low exhausts. One way larger than normal, let say one meter wide by 40cm high or so, going outside through a tunnel of the same size. Being the kind of low flue for condensing gases. And above that 40 cm height, an elbow and an insulated chimney going through the house. As usual. A heat riser exhausting directly in this room. Since the hot gases are being pushed upwards by the cold ones. Even if it's opened to the great outdoors, it would transfer heat to that room's walls, and warm the surrounding rooms. But it might act as an outside rocket.
Just a thought!
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Post by pyrophile on Jan 4, 2015 8:43:34 GMT -8
About condensation, I think that it is true with metal bells. Then maximum ISA makes sense for metal bells. I would add that, for metal bells, an important thing is to know if the heat riser is in the bell or not. If it is in the metal bell, it will radiate towards the walls and you will need a larger ISA.
In a brick bell, I think that bricks partially absorb humidity. Then I think that there is no practical bundary. That's why I gave the example of a high ceiling workshop being the bell : this huge surface of walls will absorb the humidity. Put a pipe at the low part of one door and the stove will work, which is our goal. When I light a stove with kindling, i.e little pieces of dry wood, the stove works even if the bell has a huge area compared to the very little fire. If it does'nt, I use the primer. That is also what potters do with their kilns, whatever ISA the kiln has.
To say it from another point of view, as the fire is burning, the bell's walls are getting warmer, the difference of temperature between walls and smoke is getting smaller, there is less exchange and the ISA should be rised to compensate the loss of transfer (exchange). It is not true with metal bells because metal transfers heat very quickly.
In a practical point of view, mass stoves can't be too big : I incorporate columns and other systems in the bell(s) to rise internal surface of transfer.
That is the way I see how it works but maybe something goes wrong!
Benoit
PS What I said about the presence of the heat riser in the bell is also right for a brick bell : if the heat riser is in the bell, it will radiate towards the bell's walls and one will need a bigger surface of walls.
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Post by matthewwalker on Jan 4, 2015 10:04:30 GMT -8
I will just add that my interpretation of the ISA recommendations for system size is that they are recommendations that should lead to a system that will work reliably in a lot of conditions; cold start, warm start, windy, cold outdoor temps, warm outdoor temps, etc.
I personally don't bother to calculate, although I do feel it is a good tool for people without experience to gauge how big of a system to build. I also know that I may be more tolerant of a finicky system than someone I build for, so my max ISA is different than what I would use for a build for someone else. I get what you are saying Benoit, and if the thing is started with a bypass, then kept warm consistently, the numbers will be very different than a system that is expected to start up from cold with no bypass.
The metal comment Benoit makes above is incredibly important. It is incredible how little bare metal surface it takes to stall a system. All the folks testing their new RMH builds prior to cobbing and wondering what's wrong are a great illustration of this. Those bare flue pipes stall the system almost every time.
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Post by patamos on Jan 4, 2015 10:23:18 GMT -8
It seems to me that much of the intensive calculating done by Russian builders over the years... has been oriented towards achieving maximal ISA whilst also making sure the system can start cold under worst case atmospheric conditions. Without a start up bypass, such calculations had best be adhered to, as Peter suggests. With a bypass, we have more room for oversizing the bells. This gets me to wondering about start up bypass flues that have some semblance of a small bell in their run towards the exit flue. This way, as the absorptivity of heat varies with the temperature of the bell's mass, the bypass (auxiliary bell?) can still harvest the heat coming through it. This could be totally unnecessary though, so long as the simple bypass flue is utilized only just enough to keep a large ISA system from stalling.
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Post by satamax on Jan 4, 2015 10:38:18 GMT -8
Well, i will try to get back down to earth! There's a limit to the ISA of bells, the usefullness limit! If it's huge and serve no purpose, it's worth nothing!
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Post by peterberg on Jan 4, 2015 13:54:31 GMT -8
In a brick bell, I think that bricks partially absorb humidity. Then I think that there is no practical bundary. That's why I gave the example of a high ceiling workshop being the bell : this huge surface of walls will absorb the humidity. Put a pipe at the low part of one door and the stove will work, which is our goal. Benoit, Brick bells are behaving much the same way as a metal bell. See my experiments with the rocket/bell project in 2008, donkey32.proboards.com/thread/40 those bells were all built out of ordinairy brick, fire brick, refractory concrete and sometimes sidewalk pavers. The times I had to heat the chimney with a paint stripper are numerous, those were the times I've found out what water vapour could do to a chimney. Of course, every chimney stack is different from another and so are the heaters. I've seen some pictures of heaters built by you, those columns are interesting. Because it's also the way Russian masons were able to close the top of a heater with nothing else than the fairly large bricks they use.
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Post by pyrophile on Jan 8, 2015 14:22:33 GMT -8
Yes, Peter, Russians used in a clever way their columns both to fill the bell for a better exchange and also to close it.
I don't use by-pass. If there is a problem, it should be sufficient to make a very little fire in the firebox with dry kindle. Then wait. Then make a bigger fire. At worse, my client could light a newspaper below the exhaust pipe but I am not sure it happened. Other stove-builders stopped building bypass finding them useless. But of course, it depends also of the kind of stoves!
About condensation, I find that it is no so easy to have a too big ISA in a masonry stove (then condensation). I find difficult to be below 70 degrees celsius (whithout a bench) till the end of the firing (the temperature inside the stove rises, then exchange goes down and exhaust gases's temperature rises) . My stoves are about 1.70m long, 0.9-1 m wide, and 1.80m high, which is not so small but too small to have very low output temperatures. Then I have to fill my bell with columns or baffles.
This year, I tried to use two bells, high and narrow, as the idea was proposed here. I find that it is not so good in the way I did it, because, as Peter already said, the first bell is not really a bell if it connected directly to the firebox at the upper part of the first bell. Above all if it is rather narrow! There is rather a flow of hot air that goes too fast towards the hole in the bottom. Then this first bell should be filled with many columns or other ways to make its job. I will try to show images and also ways to fill bells (as potters or brick makers do) but I can not the following days.
I add that I don't put anymore one bell upon the other. Before, my stoves were 2.20 - 2.40 meters high which, in my opinion, is too high in comparison with people's height. But sure, it takes less room! And it is true that, in this case, the first bell were more acting as a bell because it had another shape.
Benoit
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Post by pyrophile on Mar 25, 2015 2:23:43 GMT -8
A bell (third image) with a huge ISA that works at very low temperature, without a fire ! Of course there is no condensation in this case which is the only (almost!) limit to bell's ISA. Bell efect appears as soon as there is a difference of temperature. As does draft. They share the same principes. In a house, there is ALMOST always an already draft in the firebox and in the stove (in bricks) BEFORE you light a fire because the stove and its exchanger are warmer than the pipe. Even, very often, if you did not make a fire for a long time. There are exceptions to the "already" draft, like at the beginning of autumn when it can be cooler outsides than insides for example and, above all, when the stove is still wet. As Peter said, the practical liùmit is condensation. Otherwise any size will work in a bell made of bricks that should stay almost allways warm enough to create a draft because the chimeny is almost allwys cooler. The picture above is, I think, a good illustration even if there are (small) differences with a stove in an house. In fact I consider that there are no real difference! Except condensation, of course! A 12 or 20 M2 (square meters)will work with a 15 cm (6 inch) batch box stove but will condensate, mor or less! Of course, I don't advise to build such a huge bell! I am just trying to explain what I mean. I prefer to fill bells and keeping the stove -rather- compact, like everybody. Especially as I avoid adding bell one upon another one and prefer to make them at the same level, one next to the other one, which takes more place, unhappily! Benoit
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Post by mkrepel on Jul 9, 2015 6:57:31 GMT -8
Pyrophile,
I know this is a relatively old post, but I am reading it for the first time. Please excuse me if I misunderstood your point.
I'm not sure I completely agree that bell size makes no difference. As we have a fire that is making a certain volume of smoke that must be pushed or drawn out of the building before they become a health issue, we have to be certain that proper draft occurs. In most households, it is not okay for the stove to allow smoke into the room for more than a few minutes. As Peter says, there is a practical limit. Our goal it to maximize heat exchange while keeping the process moving in a positive direction. That means we have to "lose" a portion of the heat to drive the draft and get the gases out of the building.
There is a guy (posted somewhere on the internet) who is working on down-draft exhaust. His goal is to remove so much heat from the exhaust that the remaining gases actually fall out of the end of the flue so that stove exhausts can be vented into the city sewers. This is a neat concept because condensation is okay if the end of your flue is in a location where condensation can run downhill. He posted a utube video in which he built a stove that exhausted down hill from the firebox. I thought it was interesting and the concept did work somewhat, but it was very finicky. He and his crew ended up constructing a normal updraft chimney in order to cook pizza and stand around in the warm glow of the stove. This is a situation which is at the practical limit of heat removal.
My point is that you cannot remove all of the heat if you want the gases to exit your building vertically (upward). Your bell size is more or less dictated by the need to make this happen. As you stated, very large bells are also not practical if they take up too much room in the building you will be heating.
You obviously have a lot of experience building wood burning devices and that is worth a lot. The conversation is a good one because it helps the group to talk through all of the theory and practical applications of heat transfer and natural draft.
Thank you for this thread,
Mike
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Post by pyrophile on Jul 12, 2015 5:52:02 GMT -8
Hi! I think that practically there is almost always enough draft in a masonry stove (not metal!), except the very first day in automn when the house can be colder than outside. But the following day, the stove is now warm and draft has increased. As you open the ashdoor, you can feel the draft! In a masonry stove it is not so easy to eject too low temperature gases because your accumulator gets warmer and warmer as the fire goes...a masonry stove is always warm inside, then your gases can not transfere their low heat so much. One musn't mix up the starting phase of the first fires, when the accumulator hasn't stored all the energy it is able to and the steady, regular service when the accumulator is charged and behaves in a different manner.
What I wanted to say with the example of a big room or even a house considered as a bell was that once the draft is primed (started, by different maners), the entire volume will warm, with time. And, once warm, this bell will works, as long as you give it more heat that it needs to keep warm.
Said in another manner, as long as your gases are hot enough, around 70 to 100°C, at the middle of your firing time (not the first minutes), in the steady phase (not the first day), you will have draft enough to evacuate your smoke.
It is my point of view but I can change it , no problem! and whatever my experience is (which is not that big!)...
As long as I realize I am wrong, I am in a learning process! Which is OK for me (even if unpleasant!)
Benoit
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