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Post by Karl L on Jan 17, 2016 5:56:27 GMT -8
I have a question:
On a batch box system, what temperature do you aim to run your thermal mass up to?
The average temperature, or the range of temperatures, top and bottom.
Thanks, Karl
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Post by satamax on Jan 17, 2016 6:35:44 GMT -8
Karl, it' s hard to control. I'm just learning. But one thing I have noticed, it's that I would like to replace the metal radiator (barrel) by a mass bell. And finish the arrangement, just before the chimney, by a metzl bell/radiator. Far gentler temperatures at this point mean that you could heat for a longer time. Charging the mass better, and not feel uncomfortable in the house.
The maximum I've had my mass at, is about 34C°, after 8 hours of burning. In general it's between 15C° and 24C°. Three burns of a 9 incher batch keep me about 9/10 celcius for the whole 24hours. It's in an uninsulated workshop, well I have a R1 euuropean measure, in the walls, and R3 in the roof. 6m high ceilings. 113m². Not too bad!
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Post by Karl L on Jan 17, 2016 9:00:18 GMT -8
Thanks :-) 34C sounds like a very low temperature, and 15-24C is below typical room temperature! I was expecting people to be answering 100C at least!
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Post by Vortex on Jan 17, 2016 10:03:55 GMT -8
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Post by pyrophile on Jan 17, 2016 13:25:27 GMT -8
Hi! I would say that the surface temperature of your stove depends on different things. How big is your room or house? How is it insulated? What are the materials of your walls (earth, wood, stone, etc)? What power do you estimate that the stove needs to counteract the losses of your house or room? It depends also on the external surface of your stove : a little stove must be hotter than a bigger one. A big house with "cold" walls (like granite) needs a hotter and bigger stove than a little wood house! Rather 80-100°C for a big cold place and 45-60°C for many more common (for me!)situations. Which inertie do you want and with wich regularity? A steady 50°C for 24h or a wider range with more heat at a particular moment of the day? 12 hours of heat or 24? I often aim 45-65°C with stoves having a surface about 10-12 m2 but it is my way of doing!
I add that I usually build two (brick) skins stoves, which give more stable (steady) temperatures. Single skin stoves react quicker : the heat produced crosses the stove faster and hotter : it can be a good thing or not, depending of your needs. You can also use different materials : using a metal part in the stove, like typical rocket mass stoves using a metal drum, allows to heat the room quickly, (EDIT) which is interesting if your stove is a bit light and has cooled when you get up and must make a fire or when you come back home in the evening and the home is a bit cold. In my opinion, rather big stove don't need metal part when they heat constantly for 24hours ... and if they suit well the house, which is not so easy!
Each soultion has advantages and drawbacks, and it is not so easy to choose!It is even the most complicated task for any stover!
Benoit
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Post by pyrophile on Jan 18, 2016 2:29:37 GMT -8
100°C are then rather a high temperature or only for a rather short time, in my opinion. With this temperature, you have a lot of heat transfert as transfert rises with the difference of temperature beetween the stove and the room. Satamax's temperature around 24°c allow little transfert but maybe it is good if the stove is big and the house quite little and very well insulated (I don't remember, I didn't read deeply the forum for a while). A heating floor has surface's temperature in this range (a bit below) but with really big surfaces. For me, those temperature (15-35°C) are too low and I aim more, as I said. Satamax, sorry, I didn't read your thread attentively...Have you another opinion?
Another thing to consider is that, at 100°C on the surface of the stove,you must remember also that the stove will loose its heat quickly if it is not big enough because there is a rather big difference beetween 100°C and the room's temperature, then the transfert is high and the stove empties quickly..
I am sorry because I don't give you usable answers but rather big principles...There are many variables, the home, the type of the stove's accumulator, your needs, our (my!) habits of thinking, etc!
Benoit
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Post by satamax on Jan 18, 2016 3:22:39 GMT -8
Benoit, I clearly admit I have failed. The temps are real low, because I have too much radiation surface, compared to accumulation surface. The fuel tank I've used is far bigger than a barrel. And the second tank, covered by mass has too much of a gap between mass and inner skin.
My latest opinion on the subject, if you want to combine direct and slow heat. Is to have the mass bell above the heat riser. Anyway, despite the high temps, all heat won't be transferred straight away to the heavy mass. So it leaves a bit of heat for the radiator. I would do it in the form of a barrel or metal radiator last in line, just before the chimney.
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Post by Karl L on Jan 18, 2016 15:19:10 GMT -8
Thanks everyone, This discussion is very useful. I have been struggling to think about all of those factors, and it's useful to know that it's not easy!
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Post by pyrophile on Jan 19, 2016 9:25:42 GMT -8
Hi Satamax! Sorry I didn't want to say anything about your results because I didn't read the thread on your realisation. I just discovererd your surfaces's temperatures which seemed to me rather low but I did't suppose you failed! Even if it is my job, I , of course, already failed and might fail again, for example with the cookqstove I finished and which is supposed to warm a maybe too big heating wall (bell)...
Benoit
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Post by patamos on Jan 20, 2016 22:02:09 GMT -8
To add to all the factors under consideration. I have lately decided that any future vortex heaters i build i will make sure the body of the bench bell chambers is at least 5" thick. Less than that and some areas can get a little too hot… I like satamax's idea of metal radiator downstream of mass harvesting. A single wall flue heading towards exit can have the same effect...
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Post by pyrophile on Jan 22, 2016 6:26:10 GMT -8
What is the goal of a metal emittor? Because of its high conductivity and its thinness, a metal emitor heats room's air quickly (speed) and strongly (power), by convection (above all, but also radiation). Warm air counteracts cold walls and cold ceiling in the sensation of heat. But warm air is not very confortable and human beings prefer a feeling of heat produced by radiating "warm" walls (also ceiling and furnitures) rather than hot convecting air.
Why using metal surface for heating? Metal for heating a room is only interesting in certain situations, for example for little inertia's mass stoves or for frequent absences. For example, when a mass stove has little inertia, the room cools quickly with time. It is also common that people don't stay at home all the day along and come back home in the evening. The cooling of the house when they were away is then not a big problem if they can have quick heat when they are back home, in the evening. That is exactly what makes the metal drum of a rocket mass heater : it delivers heats quickly and strongly by convection (but also radiation) thanks to its metal part.. After the firing, the earthen or brick accumulator takes over. The standard rocket mass heater can be often a bit short in accumulation (and also in power) in our homes here in France, made of earth or stone. The standard little rooms have been enlarged by new young inhabitants. Then the metal drum is a good solution to give more heat for a short time if the rocket mass stove lacks a bit inertia (and/or power). A metal emitter is also usefull when people are often absent for one days or two. The metal part heats quickly room's air and the accumulator will take over later.
I think that a metal emittor in a mass stove is not necessary if people don't need quick and strong convective hot air. If the mass stove has inertia enough (24 hours) and power enough, I don't see the interest of a metal emittor. Even to heat a room upstairs, one must not forget that heating upstairs by convection will only last the time of the fire...not a lot! The natural convection of a mass stove will, in contrast, last for hours.
Where to put the metal radiator in the stove? If metal is usefull, the standard place, that is at the very hot place above the riser, induces that the metal emitter must not be too big. If it is too big, a lot of heat will be sent in the room and there won't be calories left enough for the accumulator! The drum can be partially surrounded by cob in order not to superheat a very well insulated house or a tiny house. The metal part can be at the end of the accumulator. Then the metal emittor has less heat to emit as a good part was allready kept by the accumulator. The metal emittor must have a big enough surface to counteract the drop in temperature of the gases and to emit enough power with cooler gases. The choice is beetween a very hot emittor with little surface or a cooler but larger surface to emit the same power. But not so big that smoke would be too cool to produce draft! Of course, one must not forget that the metal emittor won't emit heat anymore when the fire stops!
Another solution is to combine brick and metal at the same place as did russian stove builders in the 20th century or as did Peter (I think) in a metal emittor made of stacked drums full of bricks forming a kind of brick cylinder insides. In this solution, it is a metal skin that keeps emitting heat (rather convective heat) thanks to the inside brick accumulator. But, thid interesting solution has also its own problems : it is a bit tricky to develop in order to find the good quantity of bricks, their best arrangement and so on. How much inertia do you get? what does look like the curve of heat restitution, etc. Moreover, the heat is mainly produced by convection (because of metal, of shape, etc.) during the entire period of heating, which is rather a drawback, being less confortable.
Benoit
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Post by peterberg on Jan 22, 2016 8:19:36 GMT -8
Another solution is to combine brick and metal at the same place as did russian stove builders in the 20th century or as did Peter (I think) in a metal emittor made of stacked drums full of bricks forming a kind of brick cylinder insides. In this solution, it is a metal skin that keeps emitting heat (rather convective heat) thanks to the inside brick accumulator. But, thid interesting solution has also its own problems : it is a bit tricky to develop in order to find the good quantity of bricks, their best arrangement and so on. How much inertia do you get? what does look like the curve of heat restitution, etc. Moreover, the heat is mainly produced by convection (because of metal, of shape, etc.) during the entire period of heating, which is rather a drawback, being less confortable. Benoit, I think you are right about all of this bar one point. That is, heat emitted by metal is always radiation. Part of this will heat the air molecules around it, converting the heat to convection. But... when the temperature of the surface (irrespective of the material which it's made of) is higher than 120 degrees Celsius if I remember correctly, it will warm up the air molecules directly so the major portion is transformed to convection. My point being, any surface will produce loads of convection as long as the surface temperature is above a certain treshold. In a conventional central heating system the heat is emitted by so called "radiators" but in effect those are designed to emit most of their warmth as convection by enlarged surface area, double plates and soforth. A simple warm metal plate at the wall with insulation behind it acts as a radiator as long as the temperature isn't too high. There exist some wall and/or ceiling devices called "infrared radiators" but the majority of those are made of metal. The trick is that those won't get warmer than 90 C or something like that. So, to come back to the barrel container with bricks in them, those won't be hot enough to produce overly much convection.
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Post by Karl L on Jan 22, 2016 10:00:51 GMT -8
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Post by pyrophile on Jan 23, 2016 7:55:52 GMT -8
If I remember well, convection is due to two factors, conduction and radiation. Thanks to conduction, air in contact with the body of the stove gets warmer and rises. Thanks to radiation, air a few millimeters or meters away, not touching the stove's walls, becomes the radiation of the stove, gets warmer and rises.
I think that there are other factors, like conductivity of the material, its thichness or the shape of the emittor that play an important role in convection., but it is just an opinion.
I think that conductivity combined to thickness are rather important. A thin metal foil (forming a barrel, for example) has a lot of capacity to extract calories from the gases crossing it and a great capacity to send those calories outside the drum. I build simple bell stoves made in a barrel (220 liters). I have the impression that they have a high capacity to enhance convection. I think so but maybe because we often prefer to believe what we allready believe! I have the impression that there is more convection with this metal stove than other cylindrical stoves I build but with a 1 to 2 cm thick refractory concrete skin. Those one are bigger, 70-80cm in diameter and higher, 170cm high. I have the impression that those stoves work more on radiation than on convection. Their confort is more like the confort given by a mass stove, even if they are light! The following day, you still feel warmth in the room instead of with metal stove, heat quickly disappears. You fill that room's walls have been warmed by radiation like with mass stoves. I have the filling that warmth with convection disappears quicker, maybe because it rises and is more in contact with cooling parts like ceiling.
Another aspect important, owing to me, is shape. I have the impression that the same big metal barrel creates more convection when placed horizontaly, 30cm above the floor, than when placed vertically, directly on the floor. When horizontally placed, 30cm above the floor, air coming from below finds easely its way up, rubbing against it, warming and rising. That is only a supposition and I don't say I am right! Maybe there is no difference or very little!
I am interested by those temperatures of metal below 120°C Personnaly, I have the impression that a metal stove is more confortable below 80°C (and a big shape to conteract the low temperature). I have the impression that at those temperature, there is less convection and more radiation in a range of frequencies more confortable for us. I read about those things (radiation frequences more confortable than other) but I don't know what is true and what is not.
I totally agree that a big metal emittor works but if it will stop emitting when you stop the fire. This big emittor will cool a lot the gases inducing potentially problems of draft. Its size must be in relation with the power you want to emit. I think it is more problematic that it seems to be. But the best, of course, is to try and see advantages and drawbacks!
About radiation, there is also a paradox which is that radiation rises with temperature! That's why we can measure the temperaure thanks to an Infrared thermometer.
Benoit
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Post by Karl L on Jan 23, 2016 8:51:30 GMT -8
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