Dear Gustavo
The diameter of a chimney should be as small as possible without interfering with the flow of gases through the stove. Once heat enters a chimney, it is important to get it to the top with as little loss as possible. The more heat that is lost on the way to the top, the more heat must be removed from the fire to keep the chimney running with enough draft to function (it functions in the same way as a fan). There is a chimney draft calculator on the Stoves website which can be found quickly by searching for that term. It also calculates the velocity in the chimney (you can change the diameter). If the velocity rises above 3 metres per second at the maximum power of the stove you should increase the diameter. Flow resistance below 3 m/s is not significant if there are no elbows. Dear Michal The optimisation of the chimney is the first step to getting the gas flow correct. It is not related to the combustion chamber dimensions, but to the gas flow it has to handle. A combustion chamber might be large or small, just as it might be long or short. 1. Is there a good rule of thumb for the minimum chimney height to have good draw from the hearth? The height is related to the temperature available. If possible try to have an exit temperature above 100 C so you do not get condensation on the inside. The height is often dominated by what is available in the community or local common practice. The only way that I have to estimate this is using the chimney effect equation (ie flow is proportional to Area, height^0.5) and I found that for practical chimney heights for my situation (ie 15-20 ft) there was not a significant change in the flow rate of the chimney. The flow rate is related to the temperature (average) in the chimney and the overall height. It is a buoyancy thing. The Draft Calculator handles this for you. If another 5 tf made no difference it was too large in diameter (very common) or so cold the additional height contained nearly no heat. However, for the equation to apply the chimney needs to be adiabatic, which I think is a bad assumption in this case. Do you think 15 ft will be tall enough or does it need to be taller? The height of 10 ft is often quite adequate for a domestic stove up to 15 kW. If they are higher, they can be cooler, but if you drop below 100 C the condensation will create all sorts of problems especially if the chimney is made of brick. A brick chimney is very rough and does not flow as well. That is partly why chimneys use clay tubes and why the clay tubes are round, not square (conserves heat). 2. I do not understand the effects of geometry on chimney draw and was curious if there is a problem with necking down the chimney significantly after the hearth? There is no problem with necking per se but the overall performance has to be considered. If there is a horizontal shelf behind the stove with the chimney rising vertically from that shelf, rain and condensation will fall onto the shelf (often a sort of tray) and not get into the stove. I suspect that my host family may copy the design of a neighbor’s chimney (since they are hiring the same mason to build theirs) which tapers down asymmetrically (ie the left wall is slanted and the right is straight) after the hearth from about 2x3.5 ft^2 at the base to about 2x1 ft^2 over about 6 ft. That is a very large opening. The issue with large brick chimneys like that (going all the way up) is that on a cold morning it may prove impossible to get the draw started. It is just too big, too cold and too wet. The solution in such circumstances is to insert a sleeve to get the velocity up and the heat conserved so it is not absorbed into the walls. Chimneys run on heat. No heat, no draft. 3. Along the lines of the previous question, I am concerned that the rain cap for the chimneys may choke the flow. The rain caps of the chimneys here is a thin slab of concrete set at the top of the chimney with 1-3 bricks at each corner. Do I need to pay attention to the size of the gaps under the rain cap? The area should exceed the chimney area which as mentioned above, is usually far too large to work properly. I say a stove today at the China Stove Expo in Langfang which had a 140mm diameter chimney – far too large for the stove. By the time the gases get to the top they will have cooled a great deal. It was of course, running with black condensation indicating both poor combustion and too low a chimney temperature. 4. I know if you build the chimney shorter than the apex of the house the chimney effect of the house may overwhelm the effect of the chimney. Do you have any intuition or advice about the height difference between the chimney and the roof apex that is needed to avoid this? The minimum height relative to the apex is dominated by the wind over the roof, not the draft within the house itself. The standard rule is 3 ft above the apex which is why a lot of chimneys emerge at the apex. It can be lower if it is at the edge of the roof. I do not know of a rule about that. 5. I feel intuitively that it would be a good idea to minimize the height of the hearth entrance to capture the smoke plume before it spreads but I am not sure if this matters if there is good draw in the chimney. Do you have an opinion on this? That approach is quite correct. The temperature in that region will be the highest anywhere in the chimney so put it to good use. If the chimney is working properly the problem will not be getting gases to enter, it will be stopping it pulling warm air from the room and taking it up the chimney. In other words the problem should not be there in the first place if the chimney is well designed. 6. Have you ever heard of Rumsford chimneys? I had a construction engineer friend of mine build one in his house and he said that the draw from the chimney was excellent. I was just curious if you had heard of it and if you had an opinion of it. Do you mean a Rumford Chimney? http://www.helium.com/items/1212589-building-a-runford-chimney It was one of the great advances in the 18th Century. Another was the Franklin stove which was an improvement on the downdraft stove first invented 50 years earlier. “The construction of this chimney is explained in, "The Third Edition of the Fireplace and Chimney Handbook," as published by the Masonry Institute of America.” That refers to a recent improvement on the 1790 model of the Rumford chimney. Best regards Crispin in Beijing Hi all this is Gustavo I´m getting into chimneys, as I was working with Dr. Larry Winiarski I learn that the diameter of combustion chamber has to be equal the diameter of the chimney, but allwas depend on the size of the diameter of the cooking area and if you are not loosing heat in the cooking proces for that reazon what do and work for me is that if a rocket stove has a 4 inches diameter in the area wher you install the chimney I all ways reduce to 1/4 of an inch this will help to hold more heat under the cooking area surface, long time ago I wanto experiment with a Rocket Stove with diferent chimneys to calculate the optimal diameter and height of the chimney, I think not from a scientist point of view a big diameter loses heat and if is the chimney is too high the acelacion would affect the gases, therefore we would be causing a deficiency in the use of the energy produced by the combustion, I would like that someone with the right equipment put hands on the issue and get some controlled tests to pass the information to stovers. another thing that I observe is that if the diameter of the chimney is smaller than the diameter of the combustion chamber, the heat go to the opposite direction of the combustion not folowing to the chimney or whether there is a heat exes not going to the right derection, it's like a water container that is overflowing by which while we add water at a higher speed to the output speed. I said trying to increase the diameter gradually until the correct flow encontral check this link is one of my stove, in this case the chimner was a bit to small, until Dr. Winiarki give the idea and keep trying to encrease the diameter gradually until I found the proper flow of gases, this a challange in every new desing. http://www.youtube.com/watch?v=MQzHBU1CI80 ps: this is not a scientific commentary, my work is based on common sense but on some occasions when the problem is more difficult I tried to get profecional advise otherwise I could not get ahead wiht my ideas. Thank you Gustavo From: Paul Anderson <[email protected] <mailto:[email protected]> > To: Michal Usowicz <[email protected] <mailto:[email protected]> >; Discussion of biomass cooking stoves <[email protected] <mailto:[email protected]> > Sent: Saturday, March 23, 2013 5:34 PM Subject: Re: [Stoves] Chimney Project in Madagascar Dear Michal, (and Stovers) I am sending this reply and your message (below) to the 500+ readers of the Stoves Listserv. Any replies from them need to be sent to you also at Michal Usowicz mailto:[email protected] because you are not subscribed to the Stoves Listserv. About chimneys, I am not qualified to respond. My only comment is to question the size (and probable expense) of what you are discussing. Dr TLUD Paul S. Anderson, PhD aka "Dr TLUD" Email: [email protected] <mailto:[email protected]> Skype: paultlud Phone: +1-309-452-7072 Website: http://www.drtlud.com/ On 3/23/2013 7:23 AM, Michal Usowicz wrote: Dr Anderson, I hope that everything is going wonderfully with you and that your TLUDs are still burning! I recently got to Madagascar for Peace Corps service and things have been progressing well. My host family for pre-service training here cooks with wood inside but they are planning to build a chimney and I would like to help them construct a good one. Large diameter (about 3-4 ft) brick chimneys similar to what you may expect to see in US homes are common in the community that my host family is in and I think a brick chimney will be the best solution to ventilating the home in this context. A large diameter, natural draft, brick chimney seems like a straight forward problem but I wanted to make sure that I was not making any mistakes. I thought you may be a good person to ask even though I know this isn’t your exact specialty. I expect the chimney will be about 15-20 ft tall and at least the hearth will have a rectangular cross-sectional area of 3.5x4 ft^2 since that it the current size of my host mother’s fireplace (although I am unsure if my host family will construct the chimney with a taper). It will primarily be made out of clay bricks and mortar. Could you please give me some guidance on the following questions? 1. Is there a good rule of thumb for the minimum chimney height to have good draw from the hearth? The only way that I have to estimate this is using the chimney effect equation (ie flow is proportional to Area, height^0.5) and I found that for practical chimney heights for my situation (ie 15-20 ft) there was not a significant change in the flow rate of the chimney. However, for the equation to apply the chimney needs to be adiabatic, which I think is a bad assumption in this case. Do you think 15 ft will be tall enough or does it need to be taller? 2. I do not understand the effects of geometry on chimney draw and was curious if there is a problem with necking down the chimney significantly after the hearth? I suspect that my host family may copy the design of a neighbor’s chimney (since they are hiring the same mason to build theirs) which tapers down asymmetrically (ie the left wall is slanted and the right is straight) after the hearth from about 2x3.5 ft^2 at the base to about 2x1 ft^2 over about 6 ft. 3. Along the lines of the previous question, I am concerned that the rain cap for the chimneys may choke the flow. The rain caps of the chimneys here is a thin slab of concrete set at the top of the chimney with 1-3 bricks at each corner. Do I need to pay attention to the size of the gaps under the rain cap? 4. I know if you build the chimney shorter than the apex of the house the chimney effect of the house may overwhelm the effect of the chimney. Do you have any intuition or advice about the height difference between the chimney and the roof apex that is needed to avoid this? 5. I feel intuitively that it would be a good idea to minimize the height of the hearth entrance to capture the smoke plume before it spreads but I am not sure if this matters if there is good draw in the chimney. Do you have an opinion on this? 6. Have you ever heard of Rumsford chimneys? I had a construction engineer friend of mine build one in his house and he said that the draw from the chimney was excellent. I was just curious if you had heard of it and if you had an opinion of it. Again I hope that everything is going well with you. I would really appreciate any advice you may be able to give. Sincerely, Michal Usowicz
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