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Post by patamos on Oct 4, 2016 9:07:38 GMT -8
Hi Folks
Pat here. So i've been writing articles for the local news papers explaining how masonry and rocket mass heaters offer better overall efficiency. Optimized combustion is one factor. Potential for conductive (body contact) heat delivery is another factor. Large (low wattage) radiating surface area, and absence of indoor atmospheric convection loops (from a smaller concentrated heat source) another factor.
But the one that intrigues me the most is high ratio of heat harvesting. I just want make sure i have my facts straight before i beak off too much about it.
So here is my understanding: Every naturally aspirated wood-fired stove needs to send a portion of its heat up the flue to help ensure proper draft. The power or 'pull' of that vertical draught is affected by a temperature difference between flue gases and exterior atmosphere, location of chimney relative to roof height landscape and surrounding objects.. pressure planes inside and outside the house… and a few other factors.
Assuming the above factors are a constant, and assuming the stove within the dwelling needs to exhaust 80c exit temperatures.
A metal box stove with firebox temperatures of 400c will be surrendering 20% of its heat to drafting function.
A masonry/rocket with core temps of 1200c will be surrendering only 6% of its heat to drafting function.
This makes for a 14% improvement in harvesting - regardless of burn time.
Does this equation and result make sense?
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Post by Deleted on Oct 4, 2016 9:45:17 GMT -8
Some combustions products require more than 600°C to burn or a catalytic converter to raise combustions rate.
Not created heat cannot be harvested. The difference may be much bigger.
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Post by peterberg on Oct 4, 2016 10:36:06 GMT -8
Karl is right, no smoke out and no soot in the chimney means more of the fuel is being burnt. Complete combustion is key.
There's something else missing in the equation. That is, the amount of excess air, which is air that won't contribute to combustion but only heats up in the process and sent out the chimney. Not only the temperature of the exhaust gases is important, the amount of air travelling through is likewise important. So, when we would be able to minimize excess air, efficiency would be much, much higher.
Surprisingly, J-style rocket mass heaters sports an open fuel opening but when hot, excess air is quite low. The sucked in air is heating up very quickly and is resisting to go down, it wants to go up instead. So, incoming air volume is restricted this way and the J-tube works in this respect as a self-regulating mechanism.
Everybody is looking at exhaust temperature and try to get that down. Decreasing excess air is not as easy but it is ramping up efficiency no end.
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Post by satamax on Oct 4, 2016 12:59:56 GMT -8
Pat, two details, it's air buoyancy which powers the draft. Not temperature directly.
And metal stoves have far higher exhaust temperatures than rockets. IIRC, there is a norm. And to limit draft, they add a draft limiter in some cases.
I've checked french sites, they all say between 200C° and 250C°. But no norm. Tho i'm pretty sure i have seen it somewhere.
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Post by patamos on Oct 4, 2016 16:28:53 GMT -8
Thanks guys Great points. I am so grateful for all the things i am learning on this forum… And always good to be reminded of the principles. In my comparison i was thinking of the most efficient metal box stoves now in mass production. Most of these have tightly controlled air and some sort of super heated reburn device. Although those thin metal catalytic converters just cannot be as durable as refractory materials Peter your comment about the downward flow of heated air automatically reducing excess air coincides with an idea i have for the vortex stove. I have been pondering ways to eliminate the user input when it comes to adjusting primary and secondary air ratios. These vortex's can slow gassify if the primary air supply (through front door bottom) once the core warms up enough. But most users won't be able to discern between that and a crash-smoulder. So i wonder… In a design something like Trev's original rectangular cookstove version, would a long tube running through the harvesting bell chamber, sloping downward to delivery points along the bottom or side of the firebox door… offer the same slow down effect that the J-tube achieves? And would 25% CSA be a good starting point to experiment with? I know the vortex isn't a 'rocket' per se, but this is definitely the forum where ideas are most welcome and creative possibilities unfold.
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Post by satamax on Oct 4, 2016 20:29:45 GMT -8
So i wonder… In a design something like Trev's original rectangular cookstove version, would a long tube running through the harvesting bell chamber, sloping downward to delivery points along the bottom or side of the firebox door… offer the same slow down effect that the J-tube achieves? Pat, i think i can "sense" what you're thinking. I'm by no way an expert. But what you're describing here, isn't exactly the same as what happens in the rocket. The effect would be buffered. Basicaly, in the J tube, this self adjusting is induced by the fire right bellow the air entrance. While in your case, the air thinning would be induced by the heat in the bell. I don't think it would self adjust as fast as a rocket. You know, when you refill a hot feed tube, and the things go crazy for a while, all the wood pyrolizing at the contact of the walls and bottom, and also, the ones not in direct contact get the same effect through radiation. This realy violent fire counteract the "reverse chimney effect" when reloading. Or at least, this is my impression. I fear that, your bell "draft limiter" might hinder the burn, at reload. It's just a gut feeling. This leads me to another question, Peter, wouldn't this be the time to try top air in a batch, instead of bottom air? I realy admit, my batch seems to have less smokeback with the accidental top air gap filled. But you know, with this vertical batch idea i've been toying with. Top air would seems to make sense. Well guys, don't mind me, this is just a brain fart!
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Post by peterberg on Oct 5, 2016 0:56:24 GMT -8
Top air and top loading isn't a neccesary combination. I have seen one crude implementation sporting top air but front loading. Air was fed low at the side between two walls and ended up in the firebox above the fire. Top loading can only been done when starting, refills are notoriously smoky doing that. Apart from all this, the p-channel is a top air device anyway.
I am slowly working on a version with a backwards sloped floor and another way to implement the port and riser combination. Because of the angled floor, it would be far easier to look down and feeding the thing. Just one of those experiments, nothing firm to publish at this time. Keeping my mouth shut until it is ready will keep other people from claiming they found out first, which I experienced twice.
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Post by Vortex on Oct 5, 2016 1:49:11 GMT -8
Thanks guys Great points. I am so grateful for all the things i am learning on this forum… And always good to be reminded of the principles. In my comparison i was thinking of the most efficient metal box stoves now in mass production. Most of these have tightly controlled air and some sort of super heated reburn device. Although those thin metal catalytic converters just cannot be as durable as refractory materials Peter your comment about the downward flow of heated air automatically reducing excess air coincides with an idea i have for the vortex stove. I have been pondering ways to eliminate the user input when it comes to adjusting primary and secondary air ratios. These vortex's can slow gassify if the primary air supply (through front door bottom) once the core warms up enough. But most users won't be able to discern between that and a crash-smoulder. So i wonder… In a design something like Trev's original rectangular cookstove version, would a long tube running through the harvesting bell chamber, sloping downward to delivery points along the bottom or side of the firebox door… offer the same slow down effect that the J-tube achieves? And would 25% CSA be a good starting point to experiment with? I know the vortex isn't a 'rocket' per se, but this is definitely the forum where ideas are most welcome and creative possibilities unfold. Hi Pat, The best self regulating system I've come across for this is a bi-metalic coil, which controls a hinged flap to gradually close off the primary air as the stove heats up. I first saw one in the early 80's when I was working in a factory that had a huge wood fired boiler for the central heating. Best examples I can find on the web are in this thread: www.wintertrekking.com/community/index.php?topic=2374.0Also there's a picture here of a commercial stove using one: i271.photobucket.com/albums/jj141/motorseven/Stove/VEcarb011.jpgThe other idea that I've thought of trying is a box section door frame, where the primary air inlet is at the front top of the frame but the outlet into the stove is on the bottom inside.
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Post by patamos on Oct 5, 2016 8:22:07 GMT -8
Just one of those experiments, nothing firm to publish at this time. Keeping my mouth shut until it is ready will keep other people from claiming they found out first, which I experienced twice. Peter this doesn't surprise me. Seems that many times the people who receive credit for innovations are the ones who invest the most energy in ruthless self-promotion. You are of a more noble nature, and you have my deepest respect for all that you have shared and taught us. Sata, ya i wonder what the differences are between vertical J-feed and sloping tubes… And would Trev's idea of a frame box section come closer to the effect in the J ? Trev, now I see two little bimetal coils at either end of the primary air sliders in the door. Might need some mechanical protection from wood knocking around. Sure glad i posted the question
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Post by patamos on Oct 5, 2016 10:20:57 GMT -8
So i wonder… In a design something like Trev's original rectangular cookstove version, would a long tube running through the harvesting bell chamber, sloping downward to delivery points along the bottom or side of the firebox door… offer the same slow down effect that the J-tube achieves? Pat, i think i can "sense" what you're thinking. I'm by no way an expert. But what you're describing here, isn't exactly the same as what happens in the rocket. The effect would be buffered. Basicaly, in the J tube, this self adjusting is induced by the fire right bellow the air entrance. While in your case, the air thinning would be induced by the heat in the bell. I don't think it would self adjust as fast as a rocket. You know, when you refill a hot feed tube, and the things go crazy for a while, all the wood pyrolizing at the contact of the walls and bottom, and also, the ones not in direct contact get the same effect through radiation. This realy violent fire counteract the "reverse chimney effect" when reloading. Or at least, this is my impression. I fear that, your bell "draft limiter" might hinder the burn, at reload. It's just a gut feeling. So the air entering down J's feed tube meets a horizon of intense radiant heat… the air wants to rapidly expand… which creates a tighter bottleneck. But when reloading the rocket booster effect accelerates draw which counteracts the bottleneck… ? Trev, would you agree that when reloading a vortex box (without opening the bypass damper) it sometimes helps to open the primary air slots first? Thinking of bi-metal coils on the sliders, it might be a challenge finding a way to bypass them. Maybe best to forget about it and just open the door slowly… or have a slider on the ash pit door.
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Post by satamax on Oct 5, 2016 10:57:33 GMT -8
So the air entering down J's feed tube meets a horizon of intense radiant heat… the air wants to rapidly expand… which creates a tighter bottleneck. But when reloading the rocket booster effect accelerates draw which counteracts the bottleneck… ? Pat, that's impression i have. No scientific evidence tho.
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Post by Vortex on Oct 5, 2016 14:33:52 GMT -8
Trev, would you agree that when reloading a vortex box (without opening the bypass damper) it sometimes helps to open the primary air slots first? Thinking of bi-metal coils on the sliders, it might be a challenge finding a way to bypass them. Maybe best to forget about it and just open the door slowly… or have a slider on the ash pit door. I very rarely reload my stove, one fill is usually enough for me. If I have to open the door mid-burn I open the bypass damper first to avoid getting smoke in the room. I expect you're doing it at the coal stage so that wouldn't be a problem. When reloading you want to maintain enough heat in the core to gasify the new batch of fuel, so I would only open the door for as short a time as possible. I don't think a bi-metal coil would work with the sliding type of primary air vents that I'm using at the moment. The boiler that I first saw a bi-metal coil on, had the coil in the middle of the door (there was no window). The coil had a hook on the end with a small chain hooked over it, which was connected to the primary air vent, the air vent was a horizontal rectangle hinged at the top (like a letter box), so as the coil expanded from the heat it gradually lowered the chain and reduced the primary air. If you didn't want the automatic control you just unhooked the chain. I would like to try that box section door frame idea that I mentioned, but it's quite difficult to construct.
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Post by patamos on Oct 6, 2016 7:53:27 GMT -8
I would like to try that box section door frame idea that I mentioned, but it's quite difficult to construct. Especially when welding off the back of a truck bed
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