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Post by satamax on Nov 28, 2012 9:13:02 GMT -8
Yep, thanks a lot Pinhead.
My aim is to heat the whole mass, and it will double as my sofa. Might have to insulate the top ;D
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dante
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Post by dante on Nov 29, 2012 14:44:33 GMT -8
Hi.. I have been reading alot about RMH this past week or two, just came upon the bell concept yesterday... in short, trying to wrap my mind around the whole concept. LOVE your half barrel system, this system makes the most sense of all I have read.
What I dont get, or at least what I know I dont get so far... is the exhaust system. Can you tell me your thoughts on how the exhaust gases exit? I LOVE the idea of the bell and bypassing all the piping, but dont understand how the exhaust gases exit without being led to do so by the whole system of pipes.
Okay.. they go out through the chimney... I get that... but dont all the gases just have a big opportunity to just hang out in the system and look for leaks?
Am going to build one this week I hope to heat a 70 sq ft mud hut.
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Post by matthewwalker on Nov 29, 2012 15:49:37 GMT -8
Oh man, I'm not the guy to try to explain it properly, but I'll maybe point you in the right direction and perhaps some other folks can chime in.
I've built my half barrel systems in two significantly different ways. The first is to basically use them like big flue pipe. In that configuration I couple the barrels all together with no restriction between them, in a linear manner. From firebox/main barrel exhaust into the first half barrel, then a run of half barrels, then the exhaust flue set basically on top of the last half barrel. This functions exactly like flue pipe. Exhaust flows from one end to the other and up and out. This configuration works well for longer runs and anywhere you want quick response.
The second is using the barrels in a "bell" configuration. There are a lot of posts on this board describing how a bell works, so I suggest searching that term and reading all you can, the info is here. Basically it involves using the barrels as a chamber, with the gasses coming in at a higher level than they go out. So, in this configuration the exhaust is coupled to the barrels low, so the gasses have to give off some of their heat to the mass in order to sink and find their way out the exhaust. There's a lot more to it than that, but maybe that will give you a starting point. In this configuration the flow of gasses doesn't need to be linear, so it's possible to have what look like "dead ends" in the exhaust path and still transfer heat to them. I like to use the analogy of filling a pond that has a stand pipe to drain it once it reaches a depth of 6". The water will flow to all available dead ends until the level reaches that of the stand pipe, at which point it exits. The bell works the same way inverted, if that makes any sense.
You will be able to heat 70 sq. ft. without even trying.
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dante
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Post by dante on Nov 29, 2012 16:16:15 GMT -8
Thanks.. Reading these threads here like a fiend right now... I think I will learn more as I start building and burning, but picking up alot of great ideas here. I want to have a solid understanding as I go into the project, but then again I have always learned best by doing. Thanks for responding.
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Post by pinhead on Nov 30, 2012 6:50:36 GMT -8
Here's a drawing that I did... Doesn't really explain much but may help you visualize the "bell" system... EDIT: Donkey's modification to my drawing shows a more optimal design than what I'm running right now. |
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Post by peterberg on Nov 30, 2012 9:29:27 GMT -8
I LOVE the idea of the bell and bypassing all the piping, but dont understand how the exhaust gases exit without being led to do so by the whole system of pipes. It's quite simple, really. Imagine a bell, like one in a church tower. Place it on the ground and imagine two holes in it, both through the lower rim and opposite each other. When hot air is fed into the bell through one of the holes, inside the bell the velocity of the gases will drop severely because the bell is very much wider than the pipe leading to it. The hot air, being lighter, will rise inside, pushing the colder gases out of the other hole. This effect is due to the existence of gravity, it's always there and doesn't need a stack draw to work. As a consequence, the bell system is virtually without friction, quite the opposite as in a piped system. A second bell will extract even more heat out of the exhaust gases. Of course, when more heat is fed into the bell it will fill up, a lot of this is taken up by the walls of the bell and eventually the air coming out of the exit hole will rise in temperature. But the bell itself will stay warmer than the exhausted air. This whole phenomenon is a side-step of Bernoulli's law, it's helpful to google this term and read about it. Okay.. they go out through the chimney... I get that... but dont all the gases just have a big opportunity to just hang out in the system and look for leaks? Surprisingly, it doesn't. I've measured the atmospheric pressure inside the bell and the chimney, and both where negative throughout 99% of the burn. In other words, only in the first minutes of the burn there will be a slight over pressure inside. The rest of the time there will be a small but consistently vacuum. No smoke is coming out possible leaks, but cool air is sucked in instead. Conveniently, don't you think? |
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Post by pinhead on Nov 30, 2012 9:42:13 GMT -8
Okay.. they go out through the chimney... I get that... but dont all the gases just have a big opportunity to just hang out in the system and look for leaks? Surprisingly, it doesn't. I've measured the atmospheric pressure inside the bell and the chimney, and both where negative throughout 99% of the burn. In other words, only in the first minutes of the burn there will be a slight over pressure inside. The rest of the time there will be a small but consistently vacuum. No smoke is coming out possible leaks, but cool air is sucked in instead. Conveniently, don't you think? I'd like to add that the bell in this configuration will be in a state of vacuum only when the exhaust inside the chimney is warmer than outside air -- in other words, when the chimney is aiding with the "stack effect." Luckily, as peterburg said, that's 99% of the time. |
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Post by satamax on Dec 1, 2012 6:08:31 GMT -8
Guys.
What is your opinion, cast the haff barrels in, and keep the nice wooden shuttering around? As i have drawn earlier. Or make cheap plywood shuttering/molds, that i remove and make a concrete base sofa? I could even wax the concrete.
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dante
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Post by dante on Dec 2, 2012 14:21:04 GMT -8
The hot air, being lighter, will rise inside, pushing the colder gases out of the other hole. So this infers, or rather states; 1 That there is a separation of hot air, cold air and exhaust gases. 2 That the exhaust gases are heavier / cooler than the hot air, consequently they exit via the lower route through the chimney more directly - provided the cooler air in the bell is still hotter than the outside air and that the route out of the bell is on a low / bottom plane of the bell. If I am understanding this correctly... very cool! |
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Post by Donkey on Dec 2, 2012 21:38:33 GMT -8
The hot air, being lighter, will rise inside, pushing the colder gases out of the other hole. So this infers, or rather states; 1 That there is a separation of hot air, cold air and exhaust gases. Yes, there will be a heat gradient, hotter at the top, cooler at the bottom. You can (also) make multiple bells that each posses their own gradient, one leading to the next, etc. Yep! Innit tho? |
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Post by Donkey on Dec 2, 2012 22:01:18 GMT -8
Pinhead, I've altered your drawing a bit. Removed the pop-out spaces, rounded the transition to the heat riser and changed intake height to the first bell. check it out.. To this, you could add a P-channel and trip-wire.. Attachments:
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Post by satamax on Dec 8, 2012 8:19:02 GMT -8
Hi guys.
Well, i was talking with Len at permies, and he said that it would be better to put the barrels the other way around. Curve down. He's done pretty much the same, exept that he didn't use a barrel, but a water boiler. Put the opened/cut side up and patio blocks on top. He said about the hot gasses being less dense, therefor taking more room, so there is need for more room on top., and less at the bottom. Well, it's getting more confusing now. What is your opinion guys?
Thanks.
Max.
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Post by matthewwalker on Dec 8, 2012 10:25:03 GMT -8
If I remember correctly he also put vent pipe inside it. If I was going to do what he did I wouldn't bother with the tank at all. Just build a box. More mass.
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Post by matthewwalker on Dec 8, 2012 10:35:24 GMT -8
I should add that Griz did that same thing, and it makes a really nice mass bench in my opinion. They are just totally different ideas. It's not really a case of which is better. Since you are using concrete you have a lot of options. The main benefit of the half barrel system is when working with cob they form the flue pipe, cob mold, and support the weight, all cheaply and quickly. You are doing something totally different.
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Post by grizbach on Dec 8, 2012 13:48:08 GMT -8
Yes Max, Don't over think it. There isn't a wrong way to place the barrels.
Curve up is stronger structurally, and easier to seal.
I personally like the flat side up. Flue gasses can directly touch the mass. You have to be more careful of sealing it though.
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