roy
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Posts: 38
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Post by roy on Sept 23, 2012 7:40:06 GMT -8
Im just about to start cobbing the RMH I've just built.
My question is what temperatures should I expect. I've got a simple magnetic thermometer on top of the barrel which gets to 200-300oC. The rocket seems to be working well. Great draw simple to start no smoke and little ash. The barrel is 2" higher than the heat riser. Even though this temperature seems lower than I expected if the heat riser should be around 600oC
Thanks
Chris
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roy
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Posts: 38
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Post by roy on Sept 25, 2012 11:08:21 GMT -8
Still a bit worried that my RMH is Not hot enough. I've started cobbing the bench and I've fired it up tonight and it didn't draw as well as it has done? I just want to make sure before I cob the barrel. Hopefully tomorrow. Any advice welcome.
Thanks
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morticcio
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"The problem with internet quotes is that you can't always depend on their accuracy" - Aristotle
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Post by morticcio on Sept 27, 2012 4:22:59 GMT -8
Could be a number of things. The shape & size of the exit from the barrel to the bench is also critical - see this post donkey32.proboards.com/index.cgi?board=discuss&action=display&thread=337. There is a formula to calculate the "optimal" gap between the top of the heat riser and the barrel: CSA/Circumference Assuming an 8" system with the same diameter riser the gap would be 2". For a 7" it would be 1.75" and 6" it would be 1.5". CSA = Cross sectional area Can you provide a few more details on the RMH, such as core size, type & size of exit from barrel to bench, length of bench and height of exhaust flue etc. A few photos are always helpful too.
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roy
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Posts: 38
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Post by roy on Sept 27, 2012 11:39:33 GMT -8
The gap between the riser and the barrel is 2.5". The gap around the barrel into the ash pit and flue is a semi circle 10" long and the max depth is 2". There is another gap the other side of a supporting brick which is a bit smaller.
I found the slow start up to be a buildup of ash in the burn tunnel. All other flues and ash pits are fine. And start is good again.
The heater works fine it's just that I thought it would be hotter on the top of the barrel. But saying that now I've cobbed 80% of the bench the exit flue is 60oC and leaving the barrel at 200oC ( I haven't cobbed the barrel yet).
I didn't want the heater to cook on. I plan to cob most of the top of the barrel. I just want to check all is fine before I Cover the barrel.
Thanks
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roy
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Posts: 38
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Post by roy on Sept 27, 2012 11:42:58 GMT -8
Any idea how to attach pictures on this forum?
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roy
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Posts: 38
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Post by roy on Sept 28, 2012 0:23:57 GMT -8
Some more dimensions I took this morning with the barrel off.
Gap udder barrel 3" Heat riser lenghth 38" taken from the bottom of the burn tunnel Burn tunnel length 12.5" Barrel diameter22.5" Inner barrel heat riser 20"
Feed tube 7x7 Burn tube 6x8 Heat riser7x7
Length of flue 29'
I think the gap between the insulated heat riser and the barrel is to small and possibly the all inportant barrel flue junction.
I don't want to cook on the top of the barrel but would the rocket work better with a smaller gap?
Any help appreciated.
Roy
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Post by peterberg on Sept 28, 2012 4:11:34 GMT -8
Roy, You've built an 8" system, so all the gaps and manifold outlet should be at least 49 sq. in. The side gap between the riser and outer barrel is 1.25", it's a bit tight but should be enough, recommended gap is 1.5". Top gap is 2.5", a bit larger than minimum, this is OK, could be anything up to a yard without hampering the stove.
With a 1.25" side gap the manifold is probably too tight, you have to have lots of space there. The before mentioned 49 sq. in. is the absolute minimum, try to widen this to 1.5 times that figure and add a spaciously ash trap. The total space you mention is 10" long and 2" deep. That's 20 sq in., another opening on the other side of the support brick is a bit smaller. There you won't have enough space at all, summed up this won't be more than 35 sq in.? Unless you meant the semi-circle is 10" diameter, in that case it's still not enough.
Your stove won't reach top-notch performance having a restriction down the line, when it's fixed you'll find it running hotter. The link morticcio mentioned is best for figuring out the required manifold space, by the way.
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roy
New Member
Posts: 38
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Post by roy on Sept 28, 2012 11:26:07 GMT -8
Peter,
Thank you for the reply. I thought as much. I've increased the manifold as much as I can without major surgery. I've also reduced the diameter of the inner barrel by 1" and increased the riser by 1".
Can you tell me what temps I should be getting from a magnetic thermometer stuck on top of the barrel and at the bottom of the barrel with no cob on it. As I said in my first post draw hasnt been an issue. I'm not sure how to tell what is a top notch performing rocket.
Many thanks again
Roy
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Post by peterberg on Sept 28, 2012 12:24:42 GMT -8
Can you tell me what temps I should be getting from a magnetic thermometer stuck on top of the barrel and at the bottom of the barrel with no cob on it. I'm sorry, but I can't. I've never had a normal RMH to test, always been fiddling with specialties. But... I'm inclined to make an educated guess. Temperatures on top of the barrel could be as high as 450 C. (842 F.) I would think, when the stove is really a good performer. Don't be mistaken about the draw, even if that's alright when cold. The heater can be a quick starter and doing not as nice when coming up to highest working temps.
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roy
New Member
Posts: 38
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Post by roy on Sept 28, 2012 12:31:36 GMT -8
Thanks peter, I will fiddle a bit more to increase the manifold tomorrow.
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Post by Donkey on Oct 3, 2012 15:45:27 GMT -8
I've seen temperatures at the barrel high enough to just about melt the barrel top and as low as around 350-400 deg. F. Temperatures vary SO widely depending on SO many factors. The main things are that the machine burns cleanly through as much of the burn cycle as possible (no, or very little smoke out the chimney), doesn't smoke back into the room and provides for your (actual) heating needs in a graceful way. Personally, I'd rather have a stove that burns cleanly inside and provides mellow heat to the space at all times. What you want is a well behaved stove in your home, not some out of control monster. So for me, I'd like to see barrel temps (at the top) somewhere between 300 and 600 degrees (F), preferably on the low side and a bench (mass) that stays warm for 24 hours or more.
Drying time varies with a huge number of factors too.. Climate, cob mix, how much straw is in the cob, how often you run the thing, on and on. It's NOT strange to have drying times of several weeks. Expect to get high levels of condensation during the drying cycle. It can be ALARMING (at first) how much water will come out of the things and for how long. Just stick with it, be patient and all will be well.
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