morticcio
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"The problem with internet quotes is that you can't always depend on their accuracy" - Aristotle
Posts: 371
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Post by morticcio on Feb 7, 2012 7:15:49 GMT -8
Been making a steel RMH with a friend for his workshop nr Bristol in the UK. See photos here picasaweb.google.com/102727395761916208682/RocketMassHeaterForWorkshop# . I need to post some more photos with the outer barrel in place. Some dimensions... Riser: 6"x 36" (diameter x height) Burn tube: 7"x 4"x 18" (WxHxL) Feed: 7"x 5" Inner barrel diameter: 12" Outer barrel diameter: 20" Gap between top of inner barrel and outer barrel: 2" Gap between side of inner barrel and outer barrel: 4" Exit flue hole diameter: 7" Initial test burns without the outer barrel were good although lighting it proved to be tricky. It has been cold here so a combination of low temperatures and the all-metal construction probably didn't help. However, once it was lit the draw through the feed tube was excellent and it didn't take long before it was up to temperature, with a wide even flame and burning smoke free. Placing the outer barrel on top seemed to choke the sytem. The draw reduced dramatically and started to smoke. There isn't an exhaust flue pipe on yet - we wanted to get the size of the exit hole right first before making/welding the spigot on to connect the flue on to. Is this to be expected? Can the rocket stove work temporarily without the exhaust flue or does it need to be connected to test it all works?
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Post by canyon on Feb 7, 2012 9:11:23 GMT -8
Be sure the exit "spigot" is sized right. I can't remember which thread Peterburg spelled it out for us on (maybe it was Donkey's friendly reminder thread?) but search and find it! And don't forget the ash pit! Also, you can't expect the thing to really work uninsulated. The insulation is key!!! Your post wasn't clear about that detail. Having a vertical stack at the end makes things work way better too. Some people apparently pull off the side exit but what a challenge! Good luck and keep us updated as to what works/what doesn't!
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Post by peterberg on Feb 7, 2012 13:02:23 GMT -8
The exit difficulties are described in this thread. However, I've studied the pictures and couldn't find anything wrong about the dimensions. Maybe you'll need an exhaust stack and start the stove with the barrel assembled. I think you've used a peter channel in your stove with a too large gap, which is probably the cause of a slow start. The c.s.a. of the feed is 35 square inch, 5% of that is 1.75 sq.in., divided by the width of the feed, 7 in., will give us a gap and overhang of 0.25 inch. Take a new flat piece of steel, and make the plate 2, or even 3 inches higher than the existent item. By this way, the air streams will be separated better. The smaller gap will make the stove less reluctant to start.
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Post by grizbach on Feb 7, 2012 13:27:57 GMT -8
A 4" gap between inner and outer barrel is too much. The gasses need to scrub the outer barrel to extract heat. As canyon said, a vertical rider helps tremendously!
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Post by grizbach on Feb 7, 2012 14:17:42 GMT -8
Rider should be "stack"
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berko
New Member
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Post by berko on Feb 8, 2012 1:09:33 GMT -8
A 4" gap between inner and outer barrel is too much. The gasses need to scrub the outer barrel to extract heat. I think it's false. In contrast to your theory, when gap is smaller, the gasses will run faster, therefore it will extract less heat and too much gap lead to "supercooled" gases.
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morticcio
Full Member
"The problem with internet quotes is that you can't always depend on their accuracy" - Aristotle
Posts: 371
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Post by morticcio on Feb 8, 2012 4:29:48 GMT -8
Many thanks for all your replies. canyon & peterberg - I'd seen Peter's thread on exhaust flue sizing. If my calculations are correct, using a 6" riser the sizes for the exit hole would be as follows: Circular Exhaust Flue (6" diameter) Gap (Inner and Outer Barrel | 4" | 3" | 2" | 1.5" | 1" | Diameter of Exhaust Flue | 2.25" | 3" | 4.5" | 6" | 9" |
Looking at the above I could have a 1.5" gap to continue using a 6" flue. Maybe your available exhaust flue size should determine the gap between the inner and outer barrel - working backwards so to speak? Rectangular Exhaust Flue (12" wide) Gap (Inner and Outer Barrel | 4" | 3" | 2" | 1.5" | 1" | CSA/Gap (sq in) | 15.07 | 15.42 | 18.13 | 21.84 | 30.26 | Width | 12" | 12" | 12" | 12" | 12" | Height | 1.53" | 1.71" | 3.14" | 4.92" | 9.13" |
grizbach - I wasn't sure if 4" gap was going to work - I'd looked at donkey's sauna RMH and the dimensions were almost identical (6" riser instead of his 8") so in theory it should. In conclusion, it looks like it will draw without the exhaust flue but as you have all said it is needed to make it work correctly. As an experiment we added more insulation and reduced the gap to 1" yesterday afternoon. It improved things slightly. Will make the spigot and attach the exhaust flue and post back the results (and upload any other photos). Many thanks Andy
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Post by peterberg on Feb 8, 2012 7:07:56 GMT -8
If my calculations are correct, using a 6" riser the sizes for the exit hole would be as follows: Your calculations are correct in respect to the stream profile only. But in respect to the exit hole itself these are of course utterly wrong! The smallest exit hole has to be no less than system size, of course. The gases are able to stream to the hole at any given sector, but by making a 90 degree turn in order to leave the barrel there is the restriction of a much smaller exit hole. And moreover, I haven't seen any exit at the pictures, how close is it to the lower rim of the barrel? Anything closer to 2" or 3" will result in a too small outlet. You seem to be right in respect to calculating backwards from the exit hole to the gap. The barrel exit is 7" at the moment, your decision to reduce the gap to 1" will make this too cramped. According to your own calculations, the outlet has to be 9" now and free all around its perimeter. The book do mention a minimum size of 1.5" for this gap, page 36, left column, close to the bottom of the page. It won't help to make the gaps smaller in my experience, making them wider will stand a better chance to end up with a good performance. How about the top gap above the riser?
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Post by grizbach on Feb 8, 2012 8:19:29 GMT -8
A 4" gap between inner and outer barrel is too much. The gasses need to scrub the outer barrel to extract heat. I think it's false. In contrast to your theory, when gap is smaller, the gasses will run faster, therefore it will extract less heat and too much gap lead to "supercooled" gases. Hi Berko, By having more volume you are increasing the boundary layer which acts like insulation. The smaller the gap, the more contact unless we're talking turbulence then it's on to the bell theory. I actually run a 50% larger gap in my barrel to free up the flow.
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Post by Donkey on Feb 8, 2012 8:21:03 GMT -8
Right.. The "scrape the walls" thing is all about heating a pot from below. If you don't "scrape the walls" with the flow in those conditions, you'll be wasting heat. This is a condition where heated air is flowing straight up and out of the gizmo and the pot is placed in the way of the flow to catch some heat before it leaves.
The barrel is reversed, in actuality the bigger you make the barrel, the more heat you'll get from it (with practical limits). In THIS case, heated gasses are trying to move up and are being pumped (in effect) down and out. A wider gap will slow the pumping action, giving the heat more time to hang out in the barrel area. I know it's not technically a bell, but it will act a bit like one. As a general rule, if you want more heat from the barrel, make the gap larger.
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Post by grizbach on Feb 8, 2012 8:25:38 GMT -8
grizbach - I wasn't sure if 4" gap was going to work - I'd looked at donkey's sauna RMH and the dimensions were almost identical (6" riser instead of his 8") so in theory it should. morticcio, 8" riser has a cross section of 50.25sq" 6" riser has a cross section of 28.25sq" That's 177% difference, not close to being identical.
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Post by grizbach on Feb 8, 2012 9:02:51 GMT -8
Donkey, It brings up a good point, when does an area become a bell? It does depend on turbulence. So if you have an operating bell system, and you slow down the flow enough to become laminar, it wouldn't be a bell anymore?? You would still get a lot of heat extraction because the volume of the flow is so slow. Sounds like we need some insight from peter. Donkey, you are confusing me. I feel if you have a lot of heat extraction off of the barrel with a bottom exit, you will have a very good "pump".
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Post by woodburner on Feb 8, 2012 14:10:42 GMT -8
..............In contrast to your theory, when gap is smaller, the gasses will run faster, therefore it will extract less heat .......... Gases will run faster only if the overall flow through the stove is maintained. Otherwise by reducing the gap, resistance will increase so the flow through the stove will be slower. The flow through the gap may be faster, but because more of the gas will have contact with the surfaces, the heat transfer will be greater. There is an optimum distance for this to occur. Rather in the same way the pot skirt on a small rocket stove works best at around 10mm Bigger and the heat transfer is not so effective and heat is lost, smaller and less heat has a chance to come into contact with the pot.
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morticcio
Full Member
"The problem with internet quotes is that you can't always depend on their accuracy" - Aristotle
Posts: 371
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Post by morticcio on Feb 8, 2012 16:30:50 GMT -8
Thanks again for all the replies - all have been very useful. Made some progress today. Andy made a rectangular exit in the barrel. This is 12" wide x 9" high (approx 4" from bottom lip of barrel) and runs into a 6" spigot. Connected a 90 deg bend and a 1 metre uninsulated stack. Lit the fire which started drawing immediately. Added another 1 metre uninsulated length to the stack which improved the draw. Adding a 3rd metre length didn't have any significant effect. Will disassemble tomorrow and try a few experiments: - Run horizontal flue pipe before 90 deg bend & vertical stack
- Widen the gap between the side of the barrels (will start with original 4" gap and decrease to 1.5")
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Post by peterberg on Feb 9, 2012 1:09:27 GMT -8
Donkey, It brings up a good point, when does an area become a bell? It does depend on turbulence. So if you have an operating bell system, and you slow down the flow enough to become laminar, it wouldn't be a bell anymore?? You would still get a lot of heat extraction because the volume of the flow is so slow. Grizbach, definition for a bell system would be like this: confined space with feed and exit openings at low level, or the feed slightly higher. The top of this confined space is completely closed, no hole at all. For example see the sketch below, a large part of the lowest barrel wouldn't qualify as a bell, but the top one does. So, the barrel as used in a rocket mass heater can't be a bell because the feed opening is quite near the top instead of in the lowest region. Hope this is clear, sometimes my knowledge of the English language is too limited.
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