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An RMH Mockup +sweet pics (and questions, help!)  RSS feed

 
Adam Stjohn
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Last night's rocket mass heater mock-up was mostly a success! (Just before the first rain hit)

I'll post some more detailed photos (of the interior, etc) in a couple minutes.

For now, I'll mention one thing and also ask a question:

First of all, the exhaust is mostly steam: low-clay, pacific-northwest mud was used as temporary mortar, and during the burn, all that mud produced a lot of steam. Eventually there were puddles beneath each seam in the exhaust pipe.

Secondly, question: there is was a lot of smoke-back. I tried removing sections of the exhaust pipe, but that didn't change anything (so I don't think it had to do with air-resistance in the exhaust). I'm wondering if it had to do with two things: 1) the heat riser is brick. The mock-up used mud as temporary mortar, so cracks in the brick were sealed; However, the heat-riser was not insulated. The final design will be insulated with clay and perlite. Could this explain the smoke-back? 2) Could there be a bottleneck at the exit flue? (Where the air moves from the barrel, to the exhaust pipes?)

I'll post more photos in a couple minutes, which will help in answering these questions.

For now:

 
Adam Stjohn
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More sweet pics!

Without barrel:

( Note with a question: The heat-riser has mud (temporary mortar) rounding out its top. This is because the number of bricks as-is was too short (4 1/2" below the barrel). But adding another set of bricks would have made it too tall (1/2" below the barrel). So I added some mortar on top to gain about 1 1/2". But it should be noted that it was then still too short (3" below the barrel). Does anybody have any input on how this can affect the dynamics of the system? Could this be the source of the smoke-back? In other words, will the final design work better when I lessen that gap? Getting it to a good 1 1/2" below the barrel?)




Close-ups of the exit flue:

(Note: that measuring tape is reading about 3". Note however, that the exit right there has some extra gap at its base where the brick drops down... make sense? Were it a 3" by 4" exit, and there being two such exits, then the total would be 24"-- too small. But there is that extra gap there, where the brick angles in and drops down... See?)





With barrel:



 
Adam Stjohn
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Oh, by the way, it's a 6" system. The CSA (cross-sectional-area) of the burn tunnel itself is a little under 28.3". Probably in the 27-28" range.
 
Peter van den Berg
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Location: +52° 1' 47.40", +4° 22' 57.80"
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Adam Stjohn wrote:First of all, the exhaust is mostly steam: low-clay, pacific-northwest mud was used as temporary mortar, and during the burn, all that mud produced a lot of steam. Eventually there were puddles beneath each seam in the exhaust pipe.

Quite normal for a fresh stove. Every first-time stove builder's got the same problem, you have to run it over and over again before it'll stay dry eventually.
Adam Stjohn wrote:Secondly, question: there is was a lot of smoke-back. I tried removing sections of the exhaust pipe, but that didn't change anything (so I don't think it had to do with air-resistance in the exhaust). I'm wondering if it had to do with two things: 1) the heat riser is brick. The mock-up used mud as temporary mortar, so cracks in the brick were sealed; However, the heat-riser was not insulated. The final design will be insulated with clay and perlite. Could this explain the smoke-back? 2) Could there be a bottleneck at the exit flue? (Where the air moves from the barrel, to the exhaust pipes?)

Smoke-back is due to:
1# Too much water in the stove and/or the fuel.
2# Too narrow manifold where the bench pipe start.
3# No insulation around the riser and tunnel.
4# Too long horizontal exhaust.
5# Too short vertical stack, or none at all.
6# No cob around and on top of the horizontal exhaust pipe.

This last one will cool down the gases too much.

There's a minimum for the top gap of the riser. However, there's no practical maximum limit, one could add another barrel on top of the first without repercussions. One catch, though, the bench will be cooler and maybe too long in that case. So please don't worry about the top gap being too large.

The drop-down at the exit won't help at all. In fact, it will make matters worse when it'll step down too steep, there will be a low pressure void which will apply a drag force to the gas stream. It's best to make this manifold about 1.5 or 2 times as large as the system size. In this way it will act as an ash trap as well. That is to say, in my view, anybody is entitled to think what they like.

The layout of the space below the barrel do look allright to me.
 
Adam Stjohn
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Thank you, Peter Berg. Very helpful.

To address some things:

1# Too much water in the stove and/or the fuel.


There was definitely A LOT of water and steam in the system. Now I realize why this would cause smoke-back: when water evaporates, it expands a lot; this would cause more pressure throughout the system and screw up the draft?

2# Too narrow manifold where the bench pipe start.


Yes, Peter Berg, as you mentioned:
The drop-down at the exit won't help at all. In fact, it will make matters worse when it'll step down too steep, there will be a low pressure void which will apply a drag force to the gas stream. It's best to make this manifold about 1.5 or 2 times as large as the system size.
And I do not look forward to re-working this-- But it best be done.

I find this to be the most un-documented and often the most difficult part in building an RMH. Ianto and Leslie's book kind of glides over the issue. The videos and podcasts mostly cut around it. (Almost deliberately so? Where are the details?) ...I guess I haven't yet seen Erica and Ernie's blue-prints. Perhaps I'm whining: Perhaps the real problem is that I'm kind of short on bricks. I think I can make something work. However, I still argue that there is a huge lack of straight-forward documentation around this recurring problem: the barrel-to-exhaust bottle-neck.

3# No insulation around the riser and tunnel.


This is definitely a problem and will be fixed in the final project.

4# Too long horizontal exhaust.


I don't think this was the problem. I tried removing most of the horizontal exhaust, but it did not affect the smoke-back.

5# Too short vertical stack, or none at all.


I'll try to get the heat riser closer to the barrel. But yeah, not too worried about it.

6# No cob around and on top of the horizontal exhaust pipe.


Again, if this be the problem, then the final project won't have that problem.
 
Peter van den Berg
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Location: +52° 1' 47.40", +4° 22' 57.80"
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Adam Stjohn wrote:Now I realize why this would cause smoke-back: when water evaporates, it expands a lot; this would cause more pressure throughout the system and screw up the draft?

This could be a reason, not getting up to temperature because of all the water could be the other one.

Here's some straight-forward information about the manifold difficulties.
Adam Stjohn wrote:I'll try to get the heat riser closer to the barrel. But yeah, not too worried about it.

I do get the feeling you're missing the point. As can be seen in one of your pictures there's no vertical chimney at the end of the bench loop. In other words, the exhaust is ending at ground level as a laundry vent. My advise: use a 6 foot vertical chimney stack at the end of your horizontal exhaust. Especially when a rocket heater is used in a house, it's advisible to have the chimney ending above the highest point of the roof. Otherwise, it's enough to have some air leaks at the top of the house to cause smoke back. See this site for a detailed explanation.
 
Adam Stjohn
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Ah, yes, I was missing the point: By "vertical stack" I thought you meant the heat-riser itself, but you actually meant, or at least mostly meant, a vertical portion within the exhaust system.

I do realize the drawing/drafting advantages of such a vertical stack; however, as I understand, the heat-riser acts as an internal chimney itself; so I think I'll try without a chimney at the end of the exhaust, and then, if there are any problems, adding one later should be relatively easy.

By the way, during the mock-up, besides removing portions of the exhaust pipe (to shorten it), I also tried adjusting the last section of the exhaust pipe so that it was vertical. Again, this did not help with the smoke-back; I think the smoke back is due to the other problems; however, during the mockup, there was no mass around the exhaust pipes: perhaps, if there had been a mass, then the vertical portion would have helped a lot..

Secondly, thank you for recommending http://donkey32.proboards.com/index.cgi?board=discuss&action=display&thread=337 --- as usual, that forum is was very helpful. I'll have to refer back a couple times before I really conceptualize the math.

You may find it interesting that I placed a support-brick (for the barrel and ceiling-bricks to rest on) directly between the barrel and the exit flue. I did this because in Ianto and Leslie's book, on page 34, when mentioning the gap between the heat-riser and barrel ("H"), it says:

For even heating from all sides of the barrell, we need to encourage the hot gases to swirl down equally around the barrel, not just short-cut to the exit tube.


Playing with that idea, I'm placing the earlier-mentioned support-brick squarely between the barrel and the exit flue, to encourage the gases to swirl down the other sides of the barrel, before making their way through the below-barrel-channel and out the exit flue. Plus, this gives the barrel more stability and provides support for the bricks that go on top (which are still fairly precarious-- any recommendations?)

Alright, so anyway, I'm going to widen the exit flue and insulate the heat-riser.

Thank you thank you thank you for the feed back
 
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