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Some tips to increase efficiency of a RMH - but are they dangerous?  RSS feed

 
Posts: 161
Location: S. Ontario, Canada
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Tip #1:
Don't waste heat up your chimney when your RMH is not in operation!
Many depend on a damper in their flue to stop hot air rising up the chimney. BUT this wastes an awful lot of your heat! I have a full disc damper which I can close but, while it does reduce the draft, it does not stop it! MY RMH flame will keep burning even with the flue fully closed. So I know that it would waste precious heat and  fuel if I depended on it to stop the draft when my RMH is not in operation. Therefore, I have installed a "T" joint with a cap in my flue just before it enters the chimney. So as soon as my fire has burned down as low as I want it and most coals are gone, I remove the cap, place some rolled up insulation into the flue going to the chimney and this totally stops the draft and keeps all the heat INSIDE the house!  You could do the same thing at the top of your burn tube, but I have done it at the far end to prevent any warm air from my mass ascending up the chimney. I also cap the cleanout and the top of the burn tube when the RMH is not in operation.

Tip #2:
Don't waste your coals left in your combustion chamber!
  You can make your RMH into a "coal burner" as well and thus burn ALL of your fuel. When you remove your cold ashes and coals from your RMH, sift them to reclaim the coals which remain from the ashes.  I have a horizontal ash cleanout with cap at the outer end of my combustion chamber where I start the fire and remove the ashes. I simply pile the collected ashes back into the combustion chamber once the fire is going and leave the cap partially open and thus burn the ashes which otherwise would be wasted. Just keep an eye on the burn tube and when any smoke begins to rise in it, just close the cap a bit more.

Any other tips on minor adjustments that can be made to increase overall RMH efficiency?
 
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I wouldn't use a damper at all while there is a flame. Run your fire HOT and FAST, with the objective to have no coals at all.
 
Posts: 167
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I have a grey water drain in my shower. I noticed that in Winter my bathroom would be a lot colder than the rest of the cabin. I started putting a small sandbag (sandwich bag filled with sand) over it when not using it and that fixed the problem.

I also put a piece of hardyboard over the feed tube when the fire is almost done. That allows the last bit of coals to burn off while blocking most of the blowback potential. Better than blocking it at the exit, I think.

And I have noticed a big difference between pine (hot and quick) and hardwood (not as hot but lasts longer). I actually prefer pine because it heats up the cabin quicker so I have to attend the fire for less time.
 
gardener
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Hi Bruce.  I am collecting materials for my RMH, so haven't built any yet, but your tip about the coals got me thinking that there is always (or at least quite often) coals in the firepit after an outdoor fire burns out.  A person could utilize these cold coals as part of initiating a hot burn in an RMH.  The thing about coals is that they catch fire rapidly into their porous membranes, and generate heat within themselves faster than solid wood.  I used to put the old coals together with my kindling when setting up a fire in my old cabin's wood stove.  Works great.
 
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Bruce Woodford wrote:I have installed a "T" joint with a cap in my flue just before it enters the chimney. So as soon as my fire has burned down as low as I want it and most coals are gone, I remove the cap, place some rolled up insulation into the flue going to the chimney and this totally stops the draft and keeps all the heat INSIDE the house!



I understand this to be a dangerous practice that creates a risk of carbon monoxide poisoning.  At basic safety principle for using any combustion heater is not to excessively impede the draft through the combustion chamber and out the chimney until all combustion is complete.  If this principle is not followed, there's a risk that carbon monoxide will escape into the room (and accumulate to dangerous levels) instead of going up the chimney.  From a WebMD article on extremely basic tips for preventing carbon monoxide poisoning:

When in use, make sure [chimneys] vent properly to allow gas to escape from enclosed areas. Don't close the fireplace or damper before the fire is completely out.



This is one of the reasons why standard commercial round stovepipe dampers that you can buy with any wood stove have such large pass-through holes in them.  It's not the optimal design for controlling draft, but it does allow enough draft to continue to carry away combustion gasses (including CO) as a fully-damped stove burns all the way down to cold ash.  Yes, warm air is lost, but the cost of keeping it is a poisoning risk, usually while people are asleep and unlikely to notice the onset of CO poisoning.
 
pollinator
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Dan has it right what the OP  is  suggesting is down right dangerous in my opinion ; Carbon monoxide is a killer no smell, no taste, no colour it can kill quickly or slowly depending on the concentration you even go to sleep its painless . I would not risk it .It destroys the haemoglobin in your blood cells one blood cell at a time !
The idea is to burn well and fast . If you must close up the RMH then work out a way of closing the door at the bottom  not trapping possible dangerous gases in the house with you .

David
 
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Further, I look at it, that fire burning with out exhaust is competing with you and other O2 breathing life forms for the available oxygen in your enclosed space.  When you seal off the chimney, flue, or what ever out draft you have, you also decrease the amount of fresh air entering the space.  In part it is the exiting of exhaust that draws the fresh air in.

I prefer the strategy of blocking where the air flow goes into the rocket stove, which will allow waste gasses to exit, but prevents the kind of draft running through that would strip all the heat out of the mass and send it up and out.
 
Roberto pokachinni
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I should have piped in about the carbon monoxide angle.  I'm glad that other did.  It is serious. 

I prefer the strategy of blocking where the air flow goes into the rocket stove, which will allow waste gasses to exit, but prevents the kind of draft running through that would strip all the heat out of the mass and send it up and out.

  Many RMH videos show people adjusting or blocking the flow of air by placing a couple fire bricks across the top of the feed tube.  By closing the feed end, you greatly reduce the amount of air that enters the system.  With this reduction, the amount of thermosiphoning that can take air through your burn area and up out the chimney is also greatly reduced, but it is enough to get any possible gasses out just as Thekla wrote.   

As Dan and Kyrt wrote, and as I have understood quite emphatically from the videos I have watched, as well as from what I've read on the subject here at Permies, the principal of the 'Rocket' in the rocket mass heater is best utilized to it's fullest potential, blasting at full speed, creating the hottest and most complete and efficient combustion possible.  This very hot concentrated process should burn off all of the harmful gasses.  But any hot or warm coals will continue to off-gas, that is why you should go for the super hot burn, and not close off the top.   You should only get ash as a waste product, not coals, not even little bits of coals.   
 
Roberto pokachinni
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I'm glad that Dan got some apples and pie for his post above.  I wasn't expecting it, at all, for mine, but thanks!  I think the idea of having a thread about increasing RMH efficiency is a good idea, and it might be good that the thread was started off with such a big one as this Do Not Do.  Efficiency should only go so far, and it's great that the OP asked in the thread title if these practices are dangerous; I think perhaps he knew intuitively that it might be.  There are plenty of ways to increase efficiency of your RMH, including adding more mass near your burn barrel to absorb some of it's radiation.  This can be done attractively in many ways using cob, stone, or even a large aquarium (though not too close to the burn barrel, obviously).      
 
Bruce Woodford
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Thank you all for your comments and cautions above. I entirely agree that an RMH should burn hot and fast. I didn't install a damper in my system to use during the burn but only to stop the flow once the burn is complete. I just found that it didn't do a very good job of that!  I should also have mentioned that we do have a CO detector installed not far from the rocket but it has never gone off.  Also I know that the ideal aim is to have only ash residue and no coals, but how many RMH builders achieve that consistently  I'd be interested to find out. Would others like to check in on this?   One suggestion above was regarding having a mass to store heat as close as possible to the core.  My integral mass, a tank containing sand surrounding my drum definitely stores the major portion of my heat. It often heats up to 140 F and the radiation from it can be felt across the room.

More ideas for improving efficiency, anyone?
 
Ron Helwig
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Bruce Woodford wrote:Also I know that the ideal aim is to have only ash residue and no coals, but how many RMH builders achieve that consistently  I'd be interested to find out. Would others like to check in on this?  



I almost always have just a pile of ashes. The piles are more than I had expected before building it as I end up cleaning it out every time I use it, but then I also burn a lot of paper and junky wood. If I don't clean it out then the pile, which sits in the middle of the burn tunnel, gets big enough that it starts blocking the flow. I have a 6" system.
 
Bruce Woodford
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Ron, you wrote, "I almost always have just a pile of ashes. The piles are more than I had expected before building it as I end up cleaning it out every time I use it, but then I also burn a lot of paper and junky wood. If I don't clean it out then the pile, which sits in the middle of the burn tunnel, gets big enough that it starts blocking the flow. I have a 6" system."

If you burn a lot of paper, that sure wouldn't produce coals at all. What do you mean by "junky wood"?  What sort of diameter are you talking about? Small twigs won't make coals either.  If you burn sticks or firewood with a cross sectional size of 1 to 4 square inches what happens? Any coals left then?  Just wondering.
 
Ron Helwig
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Bruce Woodford wrote:Hi Ron, If you burn a lot of paper, that sure wouldn't produce coals at all. What do you mean by "junky wood"?  What sort of diameter are you talking about? Small twigs won't make coals either.  If you burn sticks or firewood with a cross sectional size of 1 to 4 square inches what happens? Any coals left then?  Just wondering.



I split my wood to less than an inch square or so cross section, but then I need to because it is only a 6" system. No room for anything bigger.

Some of the wood has started rotting. Some of it is old pallet wood. Some is 2xX leftovers from building stuff that I split.
 
Bruce Woodford
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Ron Helwig wrote:

I split my wood to less than an inch square or so cross section, but then I need to because it is only a 6" system. No room for anything bigger.

Some of the wood has started rotting. Some of it is old pallet wood. Some is 2xX leftovers from building stuff that I split.



I think that explains why you don't have coals.  I too have a 6" system but most of my wood is just split so 4 or 5 pieces fit into my burn tube at once.
 
Roberto pokachinni
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My integral mass, a tank containing sand surrounding my drum definitely stores the major portion of my heat. It often heats up to 140 F and the radiation from it can be felt across the room. 

  I'm not sure I understand what you mean here, Bruce.  If I'm visualizing this right, then you have placed a large amount of mass in the form of contained sand directly around your barrel (drum); Is that right?

From what I understand, and I might be very wrong because I'm just running on my limited understanding of the theory instead of experience here, you might not want to have the mass at that specific location.  The barrel should radiate into the room at high heat, not be part of the absorbing mass in the system.  What I figure might be happening in your system is that your gasses in your barrel are actually cooler (and not as fast of a rockety draw as a result) than they would otherwise be if you did not have the mass against the barrel absorbing that heat.  From what I understand, the major mass in the system should be after the manifold.  This might be why you are getting coals instead of ashes.    

One suggestion above was regarding having a mass to store heat as close as possible to the core.

If you are referring to me writing about increasing the efficiency of an RMH, by storing more heat in mass near your barrel, then it seems I need to elaborate.  Again this is the way I personally understand it: the mass should be nearby, but not against the barrel.  The barrel will do the radiating from all exposed surfaces in all directions outward from it.  A mass of stone against a nearby wall or a couch-like back on a cob bench will absorb this radiated heat, and later slowly release it into the room.  If you were to have a tank of sand near your barrel, but not touching it, then I think you would gain more efficiency that what you presently have.  Again, I might be wrong about this.  Someone with more experience would know though. 

I'm really curious about your build.

Do you have photos of your system?  
 
Roberto pokachinni
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I think that explains why you don't have coals.  I too have a 6" system but most of my wood is just split so 4 or 5 pieces fit into my burn tube at once.

 

Is this when you are starting the burn or after it is already really rocketing?  The reason I ask, is that I thought that once the system was rocking it out with a good draw, a person can feed larger pieces than what Ron is using and still get a complete burn without producing coals.  The smaller bits and paper are used by most people to start the system up, but then they progress to larger pieces with no loss of burning efficiency. 

Certainly the fuel Ron is mentioning: rotten low quality wood and paper, will produce a lot of ash and will likely burn fast without producing coals.  From my experience of burning wood most of my life, that is a given regardless of the size of such low quality wood, as long as it is properly dry when going in.    
 
Thekla McDaniels
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Bruce Woodford wrote:  Also I know that the ideal aim is to have only ash residue and no coals, but how many RMH builders achieve that consistently  I'd be interested to find out. Would others like to check in on this?   One suggestion above was regarding having a mass to store heat as close as possible to the core.  My integral mass, a tank containing sand surrounding my drum definitely stores the major portion of my heat. It often heats up to 140 F and the radiation from it can be felt across the room.



I had consistently ash without charcoal.  I burned hard wood from twig to 4- 5 inch greatest cross distance conventional split cord wood, and construction scrap.

About the tanks containing sand, which surrounds your drum, I know Robert has talked about this. The one thing I may have missed and want to be sure is mentioned is the fact that the exposed sides of the drum are a big contributor to the rocekty-ness.  The cooling of the gases- as the drum radiates heat- makes the gases shrink in volume, which pulls the air current coming through the fire.  I could be wrong, but I think this is actually the biggest contributor to the rocket, this rapid cooling inside, which seems like it would be lost if the drum when the drum was encased in sand.

 
Roberto pokachinni
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About the tanks containing sand, which surrounds your drum, I know Robert has talked about this. The one thing I may have missed and want to be sure is mentioned is the fact that the exposed sides of the drum are a big contributor to the rocekty-ness.  The cooling of the gases- as the drum radiates heat- makes the gases shrink in volume, which pulls the air current coming through the fire.  I could be wrong, but I think this is actually the biggest contributor to the rocket, this rapid cooling inside, which seems like it would be lost if the drum when the drum was encased in sand.

I agree with this except that I think that the biggest contributor to the rockety-ness is the insulated burn tunnel and heat riser with it's vortex of concentrated heat working off of itself.  The drum does contribute to the rocket effect, as does having the vertical chimney right near the barrel so that the draught is increased by the radiant drum heat.  
 
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I agree that the greatest contributor to rocketyness is the insulated burn tunnel & riser. Per statements by Erica Wisner, people who have cobbed around the barrel have sometimes found a reduction in draft, which is restored when the cob is removed. The same would go for a tank of sand. Sand is actually more insulating than cob because of the millions of tiny air spaces.

A bare barrel radiates maximum heat; any mass touching the barrel (aside from maybe solid plate steel) will heat up and reduce the rate of heat transmission from the barrel, reducing the temperature difference from riser to barrel and thus the rocketyness.
 
Bruce Woodford
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One suggestion above was regarding having a mass to store heat as close as possible to the core.

If you are referring to me writing about increasing the efficiency of an RMH, by storing more heat in mass near your barrel, then it seems I need to elaborate.  Again this is the way I personally understand it: the mass should be nearby, but not against the barrel.  The barrel will do the radiating from all exposed surfaces in all directions outward from it.  A mass of stone against a nearby wall or a couch-like back on a cob bench will absorb this radiated heat, and later slowly release it into the room.  If you were to have a tank of sand near your barrel, but not touching it, then I think you would gain more efficiency that what you presently have.  Again, I might be wrong about this.  Someone with more experience would know though. 

I'm really curious about your build.

Do you have photos of your system?  

Hi Robert,  See attached pics of my system recently set up for the winter.

Also some more comments to follow in response to other posts about it.
(21)-Finished-Rocket-with-integral-sand-mass-connected-to-external-mass.JPG
[Thumbnail for (21)-Finished-Rocket-with-integral-sand-mass-connected-to-external-mass.JPG]
(23)-Completed-steel-RMH-(Notice-space-for-airflow-under-and-up-the-back-side-of-the-external-mass.).JPG
[Thumbnail for (23)-Completed-steel-RMH-(Notice-space-for-airflow-under-and-up-the-back-side-of-the-external-mass.).JPG]
 
Bruce Woodford
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Thekla wrote: "I had consistently ash without charcoal.  I burned hard wood from twig to 4- 5 inch greatest cross distance conventional split cord wood, and construction scrap.

About the tanks containing sand, which surrounds your drum, I know Robert has talked about this. The one thing I may have missed and want to be sure is mentioned is the fact that the exposed sides of the drum are a big contributor to the rocekty-ness.  The cooling of the gases- as the drum radiates heat- makes the gases shrink in volume, which pulls the air current coming through the fire.  I could be wrong, but I think this is actually the biggest contributor to the rocket, this rapid cooling inside, which seems like it would be lost if the drum when the drum was encased in sand."    (Sorry I'm still not sure how to "quote" from posts above!)

Thanks for your contribution Thekla.  As I understand it, the purpose of the drum is to cool the flu gases as much and as quickly as possible.  I am doing this with the surrounding integral mass of sand. When I ran it totally exposed to the air my temps at the horizontal tube coming out of the drum were always 400F or higher once the unit got all heated up. But with the sand mass around the barrel it runs between 300 and 375F and has never reached 400F. So I know that the sand mass cools more efficiently than air would.  My barrel is about 1/3 exposed to the air (which is plenty to heat the surrounding space fairly quickly) and 2/3 in the sand.

Thekla seems to be the only one so far who has had a system burn good sized pieces of hardwood with no coals at all left behind. Are there others who have had this experience consistently, or do most of us have at least some coals to contend with?  I want to learn and need to know what others are experiencing and try to find what the answers are. Care to check in here and tell about your own RMH and whether it yields some coals in the ash or all ash and no coals?
 
Bruce Woodford
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Glenn Herbert wrote:I agree that the greatest contributor to rocketyness is the insulated burn tunnel & riser. Per statements by Erica Wisner, people who have cobbed around the barrel have sometimes found a reduction in draft, which is restored when the cob is removed. The same would go for a tank of sand. Sand is actually more insulating than cob because of the millions of tiny air spaces.

A bare barrel radiates maximum heat; any mass touching the barrel (aside from maybe solid plate steel) will heat up and reduce the rate of heat transmission from the barrel, reducing the temperature difference from riser to barrel and thus the rocketyness.



Hi Glenn, Thanks too for your contribution and suggestions. I've read your comments about sand being insulation, but my own experience with this RMH seems to disprove that theory. Let me explain.... While setting this RMH up I often ran it with no mass around the barrel and once it got warmed up would consistently burn with temps 400F plus at the horizontal tube coming out of the bottom of the barrel.  It radiated a lot of heat and the room space around got excessively hot very quickly.  But once I filled the space around the barrel with sand (about 2/3 the way up the barrel), the horizontal tube temps never reach 400F. They get up to 300 fairly quickly (maybe 20 minutes) and may get as high as 375F but have never reached 400F with the sand surrounding.  I conclude from that that sand absorbs heat better and faster than air.  It also stores it well and distributes it slowly to the air of the house over the following 12 hours.  So if cooling the drum makes a system more "rockety", it seems to me that sand accomplishes the purpose better than air. Am I wrong?
 
pollinator
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Thanks for your contribution Thekla.  As I understand it, the purpose of the drum is to cool the flu gases as much and as quickly as possible.  I am doing this with the surrounding integral mass of sand. When I ran it totally exposed to the air my temps at the horizontal tube coming out of the drum were always 400F or higher once the unit got all heated up. But with the sand mass around the barrel it runs between 300 and 375F and has never reached 400F. So I know that the sand mass cools more efficiently than air would.  My barrel is about 1/3 exposed to the air (which is plenty to heat the surrounding space fairly quickly) and 2/3 in the sand.



But let's look at this a different way. What if, instead of absorbing more heat then just air would, and thus lowering the temperature of the outflow, the sand is actually lowering the flow of air through the system, (by not releasing as much heat, giving less shrinkage of gases) cutting down the speed of the burn, resulting in lower temperatures in the burn tunnel and thus lower temperatures at the barrel outflow?

I have no idea which is correct. Just thought I would throw that out there.
 
Roberto pokachinni
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So when I quote, I click on the quote button that is above the box that I am writing this reply in.  There are other icon buttons like b and i and u there as well.  So after clicking quote,

appears, and I just cut and past into that space, and it goes white like

This

. I hope that's helpful. 

I really don't understand your system.  Perhaps I'm missing something, but is your entire system steel?  Including your core?  Or is it an insulated brick and clay mortar core that is  just encased in sand and steel? 

It looks like your mass bench is mortared stone surrounding a bin of sand.  Is that what you meant by the sand tank?  Does the exhaust from your manifold go through the sand bed?  If so, that's a lot different than what I thought.

I conclude from that that sand absorbs heat better and faster than air.



So if cooling the drum makes a system more "rockety", it seems to me that sand accomplishes the purpose better than air. Am I wrong?



I think that the barrel throwing heat into the air is not the same as the sand absorbing the heat.  Certainly the air absorbs the radiated heat into it's mass (which is small particles dispersed in the space with large spaces), and the heat excites the air and the air reacts to the heat by expanding the spaces between it's particles and creating convection currents, and at the same time waves of radiation moves through the air to objects beyond, where it is absorbed and later re-radiated.  The radiant heat, dispersed rapidly outward from the barrel  (which is what you want, optimally with a rocket stove barrel, from what I understand), so that the barrel is consistently losing heat by throwing it off in the form of radiation, which is "cooling" the barrel.  Not really cooling it, but the air is definitely cooler than the barrel, and the barrel is throwing heat into the air.  What you have described, if I understand, is that the sand surrounding your barrel is conducting the heat away from the barrel, which is not very efficient because the sand will also be holding heat to the barrel, so it is not cooling the barrel as fast or consistently as a traditiionally built RMH.  It seems to go counter to RMH theory.  I'm not saying that it's wrong, (you may have an innovation worth looking into) but it seems like it's not done the way that I understand works best for RMH. 

If it is your intention to have a mass absorb heat from your barrel, it would be better to use a denser material than sand (like clay, or stone, or concrete), but the sand in your case is also a thermal mass, for sure, and it will hold that heat directly to your barrel system. 

I don't think that Glenn was saying that sand was insulation, but that it was insulating... when compared to denser mass materials.  If you are looking for mass, sand is good, but... there are better choices.

I may be missing something about your build that throws a bunch of what I wrote in this post out the window.  Still curious.  More questions in my mind, but need to bounce them around in there for a bit first. 
 
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the sand is actually lowering the flow of air through the system, (by not releasing as much heat, giving less shrinkage of gases) cutting down the speed of the burn, resulting in lower temperatures in the burn tunnel and thus lower temperatures at the barrel outflow?

  I think that this is the case.
 
Bruce Woodford
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Roberto pokachinni wrote:So when I quote, I click on the quote button that is above the box that I am writing this reply in.  There are other icon buttons like b and i and u there as well.  So after clicking quote,

appears, and I just cut and past into that space, and it goes white like

This

. I hope that's helpful. 

I really don't understand your system.  Perhaps I'm missing something, but is your entire system steel?  Including your core?  Or is it an insulated brick and clay mortar core that is  just encased in sand and steel? 

It looks like your mass bench is mortared stone surrounding a bin of sand.  Is that what you meant by the sand tank?  Does the exhaust from your manifold go through the sand bed?  If so, that's a lot different than what I thought.

I conclude from that that sand absorbs heat better and faster than air.



So if cooling the drum makes a system more "rockety", it seems to me that sand accomplishes the purpose better than air. Am I wrong?



I think that the barrel throwing heat into the air is not the same as the sand absorbing the heat.  Certainly the air absorbs the radiated heat into it's mass (which is small particles dispersed in the space with large spaces), and the heat excites the air and the air reacts to the heat by expanding the spaces between it's particles and creating convection currents, and at the same time waves of radiation moves through the air to objects beyond, where it is absorbed and later re-radiated.  The radiant heat, dispersed rapidly outward from the barrel  (which is what you want, optimally with a rocket stove barrel, from what I understand), so that the barrel is consistently losing heat by throwing it off in the form of radiation, which is "cooling" the barrel.  Not really cooling it, but the air is definitely cooler than the barrel, and the barrel is throwing heat into the air.  What you have described, if I understand, is that the sand surrounding your barrel is conducting the heat away from the barrel, which is not very efficient because the sand will also be holding heat to the barrel, so it is not cooling the barrel as fast or consistently as a traditiionally built RMH.  It seems to go counter to RMH theory.  I'm not saying that it's wrong, (you may have an innovation worth looking into) but it seems like it's not done the way that I understand works best for RMH. 

If it is your intention to have a mass absorb heat from your barrel, it would be better to use a denser material than sand (like clay, or stone, or concrete), but the sand in your case is also a thermal mass, for sure, and it will hold that heat directly to your barrel system. 

I don't think that Glenn was saying that sand was insulation, but that it was insulating... when compared to denser mass materials.  If you are looking for mass, sand is good, but... there are better choices.

I may be missing something about your build that throws a bunch of what I wrote in this post out the window.  Still curious.  More questions in my mind, but need to bounce them around in there for a bit first. 



Robert, just to clarify, my entire core is steel as is the container of the integral mass surrounding the barrel. See the topic "Think outside the box re metal RMH cores " for more info and actual pictures of the whole thing. Also the recent thread "Steel RMH up and running for this heating season."  Maybe that will make things much clearer before we go further. I hope so! Look forward to more comments and discussions as I believe my steel RMH challenges a lot of firmly held traditional thoughts on RMH's.
 
Roberto pokachinni
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Cool.  I will check out those threads in the next few days.  Like I said, you may have some innovations worth exploring.  I do not mean to challenge your project, but to understand it.  Thanks for the extra directions to explore this.
 
Bruce Woodford
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For those who have been following this thread and the discussion about coals remaining in an RMH burn tube...here's some more info coming from my search for ways to eliminate coals which eventually were starting to throttle the draft in my burn tube. I was also motivated by Thekla's assurance that it can be done!

Years ago we observed charcoal production in the West Indies - a heap of hardwood would be set afire and then buried under a pile of earth and left to smoulder for weeks. Then the coals would be dug out and sold. Thus coals are produced when burning wood is starved of oxygen. SO..... as I was getting far too much coal piling up in my burn tube, I figured I needed to get more O2 to the very base of my fire where the coals collected and got smothered in ash.

After pondering this for a few days, I decided to try a little innovation in the mouth of my steel core over which I had a steel cap which is removable to clean out the ashes.  I inserted an air baffle (held in place with magnets) which allows air to be drawn into the very bottom of the burn tube where the coals would normally begin to pile up. Thus oxygen is fed to the base of the fire and the coals are burned and do not have a chance to accumulate.  Secondary benefits which I did not expect include (1) a much hotter burn (almost white flames instead of orange), (2) a much stronger draft and (3) a much more pronounced "rocketing" sound!

And the coals?  What is left after my last few burns is almost totally ash with only a very few very small coals!

I've included a couple of diagrams and a picture to explain visually.
(32)-Original-design-produced-lots-of-coals.jpg
[Thumbnail for (32)-Original-design-produced-lots-of-coals.jpg]
(33)-New-design-burns-most-coals-with-hotter-flame.JPG
[Thumbnail for (33)-New-design-burns-most-coals-with-hotter-flame.JPG]
(34)Burns-with-almost-white-flame-with-end-open-and-air-baffle-in-place.JPG
[Thumbnail for (34)Burns-with-almost-white-flame-with-end-open-and-air-baffle-in-place.JPG]
 
Roberto pokachinni
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By the looks of your drawings, your air baffle also serves the purpose of taking up the space where a cooler vortex or eddy would happen in that space towards your clean out cap/away from your burn vortex direction.  This alone, is probably very helpful from a fluid dynamics/thermodynamics point of view.   There is less space for coals to accumulate.  There is less space for cooling where you are initiating your burn.
 
Bruce Woodford
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Roberto pokachinni wrote:By the looks of your drawings, your air baffle also serves the purpose of taking up the space where a cooler vortex or eddy would happen in that space towards your clean out cap/away from your burn vortex direction.  This alone, is probably very helpful from a fluid dynamics/thermodynamics point of view.   There is less space for coals to accumulate.  There is less space for cooling where you are initiating your burn.



Yes, you're right Roberto. There is no place for them to accumulate except right in the air flow which is the catalyst to their combustion.  I've been puzzling for a few weeks now on just how to accomplish this and this simple adaptation seems to do the trick.

The spark deflector is inserted as a safety measure to prevent hot sparks "exploding" off of igniting wood, passing under the baffle and ending up on the floor.  Hope such an idea may prove helpful to others.

Any other suggestions for efficiency improvements of RMH's??
 
Glenn Herbert
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The real thrust of Roberto's comment, I suspect, is that in a RMH core built of insulated refractory material without the cleanout port extending several inches back out of the airstream, there would likewise be no coal buildup, and no modifications needed. I know that in 6" J-tubes I have fired, there is next to no coal buildup.
 
Bruce Woodford
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You are probably right Glenn.  It's just that a cleanout port makes ash cleanout so much easier.
 
Bruce Woodford
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I’ve redesigned my air baffle and spark arrester described in a previous post. The first prototype air baffle was made of aluminum and melted out quite quickly! So I changed the design and made one of heavier gauge steel. This one requires no magnets to hold it in place but is free standing. It also works much more efficiently!
-This one not only effects the TOTAL consumption the coals, but it also increases the strength of the draft and raises the operating temps throughout the system!

Since I’d set the unit up in the house with sand in the integral mass around the barrel, my maximum barrel top temp had been about 580F and the maximum temp of the horizontal tube exiting the barrel had been about 350F. But the barrel top temp now rises to 675F and the horizontal tube temp rises to 450F!

Originally, my flame path temps at the base of the heat riser were over 1225F. I knew this because it would consistently melt aluminum cans. That temp must go considerably higher now (although I don’t know how high). But, with the cooling fins and cooling tubes I’ve attached to the outside of the burn tube, the burn tube temps usually remain below 900F (the point at which the steel begins to glow in the dark.) I always monitor the exterior burn tube temps and occasionally the temps on the side of the burn tube have drifted just over 900F (into the “danger zone” for steel).  Whenever that happens, I simply reduce the air flow until the temps drop back into the “safe zone” below 900F.
The following pics show the second prototype of the air baffle and accompanying spark arrester. 
(35)-Second-air-baffle-made-of-steel-insures-100-reduction-of-coals-to-ash.-(With-spark-arrester.).JPG
[Thumbnail for (35)-Second-air-baffle-made-of-steel-insures-100-reduction-of-coals-to-ash.-(With-spark-arrester.).JPG]
(36)-Another-view-of-air-baffle-and-spark-arrester..JPG
[Thumbnail for (36)-Another-view-of-air-baffle-and-spark-arrester..JPG]
(41)-Air-baffle-feeds-air-to-base-of-the-fire.JPG
[Thumbnail for (41)-Air-baffle-feeds-air-to-base-of-the-fire.JPG]
 
So I left, I came home, and I ate some pie. And then I read this tiny ad:
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