Think outside the box re metal RMH-style cores

I am a newbie here having just discovered Rocket stoves and RMH's a couple of months ago. I built an RMH with a core made out of steel BEFORE I discovered the thread on "Using metal"! Not wanting to scrap all the work I'd put into mine, I have been reading everything IO can get my hands on re RMH's and to be honest, I am questioning some pretty firmly established traditional RMH beliefs.
(1) Seems many believe that the hotter your system, the better the draft through it. But I wonder if the real mover of draft is "the taller the heat riser" and "the tall the chimney". With a simple 4" duct work rocket stove, I have proven that the draft is affected primarily by the length of the riser, (not by the extreme heat of the burn). With the chimney on our house (It's about 20-25' tall) there is a strong draft all the time as long as there is just a few degrees of difference between inside and outside temps. So I built my RMH core with a 66" riser instead of 47". Even starting from stone cold, it has a very strong draft with the very first flame. I have built my core with a front cleanout (compromise between an "L" and a "J" configuration. I have often run it with both front cleanout and top feed tube wide open with no smoke rising in the vertical feed tube.
(2) Seems that many believe that insulation of combustion chamber and heat riser is absolutely essential to efficient RMH operation. I questioned this and could not find any discussions on actual tests and comparisons of the same systems tested with and without insulation. Have I missed something here that has discussed this issue? I first ran my system with insulation of both combustion chamber and heat riser and then removed all insulation and run it without. I don't see any difference in the performance of my core. Which brings me to another common belief...
(3) Seems that many believe that use of metal in the core of an RMH is a sure invitation to disaster (as many posts and photographs clearly testify!) because it is assumed that "the hotter the better" and that metal just won't stand up to the high temps produced in an insulated system. But I wonder what would have happened to the internal combustion engine if it's early designers had assumed that "the hotter the better" and that metal simply would not cut it in the heads of internal combustion engines??? I'm sure they experienced quite a few failures from overheating, BUT rather then insulating to raise the temps, they went the opposite route and found effective operation of their engines when they cooled them with air via cooling fins added to their heads or cooled them with water via water jackets and radiators.

I am well aware of the destruction that heat produces in metal as I have built many waste oil burners which burn with white hot flames. I have seen burners, air supply tubes and tanks which enclose these heaters disintegrate from the intense heat which they produce as these components themselves glowed red or even bright orange! I have also heated my home with a woodstove as it's primary heat source for 27 years and have never run that stove with firebrick or any refractory material in it! (Such simply insulate and send most of the heat up the chimney! I have seen spots on the sides of our woodstove glow red occasionally, but I knew that was too hot so just reduced the air until things cooled down a bit. After 27 years, there is no heat corrosion evident in any part of our woodstove. So I know that heat corrosion of metal only happens at temps where the metal glows with the excessive heat!

I have run a test to run my core (without insulation) as hot as I could possibly get it to run with all the air and all the fuel it would devour. The only place it would glow in the dark was on the sides of the combustion chamber from the centre line of the freed tune and extending about 6-8" toward the heat riser. So now I'm simply working to cool that crucial area so it never glows. Just a few days ago I discovered the folks who built the "Liberator" have done exactly the same thing by adding welded on fins for air cooling of that crucial area!

Could write much more and will send pics. But what are your thoughts?

Bruce Woodford, Norwich, Ontario

Hi bruce: welcome to permies! I'm wondering how long you had your rmh in use this year ? Is it just a core hooked into your chimney, or is it heating a mass? What comes out your chimney, steam ? smoke ? heat shimmers? I'm thinking you may not be getting a complete burn (hot enough to burn all the particulates) How much ash is in your horizontal piping? Everybody cleans out the burn tunnel regularly, but the horizontal should only need minimal cleaning once a season,mainly from your ash pit. If you have horizontal piping and you are not creating the temps necessary for a complete burn and afterburn, then your gonna plug up with ash ! 2 years ago I built a version that had a long burn tunnel... seemed fine, burned hot ... but not hot enough. Late in the season it just didn't seem to be drawing like it had been... sure enough pop open my cleanout door and its full of ash ... the 8" horizontal pipe was 2/3 full ! No wonder it wasn't drawing as well ! my point here is, if you have only been burning for 3-4 months you may not be seeing the ash issue yet. I burned mine hard for 6 months that winter, bragging about how well it was working the whole time. Then one day it just wasn't rite... Nothing wrong with thinking outside the box... that's how they invented the wheel !

The reason high temps are the goal is to induce the full combustion of gasses and particulates that other wise leave the system unburnt.
Leaving gasses and particulates unburnt is inefficient and dirty.

Hi Bruce. Chill man, don't get so tense!

Draft is induced by a difference in air presure. Since cold air is denser than hot air. The hotter you go, the stronger the draft. This can be countered by many factors. Friction, whole house stack effect, hotter outside than inside.

Basicaly, what happens in a burner/chimney couple. Air is raised in temperature by fire, then, that air being less dense than the surounding air, it is pushed upwards by denser air, comming in it's place to replace it, and try again to reach a state of equilibrium.

https://en.wikipedia.org/wiki/Stack_effect

About the metal in a firebox and heat riser. Peter is right, i often lack to explain that it is not only the heat itself, but also the high oxygen content of the flue gasses coupled with high heat which kills metal. So a lower temp burning wood stove, which usualy results in a way sootier burn, doesn't have the same problem. At the cost of producing an unburned hydrocarbon, soot. When i say hotter is better. This is from an efficiency and environemental point of view.
For exemple, creosote is completely burned above 350/400C°. Dyoxin is completely burned above 900C° Both are carcinogens. And it's a good idea not to impose these in the chimney gasses, to nearby people or animals. Any other gasses involved in wood burning and probable polutants, i don't know of. I haven't studied the subject enough. Few of the people playing with rockets, have the basic testing equipement. And so far, i don't know of anybody who would be capable to do a full side by side test, between a normal stove, and a proper rocket. Testing for unburned VOCs and PAHs, is beyond my reach and the reach of many others here. So you will have to find answers by yourself. But i'm prety sure that the hotter the better

Take a heavy metal flame-path combustion core, slow down the burn so the metal doesn't "glow in the dark", takes away the ability of it to ever reach freaky hot true rocket stove clean burn temperatures, and what you have is something that may resemble a rocket, but burns like a steel box stove.

There is a big difference between an "insulating" and an "insulated" combustion core. A heavy metal combustion core (burner and heat riser) is already "shot in the foot" as it were, due to its really good thermal conductivity, one big disadvantage. Adding insulation to such a core will gain little. The heavy metal is already conducting heat away from the flame and preventing the core from achieving full rocket-stove clean burn potential, the advantages of such that Max has eluded to.

Increasing air and over-fueling is actually a hindrance, from the rocket stove point of view. Excess air will have a cooling effect even when preheated. I.e. in a properly working RMH constructed of true insulating refractory materials, the heat riser is operating at several thousand degrees. Even preheated "excess air" will cool that down enough to degrade re-burn efficiency.

Successful metal designs use "insulating refractory" materials in the flame path, and metal only for the outer casing. The outer casing may be in the exhaust path and that's okay. There are some good examples of metal encased refractory cores on Walker Stoves website, both J and Batchbox versions.

Bruce Woodford wrote:Seems that many believe that insulation of combustion chamber and heat riser is absolutely essential to efficient RMH operation. I questioned this and could not find any discussions on actual tests and comparisons of the same systems tested with and without insulation. Have I missed something here that has discussed this issue? I first ran my system with insulation of both combustion chamber and heat riser and then removed all insulation and run it without. I don't see any difference in the performance of my core.

Bruce, I did lot of testing between 2008 and 2016. I used a gas analyzer, a Testo 330-2 to determine what the level of residue was in the exhaust gases of a J-rocket, among others. It is no wonder you couldn't see a difference in the burn with or without insulation. Simply because a lot of the released gaseous hydrocarbons aren't visible most of the time. Using high temperature combustion isn't something to believe in, it is just physics and physics are still there, irrespective whether you are questioning it or not. A given combustion core can be heated up to a certain point and then there's still unburned hydrocarbons coming out. When another higher threshold is reached nearly all of the hydrocarbons will be broken down to CO², water and heat. There's more to it than just heat of course, but that is another discussion.

Most of the testing is published, here's the link to another forum. It is quite a read so take your time.
Another article about gas analyzers which clarify a lot can be found here.

Please pardon me as I haven't yet figured out how to respond to a particular reply above. But I have a question for Byron Campbell... If most RMH's operate with heat riser temps of "several thousand degrees", what happens in the two inches above the riser? Erica Wismer tells us that temps at the top of the riser range between 400 and 700 degrees F. Most have steel drum barrels which would not withstand thousands of degrees of heat! Are Erica's and Ernie's RMH's horribly inefficient?

What am I missing here?

My flame is plenty hot to burn creosote and Dyoxins and there is nothing visible coming from my chimney.

More help and info is appreciated. Bruce

Bruce Woodford wrote:Please pardon me as I haven't yet figured out how to respond to a particular reply above. But I have a question for Byron Campbell... If most RMH's operate with heat riser temps of "several thousand degrees", what happens in the two inches above the riser? Erica Wismer tells us that temps at the top of the riser range between 400 and 700 degrees F. Most have steel drum barrels which would not withstand thousands of degrees of heat! Are Erica's and Ernie's RMH's horribly inefficient?

What am I missing here?

My flame is plenty hot to burn creosote and Dyoxins and there is nothing visible coming from my chimney.

More help and info is appreciated. Bruce

For quotoing, you use the quote button.

Ernie and Erika's rockets are prety much the same as others, and are quite efficient.

What you haven't grasped here is they're talking about the top of the barrel temp, not inside temp. Despite the metal which carries the heat very efficiently, all heat can't be shed to the atmosphere above the barrel top. So that temp is lower than inside the top of the heat riser. Plus, by that stage, usualy the burn has completed, there is no more an oxydizing flame to kill the barrel top.

And about your claim that creosote and dyoxins are all burned. Are you absolutely sure? Have you chemicaly tested your exhaust gasses? When you reach the 700C°/800C° range, there is no smoke anymore. But that's far from being complete combustion. If you can't see anything, doesn't mean that there is nothing. You don't see microbes for example. The smell can be a giveaway. If you smell something like a mix of clothes dryer with a small very faint acrid smell behind, there's still unburned hydrocarbons. Very faint bleach smell tells you there's dyoxin iIRC.

Bruce, a daft question, what is your exhaust température, at the entrance on the vertical chimney?

Hi Bruce, for a standard by the book build J-tube RMH, the hottest portion of the re-burn is concentrated at the back of the burn tunnel / bottom one third of the heat riser. After re-burn the rising column of super hot exhaust gas begins cooling rapidly, and at the top of the heat riser where the gases fan out and make a 180 degree turn towards the manifold, only a portion of those gases actually brush the barrel. Technically, the barrel is not in the flame path, only the exhaust path, as Max mentioned. The barrel survives by quickly dissipating heat through convection and radiation.

Reminds me, back years ago as a teenager, holding a soldering propane torch to the center of a tin sheet to see if I could melt a hole through it:) Didn't happen, the tin sheet conducted the high temperatures away to quickly. Very similar to the barrel on a RMH.

thomas rubino wrote:Hi bruce: welcome to permies! I'm wondering how long you had your rmh in use this year ? Is it just a core hooked into your chimney, or is it heating a mass? What comes out your chimney, steam ? smoke ? heat shimmers? I'm thinking you may not be getting a complete burn (hot enough to burn all the particulates) How much ash is in your horizontal piping? Everybody cleans out the burn tunnel regularly, but the horizontal should only need minimal cleaning once a season,mainly from your ash pit. If you have horizontal piping and you are not creating the temps necessary for a complete burn and afterburn, then your gonna plug up with ash ! 2 years ago I built a version that had a long burn tunnel... seemed fine, burned hot ... but not hot enough. Late in the season it just didn't seem to be drawing like it had been... sure enough pop open my cleanout door and its full of ash ... the 8" horizontal pipe was 2/3 full ! No wonder it wasn't drawing as well ! my point here is, if you have only been burning for 3-4 months you may not be seeing the ash issue yet. I burned mine hard for 6 months that winter, bragging about how well it was working the whole time. Then one day it just wasn't rite... Nothing wrong with thinking outside the box... that's how they invented the wheel !

Hi Thomas, Thanks for the welcome and the info! I've just been building and testing my RMH the last 2 months. So far it has not been connected to a mass as I've been testing and tweaking the core first but plan to have an integral mass of rock and sand right around the unit. (see pics below) When I first set it up with just a cobbled up 4" flue, the air moved well but there was not enough of it and coals did not burn completely and piled up in the burn tunnel within about 5 hours. But connecting it to a proper 6" flue made all the difference in the world.

P.S. still trying to figure out how to attach more than 1 picture here!

Satamax Antone wrote:Bruce, a daft question, what is your exhaust température, at the entrance on the vertical chimney?

It's not a daft question at all, but a good one. I'm assuming you mean the temperature as it exhausts from the base of the barrel. (I have not yet made or connected to my mass of rock and sand.) But once the heater gets warmed up (20-30 minutes) it runs about 500F to 550F at the top of the barrel and the flue temp entering the chimney to the outside is 400F. That is the temp which would be entering my mass. How do these temps compare to others'? Any way to figure out beforehand how long my mass ductwork should be at this starting temp?

Hi Bruce; Here's some numbers for you. All my cores have been 8", they have all used the same mass. My first core was built following matt walkers plans using fire clay and perlite. This core was by far the fastest & hottest one i've built to date ! Commonly 900F. + at barrel top and easily went to just over 1100 F when I wanted it to! Temp where my pipe leaves the building was 130 F .My horizontal piping runs 14' makes a 180 degree turn and comes back 11' at that point it goes vertical and does not count against distance limits. An 8" system is said to be able to push 50' horizontal with every 90 degree turn a 5' deduction. a 6"system is limited to 30'. That first core only lasted one season before feed tube abrasion made a 7.5" square hole into a round hole the size of a 5 gal bucket. My next core was built using insulated & heavy fire brick. It regularly ran 800 F at barrel top and I could baby it up to 1000F exhaust temp was 130 F. That core also failed for feed tube abrasion. Next core was all heavy fire brick , this is the one with a long burn tunnel(18") Regularly ran 650 F - 700 F at barrel top and I could baby it up to maybe 850 F, exhaust temp 130 F This is the core that filled my horizontal with ash !!! However, I had no feed tube abrasion issues . My current core is also heavy fire brick has a 12" burn tunnel roof and a 51" riser. Regularly runs 800 F at top and can be babied up to 950 F again it is 130 F at exhaust (notice the common exhaust temps) I have not opened the cleanout door yet this year as I am still lighting it off from time to time. When I do, I expect to see maybe half an inch of super fine fly ash in the horizontal and a third of a 5 gal pail from the ash pit. Hope these numbers help. I should ask if you have a copy of ianto evans book rocket mass heaters ? If not you should have one. Another book very soon to be available is the builders guide by the Wisners. Build what you want... if it doesn't last then build another... even if it does last build another anyway just because they are so very cool ! Oh you can add 3 pictures , hit attachment , choose file, add the first picture then look for the add another file button located bottom center.

Bruce Woodford wrote:

Satamax Antone wrote:Bruce, a daft question, what is your exhaust température, at the entrance on the vertical chimney?

It's not a daft question at all, but a good one. I'm assuming you mean the temperature as it exhausts from the base of the barrel. (I have not yet made or connected to my mass of rock and sand.) But once the heater gets warmed up (20-30 minutes) it runs about 500F to 550F at the top of the barrel and the flue temp entering the chimney to the outside is 400F. That is the temp which would be entering my mass. How do these temps compare to others'? Any way to figure out beforehand how long my mass ductwork should be at this starting temp?

Bruce, i meant the temp entering the vertical chimney. Not right after the rocket. But nevermind. What i wanted to explain, is that having verry strong drafting, hot, insulated core rockets has another reason besides trying to burn all the combustibles from the wood. There's also the recovery of the heat which is important in a rocket. If you shed your heat too fast into the room, it will get too hot too fast, and you will have to stop burning wood. When you add mass, it can gather lots of heat; so you fine tune your rocket to be able to heat, for example four hours a day, and for that period of time it's comfortable, and the mass gets charged for the remaining 20 hours. But mass has an adverse effect. It can hinder the draft. So you build the strongest drafting, most violent rocket you can. You don't cool down the metal heat riser, so it can survive You build a mad machine, out of the proper materials, which might cost even less than the metal. And have a raging rocket.

Building rocket stoves is not rocket science. But tuning them can be like car tuning. When you start playing with batches, cooking plates, ovens, and keep heating your place. Then it get a bit complicated.

I'll pass this thread link along in case it may be of some help:

https://permies.com/t/52476/rocket-stoves/compact-RMH-hybrid-cook-top

Satamax Antone wrote:Bruce, i meant the temp entering the vertical chimney. Not right after the rocket. But nevermind. What i wanted to explain, is that having verry strong drafting, hot, insulated core rockets has another reason besides trying to burn all the combustibles from the wood. There's also the recovery of the heat which is important in a rocket. If you shed your heat too fast into the room, it will get too hot too fast, and you will have to stop burning wood. When you add mass, it can gather lots of heat; so you fine tune your rocket to be able to heat, for example four hours a day, and for that period of time it's comfortable, and the mass gets charged for the remaining 20 hours. But mass has an adverse effect. It can hinder the draft. So you build the strongest drafting, most violent rocket you can. You don't cool down the metal heat riser, so it can survive You build a mad machine, out of the proper materials, which might cost even less than the metal. And have a raging rocket.

Building rocket stoves is not rocket science. But tuning them can be like car tuning. When you start playing with batches, cooking plates, ovens, and keep heating your place. Then it get a bit complicated.

Thanks Satamax, Yes, I realize that RMH's produce a lot of heat which needs to be stored in a mass. I am building an integral mass right around my barrel and then another mass to feed between the heater and the chimney. See the following pics...

heres a good question I have about the heat and draft, I built a L, J STYLE , refactory/perlite core and heat riser about 2" thick, 6 inch system, burn tunnel is 20" heat riser 41.5", 55 gal barrel heat exchanger, about 6 ft of exhaust pipe connected to a 20-25 ft masonry chimney,pellet basket is made from 309 stainless steel (service temp is rated at 2000F) burning wood pellets fed from the j tube, and the L tube is the air intake, built a plenum around barrel and have 2 fans connected to plenum to blow the heat into my duct work to heat my house, my testing results have been good, without the fans running on plenum,in about 15-20 minutes I can burn wood pellets and top of barrel gets to around 600-660F, a rockety sound, the exhaust coming out barrel gets to around 100F, now when I turn on fan in plenum and barrel starts to give up its heat to the air faster , it really gets a rockety jet sounding burn going on its amazing.... though I cannot get a accurate temp at barrel top because of the fan blowing on barrel...... but the exhaust temps are still around 100F maybe sometimes 20 degrees hotter... anybody have a explanation for this see photos of system attached

Anthony Donner wrote:... now when I turn on fan in plenum and barrel starts to give up its heat to the air faster , it really gets a rockety jet sounding burn going on its amazing.... though I cannot get a accurate temp at barrel top because of the fan blowing on barrel...... but the exhaust temps are still around 100F maybe sometimes 20 degrees hotter... anybody have a explanation for this see photos of system attached

Anthony, I love your innovation (putting a plenum over your barrel for your furnace to draw heat through the house! Great! As to the increase in the "rockety jet sound" when you start drawing heat off the barrel... I'd suggest that you are causing the air to rapidly contract as it cools which draws even more air through the system and produces more heat. The more you take the more you get!!!

What happens when you turn the fan off? Does the "rocket" slow down too?

Let us know. That is really intriguing!

I would also suggest that if you have the vertical space, increasing the height of your riser by 50% (and the height of your barrel to match) would dramatically increase your draft as well!

Bruce

Bruce,
I will have to let you know when I see what happens by turning the fans off, though before the fans are turned on it still has the rockety sound, it just seams to get more rockecty when the fans are turned on,
I have been experimenting with it 2-3 hours in the evenings when it was colder out this spring,but now it is summer and I don't know how often I will be running it...till fall, yes you are correct about the height of the heat riser, I think I could have made the burn tunnel a little shorter but due to the plenum I needed a little more space, though with the demensions its at it still runs better than I expected, has great draft even when cold at start up, have never had any problems as of yet, it basically burns up all the ash or sucks it up the exhaust not sure yet but I am happy with it so far...the real test will come this winter...cause I removed my woodstove and replaced it with this,this spring... though I do need to find a way to get a little more btus out of it ..cause it burns about 5lbs of pellets a hour whitch ='s about 40,000 btu's,this wont be enough to really heat my house completely, I have a 2500sqft to heat, also this fall I am going to experiment with burning rice coal in it which has almost twice the Btu's as pellets per pound.

Bruce Woodford wrote:I have run a test to run my core (without insulation) as hot as I could possibly get it to run with all the air and all the fuel it would devour. The only place it would glow in the dark was on the sides of the combustion chamber from the centre line of the freed tune and extending about 6-8" toward the heat riser. So now I'm simply working to cool that crucial area so it never glows. Just a few days ago I discovered the folks who built the "Liberator" have done exactly the same thing by adding welded on fins for air cooling of that crucial area!

I wrote the above last week and now here are some pics of the solution to my overheating of the burn tube.

what does that pipe and step down under burn tunnel and riser do? in the top first picture, does it add air?

Anthony Donner wrote:what does that pipe and step down under burn tunnel and riser do? in the top first picture, does it add air?

Hey Anthony, No, it's simply a vacuum cleanout for the ash pit at the base of the riser. See the pics with my original post above.

ok , I see ,what is the pipe sticking out of the side of the blue tank with a cap on it do in your previous pictures?

Anthony Donner wrote:what is the pipe sticking out of the side of the blue tank with a cap on it do in your previous pictures?

The blue tank will enclose an integral mass of rock and sand and the drain with the cap is simply to drain the sand when it needs to be dismantled.

Heat corrodes steel only at temperatures where it is so hot that it glows red or hotter. However, there are very sound physical reasons (based on the nature of steel heat corrosion) why steel RMH’s can function indefinitely and with a stronger draft than traditional RMH’s if some things are understood about (1) heat risers and (2) the nature of heat corrosion of steel itself.

A chimney 15’ tall will create a much stronger draft than a similar 10’ chimney. Similarly, a hot air balloon which is 50 feet tall and 30 feet in diameter will have much more lift than one which is 30 feet tall and the same diameter even though the internal temperatures of the two are identical. So too, a 66” heat riser in an RMH will create a much stronger draft than a 47” riser all other things being equal. Once you understand that the draw, draft or upward movement of warmed air in heat risers and chimneys is increased the most by increasing the height of the heat riser and thus the volume of the warmed air rising within it (NOT primarily by increasing the temperature of the warmed air within it) you can build a heat riser considerably taller than traditional RMHs which will produce much more draft . Thus, even when initially starting a fire in a stone cold RMH, one with a 66” riser will draft more vigorously than one with a 47” riser! And it only gets better as the core heats up!

Once you understand that steel does not corrode from heat at temperatures cooler than its initial glowing point, you can build a steel core with an exposed, non-insulated, air cooled burn tube! (The secret is to design the core so the flames within it are hot enough to entirely consume your wood gases with no remaining smoke, but never hot enough to cause the steel of your combustion chamber to glow red.)

Another reason why the draft in this design is better has to do with the added 45 degree gussets at the intersection of the combustion chamber and heat riser. We know that 90 degree bends in a flue dramatically increase the drag or resistance to a draft through that flue. As these two gussets increase the CSA (cross sectional area) of that bend the resistance to the draft is reduced.

You can build a STEEL CORE with a longer feed tube than most traditional RMH’s (24” rather than 12-15”) and a horizontal 6” long cleanout port intersecting the bottom of it. If you have a tall heat riser and a tall chimney you will have a natural draft through your system BEFORE YOU EVEN IGNITE A FIRE (as long as the inside temp of your house or shop is just a few degrees warmer than outside.) This is why we often light our STEEL RMH just below the top of the feed tube rather than at the bottom! We place a couple of handfuls of pine cones and small twigs in the bottom, then a few small sticks and place a ball of loosely crumpled paper on top. With just one match this ignites a descending flame and away it goes! With this core configuration most combustion takes place in the lower portion of the feed tube which is air cooled by incoming combustion air and in the front end of the combustion chamber between the feed tube and heat riser. The sides of the front portion of the burn tube (adjacent to the most ferocious burning and hottest temps in the system) are the hottest spots of the system and the steel at these points glows red. For that reason, these sections of the burn tube are equipped with external cooling fins which increase its surface area to dissipate excess heat and thus maintain the temperature of the burn tube walls below the point where they will glow and begin to corrode. Flames do not “back fire” or rise vertically in the feed tube because the heat riser is so much taller and creates a stronger draft.

Steel RMH heat tests #’s 1,2 & 3: I have wanted to see how my temps compare with Erica and Ernie’s RMH’s which they give as follows: barrel top temps 400-700F and burn tube internal temps approximately 1100F.

Conditions of Test # 1: My Steel core was entirely insulated with vermiculite given all the fuel and air it would draft and eventually the barrel top temp rose to 1200F. (Aluminum would have nearly melted on top of the barrel!) The temp of the flue at the base of the barrel exiting to the chimney was 400F. That was a cool night (mid 60’s) with a good initial draft up my 8’ temporary through-the-wall chimney. The core being prevented from radiating or conducting heat out of itself could only do so into the air passing through it. So the heat from the hottest spot (the entrance to the burn tube no doubt was conducted through the entire core and although not visible, likely made the entire core glow bright orange. (It’s no wonder insulated steel cores fail very quickly!)

Conditions of Test # 2: Steel core (totally uninsulated), not encased in the barrel and connected directly to the house chimney (20’-25’ masonry) so I could observe it burning in the dark to observe “hot spots” where the metal would glow red. The only places where the steel glowed at all (when enabled to cool naturally via conduction, radiation and convection to surrounding air during its burn) was a spot on either side of the burn tube starting at the centre of the feed tube and extending about 6” toward the riser. My thermometer had “bit the dust” at this point so I had no way of checking the temps! (But test #3 sheds more info.)
After this test, I installed cooling fins on both sides of the burn tunnel to conduct heat away from those hot spots.

Conditions of Test #3: Entire steel core, barrel, exit flue and integral mass tank were assembled with no insulation and with cooling fins installed on sides of burn tunnel but with no mass yet in the of the integral mass tank and not yet connected to an external mass. I wanted to get some indication of my internal temps so put 2 aluminum pop cans in the burn tube… one on the grate over my ash pit right at the base of the riser and one halfway between feed tube and riser. (Aluminum melts at 1220F or 660C.) It was a hot night (80F) with virtually no temp difference between inside and outside of my shop so there was no initial draft at all with my 8’ temporary chimney! Smoked a fair bit till I got fire rising in the riser with a propane torch! Within 25 minutes my barrel top temp rose to 600F. At that point, I opened the cleanout in the end of the burn tube and cleared a line of sight with a metal rod to see what shape the aluminum cans were in. The top of the one at the base of the riser was being licked by the flames ascending into the riser and was sagging as in the pic below. The one in the middle of the burn tube was no longer visible! See remains of it recovered this morning after all had cooled off. 30 minutes from startup the barrel top reached its maximum temp of 650F. And the maximum flue temp entering my temporary chimney was 460F.

My cooling fin apparatus seemed to have done its job as the burn tube no longer glowed there. However, because the draft was not as vigorous (with no inside-outside temp difference) the combustion was now taking place slightly higher in the base of the feed tube. No smoke or flame began to rise out of the feed tube, but the area of combustion was slightly higher than it has been before. So when barrel top temps reached 600F this area began to glow red (I’m assuming because of less draft and cooling of the base of the feed tube. I briefly put my thermometer on this glowing spot and it read 900F.

So by observation, I have learned a few things about my steel RMH:
-The hottest place by far is always where combustion begins (fuel, heat and oxygen combine). The high rate of radiation heat from the combustion causes the hottest spot in the whole steel core. Thus looking into the feed tube where the fire is visible, I can see the very hottest place in my system. The flame temps exceed 1220F and the radiation at that point produces the external steel temperature of 900F.

-The temperatures within the flame path (extending from the base of the feed tube into the riser) exceed 1220F. I know this for aluminum melted in the middle of the burn tube and at the base of the riser.

- Draft from inside/outside temperature difference or lack of it affects where combustion takes place. With a strong initial draft it all takes place in the forward part of the burn tunnel. With no initial draft, and thus weaker drafting while burning, the combustion rises a few inches into the base of the feed tube.

Comparing my noninsulated steel core internal temps and barrel top temps with Erica’s and Ernie’s with heavily insulated refractory cores, I would say the performance of mine is comparable and quite in line with theirs.
Therefore, if I can eliminate any possibility of my core ever glowing from excessive heat, is there any reason why a noninsulated, air cooled steel core could not be used indefinitely and just as efficiently as an insulated refractory core?

Next steps to insure that my core will never glow while in operation:
-Install an “L” shaped 2 ½” pipe to the back of each of my air coolers which will rise through my internal mass of rock and sand and exit at the top of the external tank. This will continually draw cool air through those coolers to insure the burn tunnel metal never glows again.
-Weld some vertical steel fins to both sides and front of the feed tube to insure that it radiates more heat into the air so it will never glow again.
Both of these measures will add more initial heat to the room while the RMH is burning but will not affect the heat of the combustion in the burn tunnel. It seems to exceed 1220F regardless of what I do.

Any thoughts, suggestions or cautions for this greenhorn RMH builder who is thinking outside the box and seeking to do the unthinkable by building an uninsulated and air cooled steel core for his RMH?

(Not sure why my last 2 pics don't show up. I tried to show them in another post but no luck! Any suggestions?)

More pics re heat tests above:

There are three factors affecting combustion efficiency and completeness, commonly referred to as time, temperature and turbulence. For full combustion, the gases must be hot enough, for long enough, and swirling turbulently so oxygen reaches all combustibles. I note that you are getting your hottest temperatures near the feed tube - burn tunnel junction, and while the base of the riser is hot, it is not nearly as hot. Your uninsulated core is shedding heat so fast that the riser may not be sustaining combustion all the way up. A well-insulated J-tube core will have its hottest temperatures in the lowest third of the riser, not in the burn tunnel, indicating that the gases are staying hotter longer.

I noticed that you mentioned your gussets at the angles between feed, burn tunnel, and riser to reduce friction. This is a good thought, except that the sharp angles serve a critical mixing function of inducing turbulence, ensuring that all gases meet oxygen.

I know that you feel you are getting smoke-free combustion, but only a high-quality (and expensive) testing instrument can determine how efficient a unit actually is, and how much carbon monoxide it puts out.

Hi,

to answer some of your questions:
1) yes, draft is affected by height differences (heat riser, chimney) as well as temperature. Both affect the density of air.
2) insulation creates a higher temperature difference between insulated heat riser and uninsulated barrel. The steel drum is there to radiate heat and create a high temp. difference.
3) you think internal combustion engines are efficient? They aren't. They are much more efficient than they used to. But there's a huge compromise to make between efficiency and portability. They don't run as hot, because it's not practical to have a red hot glowing engine in your car. If You look at f1-engines on test rigs (youtube), You see red-hot glowing exhausts.

Now, why are high temperatures important? To burn fuel to close to 100%. The higher the temp., the higher the probability to burn 100%of fuel in all conditions. The compromise between efficiency and material stress is usually balanced towards efficiency, as the material stress isn't as important if you don't use steel. Sure, You already built it, so I would use it, too. But I don't see the relevance if you use the right material to start with

Regarding your cooling solution and the shift of the highest burn temp: There's 2 variables changed between test #2 and #3: outside temp difference and burning room temp (cooling). The result was that the red glowing appeared after the cooled area. But this could well be due to the cooling area! If you expand the cooling area, you might solve the problem. Or maybe the RMH doesn't overheat anymore, but the temps are too low for 100% combustion.
That's a common problem in engineering: You try to fix a problem, but instead, you just shift it to another place and maybe even worsen it. But you think you solved it, because you are looking in the wrong place...

All the best,
basti

Bastian be wrote:
Now, why are high temperatures important? To burn fuel to close to 100%. The higher the temp., the higher the probability to burn 100%of fuel in all conditions. The compromise between efficiency and material stress is usually balanced towards efficiency, as the material stress isn't as important if you don't use steel. Sure, You already built it, so I would use it, too. But I don't see the relevance if you use the right material to start with

Regarding your cooling solution and the shift of the highest burn temp: There's 2 variables changed between test #2 and #3: outside temp difference and burning room temp (cooling). The result was that the red glowing appeared after the cooled area. But this could well be due to the cooling area! If you expand the cooling area, you might solve the problem. Or maybe the RMH doesn't overheat anymore, but the temps are too low for 100% combustion.
That's a common problem in engineering: You try to fix a problem, but instead, you just shift it to another place and maybe even worsen it. But you think you solved it, because you are looking in the wrong place...

All the best,
basti

OK, help me out...what am I missing? Ernie and Erica's RMH's are deemed to be efficient and their insulated temps and my uninsulated temps seem comparable. Their barrel top temps run from 400F to 700F and mine are about 650F. Their burn tunnel temps are estimated at 1100F and mine are at least 1220F and probably higher. Why would theirs be burning more efficiently than mine?

Bruce

Glenn Herbert wrote:There are three factors affecting combustion efficiency and completeness, commonly referred to as time, temperature and turbulence. For full combustion, the gases must be hot enough, for long enough, and swirling turbulently so oxygen reaches all combustibles. I note that you are getting your hottest temperatures near the feed tube - burn tunnel junction, and while the base of the riser is hot, it is not nearly as hot. Your uninsulated core is shedding heat so fast that the riser may not be sustaining combustion all the way up. A well-insulated J-tube core will have its hottest temperatures in the lowest third of the riser, not in the burn tunnel, indicating that the gases are staying hotter longer.

Thanks for your comments Glenn. You said that a well insulated J-tube core will have its hottest temperatures in the lowest third of the heat riser. Can you tell me who has measured their temps throughout the core while it is in operation and how they did it? Or is this an assumption or a guess? Mine is based on observable and meaureable data. Glowing spots in steel are obviously the hottest and melting aluminum in burn tube and at base of the riser gives the info that the temps are at least 1220F in the flame path in those places.

I know I have lots to learn so like to find the basis for claims which are made.

I look forward to hearing from you.

Bruce, most of us have started with steel, so the first temperature data were easy to obtain. Then, some have used firing cones to measure.

I know i have reached at least once metal melting temps, in the cyclonic rocket, as part the top of the gas bottle has sagged under it's own weight.

Now i go by a clay colour firing chart.

http://www.ceramicartdaily.net/PMI/KilnFiringChart.pdf But that's using mostly batch rockets.

I know that i have reached more than a 1000C° in one of my batch rockets. Far from 1100F° We're talking of about 1832F°

At 1100F° you haven't burned all the dioxins, so the flue gas could be potentialy dangerous. Far less than smoke. But far from having burned all hydrocarbons.

Barrel top temp are "erratic" i would say. Half inch change in gap can cause a great change. The dome shape of your gas bottle could also change things, since the gases are forced to change direction more and faster than bellow a barrel top. Speed of gases is also a factor, the more time gases stay in contact with the top, the more energy they will release. So, i think this is not a good indicator of how a rocket works inside. Just a way to guesstimate.


The ultimate test so far, let a Testo sniff your stove, and check it's efficiency.

Satamax, My point is that if claims are made about what really happens inside a rocket mass heater (highest temps in the lower third of the riser, for example) there should be verifiable proofs and actual tests to demonstrate them. Otherwise they are no more than guesses, estimates, hopes or false claims. None of us can learn anything from such but such ideas can become deeply set and believed by many when repeated often enough, even though they may be entirely untrue!

I cannot disprove such but I only ask those who make such claims to verify them with substantive proofs.

I am curious though, how do you use the clay firing chart to determine temps within your RMH? Can you observe the colour of the glow inside your core? If so, how do you do it?

Bruce.

Measuring temps in a rocket is not easy, but can be done.

I don't know if you have noticed, but i said i use batches, and not J tubes. I find the J tube not powerfull enough for my needs, and fussy as hell. So yes, i have easy view on the lower third of the heat riser, and the firebox. And i have checked a few times on my testing cores, looking down the heat riser. There's a trick for that, you look in there with a miror held up high, at an aproximate 45° angle.

For J tubes, i have tried the thermocouples on the DMM. But that's killing the ones i can afford. The best way is to have a tiny hole in the barrel, so you can lower the thermocouple inside the heat riser at different heights. No reccord of this. I didn't do it a scientific way. But you're more than welcore, if you have time and money on hand, to do better testing.

Few guys at donkeys have used IR thermometers. It can be done via the feed tube and heat riser. Some have used bare metal cores for primary testing. And measured outside. Some have used clay firing cones. Which gives a good indication of max temps reached, and the time they lasted.

http://www.bigceramicstore.com/tools-supplies/firing-supplies/cones.html

Then, chalenging others is good sometimes. But it gets tiring. Metal tinkerers, even argumentitive ones, we've seen aplenty over here. As i say every time. Prove us wrong. You make a metal rocket, which is working. One rule tho, the ultimate test is getting it sniffed by a testo 330 2. Checking for excess O², CO CO², and temps and efficiency. You can even, if you realy wish, use an uninsulated core. Actualy, that would be intresting to see what it does. Otherwise, please don't argue. It gets old fast.

About the argumentitive ones. You know, people come here, and have claims that their thing is working soo beautifully, and blah di blah di blah. Three month later, they come back, asking what the problem is with theyr rocket; when they dare. And discover that a burn tunnel needs to be cleaned, or that the transition from barrel to flue is way too small etc.

Ernie, Erica, Peter, Glen and I are here to help with the little knowlege we have gathered. There's a few others who are not in the staff whic are of great help. And it's what we try to do. Give help. If you want more or better quality help, may be it's time to find an expert. But i'm affraid to say; you might have to pay. And even may be; you might have to pay for help from somebody who knows less than the sumn of the guys here.

Max,
In response to your comments above...first thanks for taking the time to respond at all!

Secondly, I fully realize that reading temps in a rocket is difficult and that is why I asked how (empirical evidence) one knows that the hottest place in a rocket core is the lower third of the heat riser. So far no one has provided any evidence for that claim. But I do know that the hottest place in my own is at the junction of the feed tube and burn tunnel where the combustion begins and radiation energy is the most intense. My core does not glow anywhere else. (Now if it was insulated, I have no doubt it would eventually get just about as hot up the riser as it does in the burn tube simply by conduction. But my main question is really where does the hottest combustion take place? i.e. Does the hottest combustion take place where oxygen first meets combustible gases or does it take place further along the flame path? Mine is the only system I've been able to observe so far and it evidently develops the highest combustion temps where O2 first meets combustible gases.

Thirdly, yes I did notice that you were talking about batches and not J tubes. Seems they are two totally different "animals" to me! BTW, what do you heat or power with your batch?

You speak of argumentative "metal tinkerers"... Yes, I am working with a metal RMH core and seeking to address the problems with such (the primary one seems to me to be insulating them and thus overheating them) I'm seeking to make one work without overheating and corroding it away. But I am NOT seeking to be argumentative. If I challenge a common perception about RMH's, it is simply because I want to know whether it is really true or not. (i.e. do RMH's really work better with insulated cores? Is the lower third of the heat riser really the hottest spot in a J tube RMH? Is it possible that a taller non-insulated riser may cause more draft than insulating and seeking to raise the temps in a shorter one? etc)

As to your challenge, I feel your condition is unfair unless also applied to the traditional RMH with a non-metallic core. My aim is not to drastically reduce emissions except for creosote and tar in the chimney and smoke coming back into my house (problems which I have commonly encountered with a woodstove.) My aim is to reduce the amount of wood that I burn and the amount of time I spend cutting, splitting and stacking it etc to heat our older home. So my aim is to see if an RMH with a steel core can accomplish those aims as well as one with a non-metallic core. And, yes, I am using and plan to continue to use an un-insulated core which will actually be air cooled. This coming winter heating season will be the acid test and I'll let you all know how it fares. Is that fair?

Then if one experiences difficulty with a metal cored RMH and can't figure out that their draft is reduced by a reduction of CSA because they didn't clean their burn tunnel... what are they thinking? It doesn't take a rocket scientist to figure that out!

Finally, I do appreciate the opportunity to discuss RMH's (and related issues and problems) that is afforded here by you all. You have built, experienced, used and tested RMH's long before I ever heard of them! So I acknowledge that I am a "greenhorn" and a new comer to the RMH scene. I have simple aims and if there is a better, simpler way to accomplish them than with a steel cored RMH I'll be thankful to those who show me that.

I look forward to learning from you all in the months ahead.
Bruce

Bruce, i heat my workshop with one. 220mm heat riser batch.

If you want to test where your core is burning the hotest, use cones. That's the best way some have found. Hang them in the heat riser with piano wire. I can't remember where on donkey's board there 's a testing being done in a normal J.

If you want to heat a house, have no smokeback, cut less fuel, and use less fuel. Go batch. Forget the J. Sure you would use less fuel with a J, but the cutting is a chore. Use mass to store energy.

Putain d'bordel de merde! It's freaking bed time for me!

Thanks again, Max.
You mentioned a 220mm heat riser. I thought that was the height but it must be the diameter (about 9 inches). How high is it?

Please pardon my ignorance, but what are the cones made of which are used for testing heat riser temps? Where would I find info on this method?

Bruce

Cones, link provided yesterday! In a post above.

http://www.bigceramicstore.com/tools-supplies/firing-supplies/cones.html

220mm, IIRC, i'ts 160cm in height. Or 169cm.

All dimensions for batches can be found here

http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions

or here

http://batchrocket.eu/en/

I've had very little time to get online for the past week or so, but regarding the location of hottest temperatures in the lower third of the heat riser, there is direct evidence from people who have built thin steel liners for insulated cores, and shown where the liners failed first. Some have posted in this forum, and some at Donkey's forum.

People who have made metal "P-channel" inserts which sit at the junction of feed tube and burn tunnel report that the metal tends to warp or slowly degrade at that point, but not to the point of corroding away like the heat riser experiences.

Glenn Herbert wrote:I've had very little time to get online for the past week or so, but regarding the location of hottest temperatures in the lower third of the heat riser, there is direct evidence from people who have built thin steel liners for insulated cores, and shown where the liners failed first. Some have posted in this forum, and some at Donkey's forum.

People who have made metal "P-channel" inserts which sit at the junction of feed tube and burn tunnel report that the metal tends to warp or slowly degrade at that point, but not to the point of corroding away like the heat riser experiences.

Thanks Glenn... just a couple of questions in this regard: (1) Did these "steel liners" line the entire core (feed tube, burn tunnel and heat riser) or just the riser itself?  (2) Are we talking about J-tube RMH's or batches?  There are tremendous differences between the two!  Can you direct me to these specific posts?  Thanks again!
Bruce