Excessive temps which destroy most steel cores are avoided in this RMH-syle core

Most RMH builders avoid building with steel because all INSULATED steel cores corrode very quickly from the excessive temps to which the steel is thereby subjected. But in a NON-INSULATED, AIR COOLED RMH with an INTEGRAL MASS OF SAND around the barrel,  a 67” HIGH HEAT RISER and an AIR BAFFLE which forces air into the base of the flame in the burn tube… a highly efficient unit with flame path temps well over 1300F is the result.

But in this same STEEL RMH the temps of the burn tube and heat riser are maintained below 900F!  (For more details and extensive pics see the following topics:  “Think outside the box re metal RMH cores”, “Steel RMH with air-cooled core up and running for this heating season” and “Tips to increase RMH efficiency”.) To access those topics just click on my name at the beginning of this post, then on "Topics" and then on the topic you wish to read.

Here are pictures of the interior of the burn tube and the heat riser 7 weeks into the 2016 heating season.  If, as it has been designed, the steel temps can be maintained indefinitely below 900F… this entire core and RMH can also be used indefinitely without heat corrosion.  

Doesn't that just reduce the efficiency though, which is rather defeating the whole object?  Surely without reaching those high temperatures, there's going to be unburned fuel left over in one form or another, or draught will be reduced.

Burra Maluca wrote:Doesn't that just reduce the efficiency though, which is rather defeating the whole object?  Surely without reaching those high temperatures, there's going to be unburned fuel left over in one form or another, or draught will be reduced.

Burra, what high temps do you speak of, and how high?   The temperatures of the flame path or of the core material itself?  The temp of the core isn't the issue but rather the flame path itself (as far as I can see, anyway.)

But I thought in your other thread you were having to force air through because the draft wasn't good enough with the uninsulated riser to get the temperature high enough to burn all the fuel to the extent that you were getting unburned coals left over.

Burra Maluca wrote:But I thought in your other thread you were having to force air through because the draft wasn't good enough with the uninsulated riser to get the temperature high enough to burn all the fuel to the extent that you were getting unburned coals left over.

No, it was the other way round! There has always been a good strong draft but coals were collecting and eventually blocking the draft! Now, with the air baffle forcing air at the base of the fire, no coals collect, all is burned to ash and the temps are higher than ever.  What sort of temps do you figure are optimal in your own RMH?

My impression has been that the high temps burn off the creosote and smoke.  If you keep the temp low, you do keep the steel from spalling or melting, but I think you don't get it high enough to burn off the creosote and smoke.  

How clean is it?

paul wheaton wrote:My impression has been that the high temps burn off the creosote and smoke.  If you keep the temp low, you do keep the steel from spalling or melting, but I think you don't get it high enough to burn off the creosote and smoke.  

How clean is it?

Very clean. Once it's going a few minutes no visible smoke.  Again, I'd ask you, What temps are optimal to burn the creosote and smoke? I think I'm exceeding them.

Bruce Woodford wrote:Again, I'd ask you, What temps are optimal to burn the creosote and smoke? I think I'm exceeding them.

At temperatures around 600º C (1110º F) the last large hydrocarbon molecules are breaking up. It's difficult to reach such temperatures without destroying the steel. Since you don't employ a gas analizer it's impossible to tell whether or not your heater is running as clean as we normally would wish for. Also, is it a j-tube rocket heater or another magazine heater with front air? I've build quite some rockets in my time and found out the burn got much cleaner when there happened to be insulation around. See my opening post from 5 years ago in the thread Small Scale Development on Donkey's forum.

Peter van den Berg wrote:

Bruce Woodford wrote:Again, I'd ask you, What temps are optimal to burn the creosote and smoke? I think I'm exceeding them.

At temperatures around 600º C (1110º F) the last large hydrocarbon molecules are breaking up. It's difficult to reach such temperatures without destroying the steel. Since you don't employ a gas analizer it's impossible to tell whether or not your heater is running as clean as we normally would wish for. Also, is it a j-tube rocket heater or another magazine heater with front air? I've build quite some rockets in my time and found out the burn got much cleaner when there happened to be insulation around. See my opening post from 5 years ago in the thread Small Scale Development on Donkey's forum.

Thanks Peter.  This is the information I need. Please see the following...

Many have challenged me, saying that by cooling the core to prevent heat corrosion, I must, of necessity, be cooling the flue gases to such a degree that I cannot be getting sufficiently hot enough combustion in my flame path to burn smoke and creosote etc.

I realize that my steel RMH core is deemed “anathema” to many and downright dangerous to not a few (due to painful, costly and disappointing failure of steel cores which many have personally experienced.)
Thus, I have asked on many occasions here, exactly what temperatures are necessary in an RMH to get a “clean burn”.  Few have been willing to answer my question.  (Thank you, Peter van den berg for your info above!)

But if Ernie and Erica’s experience is of any value to folks on this forum, would anyone challenge them suggesting that their RMH’s run “too cool to get a clean burn”?

Ernie and Erica have estimated that their RMH’s run about 1100F in the burn tunnel and into the heat riser.

In an effort to dispel the myth that an air cooled, non-insulated steel core cannot possibly operate “hot enough”, I determined to do an observable test to demonstrate that my Steel cored, air cooled, non-insulated RMH runs considerably hotter than Ernie and Erica estimate for their own.  That test is displayed in the following pics.

As you consider the following, keep in mind that aluminum melts at 1225F.
I found that I could get a length of steel coat-hanger wire to stand up in my heat riser, so I hung some aluminum pop cans on it and placed it there prior to a normal twice-daily burn.
The first picture is my steel wire and aluminum can “contraption” outside of my RMH.
The second picture is of the bottom end of that contraption as it appears when the whole is placed into the heat riser.
The third is what remained after the burn.
I heard the first pop can fall just 3 minutes after I first lit the fire. Not sure how long it was till the others fell.
I hope this evidence speaks for itself and verifies the temps at least in the lower half of my heat riser.
This also verifies what I’ve been maintaining all along – that flame path temps are far higher than the actual temperature of the core which contains the flame.  I know that there is a temperature difference of at least 325 degrees F between my flame path in the heat riser (at least 1225F) and the hottest part of my burn tunnel (less than 900F).

Bruce Woodford wrote: flame path temps are far higher than the actual temperature of the core which contains the flame.  I know that there is a temperature difference of at least 325 degrees F between my flame path in the heat riser (at least 1225F) and the hottest part of my burn tunnel (less than 900F).

Yes this is the critical point that seems to be missed by the "Don't use metal in a RMH" dogma.
Internal combustion engines are made of metal and can last for decades.

And the engine analogy breaks down due to the fact that the fuel combustion is only intermittent, and designed to occur so fast that there is no time for significant radiation or conduction to cool it. The RMH configuration has the gases in contact with the core walls while burning, and turbulence needed for best combustion would bring much of the gas into contact with the walls at some point. You may get 1300+F in the heart of the riser, but much of the gas will be lower temperature, and will definitely not reach the fairly uniform 1600-1800+F that clean J-tubes are capable of to destroy the last pollution compounds.

(Ernie and Erica made statements years ago about 1100F, but more recently they are claiming well over 2000F maximum, with a caution to not exceed 2300F so that NOx pollutants are not created.)

Glenn Herbert wrote:And the engine analogy breaks down due to the fact that the fuel combustion is only intermittent, and designed to occur so fast that there is no time for significant radiation or conduction to cool it. The RMH configuration has the gases in contact with the core walls while burning, and turbulence needed for best combustion would bring much of the gas into contact with the walls at some point. You may get 1300+F in the heart of the riser, but much of the gas will be lower temperature, and will definitely not reach the fairly uniform 1600-1800+F that clean J-tubes are capable of to destroy the last pollution compounds.

(Ernie and Erica made statements years ago about 1100F, but more recently they are claiming well over 2000F maximum, with a caution to not exceed 2300F so that NOx pollutants are not created.)

The exhaust manifold is getting no break from combustion temps. Look at the egt gauge of a V8 turbo diesel. 1000F+ but the manifolds and turbochargers can last for decades.

At these low temperatures the metal uninsulated stove is probably synthesizing dioxins and furans.