hottest burn area and temps in RMH

Hi can someone tell me where the hottest part of the burn is in a RMH and what temperatures to expect there ?

Bobby Wynn : Initial temperatures in the earliest Rocket Mass Heaters using basically all recycled materials have always exceeded 2000 degreesF

With todays increasing use of highly Insulating/Refractory Ceramic materials we are well above that!! With all pyro-lysed wood gases having ignition
temperatures in the 400dF to 1100dF range we get a very clean burn !

The hottest part of the burn should be within the Heat Riser, The radiating surface of the upper 3rd of the barrel should be 800dF with the top of the
drum reaching 1000dF on some well crafted builds ! For the Good of the craft ! Big AL

My understanding of your question is: at what point in the combustion system is the temperature the hottest? Is it at the top of the heat riser? The bottom of the heat riser? The end of the burn tunnel?

In order to really say what the temperatures are inside a j-tube combustion system, someone would have to build it with thermocouples wired into place. The thermocouples which can handle the temperatures inside a burn tunnel are relatively expensive. One must also have a display/recording device on the other end of the wire.

On the other hand, if a particular combustion system does not have visible smoke that would indicate that somewhere in the combustion system, the temperature is reaching 1,200°F. This is the temperature required to consume the volatiles which normally appear as smoke.

Would you like to share with us why you are asking this question? Perhaps someone can help you with the issue without having these particular facts.

First many thanks for your speedy replies. Have had about 30 years experience burning wood in the conventional way (started splitting wood behind my grandfather's woodworking shop at 6 or 7 yrs of age) ; and so my initial response to a RMH design on the screen was " that's not going to work, there's WAY to much horizontal run in proportion to the rise" (and etc , etc ) However, understanding the theory and principles involved, and that, apparently , the things actually work, this would be the greatest advancement in wood heat in a long time. And so I have already started to build one since seeing this 5 days ago. This led to my next concern that the top of a common 55 gal drum, at those temperatures, would soon burn out. I saw a utoob vid where they used sand between two pipes to construct the riser. While this would provide excellent insulation, these pipes would burn out too, leaving you with a pile of sand for a riser. In order to resolve these issues, I plan to make the outside(vertical) part of the barrel of masonry and cap it with a round heavy steel plate. This would allow for some quick heat and allow the "lid" to be lifted for inspection,cleaning, or repair, should it be needed. I've been told by a welder that the melting temp of steel is about 1200-1300 degrees, which conflicts with some of the estimated temperatures I've been seeing online, hence my questions. I also discovered that a piece of 6" chimney flu liner, sold at masonry supply, fits nicely inside a piece of 8" flu liner giving a 2" thick column with an ID of 6" and OD of 10" which should work nicely for the riser. Again thanks for being there. I'm really stoked (!?) about RMH's and have interested friends.

Bobby Wynn : You are right to be very careful to marshal your thoughts a measure what you are seeing against what you already Know !

Try it this way, lets take a conventional Fossil oil Fired, Forced-air Furnace the combustion chamber is lined with ceramic type stiffened
Refractory Blanket material and the core temperatures during firing exceed 2000 degreesF !

Immediately above the Combustion chamber is the heat exchanger, this is subjected to wide (climbing)temperature extremes easily
seeing 700dF as the fan/limit switch turns on the forced air fan to cool down the Heat exchanger and incidentally provide heated air to
the rest of the house !

Depending on manufacturer the remarkably thin shelled heat exchanger is rated for 20, 25, even 30 years! A main requirement of the
heat exchangers make up is its need rot take the temperature swings and be a ductile steel that can be easily shaped to its function !

Though the 55gal barrel is considerably a much lighter metal than the 55 gallon drums made before/during WW 11 they are made of a
thicker gage of very similar Ductile steel, which perfectly matches the Barrels strength and ductility requirements when used as the
Rocket Mass Heaters RMHs Heat Exchanger !

''Ask the man who owns one" While some rocket stoves dating back to the 70s do still exist and are used seasonally, THe reports of
Cold weather climate severe service is with out a report of a barrel failure at 25+ years !

My advice would be to prepare your installation both ways, so that you have a back-up plan just in case, however the barrel will hold
up to its duties easily, while the exposure to temperatures of 1000 with expansion and contraction will cause cracks in the Cob that will
serve to keep your Cob and Clay slip skills sharp !

Actually sand is only a so so insulator, we like to make square Heat Risers of firebrick or round Heat risers out of a clay,builders sand
and Perlite and cover the outside of the Heat risers with Rock wool or more refractory Ceramic batt materials to ensure that here is a
sharp heat difference between the inner wall of the Heat Riser and the outer wall of the heat riser !

As the Hot exhaust gases rising in the Core of the heat riser Slam into the Barrels top they lose heat energy that is radiated off of the
Barrel. These now much cooler and denser gases create the pull-me push-you magic that creates the 'Draft' that flows the hot exhaust
gases 50' horizontally through the thermal mass before exiting via the Final vertical chimney !

Cindy Mathieu is an expert on what is possible with a Clay Flue Tile, and she states that any temperature swing of 50degrees F occurring
in one hour is generally fatal to the integrity of the tile !

She is a Partner and spokesperson for the only manufacturer of several different DIY kits that save time and frustration by direct purchase
and installation of these superior kits Check them out at Dragonheaters.com

Finally I can also highly recommend that you go to Rocketstoves.com to Download Your PDF copy of the brand new 3rd addition of Rocket
Mass Heaters. This is "The Book', with it you can come here to talk with your fellow members and be sure that you are using the
same words to describe the Size(s), Shape(s) and orientation of the RMHs parts to themselves, each other and the whole!

Please, Get the Book, read the Book and come back often, it will be fun helping another Fellow Member to become a Rocketeer !

Big AL

THanks Al will definitely read the book and check out dragonheaters. If there are any doubts about flu liners, I'll build riser out of firebrick. Have only got dry stacked "J" at this point to see how things will lay out. I live in Western North Carolina and would be very interested to know if there is anyone in this area with a functioning RMH willing to let me have a peek at it. Thanks again!

It might be that your welder friend was speaking in Celsius about the melting point of steel. That would be weird in North Carolina, but I wasn't there, so what do I know?

The melting point of steel depends on it's recipe, but it's around 2,500 F.

http://education.jlab.org/qa/meltingpoint_01.html

Now, that said, the higher the temperature steel is held at in the presence of oxygen, the more quickly it will oxidize. So there's plenty of trouble to get into at 1,000F, with steel parts scaling/rusting/corroding... but they won't be melting.

Mike C. : An even faster acting form of iron steel disruption and failure occurs as a Result of High temperature Hydrogen attack, a.k.a. Hydrogen embrittlement.
I have seen high temperature Hydrogen attack occur in the area of a Fossil fuel fired Forced-air Furnace's ductile steel walls of the combustion chamber and/or
just above in the heat exchanger when the refractory ceramic shield was damaged !

The crystalline structure of the steel is so radically changed it starts shedding little cubes of steel looking like what happens when a side windowing a car shatters !
This is followed by warping, a kind of slumping, and a frank hole through the heat exchanger!

A couple of years ago we had a minor kerfluffel involving the melting point of materials to make common virgin glass, (high) and the melting point of glass during
re-cycling, I think that there is a similar characteristic happening here but I too am at a loss to explain the Temp quoted! for the Good of the Craft! Big AL

Thanks, Mike and Al; welder is from Michigan so that might explain it-I was thinking in F* and didn't ask. 1200-1300 F did seem a little low. I've seen stove and pipe get red hot in wood stoves and knew that structural damage can occur by repeated overheating, hence my original questions. I THINK that the temp of iron/steel can be gauged by it's appearance(color) and that's how blacksmiths do it. I am going to a renaissance fair tomorrow where they have blacksmithing and glassblowing demonstrations, so I'll be asking questions. Let you know what they say. Got to run.