I've been reading about RMHs for a few years now but can't wrap my head around the use of a thin barrel within inches of a high temperature riser where metal is taboo. Does the barrel burn out? If so what is the life span of a typical barrel?
I've considered ideas like attaching ceramic fiber board bolted to the top of the barrel, but it seems that might diminish the heat sink properties of the barrel. Also if the fasteners failed it would block the riser, snuff the fire and cause smoke inside the house.
I've also seen a steel plate set inside the rim on the top of the barrel to add an extra layer.
Is there any value in this, or is barrel burnout not a real world concern?
I don't know if my first RMH will be this summer or 20 years from now, but I'm trying to plan ahead...
Barrels have been used on RMHs for many years now and reports that come back from early builds aren't typically talking about barrel burnout. I think the main things that are at play here are the reducing environment inside the barrel, which keeps the steel from being eaten, and the way the top radiates heat away as quickly as it's imparted by the gases from the heat riser. There is an upper limit to how hot the barrel top and sides will get in operation, and although it varies from build to build, no one has had problems with molten steel. We're talking about 300 C or so, far short of 1500 C (although by the time carbon steel reaches 1200 it is subject to deterioration well before it flows).
In the burn tunnel and heat riser, we are trying to keep the heat in as much as possible, so we insulate and hence the temps those materials withstand will quite easily exceed 1200 C. This is the main difference: we're retaining heat in the burn zone to support complete combustion, but once it reaches the barrel or bell, we've flipped the goal and now we're trying to shed heat from the gases as efficiently as possible. Thin steel does this well, and it's cheap and readily available in an extremely useful configuration, so there you go.
Phil did a good job of explaining the thin steel is shedding heat rapidly.
Try to use just plate steel and it will spall away in a few years.
My first rmh is now on its 7th season , 8" J tube. I can get 1100F in a glowing 8" circle on the lid of my barrel! Same barrel no changes. Other's report of 20 years plus on their barrels.
Ceramic boards for your burn tunnel and ceramic blanket for your riser are the current best suggested configuration to build super hot rockets.
Ianto Evans' RMH in the Myrtle was installed many years ago, I think he said in the '90s, and is still using the same barrel. That's in the coastal Oregon rain forest of the Cob Cottage Company. I've heard people suggest that you could lift the barrel a bit so it's more than the 1.5-2" above the riser (which also helps limit ash blockage in that torus CSA) which will spread the heat further across the top of the barrel. Peter has added an entire second barrel on top of the first for the heater in Paul's shop/classroom, so a true limit isn't normally going to be encountered. My own heater will use 2 barrels, with the second being cut out to form the manifold around the burn tunnel bricks, and I'm aiming for "about" 6" or so, maybe... perhaps... I'm not sure yet!
I'm not quite a lumberjack, but that's OK, I sleep all night and I dream all day; I'll coppice trees, I'll grow my food, and compost poo and pee! With a well and off-grid solar, it's a permies life for me! https://www.youtube.com/watch?v=FshU58nI0Ts
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