preventing the barrel top from melting

[michaelegan]

The only rmh I currently have is an 8in batch in my shop, this is my 3rd season with it. When I load up the firebox with, say 40-50lbs of dry firewood the barrel top gets over 1000F according to my cheap thermometer and gets a cherry red circle in the middle, about the same diameter as the 8in stainless steel double wall insulated riser ($100 from Menards). Is there a danger of the top ever melting? I put a stack of firebrick splits on the top to absorb heat and also have not run the stove over around 900F lately but i'm going to build a few more rmh's at our farm and would like input on this. I know steel melts at around 2200F so am thinking about putting a cap a few inches thick on the tops of the rmh's. I'm considering using the recipe Manny and Hpmr have been experimenting with: I made some samples with ashes and clay from our ground (50-50) and was very impressed.

[pa_friendly_guy]

I read somewhere, not sure if it was on here or that other permie site about someone who had welded a sacrificial metal plate to the inside of the barrel lid to extend the life of the barrel . I believe they used an 1/8 inch plate cut to size to fit inside the barrel lid and just welded it in place . Their theory was that the plate would protect the thin metal of the barrel lid which as you say takes a lot of beating from the intense heat, and therefore extend the useful life of the barrel. Matt and others on here seem to feel the barrels last a good long time just by themselves. My feeling is that the extra metal plate adds a level of safety from the exhaust gases leaking into the house some night and asphyxiating everyone in your family. The extra cost is minor, the added safety factor is a comfort, the cooking surface at the top of the barrel is still intact, it does not effect the efficient operation of your stove, I guess my feeling is, why not? Safety First, always error on the side of safety.

[matt walker]

I believe that the top can shed heat fast enough it's in no danger if it's left as is. I do think that putting anything on top of the barrel, especially over the riser, will potentially lead to failure as the top can't shed the heat. So, I don't think firebrick right on the top is a good idea to help longevity, but it probably won't instantly cause the barrel to fail either. The idea of a sacrificial plate is okay, but it is unnecessary materials and work in my opinion. It would make a nice cook top though!

[matt walker]

Oh, forgot to say that I think the use of masonry or refractory caps is a good way to reduce radiant heat and clearances to a ceiling, add some mass, move more heat out the sides and downstream, and all sorts of functions. I don't think it will save the barrel, rather I think it may break down faster, but again, I don't think that is really a design concern. They will last a good long while either way and are replaced easily, cheaply, and quickly. Dragon Heaters makes a preformed refractory cap for 55g drums that I've seen in use and it is nice.

[pa_friendly_guy]

I think their idea of putting in the sacrificial plate on the inside of the barrel did improve the barrel top as a cooking unit. It distributed heat across the top a bit better, and more evenly. Its main purpose was to extend the life of the barrel I believe. How many years it would add to the barrels life though is an unknown, and as you say Matt, it is easy to replace the barrel, so it may be an unnecessary step, and an unnecessary expense.

[ByronC]

[michaelegan]

thank you for your inputs. One follow up question about the space between the riser and barrel top: i assume the standard distance is roughly 3 inches as per ianto's book and that's what this one is, but i haven't seen much about increasing it to dissipate the intensity of the stream hitting the lid. Would that cause the flow to decrease and weaken the overall draw?

Regarding the stainless steel chimney, yes it is a double wall insulated pipe I bought from a big box lumber store (Menards). The rmh is in my shop and I run it a couple of times a week in the winter so it doesn't get daily use. When I built the current version 2 and a half years ago some people were still experimenting with them. Ianto even mentioned it in his book but also said he wasn't sure how steel would hold up. My two previous rmhs had the square firebrick risers from the book and i wasn't satisfied with the long time they took to heat up. since then (or I think right about the time I built this latest rmh) Matt and others started posting their cast cores which I believe are the way to go. I plan on tearing down the rmh in April so i'll take pictures and video and post it to show the different parts of the unit.

[mannytheseacow]

Hi Michael. I wouldn't hesitate to make that gap 6" or more. It will still run just fine and also take some pressure off the top of the barrel.

[matt walker]

It will be interesting to see what you find in there Micheal. It's important to note that your heater is quite a bit different than the normal J, so the highest temps just might not be right at the base of the riser. I was able to wreck a SS class A chimney section in an hour when I used it to dry out a core, but of course that's going thermonuclear due to the extreme draft from no barrel.

Anyway, larger top gap is almost always helpful in my experience. I don't think it effects the velocity at all, overall. I believe that is strictly a function of end temps and draft, although you can restrict it and slow it down. Hmmm, so maybe it will go a bit faster, if that was a choke point. I do think that it helps in that it just removes one more small restriction so the thing can breathe the way it wants to. So, I suppose my position is that if it has any effect it should be to increase draft.

[michaelegan]

ok, thanks Matt, Manny, Byron and Pa for your perspective. As I said, I'll let you in on the autopsy. I've become more a fan of the batch heaters and less of the Js because the batch heaters can be bigger and heat a larger room. The other advantage for me is that I don't need to "babysit" it so much: once it heats up (15min) I can load it every couple of hours or so. The third is the ability to use larger pieces of wood.

Just to double check, I got out my dog-eared, well-worn copy of "Rocket Mass Heaters" and found a couple of quotes that pretty much line up with comments here. On page 32 of my edition, at the end of the section on barrels Ianto says "Can a barrel melt in these temperatures? We haven't seen it happen yet. the melting point of the steel used in the typical oil drum is a much higher temperature than the stove will achieve." His comments on steel pipe might be a bit more dated and probably subject to revision after 8 years (this is the 2007 edition). On page 33 under the section on steel pipe he says: "If you choose steel for the burn tunnel or feed tube, be aware that where oxygen is plentiful it will gradually burn away... Several people have experimentally used triple-wall stainless steel pipe. Its durability in Rocket Stove use is not known but its thermal mass is low and its insulation value is high. So it could be a valuable material to use."

At this point I have no plans on using steel in the firebox or riser anymore.