using metal in the burn tunnel and heat riser

I have a cast refractory J-tube core and perlite/clay slip riser which is quite insulative, and coals burn to ash every time. I also have a positive natural draft, so if I don't cover the feed after burning I will get constant cooling airflow. I think you may be getting too strong airflow during coaling which is stealing the heat. Wood that is not dry enough could also be an issue, though if you have the same wood you used to that becomes less likely.

Glenn Herbert wrote:I have a cast refractory J-tube core and perlite/clay slip riser which is quite insulative, and coals burn to ash every time. I also have a positive natural draft, so if I don't cover the feed after burning I will get constant cooling airflow. I think you may be getting too strong airflow during coaling which is stealing the heat. Wood that is not dry enough could also be an issue, though if you have the same wood you used to that becomes less likely.

Right. When I had the more massive perlite clay slip riser, the coals burned to ash every time, and yes, I also had to put my feed tube lid on over night to damp. I've reached the opposite conclusion now, though, on the airflow. Now the air flow seems to stop on it's own, as the riser cools so quickly after a burn. Sure the mass bench is warm, but not enough to create a draw it seems. And my water kettle cools after a few hours, too. Result of such a light weight core.  I have a small barrel on my feed tube, and I use the lid to slow air flow and usually close it off almost tight at night.
I tried an extra long, hot burn  yesterday, and also forgot to lid the feed. I had much better results. Some ash formed and it insulated the still glowing embers all night. Kettle was even slightly warm.
So my conclusion is that the 5 minute riser lets me cook more quickly, and therefore I've been able to have shorter fires and still have a hot meal and warm house. But the riser is so much less massive that in order to store enough heat to complete a fuel burn, I should keep her full throttle for at least an hour.  

Satamax Antone wrote:Laura, you might be missing some mass behind, and your draft is too strong. Maybe. Your firebox and burn tunnel still are made of bricks?

There is another thing which can do that, wet wood.

Ooo, damp fuel is a harsh teacher for sure. I have cooked and heated exclusively with wood for over 7 years now, and I'm still in love with fire. If there is one thing which I've learned:  use well-seasoned wood.  
As I write in my reply to Glenn, it appears lack of draw from too small of a burn may be my issue. Yes, the firebox and burn tunnel are brick, inside cob. Tons of mass in the bench around the exhaust. Old riser was especially massive, though, so it appears I'm just learning how to handle this lighter, more responsive one.

I realize this is an old thread, even if its been commented on recently.
I work for an industrial contractor.  one of  our primary clients is an aluminum foundry / casting plant.  we regularly receive orders from them for new flues, dampers, furnaces, doors, burners, tuyeres, crane buckets, etc.  even where items are made of stainless steel to help resist the heat, the steel just does not hold up long term to high heat without need for replacement.  most applications involve being clad in refractory, either castable or ceramic fiber, but even still wherever refractory fails or falls away the steel is promptly eaten away from prolonged exposure to high heat.

I would imagine that with the right engineering  and resources that something  could be made with steel in a way that could resist much longer, but its not on the kind of DIY budgets that I would imagine most viewing these forums would be restricted to.
the rate at which steel prices have been climbing lately is making getting hold of good scraps ever more difficult.

I suppose that metal rockets could be fine if they were made easily accessible to replace, or if they were not otherwise enclosed in an elaborate indoor RMH covered in Cob and pleasantly finished.  

even having seen this process for years, I found myself eyeballing scrap steel here that I might use to break into rocket stove experimentation, only recently having put together my experience with what I was planning.
even after digging on our property and finding what appeared to be good soil for making of Cob, I popped my own balloon.  so now I am reworking ideas to go with firebrick.

Trevor.  Do you know anything, besides metal monel, and iconel,  which would withstand the continuous 1200C° of a batch rocket, for a whole winter heating season?

I work mostly around steel, just a lowly draftsman at that, and I am new to the rocket heaters, but i've been hearing about ceramic fiber tubes for the burn and riser.  they can take high heat.
I was lucky enough to take home some ceramic fiber batts from work that were going in the trash after refitting a huge furnace door.  
good lord!  I just looked it up:  https://www.amazon.com/Inswool-HP-Blanket-Refractory-Ceramic-Fiber/dp/B06W53C9HB
I made of with $200-$300 worth of "trash"inswool insulation rated at 2300 degrees.
I was fiddling with a little piece of it last night and noticed that it tends to compress.  then I was looking at some Ceramic Fiber tubes online today and noticed that some look as though they were in fact compressed.

might be an option.

I was wondering if I could just make the core from the roughly formed uncompressed batts, but it might lead to too much accumulation of "stuff" inside over time.

Careful with inswool.

https://thinkhwi.com/datasheets/1214/sds/5828-INSWOOL-HP%20BLANKET%208)_(USA).PDF

What you saw might be a tube made of insulating ceramic fiber, and binder.

Stuff looking like this?

https://www.google.fr/search?q=ceramic+fiber+insulating+riser+sleeves&tbm=isch&ved=2ahUKEwjenpPd9r3vAhUVMRoKHdAaBOYQ2-cCegQIABAA&oq=ceramic+fiber+insulating+riser+sleeves&

So...
Bear with me on this one...

There seems to be a consensus that metal in a rocket combustion area is bad due to high temps.

However on the metal properties list near the top of the thread,  at the bottom of the list it says that titanium maxes out at 2000 deg F.
Oh sure, you might say.  How much does that cost?
Surely it is through the roof.  Right?

Or is it?

I found this on amazon for under $20.

Just enough perhaps for a burn "basket" under a self feeding pellet hopper...

That just _might_ be durable enough, for someone who wants to make a pellet-fueled RMH instead of the standard wood-fueled variety. Temperatures in a good RMH combustion core (J-tube or batch box) can get well over 2000F in certain areas, which might not include the feed area. If the application is of real interest to you, try it out and let us know how it works.

well,  I made a mistake in the choice of metal screen.  I meant to look at tungsten for its super high melting point.  problem there is that it is super expensive and I couldn't find a screen that  I felt was open enough for airflow and ash etc.  but while I was looking I kept bumping into welding rods.  at first I ignored them thinking that those are meant to melt.  but some are not.  TIG welding uses Tungsten welding rods to deliver an arc that works a separate consumable filler rod.  there was a pack of tungsten rods, 7" long that was affordable and bendable.
I hope to weave a screen of bent rods that can act as a burn surface that permits air flow and ash drop.  

even with the durability of tungsten, I am going to consider this to be a small, removable and ultimately consumable component of the overall heater.  I think that should be the take away for this whole thread.  any metal in a RMH needs to be considered consumable and if used should be made to be replaced.

before I buy the rods though, I have to practice making a screen from "zig zag" bent wires.  I was blinded by curiosity enough to try out making "ring mail" once upon a time.  surely this can't be as tedious as that was.  If I can't make that work, then its back to the drawing board.

From this layman's point of view, EXACTLY:   All the stoves called rocket stoves and sold on Ebay, for example, throw all but a little of their heat out the top and into the air, along with the poisonous gases we don't want in our houses and shops.  The average wood stove sold to heat a house seems only a bit better. We want heating equipment that grabs every morsel of heat otherwise lost out the chimney and, CERTAINLY, little to no smoke.

Satamax Antone wrote:The fumivore, not a rocket!

using the term Rocket on a wood stove regardless how fancy looking or professional it may be constructed dilutes the definition of our goals and our goal at permies is to be efficient, earth friendly, sustainable and you can not do all that by not burning almost all the fuel and gasses. Permies members seek efficient and environmentally built Rocket stoves. Allowing unconsumed fuel escape out the exhaust defeats those desires and goals.

http://donkey32.proboards.com/post/20514/thread

Somewhere, way back when, an article claimed soapstone was superior to common fire brick, because it could take around 1,000 degrees more heat.  

Also in the article, it talked about converting common talc to soapstone by mixing it with waterglass, then cooking it.

When my son was building his forge, I noted the material it used incorporated waterglass in the manufacturing process.  Here is a tidbit about waterglass some might find interesting:

https://www.youtube.com/watch?v=IXQM8nheRv0

It would be interesting to learn how adding waterglass to cob and other mixes affected their reliability for burn chamber insulation longevity.