8" Batch Box RMH Design/Build Looking for Feedback

I'm planning on building this system 1 week from today. Can anyone see red flags or issues I might run into based on the following images?

a couple more...

Enlarge your transition area.

Transition area = space behind the port at the bottom of the heat riser?

Max, does this area need to have a greater CSA than the riser? If so what should I shoot for?

Jesse, I believe you'll find this of interest:

http://donkey32.proboards.com/thread/337/helpful-hint

Thank You Byron,

So the place where I need more space is at the bottom of the bell before it exhausts out into the mass.

It seems like Peter mentioned that to me in passing. Duly noted!

Jesse,
Max is right, you need a larger hole in the back of the lower barrel. Take the side gap, between barrel and riser insulation canister and multiply that by the circumference of the pipe in the bench. The resulting figure should be 150% or more of the cross sectional area of the system size. When it isn't, you have the choice to enlarge the side gap by placing the riser off-center or use an adapter, round 10" to 8" or a much larger rectangle to round 8". Keep the hole in the barrel at least half the system size off the bottom of the barrel. I know, the set up in the auditorium at richsoil had only 2" beneath the exit pipe due to space constraints and I got away with it, thanks to the straight chimney among other things.

Further possible smaller niggles: the floor of the fire chamber is a small strip of horizontal the width of the port. Works better in the glowing phase in my opinion.
The back of the firebox need to be a structural part of the the rest of the firebox. Your pictures aren't clear about that.
Please use refractory castable for the top of the firebox, or ceramic glass if you like. I'm not convinced calcium silicate board will be a permanent solution.

This 8" version is a very, very powerful stove, probably over twice as much as an 8" J-tube. Are you sure you need such a dragon?

No sweat Jesse. I was just looking at that info. the other day, and currently planning a residential RMH build myself.

In case you haven't yet sized your batch-box's firebox to port to P-channel to the desired riser size, I'll pass that link along too. For a batch-box, this stuff is indeed akin to "rocket science" :->

http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions

Peter,

Thanks for looking this over.

Let's see if I can do this right.

I've switched the transition to be a 10" to 8" reducer:

As for the lid and door of the fire chamber... I was thinking about using half fire bricks and casting them in 1-2" of concrete so that the refractory brick side is facing the fire.

This system will be going in an out-building even bigger than the one at Paul's so I'd like to go with the 8" batch box.

Jesse Biggs wrote:I've switched the transition to be a 10" to 8" reducer:

Can be done this way, but is it enough? Don't forget to do the calculation to make sure there's space enough.

The lower part of the riser isn't sound construction-wise, the bricks are just stacked on top of each other. Since this is the hottest spot of the stove and the expansion of the bricks the largest this will be prone to leakage.

The firebox floor is OK, but why not make a smooth transition from the sloped sides to the floor, are the firebrick splits larger in your area?

Jesse Biggs wrote:As for the lid and door of the fire chamber... I was thinking about using half fire bricks and casting them in 1-2" of concrete so that the refractory brick side is facing the fire.

Can be done this way provided you are using refractory concrete, preferably with the same expansion specification as the bricks. Normal concrete isn't suitable because this will desintegrate at quite low temperatures and your stove top will fall apart in mid-burn. You don't want that happen, believe me.

Jesse Biggs wrote:This system will be going in an out-building even bigger than the one at Paul's so I'd like to go with the 8" batch box.

Fair enough, what's the length of the bench and what is the filling of this? Maybe I can roughly calculate what length the stove would be able to serve.

Edit:
Your riser is square inside, this will work, but it won't be a quick starter. The round riser is definitely better, octagon a close approximation. Which of the two is better I don't know yet.

Thanks again Peter.

I'll try to solve the issues you've pointed out.

Your input on bench sizing is much appreciated. I've calculated a barrel to be roughly 1.5 square meters of surface area, and the horizontal duct to be roughly half that or .75 square meters per 5' length. I was going to shoot for an overall surface area of between 9 & 10 square meters.

The bench will be filled with cob.

Jesse Biggs wrote:I was going to shoot for an overall surface area of between 9 & 10 square meters.

That's the right figure, yes. Have you thought about omitting the lower quarter of the pipe in the bench? My calculation assumed this wouldn't add much to the heat extraction since hot gases are always in the upper three quarter of the pipe.
The bench filling will be monolithic, that's a good thing.

If i may add, looking at the numbers in this MHA paper; http://heatkit.com/docs/course.PDF

It would seem that you should shoot for 16m² of external surface for the mass, and 8.6 metric ton. If that possible.

By way of an update... I decided to follow Peter's advice and fab a lid and door out of castable refractory. This way I have a material that can withstand the temps and I'm not worrying about different expansion rates between brick and concrete.

One 17 hour day later we have the engine all put together. It started drawing immediately.

Morgan Bowen came and took some video during the build... now everyone can point out any false information I've shared far and wide:

Thanks for posting the video Jesse, very illustrative.
Looking forward to the sequel.