Feedback on my RMH concept wanted

[PNW Dave]

Hi all, I have spent quite a while reading here, at Permies, "The Book" 3rd Ed. etc. and I'm finally collecting my thoughts and coming up with a design concept for my backyard RBH&C (Rocket Bell Heater & Cooker). I'm looking for some general opinions on my proposed design. Many thanks to everyone who has so freely shared their ideas and real world experiences.

This outdoor RBH&C is mainly for entertaining. Cooking, warming butts, looking cool during outdoor movie night, a conversation piece. It is not for maintaining comfortable living temps in a home during the winter. 


The bench-

I intend to use the half barrel bell system for the bench flue (similar to what Matthew Walker did in this thread- donkey32.proboards.com/thread/596/temporary-fire-pit-outdoor-kitchen ). Instead of an all cob mass bench, I'll build a wood bench similar to an Adirondack chair over the barrels, with a good gap between the barrels and wood (will probably need to sink the barrels into the ground a bit to allow the seat to be at the desired low-height). The bench sides will be enclosed all around the barrels with air able to rise up between gaps in the seat boards. The back rest is also part of the airspace within the bench, acting as a secondary bell. Fresh air into the bench will come in from slots down by where your heels would be when sitting on the bench.



Horizontal orange lines are indicating the single air space within the bench, green line is ground level. Enclosing side walls not shown.




Core and Riser:

The core will be fire brick splits held in place with a fireclay and perlite mix, either hand packed & sculpted or rammed & vibrated in a form surrounding the bricks.






The heat riser will be a fireclay and perlite mix, cast and vibrated in about 10" tall rings, which will be fired in my kiln. Then stacked with fireclay slip as mortar to form the riser. This photo shows the form tubes in place of the riser, which will be cut to size for casting the riser sections. You can ignore the black and the blue barrels.




Dimensions:

Feed tube- Width= 7 1/2", Length= 8", Height= 13"

Burn Tunnel- W= 3", L= 7", H= 9" (or 4 1/2")

Heat Riser (round)- Height= 40" (), ID= 8 3/4", OD= 11 1/4" thickness= 1 1/2"

Barrel- H= 28", ID= 18", Top Gap= 2 1/2", Gap from Riser= 3 1/4"

Manifold- To be determined...

Bench flue- 4 Half barrels, single large bell design

Small barrel x2 (one for down draft & one for core surround)- H= 28" x Dia= 18”

Large barrel x3 (two for half barrel bell & one for exhaust warmer)- H= 38" x Dia= 22"


Questions:
1) Is the vortex induced by the off center entrance to the riser better/worse than a centered "rams horn"?
2) Will the bell get too hot for the wood bench? Will it need a little mass covering the barrels?
3) Is the four half-barrels bell way too large?
4) I haven't calculated any dimensions for constant CSA etc, any help with that?

Thanks for any input on this concept!


Other ideas to be detailed later:
- The top of the barrel being able to accommodate various attachments besides a simple cooktop. Like a pizza oven, charcoal retort or a startup bypass.
- Final exhaust re-warmer if exit temps are too low with this concept
- Peltier powered LED lights inside the bench for a "warm seat indicator" and cool ambient light
- Fire viewing windows in the core
- Preheated primary air and/or secondary air
- Inline bell vs dead end bell
- Bell intake and exit "shelves"
- more stuff
-

[PNW Dave]

And just for fun, some other pics.


The RBH&C will be enjoyed while viewing this screen-



Which is down in this yard-



To cook things like steak and veggies, stuffed jalapeños and mushrooms on cast iron-




Keeping us warm on nights like this-

[satamax]

Well, the concept could work.

Tho, your core seems a bit unconventional to me. Having a cyclonic action or turbulator can be advantageous or not. But the narow burn tunel, if it's not a batch rocket is rather bad imho. A little narowing is good, but that much, i'm dubious.

[matthewwalker]

David, I love it! As Max said, your core layout is unconventional and will most likely not perform as well as it could. If you are going with a vertical feed J I would suggest you start with standard dimensions. You are awfully close anyway, just go with an 8" J with square sections 7.25" or thereabouts. Your slightly larger than 8" riser dia. will only help the thing draw. Inducing vortex is unnecessary, and usually hurts more than it helps in my experience. We don't need to help the toilet or the sink drain form a vortex. Same deal.

Leaving the barrels naked will really put a load on the stove. In other words, they will shed a ton of heat and depending on how you run the exhaust it may or may not draft very well. However, that's an aspect of the build you can tweak as you go. I do think the naked barrels in the ventilated surround would provide some nice warm seating. Better than sitting on mass, I don't know, but you could try sections of each style and see what you prefer.

Man, it looks like it's gonna be great. Where are you? Looks like here. If you are close enough, let me know if you want some help some day. I'm running out of room for new builds here.

[peterberg]

Jul 20, 2014 1:59:48 GMT -8 PNW Dave said:

1) Is the vortex induced by the off center entrance to the riser better/worse than a centered "rams horn"?

Usually, there's no ram's horn in a J-tube rocket heater. This cyclone type riser entrance will almost certainly slow the air speed down which is a bad thing.
Your tunnel seems to be too narrow and the feed too wide. Start with a consequent system size, the same cross sectional area throughout. And not even that, also the shape matters. Best to keep the shape the same as well.

[PNW Dave]

Thanks for the replies guys.

Matt, I'm in Edmonds Washington. I would definitely welcome the opportunity to collaborate with you in person on this concept and build.

I had a suspicion that the vortex would be more of a hindrance than a benefit. Part of the reason I laid it out that way was for the placement of a viewing window. I think the curling flames whipping around would be especially impressive to guests that are not familiar with any type of RMH. That and "vortex" "cyclone" and "turbulator" just plain sound cool. No biggie, the vortex idea can be scrapped and still have an awesome sight through the viewing window.

In the event that exhaust temps are too low, I envision a larger barrel over the downdraft barrel. I know it will steal a lot of heat from the system, but I figure this will increase the temperature difference, improving downdraft after the riser and at the same time improve draft out the final exit. Hopefully ensuring there is sufficient pull through the burn tunnel. The downdraft barrel may need a little insulation around it to not lose too much heat..

Orange is the path of gasses, purple barrel increases exit temp, green area is where barrels will be mated together so it can still be used as a cooktop.



About the unconventional feed and burn tunnel. I will definitely be setting up and playing with different configurations before building the wood bench. Nothing permanent until it functions correctly (or ever!). To your points, Max and Peter, the idea is to make it much more like a batch box than a J-tube (a J-Batch?). A cover over the feed tube with adjustable air intake, burning cord wood that is standing on end, secondary air enters through a channel under the batch box and feeds into bottom of the burn tunnel (like a P-channel but from underneath). It will be fun to play with during test setup and burns. If it doesn't work, I can easily switch to either a standard J-tube or batch box dimensions. Always open to suggestions.

As a primary air warmer, a small barrel laid on its side will be cut to fit as a core surround. There will be a gap between the top of the feed tube and the opening in the surround barrel. Once a good fire is established, a smaller 5gal barrel will sit over the feed tube opening, then 100% of the primary air (except air from adjustable baffle on top of the cover) will pass through the core surround to be warmed prior to entering the feed tube. These are the blue and black barrels i said to ignore in the last pic of the original post.






Alternate, more conventional, J-tube design. Symmetrical 8” feed and riser, widened burn tunnel. Would the barrel be the right size for an 8” system? I've never got a test build constructed to the point of setting the barrel "when the vertical chimney MUST go up" (except that one time when I proved the point). I've just made J-tubes and burnt scrap wood for fun.





and of course I have several other unconventional ideas, to be brought up as the design progresses. thanks for your input so far.

[PNW Dave]

Further details on my ideas for second barrel. Raising exit temps, startup bypass, cooktop and pizza oven.

The holes in the top of the barrels will be somewhere near the system size of 8", allowing various accessory inserts to perform as desired. The top surface of the green barrel will be covered with fired fireclay panels for a nice, solid, flat top that will accurately fit the inserts.

Green shapes:

1) Bell bypass for startup. A flat disc of fired fire clay that blocks gasses from exiting the hole in the outer (green) barrel. Gases may travel up through the hole at the top of the inner barrel and get to take a shortcut right out the exit chimney. Some gasses may also go down into the bench before reaching the exit, if they so desire.

2) Solid plug of fired fire clay and perlite for normal bench-heating operation. All gasses must go straight to the bench and cool off in the bell before exiting. Mandatory.

3a) Short hollow fire clay cylinder for Cook Top operation (see #4). All gasses must go through the bench before exiting but some must first heat my Griswold Slant Large logo EPU HR.

3b) Long hollow fire clay cylinder (not pictured) for pizza oven operation. Access to both barrels, bench and exit chimney is denied. All gasses must make me PIZZA! Custom pizza oven is simply set on top of barrel.

3c) Many other variants of the cylinder can be made to adjust the cooktop temperature by varying the access to bench and bypass. A pot skirt could be added to the long cylinder setup as well.

4) Cook Top. At first it will probably be a large Lodge cast iron skillet set upside down over the hole. Another skillet is placed on the cooktop for actually cooking food in.




How do these ideas sound to you guys?

[Dan (Upstate NY, USA)]

I am not worried about your other ideas, if they don't work you can fix them, your dimensions in the feed burn tunnel and riser seem "off" to the point of poor performance.

I'd build them to what works and then try all your experiments with the other stuff.

Its like your are trying to manipulate too many elements at once.

Have you had to live with a RMH for one heating season yet?

[PNW Dave]

Jul 21, 2014 1:00:07 GMT -8 Dan (Upstate NY, USA) said:

I am not worried about your other ideas, if they don't work you can fix them, your dimensions in the feed burn tunnel and riser seem "off" to the point of poor performance.

I'd build them to what works and then try all your experiments with the other stuff.

Its like your are trying to manipulate too many elements at once.

Have you had to live with a RMH for one heating season yet?

Hi Wolf,

Are you referring to the feed, tunnel and riser from my original post? Or are you referring to couple posts down where I mocked up the shape for an 8" system with symmetrical round feed and riser with much less narrowing at the tunnel?

I agree with your comment "trying to manipulate too many elements at once" but thats why I'm here fleshing out some ideas, and abandoning others, before actually starting the build. I'm going to take the advice everyone has given, build a conventional core and riser that is bound to work well. Then I can focus on what is actually important, tuning the performance of the bell and then work on creating the whole second barrel cook top and startup bypass.

I have not lived with an RMH for a heating season, though I'm sure a good by the book RMH would vastly reduce the amount of fuel my wood stove insert uses through the heating season. This "RBH&C" that I am working on is going to improve the seating conditions for my outdoor movie theater, not heat a home.

[PNW Dave]

Last night I did a test burn of both core configurations pictured above, with a 37" tall 8" ID triple walled metal riser. This set on top of the fire bricks made a 46" riser. Bricks just set in place, little air leaks everywhere.

As everyone here said, my whacky J-Batch feed and burn tunnel was not a good design at all. Even after the system heated up a bit and I restricted primary air in various ways, it was obviously functioning poorly. Teetering the edge of burning like a pile of wood in a hole, and merely decent rocket action. It got going for sure, with clean looking exhaust, but was like herding cats into a running shower.

The second core layout, symmetrical round 8" feed and riser with slight narrowing in the burn tunnel (even less than pictured above) took off right away. Of course the bricks were still quite hot, but without a doubt it is a far superior design. The fire "wanted" to go through the tunnel and riser. Awesome.

It was fun seeing the difference first-hand and it gives me confidence in moving forward. No need to try and do anything fancy and new with the core design, just follow the tried and true. It just plain works.

Started cutting my barrels in half, looking forward to doing some test runs with the bell in place soon.

[PNW Dave]

Burned out four half-barrels at once (though it took two burns to do evenly) a couple weeks ago. Just got around to putting some videos together. Here's this one-

[Dan (Upstate NY, USA)]

Cool keep learning...