6.5" J-feed masonry downdraft with bell/flue-bench

[patamos]

Hi All,

For those who have been following this build and questions on other threads, i have gotten around to finishing the light clay house, and the rocket mass heater along with it.

The latest photos (below) are of the heater drying after finishing plastering.  The first photo shows where the residual heat was from a last firing two days earlier.  The next shows drying after about 3 hours of firing.  

s895.photobucket.com/user/patamos/library/?sort=3&page=1

I know a few people stated that bell chambers are much more effective at absorbing and distributing heat than flue runs.  And i think the slower drying to the left of the second pic proves the point.  This is where the flue run and return are located.
Also, the relative constriction that the beginning of flue the run creates (just to the right of the big clean out) possibly slows the distribution of heat away from the downdraft to bench transition on the right.
Ironic that i put extra energy into installing the funky shaped flue run to the end of the bench to try to ensure enough heat would get down there.  This also made for a less optimal clean out strategy...
Oh well, live and learn.  The clients are super happy, and they appreciate the variations in heat for various warming purposes.

The griddle works a charm.  1/2" mild steel.  sits loose on top of two 1/8" layers of ceramic felt.  No warping yet.

The fiberglass mesh embedded in (old finish plaster) mortar also works super well.  That down draft wall has now taken at least 20 days of 12+ straight hours of full tilt firing.  It has often been too hot to touch for more than a split second.  The only sign of cracking was where i didn't (at first) put the mesh.  A few more strips of mesh and the problem was solved.  Time will tell how the few old red fire bricks in the wall will hold up...  

All in all, i'm sold on bench bells with bypass flues.  And after seeing how this downdraft chamber lay up of brick, then cob (left to dry and shrink), then mortar with mesh performs... i think it could satisfy expectations/intentions of a 'double skin' .

well being

pat

 

[matthewwalker]

Pat, it looks so damn good man. You killed it! I have a question or two, if you don't mind.

The oil, what did you use and how do you feel about it's performance on the heater? I've been wary of linseed oil on the heaters thinking it may stay soft with the heat cycles and stain stuff like pants. Any concern over that? I imagine it's fine and I'm just showing my inexperience with that material.

The 1/2" mild steel. Did you go to a machine shop for that, or find it in scrap, or..? Did you cut it to size yourself? How?

Okay, sorry for grilling you there. Love the heater man, and yeah, that bypass feature is great isn't it?

Oh, care to give your impression of the window? Install tech and overall impression on location and performance. I know I love 'em, and one thing about the configuration of forcing the flame against the glass is I've found they throw an amazing amount of heat down low. Could be dangerous if you weren't planning for it, but I think it's a great feature, that heat output way low in the space.

[patamos]

Thanks Matt,

Ya i wasn't sure about the oil and heat dynamic. Did a fair bit of researching around. I think Kari pointed out that linseed is stable up to 200c. So that seemed viable enough. I have used raw and boiled (aka chemical dryers) on many a non-heated bench and floor, and noticed there that once it polymerizes and hardens off there is no leaching into fabric etc... The clients oiled this after i left with a gallon+ of boiled linseed, which was all they could find at the local building supply stores. They had to do it quick before the dogs took ownership

We'll have to see how this oiling holds up. If anything goes sideways i will be sure to let y'all know.

The 1/2" mild steel plate was cut and cleaned up by a metal worker friend. I gave him a cardboard template and he picked away at it. Interestingly, for the interim time before the plate was ready i had a 1/8" sheet of stainless steel on there. I would have thought the thinner metal would be putting out more heat, but the opposite was true. Does stainless have a lower specific heat capacity and conduct less? Maybe the 1/2" absorbs more and thereby has more to release/conduct on contact.

Ya the bypass is such a valuable feature. Seems a very good way to boost (relax about) the ISA without having to worry about cold starts.

For the window i cast an arch with legs. I think there are some pictures of it further back in the photobucket series. I opted to set it 2" outboard of the inside wall surface of the burn tunnel, and 1" or so away from the nearest corner of the feed tube. The idea was to reduce risk of mechanical damage from wood getting rammed in there (eg kids playing with it...). The window does build up a thin coat of light grey soot that is easily wiped off during clean out. I think this has something to do with the set back from inside surface.

The window does throw out a lot of heat. So it is serving as good lower leg warmer and wet wood dryer. I was hoping that this and the relatively thin front wall of the bench would radiate a lot of low heat horizontally. I think it is working well.

I'm looking forward to building a batch box with a larger window such as the ones you place in your stoves.


well being

pat

[matthewwalker]

Thanks Pat, I appreciate the follow up greatly. Yeah, stainless has pretty bad emissivity numbers, although you can bring it up considerably by sandblasting or otherwise roughing it up. Here's a good quote from wikipedia regarding a great thing to remember about emissivity.

When dealing with non-black surfaces, the deviations from ideal black body behavior are determined by both the geometrical structure and the chemical composition, and follow Kirchhoff's law of thermal radiation: emissivity equals absorptivity (for an object in thermal equilibrium), so that an object that does not absorb all incident light will also emit less radiation than an ideal black body.

Check out the chart here. 1 is black body, or the best at emitting radiant energy, and the lower the number, the worse it is. Check out copper for a terrible radiator material, until it becomes oxidized. Also, look at how adding lampblack raises it. There is a reason box stoves are usually flat black.

www.infraredtraininginstitute.com/emissivity-of-various-materials/

[Robert]

wow. super nice. patamos great build.
i have a question. if you could please describe how you placed the glass in your stove? and please use a very simple english since i have problems with understanding technical english:) and with my next project my client wants a glass. i was thinking making some metal frame, but from your description i understand i can just use a ceramic wool and build with cob around?

[patamos]

Thanks Matt, thanks Robert,

I'll get back into connecting down the road here. Feels like i could use a few more days to get my ducks in a row back home... shaping up for the next build/chapter

well being

pat

[klorinth]

I'm with Robert on this one, I would like to hear a bit more about how you installed the window in this project. Placement looks good for that room. Would it be possible to change the location depending on the orientation of the stove in the room?

[Robert]

Apr 22, 2014 21:55:11 GMT -8 patamos said:

Thanks Matt, thanks Robert,

I'll get back into connecting down the road here. Feels like i could use a few more days to get my ducks in a row back home... shaping up for the next build/chapter

well being

pat

Hey Pat.

Can you please write about this window issue please...

[patamos]

Hi Robert

Apologies for taking so long.  Meant to post this yesterday, but had trouble with our old camera.
Unfortunately i did not take many pictures during that stage of the build, but i did manage to find most of the wooden pieces i used to make the mould for the arch over the window

s895.photobucket.com/user/patamos/media/arch_zpsa1b670bd.jpg.html

I remember making the burn tunnel 7" tall by combining 4.5" bricks with half cuts at 2.5".

So the beam is 4.5" tall x 2.5" thick x 22" long, and has a ledge built into it along the top which carries the bricks that span across the top of the burn tunnel.  The taller side of the ledge faces outward towards the room.  I included this extra height to the beam for added strength across the open span of the arch.

I do not recall the dimensions of the ledge.  That is the one piece of moulding that i could not find.  But i know i was going for as much window height as i could without compromising the strength of the beam.  At the highest point of the arch there is probably 1" of beam thickness to the ledge.  And 3" of thickness across the outer face.  

I set the beam flush with the rest of the burn tunnel bricks so that the outside surface was smooth.  I then cut 1/8" inch ceramic felt to mimic the shape of the arch opening and side walls, with enough material to back the glass which was about 1.5" taller than the arch and 1.5" wider each side.
I also ensured that the felt could wrap around the edges of the glass.
Across the bottom i made sure the base was flat, then placed a 3/4" strip of felt to eliminate point loads.  

I placed the felt onto the burn tunnel with some clay slip.  Then placed the glass and folded over the felt.  Then added more strips of felt to the outside of the glass that was to be covered.  I then added cob with perlite in it around the edges to hold the glass in place.  I added the perlite to keep the overhang light and stiff

The idea was to have the glass fully floating in the felt, so that any expansion contraction in the body of the heater would be buffered.

ONce i knew what the final shape of the window was, i cut away any bits of felt that were overhanging.

Hope that makes sense.


BTW.  Any time i go to upload a pic into this or other new threads i get the message that i have exceeded the size limit.  Could this be due to too many/large pics in earlier threads? 



 

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Regarding the stainless, besides not emitting much heat with its' reflective nature it also has very poor thermal conductivity which is why cast iron and copper pots tend to have much more uniform interior temperatures than stainless for cooking. I run a small machine shop and when machining SS it's easy to burn the tip off your tool and welding doesn't require as much heat input (except that the melting temps are high) compared to many other metals. With stainless you find the heat really builds up near the source, unlike aluminum which quickly wicks it to every corner. Steel and cast iron are someplace in between.