Vortex Stove

[Vortex]

The 10mm was fine for the first few months then suddenly twisted lifting the front corner up about 30mm. That's why I cut a square out the top and made it into an expansion plate. I think the thicker the metal the better, but without an expansion plate you'll get twisting because you're heating one end more than the other.

If you make an expansion plate where the top is going to get the hottest, your giving it some room to stretch without causing the whole thing to twist. Get a bunch of those really thin stainless cutting disks and use them to cut out a big square or rectangle above the hottest area (I think I did 300mm X 360mm), then cut a 5mm thick piece of sheet steel about 10mm bigger and weld that on the top, so the piece you cut out will sit nicely in the hole it came out of. Then you'll have no problems.

[Vortex]

I've been building a couple more Vortex stoves. I Improved the door design so it's easier to make and the window's are a bit bigger. If anyone's interested in the details I'll post the pics of the various stages.

[dkeav]

Well I for one would like to see them. The doors look great for what you can do with some flat stock and angle iron. Do they have an airwash I'm not seeing or just have to wait for the fire to clean them?

[Vortex]

No Airwash as the glass doesn't soot up. You can get a bit if the wood is too close to the glass, but even that burns off as soon as the stove gets up to temperature.

The new door design is all welded from the back so no one has to look at my ugly welding

The door Frame is made from 40mm x 6mm angle (wanted 5mm but couldn't get it). Door is made from 30mm x 5mm angle. Note how the corners are cut, this is the easiest way I found to do them. If you grind down the two facing edges that you're going to weld so it looks like this: \/ then you can weld into the top of the V and grind it flat afterwards. That way there's no weld sticking up to get in the way of anything.



The primary air inlet. Backplate is cut out of a piece of 40mm X 3mm box section.





A mock up of all the pieces cleaned up ready for the welding. (Threaded part of the hinges gets cut off before welding). 20mm x 5mm steel bar gets welded onto the front of the door with a 10mm overlap so the glass can sit behind it flush with the door.



There's the option of having a couple of horizontal bars behind the glass to protect it from falling logs, this customer didn't want them, they get welded to the vertical bars that hold the glass in place.

[dkeav]

Very nice, thanks for posting in detail.

[patamos]

Thanks for sharing all this Trev.

I'm T-ing up a couple of more builds and these details help greatly

An update on the vortex i built into the niche. My client/friend mentioned that the downdraft wall of bricks and cob was getting a little too hot to touch when fired in longer burn mode. So i applied a layer of 1/4" ceramic felt to the inside wall with sodium silicate as the binder. This solved the issue nicely.

Also, the bench was getting hotter on that end, so i accessed the column/baffle via the first bell clean-out and adjusted it to allow the gasses an easier route to the far end of the bell. This also makes for an easier route to the exit port. All in all these adjustments have evened out the heat in the bench and offered more to the backrest bell.

On another note: I have noticed that since closing up the 1/4" gap above the door frame the 'slow burn mode' with primary air closed has become less efficient. Leaving chunks of unburned wood when a biggish log was placed on the fire towards the end of the burn.
This coincides with a conversation i have been having of late with Alex Chernov, who opines that a hot fast burn is always the way to go; citing that clients will often not follow instructions if you give them options. The more i think about it the more i see his point. People around here are so conditioned to having a fire going steady that they will gravitate towards the slow burn as default.

I am still hopeful that we will find some new alchemy of throat design and air flow that will achieve very efficient slower burn. But in the mean time i am feeling like i had best ensure there is always enough secondary air running to make sure it doesn't operate as just another smoker.

thoughts for now

[Vortex]

Hi Pat, Glad to hear you got it working nice. Thanks for the feedback.
I think I would have originally gone for a slightly longer route from the firebox to the bench - probably down the right-hand side and under the ashbox - That would have shed a bit more heat before it got to the bench. Glad you found a solution anyway.

If it's making charcoal after filling the gap above the door frame, then I suspect your door is too good a fit.
The door should have at least a 2mm gap between it and the door frame - this is the secondary air - feeding hot air in just in front of the throat. Pure alchemy When I make the door and frame I place the door into the frame with cardboard in between all 4 edges, and then weld the hinges on, so that it maintains the equal air gap all around.

Only other way it will make charcoal is if it's being closed down too early, before the stove has stored enough heat in the mass of the firebox to maintain the slow burn. It needs to be really hot. Most peoples experience comes from metal-box stoves, and so they think it's burning to fast, and close it down to early and to much. Also you can't add wood to the fire during the long slow burn. Opening the firebox to cool air and adding cold logs will cause it to crash and smoulder.

Something that seems to help get the fire into the long slow burn, is to place the wood all around the inside edges of the firebox, with the kindling in the middle. This seems to act like insulation between the fire and the firebrick until the stove is up to temperature to gasify.

[patamos]

"Also you can't add wood to the fire during the long slow burn. Opening the firebox to cool air and adding cold logs will cause it to crash and smoulder."

I think this is what happened.  If such is the case and people do want to reload… then i suppose it makes sense to open the primary air back up to fast burn for a while.

I think i have close to 2mm between door and frame, but the ash box door is rather snug, as is the rest of the perimeter.

regarding downdraft route, yes i agree, but man, there was so little room to work with in that 6' of space, and i was quite determined to get at least a two person bench in there too.  Maybe a downdraft at the rear could have done well.  
living and learning...





 
 

[Vortex]

In that case down the back and under the ashbox would probably have been the best route. I wasn't criticising your design by the way, just the way I tend to work is to try and spread the heat around the mass as much as possible in a small space.

Yes, if you want to re-fuel during the long slow burn you have to open up the primary air and run it fast and hot again until the firebox and new fuel is all back up to gasifying temperature.

Type of wood you're burning also affects the 'long slow burn' as well. If it crappy pine for instance, then it wont have enough potential heat in it to heat up the mass and still have anything left to gasify. I find hardwoods work the best for it.

I haven't tried this yet, but I suspect a firebox of the same mass as these, but well insulated would make for an easier 'long slow burn', as it would have enough mass to store the heat for gasifying but not have the heat being leached away from it constantly into the mass around it.

[patamos]

Hi Trev,

No offense taken. I see the value in spreading the heat around the mass. Yes i think rearward downdraft is a great idea, especially in an open layout.
In this case, i was wary of sending too much heat towards the rear combustible wall… in order to help the building inspector feel safe about it.

All in all i think this layout has the fire chamber well insulated on back and sides. The issue is more one of reloading in slow burn mode. Almost all softwood, hemlock, fir, alder, poplar. A bit of maple though.

Also, this is yet another home in a damp valley with little winter sunlight. So many homes around here are surrounded by tall trees and humidity is 100% for weeks at a stretch. Given this climate, i feel inclined to build in (whenever possible) a wood drying niche. Most likely in an arch under the ash pit. This will raise the griddle height 1' or so which may not be a bonus, but it also moves the deck height further away from toddler's curious hands. pros and cons...

[stoker]

Feb 14, 2016 15:50:58 GMT -8 Vortex said:

Only other way it will make charcoal is if it's being closed down too early, before the stove has stored enough heat in the mass of the firebox to maintain the slow burn. It needs to be really hot. Most peoples experience comes from metal-box stoves, and so they think it's burning to fast, and close it down to early and to much. Also you can't add wood to the fire during the long slow burn. Opening the firebox to cool air and adding cold logs will cause it to crash and smoulder.

That's one of the nice things about the classic J rocket mass heater: it's meant to burn fast, and there are no controls so people are less likely to choke its air supply, which would produce smoke and tar (and charcoal).

In the stoves being discussed in this thread, the actual fireplace part is essentially the same kind of shape as in the traditional metal box stove with a baffle:
A box with a door at the front
Air entering through the door
That chamber has an exit (throat) just above the door.
From that throat the flow goes toward the back of the stove, under a metal surface for cooking/heating.

E.g.
jotul.com/int/products/wood-stoves/jotul-f-602
See the "exploded view" PDF. (A refinement there: the baffle is hollow and feeds in hot secondary air through two rows of little holes where it mixes with the gases under the baffle and in the throat.)

Those metal stoves can run quite efficiently if they're used right, but if they are operated wrongly then they'll make smoke, tar and charcoal. So it's not surprising we see the same with these stoves made from ceramics, though these have the advantage of being less conductive than the iron, with the fire losing less heat through the sides and back.

[stoker]

Have any of you experimented much with difference shapes and sizes for the batchbox and throat?

What happens if you halve the width and leave everything else the same? That could be easy to try out by just stacking some bricks in the firebox temporarily.

[Vortex]

Those Jøtul's were great little burners, had one for a few years. Only thing I disliked about it was the primary air hole was intentionally choked right down so you couldn't get a good roar going. I guess it was designed that way in case it was forgotten about and went into meltdown. Not a problem on a masonry stove though.

Haven't experimented with a narrow version. I think I see where you're going with it though. Would probably be best to line the box with ceramic felt and then stack firebricks on their sides on both sides. That would leave just over a 4 inch wide box, 16 inches deep. Might give it a try sometime.

[patamos]

I think another difference with these vortex fire chambers from a metal box stove is their ability to accumulate a lot of heat in the mass of the chamber. When this is combined with reduction of excess air the gassification process is more ideal. Kind of like a vaporizer…

Also, this being a masonry heater, the combustion is intentionally decoupled from the heat distribution by the flywheel effect of the whole heater's body. Thus there is no danger of smouldering so long as we treat it right.

I checked in with Aukje about the unburnt wood. She said she reloaded with a largish log, then forgot to make sure it had taken before she went out to run some errands. Still, to my mind, worth opening the primary back up for at least a few minutes no matter what after reloading...

[pinhead]

Feb 15, 2016 15:08:27 GMT -8 stoker said:

Have any of you experimented much with difference shapes and sizes for the batchbox and throat?

What happens if you halve the width and leave everything else the same? That could be easy to try out by just stacking some bricks in the firebox temporarily.

EDIT: Nevermind; I was in the wrong thread and thought you were referring to Peterberg's Batch Box.