My take on the DIY evaporator thing: Pellet fueled rocket stove

[adatesman]

I've been playing with the DIY evaporator thing a couple years now, scaling up from pots on turkey fryers to steam trays grafted onto a charcoal BBQ to a purpose-made insulated tub for steam trays fired by a wood pellet fueled, natural draft rocket stove.

In a nutshell, this year's evaporator is a sheet metal tub that holds 5 restaurant steam trays and is lined with 2.5" of 2300 deg F rated ceramic insulation. The first three trays are directly fired by the attached rocket stove (well, direct once the flames pass by the horizontal diverter shown in the pic below...), and the last two are set up as preheaters to scavenge any heat remaining in the exhaust stream in an attempt to boost efficiency.

The rocket stove is of similar construction, with 2.5" of high-temp ceramic insulation wrapped in sheet metal. The pellets self-feed to a stainless steel burn screen and the draft is natural (no forced air/fans).

While running it consumes just over 20 pounds of pellets per hour (~170,000 BTU/hr) and boils off ~8.5-9.0 gallons per hour on an effective 6 sq-ft of pan area. The first 3 pans will reach a very hard boil quickly, and the 4th pan will reach boiling just as the sap in the first three is ready to be moved pan to pan.

Overall I'm quite pleased with it, so figured I'd share. :)

The tub:
http://shariconglobal.com/misc/maple/tub.jpg

The first three trays are direct fired, and baffles before the last two to create turbulent flow to scrub more heat out of the exhaust:

EDIT- there's one more baffle at the end, which was left out so the exhaust port could be seen.

http://shariconglobal.com/misc/maple/baffles.jpg

[adatesman]

The rocket burner:
http://shariconglobal.com/misc/maple/rocket.jpg

And the boil in process:
http://shariconglobal.com/misc/maple/boil.jpg

[1ruralmailman]

verry interesting,thanks for sharing.

[sams64]

It looks like you have the flue gasses running through passages at the bottom of your arch. I bet if you out in a ramp and moved up your exhaust outlet you would get those back pans boiling pretty hard too.

Sam

[adatesman]

Thanks, 1ruralmailman.

Sams64- I tried the ramp idea last year with 4 pans in 2x2 configuration, and could only get a low boil on 2 pans. Admittedly the rocket burner for that was much smaller (20 sq-in cross-sectional area vs 49 sq-in this year), but it also had much, much higher exhaust temps which means much lower efficiency. Even though I more than doubled the heat output this year, converting to a linear setup dropped the exhaust temps by half compared to last year, so my guess is that the baffles worked well to slow and mix the exhaust stream so that more heat could be extracted. From the look of it I could have had a 6th or 7th pan (exhaust was over 500 deg F), but space constraints limited it to 5 pans. Over the summer I'll likely play around with reconfiguring the baffles to see if I can tweak it a bit more efficiency from it. The 4th pan would come up to a decent boil, but not before the first three pans needed to be replentished. Heck, while finishing up I even had the 5th pan boiling since I was no longer adding cold sap to it every 15 minutes.

Anyway, the chambers and baffles thing is what I gleaned from spending the past year reading everything I could find regarding rocket burner based cook stoves and water heaters.... if you have a strong draft and a straight exhaust path the heat will escape right up the chimney right quick rather than swirl and stall, thereby providing time for heat to pass into the pan. I suspect I could remove the first set of baffles and have the first 4 pans direct fired, but didn't have time to try that out before I had 100 gallons of sap sitting for longer than I'd like.

BTW, the insulation is 1" of rigid 2300 deg F ceramic board backed with 1.5" of 2300 deg F blanket. Both were sourced relatively cheaply as industrial surplus on Ebay, and the end result is overkill for the evaporator tub. That said it's absolutely needed in the rocket elbow... I used steel for the core of last year's build, and it burned a softball sized hole through 1/8" of steel in under 10 hours of burn time. The ceramic board/blanket held up just fine and kept the external temps of the burner below 500 deg F (mind you, it was over 2000 deg F inside the rocket elbow). Inside the tub it was definitely over 1500 deg F, but the exterior metal didn't get above 130. Did not expect it to insulate that well.

[Dennis H.]

Very cool concept. I love it.

[Toprecycler]

And I was just talking with my partner about next years stove. Mentioned a pellet burner and he thought it might not get hot enough. I guess this thread solves that issue. It just has to be done right. Thanks for sharing.

[adatesman]

....he thought it might not get hot enough.

As a point of reference, I made the rocket burner in two parts.... The insulated elbow and the pellet hopper/burn screen/ash pit. The front of the ash pit (directly under the burn screen) melted within the first 45 minutes of running it (22ga sheet steel), and in theory that's one of the colder parts of the rocket burner. Will get pics tomorrow while tearing everything down for the season. As a point of reference, for a bit I was using a piece of 1/2" x 1/2" x 3/16" angle iron to support the stainless burn screen. Within 5 minutes of inserting it into the firebox the end would be well into steel forging temps and around a yellow-hot.

http://www.stormthecastle.com/blacksmithing/images/blacksmithing-color-chart.jpg

The key, from what I can tell after a building a second one, is to find a way to convert the insanely high temperature, high CFM output of the rocket stove into heat transfer into the pans by way of separate heating chambers and turbulent flow. Lots of good reading on aprovecho.org and related sites concerning rocket based cook stoves and water heaters.

[beaglebriar]

Looks like a neat set up but I really don't see the benefit. I can hit all the stats you mentioned on my 2x3 and probably not burn much over 20 lbs of wood per hour. If you hit double the gph I could see going to all the trouble of building this but to each their own. I assume it was a fun project but looks like a headache.

[DaveB]

Looks like a neat set up but I really don't see the benefit. I can hit all the stats you mentioned on my 2x3 and probably not burn much over 20 lbs of wood per hour. If you hit double the gph I could see going to all the trouble of building this but to each their own. I assume it was a fun project but looks like a headache.

There's nothing wrong with experimenting and trying something. How can you improve if you don't try something new?

I wonder how that design might work on a traditional arch if you replaced the door with the burner unit and used forced air? I know there are some commercial pellet evaporators and the idea interests me as there are pellet mills and it would be relatively easy to make your own pellets or buy them. I like the idea of a constant feed too.

[CampHamp]

Awesome work and post! If you put siphons between your pans, you'll be able to automate your boiling considerably. There's a specific design with end caps that prevent air bubbles from breaking the siphon. If you can't find info on it and are interested, I can find my old set and send some pics...

[adatesman]

Looks like a neat set up but I really don't see the benefit. I can hit all the stats you mentioned on my 2x3 and probably not burn much over 20 lbs of wood per hour. If you hit double the gph I could see going to all the trouble of building this but to each their own. I assume it was a fun project but looks like a headache.

Well, true. But the end result I'm working towards is being able to simply load a couple hundred pounds of pellets into a hopper, fill a tank with sap, light it and walk away for a couple hours. Can't really do that with cordwood. :)

[wildwood]

Awesome work and post! If you put siphons between your pans, you'll be able to automate your boiling considerably. There's a specific design with end caps that prevent air bubbles from breaking the siphon. If you can't find info on it and are interested, I can find my old set and send some pics...

Along with that, it sounds like this setup really needs a preheater based on the comments about having a better boil once you were able to stop adding cold sap to the pans. Even the smallest evaporators benefit greatly from a preheater, nothing stops a boil like a bunch of 40*F sap

Keep up the good work, experimentation is wonderful!

[pennslytucky]

love this stuff. nice fab work. the more i fire my block arch, the more i think a large rocket style front with 4' slabs in it auto-feeding vertically would be a great evap. ill build one this summer for next year and see how it works. our traditional arches are very close to a rocket stove anyway, just doing the work before the flue instead of after

[adatesman]

Pulled the burner a bit ago, and here's how it looks after ~20 hours of use. The detached piece is what's left of the front vertical part of the ash box, and is located where it was originally standing. It burned through/fell off in the first hour of use. The rest of the ash box simply burned up and mixed in with the ash from the pellets. I expected this to happen, hence designing the burner to be a separate, replaceable piece.

http://shariconglobal.com/misc/maple/burned.jpg

[beaglebriar]

Looks like you're going to have to build it out of ceramic refractory. Not much else will handle the heat. I'd cast something similar to a nozzle in a gasifier. Won't be cheap but it will hold up.