Evaporator from small wood-burning stove - cheap!

[berkshires]

Hi Folks,

I've gotten so much help from everyone here! Thought I'd pass along how I've built a small hobby size evaporator, in case it might be helpful to someone in the future.

By the way, as you read through this, be aware that I don't know how to weld. Oh, and I'm cheap. So all of this probably cost me well under $100. Okay, so here we go...

I started off with an ancient small, cheap, sheet metal woodburning stove. It had been retired for some time, so I didn't feel bad about chopping it up.

To give you a sense of scale, it's only around 13" wide. And the top is not long enough for even one steam tray pan to fit on it.
I first tried boiling sap in a pot on top.

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You can guess how well that went (not!)

So next I cut a chunk out of the top of the stove so I could fit a steam tray pan into it. I had to fit the pan sideways to have room in front of the smokestack.
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Now I could at least start boiling sap. But I wanted to fit a second steam tray pan on the "arch". But the stovepipe was in the way. I needed the stovepipe to come out the back of the stove instead of the top.

So next I cut out most of the rest of the top. This includes the part where the stovepipe came out. I also cut a chunk out of the back of the stove, and put a new flue in (using sheet metal screws and furnace cement).

Here's the new duct/flue connector: https://www.lowes.com/pd/IMPERIAL-6-in-x-4-in-Galvanized-Steel-90-Degree-Register-Duct-Boot/3134369
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And here's how it looked after the work was complete:
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That worked fine for the rest of the season. After I got my systems dialed, I was able to boil off two gallons per hour. But there were still some big issues.

Problems:
1 - Sides of pans sticking off the sides of arch. Not only was that wasted square inches on the bottom of the pan, but the cold air under the part of the pan not over the stove wicked away heat from the sap and the pan. Not good.
2 - No grate means the fire burns inefficiently.
3 - To get enough air to the fire, I had to open the door a crack. This made the fire cooler. Especially since the air was coming in right under the front pan!
4 - The fire doesn't hit the bottom of the pans well. Unless I really worked to arrange the logs right, nearly all the heat was in the middle/back, and very little under the front pan.
5 - Needed a way to pre-warm sap.

In my next post, I'll talk about what I did to solve these problems.

[psparr]

You've got the bug!

[Haynes Forest Products]

Berkshires Nice use of available materials. I bet when its said and done you might be more efficient if you ran the deep pan only and set it down into the arch/stove and ran the sap deeper. Having a single pan that rips and roars is better than 2 pans just simmering. Now if you get a nice fit it will help. You might consider a few holes under to firebox so air can come from below so the fire goes straight up to the pans and keep the blanket of cold air out of ther arch. Now about that old hair blower and some simple duct work.......................never mind.

[barnbc76]

I always admire what people can improvise with what they have. A few years ago before I moved in my house i took an 275 gal fuel tank and a woodburning/electric stove to the junk yard, needless to say a coulple years later i wish i had not, o well never know what you are going to need and the possibilities are nearly endless with what you can build to make maple syrup.

[Zucker Lager]

Hey Birkshires:
What about adding just thin sheet metal sides to the stove that start at the bottom of the stove and run up to the sides of the pans on each side also closing off the front and back of each side so that the heat from the sides of the stove stays under the pans that are hanging over. Every little bit will help. Like Pspar said "youve got the bug" he he Don't we all. Jay

[Shaggy acres]

That looks almost exactly what I started with. I cut off the top and my father in law made me a new pan custom fit to sit 2" deep on the top. I also bent cheap flashing to run up the sides to keep in the heat. I found that pan is a big heat sync in the cold weather so the sides kept the heat on the pan.

I have since retired the stove and built a "rocket" stove base for the pan out of a metal file cabinet and fire brick. My first attempt at the rocket worked great with 6" square steel conduit 1/4" thick. The intense heat carbonized the steel and it disintegrated in the fire box. That is why it is now made of fire brick!

Most of all have fun!

[berkshires]

Where we left off, I was saying there were some serious issues.

Problems:
1 - Sides of pans sticking off the sides of arch.
2 - No grate means the fire burns inefficiently.
3 - To get enough air to the fire, I had to open the door a crack. This made the fire cooler. Especially since the air was coming in right under the front pan!
4 - The fire doesn't hit the bottom of the pans well. Unless I really worked to arrange the logs right, nearly all the heat was in the middle/back, and very little under the front pan.
5 - Needed a way to pre-warm sap.

So now on with the story. Here's what I did to address the above.

Solutions:
1 - Added "wings" on the sides of the arch to go under the sides of the pans. I cut out the side of the stove about three inches down below where the pans stick out, and added stainless steel "wings". These essentially extend the arch out to the sides of the pan. Here's a couple photos (with fire blanket as a seal).
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2 - Made a grate by chopping up fencepost, drilling holes in it, and screwing it together. Here's a pic where you can see the grate. You can also see the inside of the "wings".
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3 - Opened up space under door so there is plenty of AUF coming in under the door and getting sucked through the grate into the fire.

4 - To force fire up under the pans, I chopped down a cinderblock to the right size to create an internal "flue" for the back nine inches inside the stove. Filled the sides around it with vermiculite to keep air from leaking around. You can see what this looks like in the photo above.

Then I added a thin capping cinderblock to close off the top of the regular one:
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5 - Added warming pan "troughs" hanging from the sides.

Here's the finished arch, showing the trough on one side:
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So that's it! Any thoughts? Suggestions? Throw the whole thing out and buy a "real" arch? LOL!

I've only done a very quick test, but the fire really roars now. Can't wait to start tapping and put it through its paces!

Of course I already have ideas for what else I want to improve my "FrankenArch", but I think I'll save most for after this season is done. I expect to boil off close to 3 GPH. Not bad, starting with a stove that was about 13" by 16" on top!

[barnbc76]

How many taps do you plan, and how many potential taps? If all you are going to do is 6 then it will do well. Once you get to 10 or 15 you will likely be thinking of something bigger and better but it will still work. 10 taps x 2 gal/day = 20 gal at 3gal/hr will be just under 7hrs of boiling. I dont think i can make any other suggestions.

[berkshires]

How many taps do you plan

15 this season. I know it's going to be a stretch, and I'll have some long nights!

The next thing on my to-do list is to add drop-tubes to the back pan. That should add a good bit of capacity, but that won't happen until next season.

[barnbc76]

Yeah you will be busy, got anything else you can use? A galvanized trash can ($20 at Walmart) with a 4 gal pot ($12 walmart) will give you just under a gal/hr Ive used mine as a preheater mainly but it will boil well if you keep the fire stoked. Cut a hole in the bottom of the trash can for the pot, a hole for the smoke stack and a small hole for wood on the bottom. It works.15045

[berkshires]

Thanks for the suggestions, but I think (hope) my mods should be enough to allow me to keep up with the sap. I'm anxious to find out! It sure is tough to sit on my hands when this week looks like a killer run. But I know March will probably be better, and if I tap now, I'll kick myself later.

[berkshires]

Oh, and if anyone's curious to see more photos, you can see them here: http://mapletrader.com/maplegallery/g649-homemade-evaporator.html

[berkshires]

Finally got to test out my latest changes. Tapped on Saturday, and boiled on Sunday and Monday. All the modifications made a huge difference, and everything worked great! Over the course of the two days it averaged about 3 1/4 gals per hour. That's nearly a 60% increase in gallons per hour over my best boil last year. Sweet!

[berkshires]

I was really impressed with how well the warming troughs worked. Even though they each don't hold much, and with three gallons per hour moving through them, they were bringing 40 degree sap to between 150 to 180 degrees (I measured a bunch of times). I'm sure that really helped my boil rate!

GO

[berkshires]

Had my second boil of the season yesterday. Had more sap to process, so it was a longer one (less warm-up and cool-down time relative to boil time). This time I got nearly 3.5 gallons of sap boiled off per hour. Very sweet!

At one point I even accidentally started to make syrup in the syrup pan before I was done (I recognized the bubbles). That's never happened to me before!
I don't have a draw-off, so I just raised the level in the syrup pan. In the future I guess I could ladle some out when this happens, but I'm not sure it's worth the trouble.

At this point, unless I add more taps, I don't think I'd want the evaporator to go any faster. If it goes too fast, I'll need to wait a while to get enough sap to boil, leading to the sap potentially spoiling on warm days.

So, all things considered, it's a funny looking creature, but for its size, it sure does eat sap!

Gabe

[wmick]

I Love projects like this... Built a couple boilers to heat my house a bunch of years back... One wood fired and a used oil boiler... Lots of fun experimenting and learning about heat transfer and combustion efficiency.

I have one idea that I'll throw out there.... Expanding on your cinder block idea.... What about creating a solid barrier from the back of the stove to almost the front.... Couple inches under the pans, That would force all the heat to pass under the full pan.
It might make more use of some of the heat going straight to the chimney now??
It would be best, made out of some sort of refractory material, if possible... to maximize the combustion.
I had something similar in my wood fired boiler.... I suspended "T" shaped rails that became tracks to slide firebrick into.

[berkshires]

wmick, interesting idea. So you're saying I take my current design:

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And modify it so there's a barrier going from the cinderblock to near the front. Like this?

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The main problem I see with this idea is that a lot of the boil for the front pan comes from radiative heating from the coals. With your design, that gets blocked.

Also worth noting is that when I fire, I rake the coals to the front of the arch and place the wood such that the flames start pretty near the front of the front pan. Like this:

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So, in effect, I'm accomplishing the same thing you suggest, but without blocking radiative heating from the coals.

Gabe

[wmick]

Actually - Now that I see your sketch, it looks like your cinder block is quite close to the back pan and forcing the heat to it nicely... Photos were little deceiving to me.. Looked like there was a wide open space from the fire to the chimney...

[berkshires]

This photo might help. The photo is a little out of date, but the only difference between this and the current setup is that here you're looking at the capping cinderblock, but I have also added a layer of half-brick on top of that. So there is currently two inches between the top of the firebrick and the bottom of the back pan.

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You can see quite clearly in this photo the one thing I really wish I had done differently. I have about three inches of the original stove across the middle there. I added insulating blanket under it to try to reduce heat loss, but still, that's the hottest spot. I do wish I had set the front pan back about two inches from where I did, making that bit smaller, and making the front pan catch more of that good heat.

But at this point it would be an enormous pain in the *** to move the positions of either pan on the arch, so I'll live with the slight loss of efficiency.

Gabe

[berkshires]

Well my biggest run from 15 taps was nearly 50 gallons. Took two days to boil that down, but I planned around it, so it was fine. Another time I had 40 gallons of sap I had to boil down in a day. That was kind of a long day. But I'm hitting 3 1/3 gph on average, so that's fine. I need to fix the door, which is uninsulated, and doesn't close properly. But aside from that, I don't think I'll make any changes for next year.

Gabe

[berkshires]

Just a quick postlude to this thread. I successfully boiled on this rig until last year, when I bought a Mason 2x3. However toward the end it was pretty problematic, mostly due to the fact that the inside was uninsulated. This meant that the whole stove warped so badly that the front door didn't close properly, and eventually the pans didn't fit on top any more. But it got me through about five years! And made 13 gallons of delicious syrup. So very happy with it.

GO