Large Outdoor Wood Burning Maple Sap Evaporator

Hello all.

I am looking into buildings ideas/materials for a large outdoor Maple Sap evaporator, for making Maple Syrup.

The general idea is:

But I have an approximately 4'x5'x1' steel ~pan~*. I guess cinder blocks work, but they are supposed to fall apart pretty quickly if exposed to heat (and possibly even  explode if wet). I was wondering if anyone had a better idea for the overall design and if we could come up with better materials (price is a huge object).


* Half of one of these:

This thread might give you some different ideas  Bengali Pit Evaporator

There is a ton of great maple syrup information on a couple forums that you may want to visit.  Sugarbush.info and Mapletrader.com.  You can spend months looking through the designs other folks have come up with.

How many trees do you plan to tap?  And/or how much syrup do you want to make?  That would help us guide you better.

So your pan would be half of an oil tank?  I'd be somewhat worried about that.  I'm hoping it would be a clean/unused one.  But doesn't it still have paint on it?  If you can get all the paint off before you cook your food in it, that would be preferable.  

The first picture you have of the cinder block arch (an "arch" is what they call the part that holds the fire and the pan) is a good way to go for outside use.  I've used a block arch for about 6 years now.  I've never had a cinder block explode, even when they're getting wet so I suspect that's a rural myth.  They do break down due to the heat.  But I've usually gotten a full syrup season out of a set-up when it isn't protected.  If you get some sheet metal and/or fire bricks and put them on the inside between the heat and the blocks, they last a very long time.  I had 4 years on my last arch without cracking any blocks by including that protection.

The key to fast boiling is lots of surface area.  A 4'x5' pan would give you a lot of surface area but you want fire under the whole thing.  And I find that you want the hardest/hottest fire you can get.  Lots of heat goes up the chimney but a lot goes into the pan.  I aim for wood to be about wrist sized and fill the firebox every 5-7 minutes.  A big oak log on the fire really slows things down.  

With a round bottom pan it would be kinda hard to keep the fire contained under it.  And as the sap evaporates and the level drops, the fire really heats up the part of the pan that isn't covered by sap and it starts to burn.  By the end when you are down to syrup, your pan would be very shallow and probably scorching a fair bit.  In the olden days they boiled syrup in huge kettles over open fires.  You could do that and save all the money of building an arch.  You'd want to be able to kill the fire or swing the pan off the heat as the syrup finishes.

Many folks use oil tanks as the arch and cut out the top to put steam pans in.  So using the pans from your first picture to hold the sap suspended in the metal oil barrel cooker.  They make door kits for barrel stoves that can fit on the oil drum.  The downside is that you can normally fit only a couple pans in them which gives you a relatively low boil rate.

Thank you, that is a far better thread than I had found by searching before this.

But, that design would need a lot of changes to work on the scale I am thinking of, and digging in the forest would be very hard and likely end with a new well instead of a working fire pit. I have some ideas on how a rocket stove like idea might work, but I am not sure what parts of a rocket stove can be changed without breaking it. For one the fire needs to be much bigger, possibly a line instead of a circle (so you have a line 3-4' long of fire). But I do not think I have much room for a tall stack before the evaporator pan, can the stack where rocket thing happens be horizontal? And how much do I have to worry about heat distribution/get a more even heat distribution.
I plan on looking at few books on rocket stoves, so they might give me a better idea. But I am thinking that it might not be possible to convert the idea of a rocket stove to heating such a large pan.

@Mike Jay:
50-100 trees (I have only be able to round up 50 pails so far), and as much as I can (I guess I might get a liter to 4 a day, 100 trees might produce up to 50 gallons of syrup if the Internet can be trusted). I am hoping that this monster pan can do a days worth in a single batch. This is my first experience with Syrup production, but I want to do enough to make spending half the day in the woods worth while.


Yes, it is a oil tank. And it was used, but I was surprised how refined heating oil is, and due to this purity and how new the tank is it hardly even managed to stain the metal. The one thing that really struck me with the rocket stoves is the idea of not impeding the flow of air. You want the same size exhaust as air input. Which means that none of these cinder block tunnel designs have anywhere near big enough exhaust stacks. We are not actually talking about building a rocket stove here, but I think the idea of fast moving air is supposed to be universal for getting a high heat. Have you tried one of those cinder block tunnel designs but with a more open back end to increase air flow? That is a great insight about the sides. Possibly I could just adjust the fire to be more in the middle at that point, or build the arch to exclude the sides. Instead of cinder blocks I might just be able to stack some earth and possibly rocks, or I am looking into what refractory cement costs.

Ok, 50-100 trees is a goodly number.  After 6 years I'm up to 50 trees even though I could tap more.  On buckets my average yield seems to be about a quart of syrup per tree per season.  

So let's do some math...  When the trees run, you can get two gallons of sap per tree per day.  1/2 to 1 gallon is more normal but you want to be able to handle a good run without killing yourself.  So if you tap 100 trees, you could have to deal with 200 gallons of sap.  Good news is you'd make 5 gallons of syrup that day.  Bad news is that you have to boil away 195 gallons of water.  

I see boil rates of around 1.5 gallons per hour per square foot of evaporator area that is in contact with the fire.  So if your oil drum is 4' by 5' that gives 20 square feet.  It's really a curve so there's more area than that but you probably won't have it full so let's just assume 20 square feet for the kicks of it.  With a good hot fire, you'd get 30 gallons per hour of boiling.  So in a perfect world, you could boil away all that water in 6.5 hours.  

Now in the real world, you have to deal with the depth of the sap in the tank.  If it's deeper than a few inches, it reduces your boil rate.  Ideal is in the 1-2" depth (depending on your pan style).  So if you have a foot of sap in your curved bottom pan, that would likely reduce your rate.

Another element of the real world is that it takes quite a while to get a huge tank of 40 degree F sap up to boiling temps.  I'd guess at least an hour.  Then finishing the syrup adds more time.  I don't think you'd be able to boil it all the way to syrup on that curved pan.  When you get down to 5 gallons of syrup (or 2-3 gallons on a normal run) the level would maybe be an inch or two in the center but where the syrup gets shallower at the sides it would probably be burning.  So you would likely want to take the syrup out early and do the last bit of boiling on a more controllable device (turkey fryer, gas range).  Draining the tank and doing that last bit of boiling will take at least another hour or two.  Then you have some filtering and bottling time.  So I'm guessing you'd be very tired at the end of the day.

I guess what I'm saying is that you may want to start smaller your first year and work the kinks out.

Now back to your questions..  My early arches had cinder blocks on their sides as simple two barrel chimneys.  My better arch had two 6" chimneys but I found I only needed one of them.  The rule of thumb I'm most familiar with is that the cross sectional area under the pans once you're beyond the firebox should equal your chimney cross section.  So for the block arch in your first photo, once you get a couple feet into the interior of the arch the floor should come up (sand/rubble) and end up an inch or so below the bottom of the pans.  That will match the chimney cross section and force the hot exhaust gasses and smoke up against the pans to give a better boil.  

If you built the block arch and had it 3' wide and set your 4' wide oil pan on it that would help a lot with the burning of the syrup on the sides.  If you use rocks you'd want to explore which ones can handle heat without exploding.  Maybe another rural myth but I've heard some types of stone will blow up if they're wet and get hot.

I'd be very leery of using a used oil drum.  As you boil down the sap, you're concentrating it.  And you're possibly concentrating anything else that's in the pan.  The steam pans are fairly affordable on-line ~$25 each.  Or cheaper from restaurant supply places or the dump.

Hopefully this made sense, sorry for the length of the post...

Mike Jay wrote:
So for the block arch in your first photo, once you get a couple feet into the interior of the arch the floor should come up (sand/rubble) and end up an inch or so below the bottom of the pans.  That will match the chimney cross section and force the hot exhaust gasses and smoke up against the pans to give a better boil.  

The steam pans are fairly affordable on-line ~$25 each.  Or cheaper from restaurant supply places or the dump.

Thanks for all the suggestions.

But did you not say that the fire should be under the entire thing? If I designed it like that the fire would only be at the front.

Yes, and  Canada products are normally priced at around 33% more than the original American price. And if I needed 20 of them...

Ah, sorry I wasn't clear on that.  For best boiling on a pan of a given size, you want fire under the whole pan.  The "firebox" where the wood is burning doesn't have to be that whole area but the smoke/gasses from the fire need to be under the whole pan.  A common size evaporator is 2' by 6'.  A firebox for that would be the full 2' width and then 2-3 feet long.  Behind that the floor of the arch ramps up to keep the extremely hot flames/smoke/gasses pressed up against the last 3-4 feet of the pan.  If you could see under the pan at the very back you'd likely still see flame shooting horizontally under the pan and possibly up the chimney.  On my block arch with separate steam pans I'd take out the last pan and the flames would keep racing past the opening up the chimney due to the serious draft that is developed.

In your case I was trying to highlight the width of the fire vs the width of your pan.  If you have a 4' wide curved oil pan and a 4' wide firebox, you'd get maximum heat transfer to the sap.  But since the pan is curved, you'd have the downside of scorching the sides as the sap level drops.  If you gave up some of that width and had a 3' wide arch, you'd burn less syrup but your boil rate would go down.  That's why most pans are generally flat.

Hmm, sorry about the exchange rate.  20 pans would be a lot.  And when you have more than a few of them, it becomes a pain to ladle the sap between them.  Commonly you have the sap flowing into the back pan and then ladle it forward so that you have syrup in the front pan.  Each extra pan equals more ladling.  Are you handy at metalworking?  Maybe you could get a pan made for you?  Or find a used one cheap?  They don't lose much value (unless you scorch them).

Here is someone who built a cheap, effective and efficient rocket stove evaporator:
Rocket Stove Maple Syrup Evaporator