Maple syrup evaporator

A few years back I had my sugaring setup using a first gen prototype of a rocket stove. I actually went through a couple prototypes of this a few different seasons and then moved to a non rocket stove set up. I want to go back to a rocket stove next year and while I watch the sap steam off this year I am doing some planning in my head.

My pan is 16”x41” long. Previously I put a rocket stove at one end and a long narrow passage under it with the chimney at the far end. This caused the flames to have to move along the bottom of the pan to get to the chimney. It worked ok but I really only got a vigorous boil right above the stove and a lazy boil about 1/2 of the way down to barely a simmer if anything at the back of the pan.

I want to build the next one with two stoves. And a similar passageway under them to a chimney at the back.

Previously I used an L shaped stove, I like the idea of a J stove because it’s self feeding. However the problem is unlike a normal cook stove or rocket mass heater, this thing can run continuously for 6-8 hours or more to get through all the sap. This creates a lot of ash. I think it’s a lot easier to clean out an L stove than a J stove and I’m pretty sure that’s why I went with this design last time.

Other than having to keep poking the wood forward, is there any other real disadvantage to sticking with L stoves?

Alternatively anyone ever build or see a J stove designed with some kind of ash pit with a cleaning door? Would love to see pictures of this as that might be the best of both worlds.

I can get a nearly unlimited supply of kiln dried pine/spruce clapboards from a local mill with really high standards and plenty of cull. These make almost the perfect fuel for a large rocket stove I think.

How about a J/L stove?  A J with a removable brick where the L would have an opening so you can rake out the ash?  

I have built two L-tube rocket fired evaporators, 8 and 6 square feet. An 8" wide x 9" high burn tunnel about 32" long, with a riser around 30" high, was barely enough for 2' x 4', but okay for a 30" square pan. With proper wood selection (as many larger logs up to 5" diameter as possible), the logs hold their shape until nearly burned and do not collapse in a choking pile of coals. I have run a 10+ hour session without raking out coals. Your fuel source may make this difficult.

This is to say that a single larger L-tube will probably be steadier and easier to maintain than a pair of smaller rockets.
I cut wood to 30 or 32" lengths - less cutting and handling, and pushes all the way to the back of the firebox at one go.

If you add some mass to it, there is less chance of burning the syrup.
The most critical time is right near when it's done.
This is my first year making syrup and my first 2 attempts could have gone worse if I didn't have the mass to give off a
sustained low heat toward the end.

craig howard wrote:If you add some mass to it, there is less chance of burning the syrup.
The most critical time is right near when it's done.

I don’t finish in the pan. I get it close as I dare and then let it cool down and drain the pan and bring it inside where I finish on the stovetop with full control. This is one advantage to the current non rocket stove set up. I can let the level in the pan get a little high and load the stove up and call it a night about an hour early knowing it will continue to boil down some while I sleep. Then in the morning I drain it into a bucket and bring it into the stove. I have gotten pretty good at knowing just where to leave it to find it where I want it in the morning. As I am generally only ever doing batches of 1-2 gallons (finished) at a time this works fine for me.

The J and L hybrid idea is interesting. I’ll have to give that some thought.

The clapboards do have a tendency to collapse into a huge pile of coals even in the normal stove this can be an issue. But they burn fast and hot and I can get them free and dry. I think a dual stove set up might allow me to clean one out while keeping the boil going. I don’t really see feeding two stoves as that big of a challenge as they will be literally feet from each other. If one end of the pan is boiling more vigorously than the other that’s not an issue for me as I don’t have baffles in the pan and I don’t do a continuous finish process like some of the bigger operations do where they are pulling syrup from one end and adding sap to the other. I do batches.

I wonder if, instead of devising ways to be able to clean coals out of a stove, it would be worthwhile to figure out how to get the coals fed air so they finish combusting. Perhaps a "grate" floor consisting of firebricks with small gaps between them, so air can come up below the coals along the length of the firebox. This would preheat the air also. The air channel could be set up to allow easy cleaning of any ash that falls through.


In woodfired kilns I have built, I have found that having the firebox about 16" wide (and 14-16" high by 4' deep) gives space for coals to burn up while new wood is added on top. This is a very intense sustained fire situation where the goal is over 2000 F exhaust for several hours.