The sap pan we were previously borrowing got repo’d by its original owner so we came up with a new version. Meet Rocket Stove Sap Evaporator 2.0:
Sap pans cost a lot of money, so we opted to base our new evaporator on steam tray pans easily obtained from our local kitchen supply store. I think these were $25/each, and a used sap pan that held the same volume of liquid cost around $300. This design will also allow us to do smaller amounts in just one or more pans (with water in the other pans to prevent them from warping), opening up options for making jams, tomato sauce, fruit butters, etc.
It’s built from two 55-gallon drums with a welded steel burn chamber and inner riser, and welded angle iron that goes across the half barrels to form a solid surface to put the pans in. We used fiberglass insulation as well as perlite around the riser since that gets too hot to use fiberglass.
Roger also welded a waste oil burner to add to the wood fire (that’s the PVC contraption to the side of the vertical barrel, which leads via tubing and a valve to angle iron that directs the oil into the burn chamber), so now we have a way to make use of our old motor oil. No smoke! He hooked up a computer fan blower attached to a Makita cordless battery, which makes for a really hot, clean burn.
We’ve gotten enough hits from people looking for rocket stove evaporator info that in response to Neal’s comment Roger put together a detailed explanation of how he built it.
I ended up writing a quasi-full explanation of the way I built it. I’ve probably over explained parts and omitted others so feel free to contact me for more clarification/confusion.
The burn tube is 8″ X 1/4″ welded flat stock to form an elbow up to the upper pans. They need close to a full bead to keep the seams from warping. As of this moment I’ve only made about 6 gallons of syrup and about that much tomato paste but it is holding up so far. It should probably be made out of some sort of refractory but it’s working for now so it will wait. I made a grate out of flat stock on edge parallel with the wood and held together by two flat stock straps on the bottom. I then modified it with a piece of 2″ box tube through the grate and fastened a 12v computer squirrel fan to the end running off of my 18v power tool battery. It really helps in keeping the heat high and the coals from building up and clogging up the works.
The upper part is just one barrel cut in half lengthwise and then cut to proper length to fit the steamer pans I bought from the restaurant supply house. I overlapped a piece of the off-cut at the seam and pop riveted them together. They sit on top of the lower barrel with a square hole where the fire tube goes through (I cut it with a cut off wheel of an angle grinder). That helps secure and align the top and bottom together. I attached a leg to hold up the otherwise unsupported end. Angle brackets should probably be used to hold the top and bottom together more securely but I did not.
There is a frame built of 1/8″ angle iron welded together to support the steam pans. The welded frame just sits down on the top of the barrels with a bit of fiberglass insulation as a weatherstripping between the barrel and the frame.The pans sit down on top of the frame itself quite nicely for a tight seal without anything else. Most of the insulation is just fiberglass, but the bottom barrel is filled with perlite with one layer of fiberglass around the perimeter to save some money– fiberglass anywhere towards the middle would just melt. Trust me… The seam where the lower and upper barrels meet, and also lining the upper barrel up to the first steam tray has a high temp refractory wool to withstand the high temps generated. I think it’s rated for 2100 degrees F. Kaowool is the brand I happened to have from building a kiln. It’s super expensive but it is easy to work with and lasts well.
I also friction fit in some insulation between the walls of the pans to try to keep the heat from going up the sides too much. The high heat on the sides of the pans was scorching the sap film above the liquid level. One also needs to make sure there is liquid in all the pans whether they are needed or not so that they don’t warp. When there is no more sap to put in the last pan I dump it into the next one and then replace it with water. I do that with all the pans in sequence and then finish in the first pan keeping the flame lower to not burn the syrup.
The chimney helps to keep exhaust gases from coming out as readily when you remove a pan when the fire is running. It drafts fine without it due to the insulated riser, and a fan seals the deal. The stove burns HOT and keeps the the gasses clear and clean without any odor. On a previous evaporator I designed a support and put a griddle over the exhaust instead of a chimney and made pancakes/bacon while boiling. It worked really well since there is absolutely no smoke!
It also works great for boiling down tremendous amounts of tomato paste and jams. I usually just boil tomatoes etc. in the first two pans and have water in the last two. Then I bottle into jars and move the water pans to the front and can the jars in the steamer trays of water with a cover on them. Keeps the house cleaner/cooler. As an aside I put our raw tomatoes through our #32 meat grinder to cut up the core and skins instead of running everything through a sieve. It’s much faster and you still get a reasonably smooth sauce without things to get caught in your teeth.