Geauga County OSU Extension
We pay a lot of attention to managing the woods during the maple season. After all success or failure depends on what happens in the woods. However, what about that shiny or not so shiny metal machine in the sugarhouse we call an evaporator. How you run that machine will make or break you in the end. There is a common joke among maple producers, that you are not really a maple producer, until you have burnt a pan. Trust me if you have never scorched a pan, or come close to burning one, consider yourself lucky. This is one place you do not want to go. Usually the most common reason for burning a pan is human error caused by some kind of distraction. When you are running a rig you are dealing with extreme heat, in up to 1000 degrees F. You are applying that heat to a relatively small volume of liquid (2 in. across the surface of the pan) separated by a thin layer of stainless steel metal. The only thing that keeps that metal from melting is that thin layer of liquid. If the sap boils out because you run out of liquid, forget to turn on a valve or you drink one to many beers, that metals will scorch, then buckle and then melt down. Your season could be over, if you cannot find a replacement. You cannot go to Walmart to buy a flue pan or a syrup pan. To prevent all of this from happening you need to understand how your evaporator works.
If you were to ask many producers how an evaporator works, the answer would be that 2 brix sap enters through a float and moves through a series of channels until it reaches the end of the syrup pan and 66 brix syrup is produced. Sounds simple enough but this is a very complex process. When sap is boiling a gradient is formed causing the heavy syrup to moves in front of the lower density sap. As long as the pan is boiling, the two will not mix, well should not mix? That is not always the case if the boil is interrupted. To completely understand the process you need to know how much water is being removed in each pan. If you understand that you will also understand how easy it is to burn up an evaporator. The sap entering the back pan or the heater pan is roughly 2 brix. Applying the Jones Rule of 86 (Now the Jones Rule of 87.1, aka 2013 research at UVM ,Proctor Research Center) where 44 gallons of sap is needed to make 1 gallon of syrup The flue pan is where all the work is done. When the sap leaves the flue pan, close to 35 to 40 gallons of water will be evaporated leaving the concentrated sap at 18 to 19 brix. That only leaves less than 5 gallons of water to be removed in the syrup pans. This makes more sense if you compare it to what happens in a Reverse Osmosis machine. If you take 2 brix sap to 8 brix you are effectively removing 75% 0f the water. This is roughly 33 gallons of water removed from the 44 gallons required to make a gallon of syrup. If the syrup represents 1 gallon that only leaves 9 gallons of water to be evaporated. How quick this happens is relative to the size of the evaporator you are running and the quality of your fuel source. The RO has made it possible to handle large volumes of sap very efficiently on a small evaporator.
There are two types of evaporators, raised flue and drop flue. With a drop flue you only have one float and it controls the entire machine back to front. You control the depth of the sap throughout the entire machine based on the depth in the front pan. This is usually between 1 1/2 to 2 inches across the entire machine. A raised flue evaporators has two floats, one for the back pan and one for the front pan. You are running one evaporator but controlling two separate processes. The level of sap needs to be between 1 and 1 1/2 inches above the flues of the back pan. This causes the back pan to boil off water fast. If the back pans are too deep the intensity of the boil will be dampened. If you put a big pot of water on the stove it takes a long time to come to a boil, however as the liquid boils off the rate of exportation becomes faster. Which style of evaporator you prefer is strictly a personal preference.
In the front pan you are taking the sap from slightly over 212 degrees F, boiling point of water to 219. F the boiling point of syrup at 66 brix. This happens at 29.9 inches of mercury on the Barometer. If the barometric pressure goes down and it will all the time in the spring of the year, the boiling point of water will vary and you thermometer will have to be adjusted. . Make sure you calibrate your thermometer in boiling water before the start of each boil. You should carry 2 inches of liquid across the front pan. Syrup should flow evenly to the draw off. If hot spots develop, that area of the pan will tend to boil faster increasing the risk of burning. The trouble usually occurs when you draw off. This is why you want to avoid removing large volumes of syrup at one time. This causes the liquid level to become very uneven, you might have 2 inches in one part of the pan and a 1/2inch in another. Removing small batches more often will accomplish this. If a heavy draw off occurs and a shallow spot develops watch the bubbles, they will become smaller and intensify in that area. As the liquid goes above 219 F the liquid will gravitates to the hot area. The steam from those bubbles will become more localized and intense. This means the sap is becoming more concentrated in that area. If this happens a long way upstream from the draw off you are headed for trouble, unless you let more liquid into the pan. However, the addition of more liquid of lessor density will disrupt the gradient. This means you will have to boil longer to make syrup. This also leads to additional large volume drawoffs. Maintaining a constant even flow of syrup in the form of low volume draw offs, stabilizes the process. Watch you r bubles they should move in one direction across the entire rig. If they start moving the opposite direction a problem has occurred and a adjustment needs to be made. Make all your adjustments in small increments and remember it takes time for that adjustment to affect the process.
Another problem that can lead to a burnt pan is allowing too much niter (mineral deposits in concentrated sap) to build up in the pan with the drawoff. Pans are designed so you can reverse the flow (reverse flow) or change out the front pan (cross flow). When you are boiling only sap you usually can boil for one day without reversing the flow or changing the pan. If you are boiling RO concentrate the change may come more often. Niter buildup tends to be worse at the end of the season and will vary from year to year. Allowing niter buildup is the same as cooking with a dirty pan the niter insulates the liquid from the pan surface cause in the surface to burn under the niter. Niter builds in the pans further away from the drawoff but the concentration is a lot less and the boiling action tends to break the niter down. This is why reversing the flow to the other side will dissolve the niter. Starting the day with a clean syrup pan is a necessity. Where you swap pans you can soak the pans in a 5% vinegar solution for several hours and then scrub and rinse the pan until the metal shines. I buy white vinegar by the case.
Technical information from the North American Maple Syrup Manual 2nd Edition