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The evaporator

Dave Klish

My evaporator is wood fired and it measures 3′ wide and 8′ long. The arch (the firebox portion and support of the pans) is older, from a 2001 evaporator I bought used in 2007. It had been used just 1 year then the original owner got injured and could no longer make maple syrup. I used that evaporator in 2008 -2011. In 2009 I added a blower and plumbing to the firebox for high pressure air over the fire (AOF). This increased the efficiency of the burn in the arch. Originally we had to add wood (a full arm load) every 7 minutes while after adding the AOF we had a faster boil while adding wood (again a full arm load) but every 9 minutes. Along with that change the stack temperature dropped. Before AOF we frequently had stack temperatures in the 1500 to 1650 F range, with the AOF and a faster boil our stack temperature seldom reached 1100 F and most often is between 800-900 F. In other words we were getting more heat into the boil rather than sending it up the stack because far more of the wood gases were being burned under the pan. In fact, before AOF we had a ball of fire out the top of the stack, plainly seen after dark, after AOF there was no ball of fire. The ball of fire had not been from fire traveling the 8′ length of the evaporator plus another foot for the base stack, then up the 17′ to the top, what was happening is the fire consumed the oxygen and then sent the unburnt wood gases up to where they got more oxygen at the top of the stack and it instantly burst into flame again.

After having to sell sap to another producer during a super sap flow period in the 2011 season, my wife and I decided to buy a reverse osmosis (RO) machine. The RO took the sap from a typical range of about 2% sugar content up to 8% sugar by removing water in the sap. I used that for the 2012 season. In that season we decided the sizes of the evaporator pans was not the best for use with the RO. The flues pan (had a bottom with flues that went down 7″, across 1/2″, up 7″ and across 1.25″, that repeated all across the 3′ width of the pan and that pan was 6′ long. Flues increase the bottom surface area under the sap, transferring far more heat to the boiling concentrated sap. Then the concentrate that had been boiled down in the flues pan flowed to the syrup pan.

The syrup pan was also 3′ wide but only 2′ long. The bottom of a syrup pan is flat to slow down the flow enough that it has time to develop a great flavor, all of the flavor is developed in the syrup pan as the boiling sap caramelizes on the bottom of the pan. The syrup made in 2012 was good, but slightly less tasty than previous years. At first we thought the RO made it have a less intense maple flavor, but in the first month after the 2012 season ended research told us that was only part of it. The length of the syrup pan was the cause. Feeding 8% concentrate into the flues pan instead of 2% sap, meant in the syrup pan the finished product didn’t have enough boil time to develop the taste desired. While sales remained strong, we were not satisfied. I then contacted an evaporator manufacturer in Quebec and had them make custom built evaporator pans. Normal flues pans in my size range had either 7″ or 7.5″ deep flues, I had them make a flues pan with 10″ deep flues and at the same time I had it made 5′ long rather than 6′ long. Then on my same arch I had room for a 3×3 syrup pan. The new flues design I had custom built sent concentrate from the 5′ flues pan to the syrup pan at slightly greater density but now with a 3×3 syrup pan rather than a 3×2 syrup pan it had time to develop that great maple flavor, while still being fired by the same arch.

We decided we really like it when a plan is successful. Now all these years later, the pans and arch remain the same. The only change has been the use of the RO, while we still concentrate 2% sap to 8% concentrate in one pass, far more often we run it for a second pass to get 12% concentrate. The 3×3 syrup pan has no problem developing the full maple flavor at that density feeding the flues pan and we burn less wood per gallon of great tasting finished syrup.

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