Boiling It Down: How to Choose an Evaporator

Concentrating maple sap into syrup couldn’t be simpler – just boil it. It’s been done for hundreds of years, by dropping hot rocks into hollowed-out logs filled with sap, hanging iron kettles over open flames, propping oversized kitchen pans on bricks over fires and many other ways.

Those are all still options for sugarmakers, but most choose to take advantage of advances in technology, purchasing an evaporator that will use fuel efficiently and process sap into syrup more carefully. But how do you know which evaporator to invest in? It’s important to find the right size, fuel type and tools to maximize your evaporator’s efficiency.

1. Size Matters

The first consideration in purchasing an evaporator is size. The best way to determine what size of an evaporator is needed is to look at how much sap you need to boil and how much time you have to do it. For example, if a good day’s run produces a gallon of sap per tap, the sugarbush has 800 taps and the sugarmaker want to work for eight hours, an evaporator that can boil off 100 gallons of sap per hour is needed. Every sugarmaker strives to get “just a few more taps” each year, so keep in mind the potential for future growth when purchasing a new evaporator.

One often-repeated rule of thumb says that a flat pan can boil off one gallon of sap per hour for every square foot of space, so a 3-foot by 4-foot flat pan would have 12 square feet of surface area and would boil off 12 gallons per hour. That rule may hold roughly true for the syrup pan, which is designed for more precise boiling of concentrated sap, but the back pan of an evaporator usually has flues. These are fins that extend either up into the pan or down into the firebox and greatly increase the surface area of heated metal coming into contact with sap, which helps boil away greater quantities of water more quickly.

The dimensions of these flues alter the calculation significantly. In addition, each evaporator manufacturer has their own standards for efficiency, which also affect how rapidly sap boils. Most manufacturers offer their own estimates for how much sap per hour their units can process, and talking with experienced sugarmakers who have used the unit under consideration will allow for some real-life analysis.

Photo by ErikaMitchell/ 

2. The Cost of Fuel

Another critical consideration when purchasing an evaporator is the fuel to be used to heat the sap. Wood is traditional and has its advantages, primarily in that it allows sugarmakers to make use of the firewood they’re most likely already creating in maintaining their sugarbush and other woodlots. The labor exerted to cut and prepare firewood results in ‘free’ fuel to run the evaporator. Wood-fired evaporators also tend to be the simplest and easiest to maintain. But evaporators that burn wood also tend to be the least efficient, using less than 50 percent of the heat generated for heating the pans and sap.

While producers who use oil-fired evaporators can’t produce their own fuel, they trade off the cost of fuel oil for tremendous savings in labor – not just in not having to cut and store firewood, but also in the operation of the evaporator, since flipping a switch replaces needing to fill the firebox with wood every few minutes. Heat in an oil-fired evaporator is easier to control and can be far more efficient, often as much as 75 percent. The downsides: upfront costs of an oil-fired evaporator are significantly higher because of fluctuating oil prices and the technology involved in these units means more systems and parts to maintain and repair.

Evaporators burn wood chips, propane or natural gas. Units that use steam generated by an oil- or gas-fired boiler are also available, but are less common.

3. Time-Saving Tools

Any evaporator’s efficiency can be greatly improved by units that capture the heat of the steam being generated, using it to pre-heat or even begin boiling the sap before it enters the evaporator. A steam hood over the flue pan that helps to channel steam out of the sugarhouse can be fitted with a preheater, a set of pipes through which cold sap is heated by steam before entering the pan, thereby reducing the amount of fuel and time required for the evaporator.

Energy recovery steam hoods, such as Piggybacks or Steam-Aways use heat from the steam and evaporated water vapor to speed up the evaporation process by injecting hot air into the sap, bringing it nearly to the boiling point before entering the flue pan. The sap is not only hot before reaching the pan, it is also already more concentrated.

These tools can greatly reduce the amount of time that it takes an evaporator to boil off a gallon of water, allowing for a smaller unit based on the amount of sap being gathered. An evaporator manufacturer or salesperson can help a sugarmaker calculate just what they need based upon the size of their operation, the amount of time they wish to spend boiling and the initial investment they are prepared to make in equipment.

Cover Photo by capecodphoto/