A rule of thumb says that sap will evaporate at a rate of one gallon per square foot area, maybe a little more or less. Our current evaporator has six square feet surface area for boiling, so when we have the evaporator really fired up well, we are evaporating at 7-8 gallons per hour rate. Our 2015 had 3 square feet of surface area. It took in excess on 12 hours maybe more (don't really want to remember that part) to evaporate 40 gallons of sap. With the new evaporator, the job is done in a bit less than 6 hours.

There are primarily two methods of sap evaporation. One is the continuous flow method that our evaporator uses. The sap drops into the boiling pan from the preheater. By the time the sap reaches the end of the last flue and we have boiled off enough water, the sap there is either syrup or near syrup and we drain it from the spigot closest to the syrup. There are many types of continuous flow methods from very simple ones like ours to very complex ones that are mostly automated and incorporate other processes such as reverse osmosis

A little about evaporators

The other method is the batch method. It is the kind of evaporator that most hobby and backyard syrup makers use. Sap is typically put in a pan or pans over a fire and sap is added as it evaporates. Eventually the near syrup is transferred to one pan and when the temperature reaches seven degrees above the boiling point of water, you have syrup or very close to it.

In 2015, our first year of maple syrup making, we had a simple 3 square foot "batch" evaporator set up on concrete blocks over a fire in the back yard. We would just keep adding sap to the pans until we had evaporated it all into one pan that was close to syrup.