THE PHYSICS OF MAPLE SYRUP MAKING
Larry R. Yoder copyright 2013
Sugaring teaches more principles of physics per unit time than any other activity I know of. The evaporator is a set of boiling pans that rest on the lips of the firebox. First-time visitors marvel at the roaring fire in direct contact with metal no thicker than a cookie sheet. Only an inch or two of sap covers the pan above this inferno. The metal holds against the intense heat, kept safe by the layer of un-evaporated liquid. Why not more sap in the pan just to be safe? There's a reason. Bubbles of water vapor (steam) form at the bottom in the sap closest to the fire. It travels upward and cools as it goes. If the path is long, sufficient vapor condenses on the way to the surface to slow the rate of evaporation, and more heat is required to maintain the vapor phase all the way to the surface. The shorter the path, the more steam released per unit of heat.
There's another technique to maximize boiling. At full heat, a thick layer of foam forms over the surface of the boiling sap. This also retards evaporation. Visitors will see the sugar maker add a few drops of fresh cream as the head of foam develops. The foam instantly collapses and releases an extra burst of steam. Cream is an excellent natural surfactant that breaks up the bubbles.
The grates where the fuel is burned are only under the front portion of the pan. The back of the firebox has a bottom that slopes upward toward the rear next to the outlet for the chimney. As the hot gasses from the fire travel under the pan, heat is transferred to the sap in the pan. The gasses cool. The upward sloping firebox concentrates the cooler gasses next to the pan and thereby maintains a higher temperature per unit of surface.
The wood we use is different from the fuel I would use in my wood stove. A hot, flashy fire is best for evaporation. The wood is finely split and dried for a year. Cris-crossing the wood in the firebox provides the maximum surface for combustion and the tall chimney, characteristic of every sugar house, assures a strong draft that supplies the fire with plenty of air. As I sit in front of the firebox during a late evening boil, I can "see" infrared. Nerve endings in my skin sense the infrared radiation emitted by the iron fire door as--- heat . Added fuel provides more energy, the wavelengths shorten, and the fire door shifts its radiation to the visible spectrum. In the quiet darkness I begin to see a dull, red glow spreading over the whole front of the firebox.
The evaporator is perfectly level. If that's the case, visitors often wonder how it is that sap moves through the pan. Forgotten is the fact that rising steam is the equivalent of dipping or draining liquid from the pan. A level bottom means that sap moves under the pull of gravity as water leaves the pan as steam. (That, of course, is the whole objective of the process!) The sugar stays behind forming a syrup of increasing sweetness to make the finished product. At that point, the sugar concentration is at 60% and the boiling point is elevated about 7 degrees. The difference in viscosity coupled with the baffles in the pan allows finished syrup to accumulate in the last section without our ever having to mechanically separate the final product. At the end of the season or if, for some reason, we wish to shut down the process and empty the pans, it is possible to draw off virtually all the syrup. One could not, of course, simply drain the pan. The fire would cause melt-down in a matter of seconds. Instead, soft water from the farm pond takes the place of the entering raw sap and supplies the necessary cooling liquid. Boiling continues, and because of their different viscosities, the syrup and pond water do not mix significantly. This same situation occurs when oils of different densities flow sequentially in a transcontinental pipeline. Very little mixing occurs at the interface between the two products. Virtually all the syrup can be drawn off before the pond water arrives at the discharge valve.
Because a shallow layer of sap yields the most efficient boiling, the operator must be constantly alert. Any interruption in the sap supply -- from an empty storage tank or a frozen supply line -- means a rapid scorch followed by melt-down as the pan goes completely dry. The pan can collapse into the firebox like foil after a hobo dinner. This is never a pretty sight. It means significant financial loss and untold embarrassment. Maple sugaring represents the perfect training for those contemplating a career as nuclear power plant operators!