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Details of the Evaporator  
  The evaporator consists of one or more pans that are placed over a firebox referred to as an arch.
 The pans are divided into sections to separate the more concentrated sap from the more dilute. The sections are not closed. Sap can move freely as the water evaporates. This arrangement increases the speed of evaporation significantly over an open, unsectioned pan. The flat bottomed pan is referred to as the syrup pan or finishing pan. Syrup reaches its final concentration in this pan.  
Flues in the bottom of the sap pan greatly increase the surface area for heating. Hot gasses from the fire pass between the flues.
The flue pan is positioned toward the back of the firebox. Raw sap enters the flue pan. The syrup pan will be placed over the front of the firebox above the grates in a wood fired arch as we see below.  
  A hot, steady fire brings about the most effective evaporation.
The syrup pan and flue pan are connected so that flow is continuous. The sap in the pans is about 2 in deep during active boiling.  
  The amount of steam that rises is substantial.