9°, BULLETIN 1370, U. S. DEPARTMENT OF AGRICULTURE 
always slightly acid and consequently gradually corrodes galvanized 
iron. When the zinc surface of the galvanized iron becomes corroded 
and pitted, the juice is boiled in contact with the exposed iron, which 
reacts with certain constituents of the juice to produce a dark sirup. 
Although a copper evaporator costs nearly twice as much as one of 
galvanized iron, the extra expense is more than counterbalanced by 
the advantages gained. Copper lasts much longer than galvanized 
iron; it can be easily cleaned without injury by the use of acid; it 
conducts heat better than does galvanized iron; and the use of a 
copper evaporator results in a lighter-colored sirup. The first cost of 
a good evaporator is only asmall item in the total cost of making sirup, 
so that the use of copper is in no sense of the word an extravagance. 
The advantages of evaporators are: (1) Rapid evaporation, which 
is essential in making light-colored sirup, is obtained; (2) the sirup is 
concentrated in a thin layer, thus increasing the rate ot boiling and 
foaming and affording better opportunity for thorough skimming; 
(3) heat is applied to the bottom of the evaporator, thus imparting 
an upward motion to the coagulated material, whereby skimming is 
facilitated. The disadvantages are: (1) More attention is required 
to maintain a properly regulated fiow of juice; (2) there is increased 
danger of scorching the sirup and altering its color and flavor; (3) 
more careful attention to firing is necessary; (4) it is difficult te obtain 
uniform sirup density. 3 
In continuous evaporation, a steady stream of juice fiows by 
gravity into the lower end of the pan and then flows slowly to the 
opposite end, at which point it reaches the density of finished sirup. 
The juice end of the pan is sufficiently lower than the finishing end 
to maintain a juice layer 2 to 244 inches deep, which should give a 
layer three-fourths to 144 inches deep (preferably only three-fourths 
inch) in the finishing end of the evaporator. In most common prac- 
tice the juice enters the front end of the pan and during evaporation 
moves toward the back or chimney end. In some instances, however, 
the juice enters at the chimney end and is drawn off at the front end 
as sirup. As water is evaporated and sirup drawn off, the juice, with 
continuous skimming, flows toward the finishing end of the evapo- 
rator, while cold juice from the supply tank is run in to maintain the 
desired level in the juice end of the pan. The heat applied to the 
juice from the bottom of the pan increases from the juice end to that 
part of the evaporator where there is danger of scorching the sirup. 
Beyond this point the heat applied gradually decreases, until only a | 
small amount is required under the iast compartment, from which ~ 
the sirup is drawn off. The hottest portion of the pan, and conse- 
quently the place where the juice boils most vigorously, is where 
concentration to sirup is a little more than half completed. This 
scheme of operating a continuous evaporator makes it easier to 
regulate the boiling. The scum is carried back to the juice end of 
the pan with the foam and may be readily removed by skimming. 
FURNACES 
The furnace for a furnace-heated continuous evaporator must be 
properly constructed and the fire must be carefully controlled. In 
direct-fire evaporation, success depends to a great extent upon the — 
construction and operation of the furnace. The capacity of a plant — 
equipped with a mill and evaporator of the best type may be reduced © 
as much as 50 per cent by an improperly constructed fire box and 
