These consist of two large closed pans provided within with steam pipes
of copper, whereby the liquid is heated. They are also connected with
each other and with pumps in such a way as to reduce the pressure in the
first to about three fifths and in the second to about one fifth the
normal atmospheric pressure.
The juice boils rapidly in the first at somewhat below the temperature
of boiling water, and in the second at a still lower temperature. The
exhaust steam from the engines is used for heating the first pan, and
the vapor from the boiling juice in the first pan is hot enough to do
all the boiling in the second, and is taken into the copper pipes of the
second for this purpose. In this way the evaporation is effected without
so great expenditure of fuel as is necessary in open pans, or in single
effect vacuum pans, and the deleterious influences of long continued
high temperature on the crystallizing powers of the sugar are avoided.
From the double effects the sirup is stored in tanks ready to be taken
into the strike pan, where the sugar is crystallized.