' In practice it is a decided advantage to have a
center air-way in all burners of more than about 2 in. diameter, as it
enables the injecting tube to be slightly shortened, and lessens the
liability of the green film to lift with varying qualities of gas. In
this class of burner I have adopted the small central air-way as a
decided improvement in the burners."
In such processes as the roasting of coffee, chiccory, grain, etc., a
diffused heat is necessary, but of much greater intensity than can be
obtained with economy from heated air. In these cases the application of
a direct flame is necessary, and it may be in actual contact with the
substances to be heated, provided these are kept in constant and rapid
motion.
The use of a revolving cylinder brings in complications with any burner
which is supplied with gas at ordinary pressures without any artificial
air supply, as the currents of air caused by the motion of the cylinder
interfere with the satisfactory working of any burner; and the air
supply must be either protected from draughts and irregular air
currents, or the air must be applied artificially from some independent
source.
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