THE MECHANICAL HANDLING OF MATERIALS 289
is already füll, the operator first starts the motor and moves the handle of his speed-regiilating rh.eost.at into the position necessary for giving the projector belt the desired speed. This speed varies according to the length of the retort, the dass of
Fig. 208.— The De Bbouwbk Pkojbctor.
coal used, and the thickness of the charge required, and must first be found by trial. With ordinary broken coal and charges of about 9 cwt., the belt speed should be about 2,300 feet per minute, but for perfectly füll charges a greater speed will be necessary. When the required speed has been attained, the hopper door is ogened and the coal falls into the space between the belt and pulley. The centrifugal force exerted on the coal as the belt passes round the circumference of the grooved dram keeps the coal in contact with the belt, so that the velocity of the coal becomes
deflected and the the coal into the
continuous
Fig. 207.— Slanting Stop jtor use with Pkojectoks.
the same as the velocity of the belt. As the coal reaches the front pulley the belt is suddenly round the pulley, momentum keeps travelling forward retort in a
stream. The distance through which the coal travels is gradually de-creased as the operation of charging proceeds, this being efEected by a gradual reduction in speed of the band. The whole operation of charging takes from 20 to 30 seconds, according to the amount of coal put into the retort.
a