Chemical Storage and Feeding Systems - Public Water Systems - page 841

size. In operation, the chemical falls by gravity into the cavity, and it is almost fully
enclosed and separated from the hopper's feed. The rate at which the cavity moves
and is discharged, together with the cavity size, governs the amount of chemical fed.
Positive-displacement feeders often use air injection to enhance flowability of the ma-
terial.

Rotary Paddle Feeder. A rotary paddle feeder is especially effective for fine materials
that tend to flood. The paddle or vane is located beneath the hopper discharge, with
the feed being varied by means of a sliding gate and / or variable-speed drive. The feed
rate can be varied easily by adjusting the variable speed drive on vane shaft. A variant
of the rotary paddle feeder is the pocket feeder, also called the star or revolving door
feeder, in which the paddle is tightly housed to permit delivery against vacuum or
pressure. Figure 24-10 illustrates this feeder type.

Oscillating Hopper Feeder. Another type of volumetric feeder is the oscillating hop-
per, or oscillating throat feeder. This feeder consists of a main hopper and an oscillating
hopper that swivels on the end of the main hopper. The material completely fills both
hoppers and rests on the tray beneath. As the oscillating hopper moves back and forth,
the scraper, which rests on the fixed tray below, is moved first to the left and then to
the right. As it moves, it pushes a ribbonlike layer of dry chemical off the tray. The
capacity is fixed by the length of the stroke, which may be varied by means of a
micrometer screw. Further adjustment is possible by changing the clearance between

VARIABLE SPEED
TRANSMISSION

PRESSUR E
PLATE

ROTOR
SCRAPER

MOTOR

INSPECTION AND
SAMPLING CHUTE

Fig. 24-10. Positive-displacement rotary feeder ( Source: Reference 7)