Page 206 - Water and wastewater engineering

CHEMICAL HANDLING AND STORAGE 5-9

TABLE 5-2
Dry feeder characteristics
Capacity Turn-down
3
Type of feeder Application m /h ratio Remarks
Gravimetric
4
Loss-in-weight Granules, powder or lumps 6 10 to 2 100:1
4
Continuous belt Dry, free-flowing granules 6 10 to 0.06 100:1 Use hopper agitator
to maintain constant
density
Volumetric
4
Rotating disc Dry, free-flowing granules or powder 3 10 to 1 20:1 Use disk unloader for
arching
Rotating cylinder Dry, free-flowing granules or powder 0.2 to 60 10:1
3
Screw Dry, free-flowing granules or powder 1 10 to 1 20:1
5
Ribbon Granules, powder or lumps 6 10 to 10:1
3
5 10
3
Belt Dry, free-flowing granules or powder 3 10 to 85 10:1
up to 3 cm size
Adapted from Hudson, 1981, and Kawamura, 2000.

The characteristics of dry chemical feeders are summarized in Table 5-2 .

Gravimetric Feeders. There are two types: loss-in-weight and belt-type. The loss-in-weight
type uses a feed hopper suspended from scale levers, a material feed control mechanism, and
a scale beam with a motorized counterpoise. The rate of weight loss of the hopper equals the
weight loss equivalent of a traveling counterpoise when the feeder is in balance. If it does not, the
scale beam deflects and the feed mechanism adjusts the feed rate.
A feed hopper and control gate regulate the flow and depth of material on the belt-type
feeder. A scale counterpoise is adjusted to establish the desired belt loading. The gate releasing
material and the speed of the belt are adjusted to produce the desired flow of material.
The loss-in-weight type feeder capacity is limited to about 400–500 kg/h. The belt-type feeders
have capacities of 225 Mg/h and up.

Volumetric Feeders. The volumetric feeders provide good overall performance at low feed
rates and acceptable accuracy for materials with stable density and uniformity. They do not per-
form well when the density of the material is not stable or is hygroscopic. They must be cali-
brated frequently.

Lime Slakers. Slaking means combining water with quicklime (CaO) in various proportions to
produce milk of lime or a lime slurry. Lime feed systems combine the addition of the chemical