Dry feeders are used to feed dry chemicals into water. In every case, the dry feeder measures a dry chemical and mixes it with water in a solution tank. Then the resulting solution is delivered into the main flow of water using a pump or injector.
This page focuses on the dry feeders used to feed solid fluoride chemicals (primarily sodium fluorosilicate) in large plants. However, dry feeders are used to feed many other chemicals in the water treatment plant, so this information will be relevant for many other processes.
There are two types of dry feeders - volumetric feeders and gravimetric feeders. We will discuss each type below.
Volumetric Feeders
Volumetric feeders feed a measured volume of dry chemical within a given time interval. The picture below shows one type of volumetric feeder, in which the chemical begins in the hopper and is measured by a rotating screw. The dry chemical is then dropped into a solution tank where it is mixed with water to produce a fluoride solution. The solution can then be pumped to the water supply.
Volumetric
feeders can also measure chemicals
using rotating disks, oscillating pans, vibratory pans, rotating
rollers, or star wheels.
Gravimetric Feeders
Gravimetric
feeders measure a
certain weight (rather than volume) of chemical within a given time
period. These feeders are more expensive, more complicated, and
require more space than volumetric feeders, but they also feed larger
amounts of chemicals and are more accurate. A typical gravimetric
feeder is shown below.
