There are many types of feeders available to handle bulk solids and they can be divided into two categories: volumetric and gravimetric. A volumetric feeder discharges a volume of material as a function of time while a gravimetric feeder weighs material.

Volumetric feeders

Volumetric feeding is adequate for many solids feeding applications. Feed accuracy in the range of 2-5% can be achieved with most volumetric designs.

Volumetric feeding can, however become inaccurate if the bulk density of the solid that is being handled varies. The feeder cannot recognize a density change because it simply discharges a certain volume per unit time.  Examples of volumetric type feeders are: screws, belts, rotary valves, louvered type, and vibratory.

Gravimetric feeders

A gravimetric feeder relies on weighing the material to achieve a required discharge rate or batch weight. This approach should be used when:

Feed accuracy of 0.25% is sometimes obtainable with a properly designed gravimetric feed system. A disadvantage of a feeder that weighs material is that it is usually more expensive than a volumetric device.

There are basically two ways to feed gravimetrically, continuous and batch.  A continuous gravimetric system controls the weight/unit time such as lbs./hr or kg/hr. A batch system controls simply the weight of material such as 50 lbs. of material to a mixer. Examples of gravimetric feeders are: weigh-belts, loss-in-weight systems, and gain-in-weight systems.

In subsequent articles, we will discuss each type of volumetric and gravimetric feed system in more detail.