There are two basic types of Feeder operation known as Volumetric and Gravimetric. Understanding the differences will help processors to select and implement the best type of equipment for their application needs.
The principle of feeding volumetrically starts with keeping the auger full. We know the volume that is displaced by the auger in the nozzle, the pitch of the auger, and the rotational speed. So an auger with a one inch pitch that is turning at 100 rpm will move the volume of material forward 100 inches per minute. Varying the rotational speed of the auger provides linear control of the volume of material being dosed.
This method provides consistent, repeatable metering with linear control with materials that are considered “good” powders. A good powder has a consistent bulk density, and manageable characteristics.
Volumetric feeding falls short of capacity requirements when a material has a varying bulk density. Material that can vary widely in bulk density within a batch will struggle to meet dosing requirements with a Volumetric feeder.
Gravimetric feeding (Loss in Weight or Gain in weight) provides a means for dosing discreetly by weight or metering continuously. It offers methods for recording and tracking materials used and can provide the means to run recipes for dosing systems.
A typical LIW system can cost double the initial purchase price of a volumetric feeder because it adds a scale, control module, and a microprocessor based HMI. However, overall control, record keeping and accuracy gained with the Gravimetric version is necessary for some applications regardless of overall cost.
Choosing the right feeder for your application is important to the cost and quality of the output of your facility. The basics are outlined here, speaking with a bulk material handling expert to design the optimum system for your application is important to achieving quantifiable savings in the long-run.