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In today’s tough global economy, the pressures for proper maintenance, increased longevity of process equipment, and optimal performance are higher than ever before. Compounding feeders that don’t feed accurately typically produce off-spec finished products and require extended periods of downtime to recalibrate or fix, resulting in decreased production rates and lost profits. In all industries, the improvement in accuracy performance by even 0.025% of ingredient feeding can result in significant overall profits.

Proper installation, ideal weighing configurations, and appropriate choice of weighing controls can result in the avoidance of a variety of future feeder problems. For a complex feeder system – such as multiple loss-in-weight (LIW) feeders feeding powder and pellets into mixers or extruders – external influences such as vibration, platform stability, and upstream/downstream equipment connections can all affect a feeder’s performance. Informed knowledge of the significance of these influences is key to optimizing its performance. By thoroughly training your operating and maintenance workers and keeping them familiar with effective troubleshooting and maintenance practices, a variety of these problems can be mitigated.

Feeders are typically engineered to address a specific material at a specific discharge rate. Changes in material, operating conditions (such as ambient or material temperatures), plant vibration levels, as well as changes in material characteristics, can all affect feeder performance. Choosing a feeder that can be easily reconfigured in your plant to handle new conditions can help you solve these problems.

By definition, most feeders can be categorized as volumetric or gravimetric. This article will investigate volumetric feeders and review the significance of a number of parameters and their direct influence on feeder performance.

Volumetric Screw Feeders
A volumetric screw feeder, feeds a certain material volume per unit time (such as ft³/hr) to a process. The volumetric screw feeder consists of a hopper, material-discharge device, and controller. This is the most common volumetric feeder, and its material-discharge device is a screw that rotates at a constant speed to meter material at a predetermined volume-per-revolution discharge rate from the hopper to the process. The controller monitors and controls the feeder’s screw speed, which determines the discharge rate of the material.

An optional agitation system located between the hopper and the screw can facilitate material discharge from the hopper. Various agitation systems and screw designs, sizes, and geometries are available to suit your application.

Because the volumetric screw feeder cannot detect or adjust to variations in a material’s bulk density, the feeder is most effective with relatively free-flowing, uniform-density materials, such as pellets, and in applications where high feeding accuracy is not crucial.

Volumetric screw feeder problems are relatively easy to diagnose. Most problems relating to the feeder’s discharge rate stem from a faulty screw speed control sensor or motor drive, a change in the discharge rate’s volume-per-revolution ratio, or material flow problems from the hopper. Precise control of the discharge rate will be impossible if the feeder’s screw-speed control sensor doesn’t register the screw speed accurately (or at all). If the feeder’s discharge rate is a problem, first check for loose sensor wiring and electrical connections. If the connections are sound, you may need to clean or replace the sensor. You can easily evaluate the sensor if the motor speed is stable.

If the screw speed sensor is not causing the problem, then the cause is probably a change in the discharge rate’s volume-per-revolution ratio. Such a change is typically caused by material buildup on the screw or in the discharge tube or by a blockage in the hopper that prevents consistent material supply to the screw. The buildup or blockage reduces the material volume that the screw discharges in each revolution at a constant screw speed. An immediate, but temporary, remedy is to clean the screw, discharge tube, hopper, or all three. To permanently solve the problem, you may have to change the screw or hopper design or add an agitation system to help move material from the hopper to the feed screw. Coating or polishing the internal metal surfaces may also alleviate the problem, but consult with your manufacturer for guidance.