Optimization of Screw Feeder Performance

In today’s tough economy, the pressures for proper maintenance and increased longevity of process equipment, as well as optimal performance, are higher than ever before. In all industries, the improvement in ingredient feeding accuracy by even ¼%, by optimizing your feeder’s performance can result in significant overall profit yields.

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 if the drive doesn’t respond as required by the setpoint. 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, depending on the sensor type and the manufacturer’s recommendation. You can easily evaluate the sensor if the motor speed is stable.

If the screw-speed control sensor isn’t causing the problem, then the cause is probably a change in the discharge rate’s volume-per-revolution ratio, typically caused by material buildup on the screw or in the discharge tube or by a blockage in the hopper that prevents a consistent material supply to the screw. The buildup or blockage reduces the material volume that the screw discharges in each revolution at the constant screw speed. An immediate, but temporary, remedy is to clean the screw, discharge tube, or 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.

Because the loss-in-weight (LIW) feeder typically uses a volumetric screw feeder to meter material, many of the volumetric feeder problems and solutions in the previous section also apply to the LIW feeder. But since the LIW feeder’s operation is based on the weight loss rate per unit time rather than the screw speed, the controller automatically compensates for material buildup on the screw or in the discharge tube or a blockage in the hopper by increasing the screw speed to maintain the setpoint. If an alarm condition occurs in your LIW feeder, check first for material buildup on the screw or in the discharge tube or a blockage in the hopper.

If you find no material buildup or blockage, check the hopper to ensure that it has material in it. If the hopper is empty, you then need to check the upstream material delivery system for a blockage or other malfunction. Since the LIW feeder’s operation depends on accurate weight measurements of the material in the hopper, make sure the feeder and weight-sensing device are isolated from any external vibration created by other equipment in your process, because vibration can impose artificial forces on the feeder that cause weighing errors. This requires installing the feeder so that the weight-sensing device is shielded from vibration effects. Do this by ensuring that the feeder has a stable mounting, using flexible connections and shock mounts, and eliminating strong air currents near the feeder.

The weight-sensing device itself can cause performance problems if you don’t select it properly for your application. Carefully evaluate the weight-sensing device’s capabilities — such as resolution, stability, responsiveness, weight signal integrity, vibration sensitivity, reliability, and data communications — before purchasing the LIW feeder. After installing your feeder, maintain its performance and find any problems such as drift (a gradual deviation from a set adjustment) as early as possible by regularly calibrating the weight-sensing device.

Other performance problems can result from a defective refill device or a leaky seal at the feeder’s discharge. If an automatic refill device loads material into the hopper, any leakage in the refill device at the hopper’s inlet will produce a feed rate error because material will continue leaking into the hopper after the refilling process has stopped. In addition, if the LIW feeder discharges material to a non-ambient pressure environment such as a pressurized or vacuum conveying line, a pressure pulse (air leaking from the downstream system through the feeder’s discharge tube to the weight-sensing device) can cause a feed rate error. To compensate for this, the controller decreases the screw speed to meet the setpoint, discharging less material per unit time. These problems are usually easy to fix but can be hard to detect. The best solution is to regularly check the feeder’s refill device and discharge for proper operation.

Sharon Nowak serves as global business development manager for the food and pharmaceutical industries for K-Tron.

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