Coperion Plastics Infocus newsletter


How to Troubleshoot Your Feeder for Optimal Performance

Part 2 of 3: Gravimetric Feeders

Basics of loss-in-weight feeding

Unlike the volumetric screw feeder as detailed in Part 1 of this series, a continuous loss-in-weight (LIW) feeder is a gravimetric feeder that directly measures the material’s weight to achieve and maintain a predetermined feed rate that’s measured in units of weight per time. The LIW feeder consists of a hopper, refill device, weight-sensing device (typically either a digital or analog scale or load cells), material-discharge device (typically a volumetric screw feeder powered by a variable-speed motor), and controller. Before operation, an operator programs the controller to discharge material at a predetermined feed rate (or setpoint) measured in units of weight per time (such as lb/hr).

The bulk material (or liquid) is discharged from a hopper with a constant weight per unit time by weighing the hopper and regulating the speed of the feeding device depending on the rate of weight loss. The weighing control system compensates for non-uniform material-flow characteristics and variations in bulk density, thus providing a high degree of feeding accuracy. When the hopper reaches a predetermined minimum weight level, the LIW control is briefly interrupted and the hopper is refilled. With Coperion K-Tron feeders, during the refill period, the controller regulates the speed of the feeding device based upon the historic weight and speed information that was accumulated during the previous weight-loss cycle. This prevents overfeeding of material during the refill cycle due to changes in headload of material and filling of material into the screws. This is also critical for maintaining feed-rate performance within specification on a second-to-second basis. The LIW feeding principle is most accurate when using a high resolution, fast-responding, and vibration- and temperature-immune weighing system.

LIW feeder performance is dependent on three factors that are closely linked

- The mechanical configuration of the feeding device and any material flow-aid used in the feeder hopper;

- The accuracy and speed of weight measurement and the immunity of the weighing system to in plant vibration and temperature fluctuations;

- The response of the control algorithm and the available features of the control algorithm.

Validating LIW feeder configuration

The mechanical feed device selected and the configuration of the feed device is the starting point in any LIW feeding system. Some typical feed devices are as follows:

  • Single-screw feeders can be applied when feeding granular free-flowing materials. When running at lower setpoints, single-screw feeders may produce a pulsating discharge that will affect second-to-second performance. Coperion K-Tron has developed software to compensate for this pulsation and achieve consistent accuracy.
  • Twin-screw feeders can be applied when feeding hard-to-flow and sticky materials and reduce the pulsating discharge even at lower feed rates.
  • Vibratory trays for granular free-flowing materials provide a uniform discharge.

Single- and twin-screw feeders have different screw profiles and configurations that can be tailored to the material being fed as well as the setpoint turndown requirements. The key is to select the proper feeder and configuration to give the most uniform and reliable volumetric feeding of the material. By doing this, the weighing and control system does not have to work as hard to provide optimal performance.

The same holds true for any flow-aid device that may be required in the feeder’s hopper. This will ensure that the material flows into the feeding device as uniformly as possible. Stability of the feeding system allows the weighing and control system to provide optimal second-to-second performance.

Several types of material flow aids available

- Flexible side-walled feeders gently agitate materials. However, these are not stainless-steel surfaces and may wear or create contamination concerns.

- Mechanical hopper agitators stir the material and break down any bridging or rat-holing of the material. However, these devices require additional headroom for the feeder and may become a cleaning concern. Depending on the material a mechanical agitator may also damage the material being agitated.

- Applying vibration to the hopper is another option, however if not properly regulated, this could pack the material in the hopper. Coperion K-Tron’s new technology, called ActiFlow™, applies vibration to the hopper using an external drive at a variable frequency and amplitude based upon the weighing and control system detecting non-uniform material flow by weight. This technique eliminates headroom and cleaning concerns and avoids compaction of the process material because only the necessary amount of vibration is applied to ensure uniform material flow.

Any LIW process controller requires accurate high-speed measurement of material weight changes in order to provide optimal feeder control and performance, especially on a second-to-second basis. The weighing system must also be able to filter out erroneous measurements due to in-plant vibrations or disturbances and must be stable over changes in ambient or material temperatures.

There are two types of weighing technology typically used in LIW feeders: analog strain-gauge technology and digital vibrating-wire technology. The key is that the higher the resolution of weight measurement and the faster those weight measurements are taken, the better the information that will be provided to the control algorithm and the better any vibration-filtering algorithm will work. Almost all weighing systems provide temperature compensation, but it should be verified over the temperature range of your application, as this can affect the long-term stability of feeder performance.