Loss-in-Weight Feeder: How to create your perfect product

Loss-in-weight feeders are gravimetric feeders that directly measures the material’s weight to achieve and maintain a predetermined feed rate.

The feed rate is measured in units of weight per time (e.g. kg/h). This setpoint is programmed by the operator in the feeder control. A constant discharge of bulk material from the hopper is maintained by continuously weighing the hopper including the contained bulk material and regulating the speed of the feeding device accordingly. Accurate regulation is therefore accomplished by comparing the rate of the actual weight loss to the desired setpoint. This enables the control system to compensate for non-uniform material flow characteristics and variations in bulk density, therefore providing a very high degree of feeding accuracy.

Coperion K-Tron offers the optimal LIW feeder system, which integrates a high resolution and fast response weighing system and also accounts for a number of additional influences on feeder performance:

1. Optimizing Screw Fill and Material Flow in Feeder Hoppers

When using single screw feeders for feeding cohesive materials, the material is often flung off the screw flights. This material then accumulates inside the outlet tube, causing buildup and, eventually, fluctuations in the consistent output of the screw feeder. In these cases, a co-rotating twin screw design will be the better choice. Twin screws are often used for more cohesive powders due to the "self-wiping" effect of one screw against the other, which not only helps to transport the material, it also minimizes or even eliminates material buildup.

In addition, there are different options available for the type of screw configuration. For example, if the cohesiveness of a powder increases due to a higher content of moisture or fat, a twin screw with a finer pitch might create a type of extrusion or squeezing effect that releases the moisture and causes additional material buildup issues. In this case, an auger type screw configuration with a larger tolerance on screw flights might be better suited.

Flow-aid devices that may be required in or on the feeder’s hopper will assure that the process material flows into the feeding device as uniformly as possible.

A range of flow-aid devices include:
- Mechanical hopper agitators that stir the material and break down any bridging or rat-holing of the material.
- Integrated vibration technologies, such as the innovative Coperion K-Tron ActiFlow device, which operates by applying vibration to the hopper wall using an external drive at a variable frequency and amplitude.

2. Weight Balancing and Isolation from Vibration Influences

Because weighing is the basis for feeding accuracy, the feeder as well as the weighing unit must be isolated from the influences of external forces, friction, vibrations or shock. This is accomplished by installing flexible connections e.g. at the feeder inlet and outlet. The feeder must have a stable mounting on shock mounts, and strong air currents near the feeder must be eliminated. Modern load cells and control algorithms, such as those supplied by Coperion K-Tron, measure the weight and apply sophisticated digital filtering algorithms to eliminate typical plant vibrations from the weighing data.

3. Pressure Compensation

If the LIW feeder is discharging material into a variable pressure environment such as a pressurized or vacuum conveying line, a pressure or vacuum pulse can cause a feed rate error. This pressure pulse affects the hopper’s instantaneous weight measurement by exerting an upward force on the hopper outlet, in effect slightly lifting the hopper so that its weight reads less. The controller interprets this as too much material delivered and slows the screw feeder down thus the actual feed rate is less than desired. Conversely, a consistent positive pressure inside the hopper can also restrict the weight loss sensed, thus the control interprets this as not enough material being fed and speeds up the screw feeder, resulting in too much actual material being fed.
As explained above, Coperion K-Tron’s innovative Electronic Pressure Compensation (EPC)can be used to automatically detect changes in pressure within a feeder and adjust the weight signal accordingly. This allows the feeder control to compensate for pressure changes in the process and accurately regulate the feed rate.

Summary

Proper installation, the ideal weighing configurations, and the proper choice of feeder controls and instrumentation can ward off a variety of future process problems. Understanding the significance of these influences is critical for achieving optimal system performance. The optimization strategies outlined above all will result in higher performance standards for LIW feeding. By understanding the returns on investment of a higher accuracy feeding technology, and ensuring the optimization of its performance, an overall increase in process profitability and quality can be achieved.