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Loss in Weight (Screw Feeders)

Anatomy of a Modular Loss-in-Weight Feeder

Loss-in-Weight Feeding Principle

In a loss-in-weight feeder, the feeding device with a hopper containing material to be fed is placed on a platform scale or suspension scale weighing system. The weight of the feeding device and hopper is electronically tared.

The bulk material or liquid is discharged from the hopper by the feeding device and the resultant weight loss per unit of time is determined by the weighing and control system. This actual weight loss per unit of time is compared to a desired weight loss per unit of time based upon a desired continuous feed rate setpoint.

Any difference between the actual and desired weight loss per unit of time results in a correction to the speed of the feeding device. When the hopper content reaches a predetermined minimum weight level, the control by weight loss is briefly interrupted and the hopper is refilled.

During the refill period, weight is increasing and the controller regulates the speed of the feeding device based upon the historical weight and speed information that was determined during the previous weight loss cycle.

The loss-in-weight feeder principle is most accurate when using a high resolution, fast responding, vibration immune weighing system combined with self tuning controls

Inside a loss-in-weight-feeder1. Coperion K-Tron Control Module or Connection Box

The Coperion K-Tron Control Module (KCM) combines the feeder control function together with the motor drive or vibratory tray drive and mounts directly on the feeder frame in an IP65 / Nema 4 enclosure. This eliminates the need to mount these components in remote control panels and eliminates expensive cabling between the feeder and the operator interface in the control room. All wiring between the weighing system, motor and KCM is done at the factory thus reducing installation time and expense. Connecting all feeders in a process line to the operator interface in the control room is accomplished using a single cable with standard serial communications protocol. For hazardous or hostile environments, the KCM can be remote mounted and replaced with a connection box.

2. Hoppers

Hoppers are available in a variety of configurations. Symmetrical and asymmetrical hoppers with and without vertical agitation are available depending on the flow characteristics of the material to be fed.

3. Modular Feeding Devices

The feeding module is supported on a swing out structure that allows for quick cleaning or changing to a variety of units. Screw feeding units with horizontal agitators as well as vibratory trays, belts, Bulk Solids Pumps and other feeding devices can be supplied.

4. Inlet/Refill

Hopper lids can accommodate manual or automatic refills, depending on feed rates. When automatic, the inlet and vent size is determined based upon refill rate, material flow characteristics and hopper size. Connection to the upstream refill device is made via a flexible bellows. Pressure compensation systems are available upon request.

5. Air/Ventilation

During the feeding process, air must enter and leave the hopper as the process material is being discharged or refilled. The vent connection is provided for this purpose and can be connected to: plantwide dust collection systems, the refill container, the atmosphere through an attached fabric filter, the atmosphere through a self-cleaning jet filter mounted on the hopper. Connection to upstream hoppers or central collection systems is made with a flexible bellows.

6. Smart Force Transducer Weighing Technology

High resolution (4 parts / million), zero deflection, vibration and temperature immune weighing system provides for the fastest most accurate weight information to be available to the control system. Available as a platform scale or three-point suspension scale system as shown here.

7. Gear Box/Motor Drives

A variety of gear box types and sizes is available to accommodate a broad range of feed rate ranges. Motors are available as DC drives with 100:1 turndown capability, AC drives with 20:1 turndown capability and stepper drives. A variety of drives are available to meet hazardous location requirements.

8. Discharge

configurations can be supplied as open horizontal, closed vertical, and closed vertical with pressure compensation designs. Separate sampling valves are available to divert materials for calibration checks.