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PBEI | March 2017 17 speed is relatively constant since the material bulk density in the feeder's metering zone doesn't vary substantially. This is because the material in a full hopper has a head load that con- denses the material in the hopper's lower section to a rela- tively consistent bulk density. As feeding proceeds and the material level in the hopper declines, the head load above the metering zone decreases, and the material becomes less dense. The controller increases the motor speed to maintain the desired feedrate until the hopper weight reaches the refill request threshold, and the refill phase begins. At the beginning of the refill phase, the motor speed is the same as at the refill request threshold, but as the hopper fills, the controller applies the stored feed factors as each cor- responding hopper weight is reached and decreases the motor speed accordingly. This more sophisticated approach to hop- per refill allows the feeder to smoothly exit the refill phase and return to true gravimetric operation. Additionally, by controlling motor speed based on the most recent perfor- mance history, gravimetric accuracy is essentially preserved during refill, as shown in the bottom graph in Figure 2. 3. Compensate for process pressure fluctuations. If your LIW feeder is discharging material into a variable pressure environment such as a pressure or vacuum pneu- matic conveying line, a pulse in the pressure or vacuum can cause a feedrate error. A pressure pulse exerts an upward force, slightly lifting the hopper and affecting the load cells' instantaneous weight measurement. The controller may in- terpret this effect as too much material having been dis- pensed and slow the motor speed to compensate, resulting in a lower than desired actual feedrate. A consistent positive pressure at the feeder discharge, on the other hand, can cause the controller to underestimate how much material is being fed and speed up the motor, re- sulting in a higher than desired actual feedrate. The Solids Flow Experts If you have a Particle, We can Characterize it Getting it Right the First Time We are dedicated to providing cost effec�ve engineered solu�ons based on the measurement of key material flow proper�es. We can combine material flow property tes�ng with nearly 40 years consul�ng experience to solve any flow and handling problems facing today's producers of fine powders and granules. We have Solved Problems with: Degrada�on • Segrega�on • Erra�c Flow • Caking • Off-Spec Product • System Failure • System Plug • Blending Issues • Scale-up C u s t o m D e s i g n s B as e d on Me as u r e d Mat e r ial F l ow Pr op e r � e s Call us: +1-352-379-8879 Visit us: www.ma�lowsol.com Figure 5 Electronic pressure compensation Pressure sensor EPC device Pressure sensor Feeder discharge