PT ZONE: Loss-in-Weight Enhancements
Mostly encountered in primary resin compounding where throughputs are high, the combination of rotary valve and a gravimetric solids flow meter offers a simplified alternative to the use of a weigh-belt feeder in controlling base resin (fluff) feed rate for the proportioning of additives prior to pelletization. If required, rotary delumping can substitute for the rotary valve to reduce clump/particle size.
In operation, a rotary valve withdraws the material from the primary resin surge hopper and discharges it to the flow meter. Falling within the meter, the material is deflected by an inclined channel and is then again redirected by a second, vertical channel. The forces experienced by these two channel sections are separately measured and combined to produce the gravimetric flow rate measurement. Based on this, rotary valve speed is adjusted to produce the desired feed rate. Downstream additive feeders are then slaved or proportioned to this rate. A bypass channel within the meter enables taring without process interruption.
The principle of operation can be likened to an inclined weigh belt feeder with constant belt speed in combination with an impact plate. The bulk material enters the flow meter through a stabilization section to reduce its vertical velocity component. It then enters an upper measurement channel where it slides across an inclined surface. The upper measurement channel is suspended by a vibrating wire load cell and continuously weighed, producing the first force measurement, F1. Subsequently, the bulk material impacts and flows through a separate vertical measurement channel, suspended by a second vibrating wire load cell, producing force measurement F2. From these two continuous measurements mass flow and velocity can be calculated according to the equation below : where ‘F1’ = the force on the inclined chute, ‘F2’ = the force on the vertical chute, ‘l’ = length of the inclined chute ‘α’(alpha) = the angle of the inclined chute, and ‘g’ = gravitational constant.
Advantages, Limitations and Considerations
Accuracy – Comparable in performance to its weigh-belt alternative, a weigh-meter / rotary valve feeding system is capable of sustained high gravimetric accuracy, typically controlling feed rate to + ½% of set rate or better.
Range – In plastics processing operations, weigh-meter feeding is typically best suited to the high-rate regime typically encountered in primary resin production (a few hundreds of thousands lb/hr).
Material Handling – Due to its dedicated mission of handling newly precipitated base resin (in its various forms, depending on wet-end process), the material handling requirements faced by this approach are relatively limited. While the weigh-meter portion of this feeding system relies only on gravity to move the material and offers an obstructionless flow path as well, no real handling concerns are associated with this portion of the feeding system. However, the rotary valve’s job of withdrawing the resin from primary storage and reliably discharging it to the weigh-meter may require careful equipment selection, and, in some instances, special flow aid measures as required. Additionally, as mentioned above, where the resin clumps or forms large chunks, the use of a delumping rotary valve is usually indicated.
Material Containment – The weigh-meter / rotary valve principle permits complete material containment.
Tare – As a result of its operating principle the weigh meter is sensitive to shifts in tare, and therefore requires periodic re-zeroing to adjust for the weight of any dust or material residue that may adhere to or become deposited on its inclined and vertical weighed surfaces.
Weighing – Unlike the other two principles presented in this section, the weigh-meter / rotary valve feeding principle is unique in that the material whose flow rate is being measured is falling under the influence of gravity. Thus, crucial to measurement accuracy for this feeding principle is not only high resolution, but ultra-fast dynamic response to the quickly changing forces applied by the falling material.
Isolation – As a direct result of its operating principle, insulation from ambient vibration is not as great a concern here as in the other feeding principles already presented. As a result, this class of feeder may be rigidly mounted at installation without any special measures taken against the effects of vibration.
Size – In primary resin compounding application, this principle offers a slightly smaller installation footprint than is typically occupied by its weigh-belt alternative. Although somewhat taller than an equivalent weigh-belt due to its reliance on gravity to move material through it, a weigh-meter feeder’s increased height is not usually a relevant factor in this kind of application.
The weigh-meter/rotary valve feeder offers a clean, cost-effective and essentially maintenance-free alternative to the well known weigh-belt approach, delivering comparable performance and obstructionless flow in a mechanically simpler package.