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How to enhance loss-in-weight performance in higher rate refills.

April 23, 2004
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Loss-in-weight feeders have evolved from mechanically ponderous devices to the sophisticated microprocessor controlled instruments of today. Weighing and control advancements over the years have made loss-in-weight (gravimetric) feeding the preferred method wherever the combination of high gravimetric accuracy, ingredient containment, and material handling capability are needed. However, loss-in-weight feeding does possess some shortcomings, especially at higher feed rates. First, during the feeder''s required hopper refill phase, weight-based control must be temporarily suspended and replaced with volumetric control. It is in this refill phase that significant feed rate errors can occur due to volumetric control inaccuracies. Second, higher feed rates have historically meant physically large and expensive systems. In some cases required space could only be obtained at the cost of significant structural changes to the plant itself. These refill challenges are especially great when feeding tough to handle powders.
Feeder companies have taken varying approaches to the challenges present during refill of the feeder''s hopper. One company, K-Tron, utilizes an approach called "Smart Refill Technology" (SRT). Smart Refill Technology ensures that intermediate to high rate loss-in-weight systems do not have to suffer from feed rate inaccuracy during refill and can use a more compact hopper offering a more economic solution to fitting into tight locations in the factory. SRT combines a refill control concept called the Refill Array with precision digital weighing to deliver improved feeding accuracy during hopper refill, substantially reduced headroom requirements, the elimination of material handling problems associated with large feeder hoppers, smooth and quick transitions into and out of hopper refill, and lower total installation costs.


The loss-in-weight principle
Loss-in-weight feeding achieves rate control by weighing the entire feeder, hopper and the material contained in it. The speed of the metering device is controlled to result in a per-unit-time loss of system weight equal to the desired feed rate. In this way high accuracy may be achieved, and nearly any metering device appropriate to handle the material at the desired rate may be selected.

As an integral part of loss-in-weight feeding the weighed hopper must be resupplied with material. To continuously resupply the hopper would destroy the very basis for control, the declining weight value itself. Hence, the hopper must be resupplied periodically rather than continuously.

Lacking any basis for gravimetric control during these brief but periodic refill phases, flow control is achieved volumetrically. Traditionally, a constant metering speed is maintained throughout the refill phase – a speed corresponding to the metering speed associated with gravimetric control just prior to entering the refill phase. If, for example, metering speed averaged 60 rpm just prior to the system sensing the need to refill the supply hopper, screw speed would be maintained at that 60 rpm for the duration of the refill operation. After refill is completed, material has settled, and the feeder senses an appropriately declining system weight, the feeder is returned to gravimetric operation where metering speed once again becomes the parameter of control.

The prime reason for this is to ensure an ever-present supply of material at the metering device so feeding may proceed without interruption. Additionally, if a sufficiently large material heel is not present, the increasing pressure applied by the impact of the incoming, possibly aerated material during refill may cause uncontrolled flooding through the feeder.

Event with an insulating heel of material in place, density within the metering zone will rise somewhat as the hopper fills. Given a constant metering speed during refill (60 rpm in our example), this increase in density causes progressive degradation (overfeeding) in feeding accuracy as more and more material enters the hopper and compacts the material in the hopper''s lower regions. How severe is this inaccuracy? The answer hinges on hopper size/geometry in addition to the compactability of the material itself. Laboratory tests and field experience involving many hundreds of materials show that, in practical terms, headload-related loss-in-weight overfeeding may range anywhere between +1 percent for relatively constant density materials to +10-15 percent for powders and other materials whose density can vary substantially.

The Smart Refill concept
To minimize feed rate errors during refill, Smart Refill Technology discards the approach of maintaining a constant metering speed. Instead, SRT enables metering speed to be gradually lowered during refill to precisely counterbalance the effects of increasing material density occurring in the metering zone as hopper weight increases. The slower rate is determined by storing in the controller''s memory an array of indices, called feed factors. These values correspond largely to material density and its mechanical behavior within the feeder, and are computed during the entirety of the gravimetric feeding cycle.

Then, on the basis of sensed hopper weight at each array point during refill, material density within the metering zone may be inferred, and a metering speed corresponding to its feed factor array value may be invoked. In this way gravimetric feeding accuracy during the brief refill may be maintained.

Beginning with a full hopper (where net hopper weight equals refill complete weight), gravimetric operation is in effect and the feeder operates normally, according to the operating principle explained above. As feeding proceeds and net hopper weight declines, the controller also determines and stores a set of up to 100 feed factors, each of which is an index of the average density of material discharged at the hopper weight associated with the feed factor. A low feed factor indicates that a higher number of screw revolutions were required to discharge a given weight, implying a reduced material density. Conversely, a high feed factor reflects higher density since fewer screw revolutions were required to deliver the same material weight.

The middle plot shows motor speed versus time. During the early portion of the gravimetric feeding phase, motor speed is relatively constant since density within the metering zone of the feeder, while higher than at later times in the feeding cycle, does not vary substantially. This is because material in the upper portion of a typical hopper is largely supported by the material below and, in turn, the tapering walls of the lower portion of the hopper. As feeding proceeds and hopper level declines, headload in the metering zone begins to lessen, resulting in a reduction in density and a corresponding increase in motor speed required to maintain feed rate. When hopper weight reaches the refill request threshold, the refill phase begins. During refill SRT begins with the motor speed that was in effect at the time of refill request, and then modifies that speed by applying the corresponding feed factor as each hopper weight ''slice'' is encountered. Without SRT, motor speed remains constant throughout the refill cycle, which results in overfeeding.

The mass flow error associated with constant metering screw speed during refill is shown in the bottom illustration of Diagram C. Note that metering speed is shown to remain constant for some time after refill completion. This undesirable effect is typical for conventional loss-in-weight systems that require several seconds for their weighing systems to stabilize before reverting to gravimetric control. Fast response digital weighing technology, such as K-Tron''s Smart Force Transducer (SFT) single vibrating wire, is needed in order to produce a low stabilization time without a big delay. Note also that when speed is held constant during refill an abrupt change in metering screw speed is required upon re-entry to gravimetric operation. SRT, in contrast, exhibits no such discontinuity, resulting in a smooth transition from refill to normal gravimetric operation.

Low or high refill frequency?
SRT provides another valuable benefit: the opportunity to substantially reduce overall feeder size and cost by enabling refill to occur at a much higher frequency than before. In earlier loss-in-weight systems a low frequency refill approach was taken. In that approach a relatively lengthy gravimetric feeding phase is followed by refill whose duration should not exceed 10 percent of the gravimetric phase. The hopper must, of course, have a capacity greater than the amount of material fed during the entire gravimetric phase. In low rate applications this requirement is not a problem; however, as feed rate increases, so must the capacity of the hopper. As a result, high-rate loss-in-weight systems have historically been large and bulky with hoppers often in the hundreds-of-cubic-feet capacity range. This, in turn, necessitated a much larger range (and hence less sensitive) weighing mechanism. The low frequency refill approach therefore translates directly into the high initial cost associated with purchasing and installing a physically large feeding system, in addition to the high continuing costs of the inefficient use of plant area and headroom.

Consider alternatively a comparably high rate loss-in-weight system with a high frequency of refill as shown in the same diagram. The metering device itself remains the same so as to provide the desired feed rate, but the hopper and weighing mechanism may now be much smaller. Both the gravimetric feeding phase and the volumetric refill phase are much shorter in duration – even up to ten times shorter than they would be under the low frequency refill approach. Note, however, that while the duration of the feed and refill cycles are much reduced, the total time spent in these cycles is the same as in the low frequency approach… the cycles are simply more finely divided than before. Similarly, while the refill device is required to resupply the weighed hopper more frequently, there is no difference in the rate at which it must deliver material.

The analogy of driving a car with your eyes closed highlights one of the most significant benefits of the high frequency refill approach: much shorter periods spent in volumetric control. In high frequency refill the feeding system operates volumetrically for a much shorter period before returning to true gravimetric control. This is analogous to blinking your eyes normally during driving. It is easy to stay in control. However, in the case of low frequency refill, volumetric control persists for a much longer time before re-entering gravimetric control. This is akin to closing your eyes for a dangerously long time when driving. By refilling more frequently, there is less of an opportunity for feed rate to deviate from setpoint before gravimetric control is re-established.

Another compelling benefit to the high frequency approach comes in the form of a smaller and more compact feeding system. Costs connected with purchase cost, installation and plant area/headroom are all cut. Specifically, depending on the application, savings on equipment costs of up to 30 percent are typical. Also, plant space savings of up to 70 percent are possible when planning a new process line or upgrading and old one. See Diagram E.

System dynamics
Let us now consider the more practical side of the issue, both from the point of view of system dynamics and the concerns of the application itself. First, system dynamics.

Precision loss-in-weight feeding is a true exercise in dynamic control. To achieve control on a moment-to-moment basis the weighing system must accurately discern the constantly declining system weight, and then the controller must, in turn, compare that weight to the known target weight at that instant, and then issue an adjusting command to the metering device. In intermediate/high rate loss-in-weight units, this dynamically based control loop may occur many times per second, placing demands on the responsiveness of the weighing system and controller alike.

During normal, constant-rate gravimetric feeding, control adjustments are typically small, and thus most loss-in-weight systems are fully capable of reasonably accurately tracking the smoothly declining weight. However, for loss-in-weight systems employing deflection-dependent weighing mechanisms, problems can arise during the attempted transition out of the refill phase. Reverberations of the dynamic loading changes encountered during refill act to delay resumption of gravimetric feeding. Deflection-dependent weighing systems characteristically exhibit high stabilization times, and no loss-in-weight system will allow itself to re-establish gravimetric operation until a valid and credible weight signal is present. It is primarily for this reason that conventional, high-deflection loss-in-weight systems have historically been developed as low frequency refill devices, utilizing large hoppers. To adopt a high frequency refill approach while using a deflection-dependent weighing system would mean that an excessive amount of time must be spent under volumetric rather than gravimetric control.

Application concerns
Turning to the application itself, four points deserve specific mention. First, as mentioned above, the maximum refill time should not exceed 10 percent of the total feed/refill cycle time.

Second, adequate venting of the hopper must be provided. Material considerations form the third area of concern. The fourth and final concern focuses on the refill device. Largely the material itself will determine the particular type of refill device.

Conclusion
A much-improved approach to higher-rate loss-in-weight feeding is made possible by the development of Smart Refill Technology in combination with advances in weighing and control systems. High-frequency refill affords substantial cost reductions at purchase, at installation, and throughout the life of the application. Intermediate-to-high-rate loss-in-weight systems can now be offered with compactness in mind with no loss in performance.

For more information, contact K-Tron at 856-589-0500 or email info@ktron.com

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