Loss-in-weight feeders have evolved from the crude devices of the late 1940s to sophisticated machines capable of feeding at continuous accuracies down to 0.2 percent of sample size at a 2 sigma confidence level (depending on material). Due to the nature of their operation, loss-in-weight feeders require periodic refill from an external device such as a screw conveyor, bin activator, pneumatic conveyor, slide gate valve, or rotary valve. Capacity requirements of the refill device and desirable refill frequencies are often overlooked by process engineers, yet they can have a major impact on the overall process accuracy. This article addresses these refill requirements and serves as a guide for specifying loss-in-weight feeders.
A loss-in-weight feeder consists of three components: a scale with a dedicated controller to weigh material; a feed mechanism (screw, vibrating tray, etc.) with a variable-speed drive to introduce material into the process; and an integral weigh hopper for material storage. As the name suggests, the unit measures a loss in weight over a certain period of time and adjusts the feed mechanism output accordingly. For example, say that the desired set point is 100 lb/hr, and at 9am the feeder has 1000 lb of material in the hopper. The feeder is started, and at 10am the weight is measured at 902 lb. The feeder has lost 98 lb of material in one hour, which is slightly below the desired feed rate of 100 lb/hr. To get back to the set point, the controller increases the feed mechanism’s output. In reality, of course, today’s loss-in-weight feeders do not sample weight every hour but approximately every 50 milliseconds to achieve the precise second-by-second control required by modern processes.
Refilling the Hopper
What happens, however, when the hopper begins to run out of material? While it is clear that the hopper must be refilled, it is equally clear that the feeder cannot control the process while the loss-in-weight scale is gaining weight rather than losing it.
In the past there were two possible solutions: shut down the feeder during refill or use two feeders, one to feed material into the process while the other is being refilled. Modern controllers bypass this problem by controlling the feed mechanism at a constant speed (volumetric control) during refill. This constant speed is determined by averaging some number of tachometer readings from the feeder drive motor just prior to refill to compensate for bulk density changes due to material head in the hopper. While this method is superior, it still does not address the fundamental problem: lack of gravimetric control during the refill period.
The feeder’s failure to respond to changes in humidity, temperature, bulk density in the screw, and screw filling characteristics during the refill period leads to a loss of feed rate control. Overall accuracy is increased when the refill time is as short as possible.
Another aspect of the refill is the repeatability of the refill device. The more that this is repeatable (say it occurs every time in 15 seconds rather than sometimes in 45 seconds, and other times in 5 seconds) the better the overall system will operate.
When a 200-lb person steps on the bathroom scale, the scale reading does not instantly go to 200 lb. Rather, it fluctuates in some random manner-say to 220, 190, 205, 198, etc., and after a few seconds eventually settles at the proper reading. The same phenomenon occurs upon refilling a loss-in-weight feeder. The refill process causes the scale weight reading to quickly go from a low to a high value. When the refill device is shut off, the feeder does not instantaneously revert to the gravimetric feed mode. The weight reading continues to oscillate for a period of time until the controller detects two consecutive readings within the preset deviation tolerance. At this point, the unit begins to feed gravimetrically. The time it takes to do this is dependent on the scale construction and the type of load cell used in the feeder. In general, high-displacement load cells take longer to stabilize than low-displacement load cells, leaving the feeder in volumetric control for a longer period of time.
In certain cases, material stabilization times should also be considered. For example, highly floodable solids may take longer to stabilize due to the action of the entrained air attempting to escape from the weigh hopper.
Accuracy Versus Refill Weight
A typical accuracy statement for a loss-in-weight feeder specifies repeatability as a percentage of the sample-size at a given statistical confidence level. For example, a repeatability of 0.5 percent of sample size at 2 sigma means that if a sufficient number of consecutive 1-min. samples are taken at a 6000 lb/hr feed rate, then 95 percent of the samples will weigh between 99.5 and 100.5 lb (assuming perfect linearity). Implicit in this statement, however is an assumption about the refill rate. Compared to the feeding time, the refill time needs to be as short as possible to maximize the percentage of time the feeder is under gravimetric control. Although higher percentages are preferable, the following example will assume that a loss-in-weight feeder needs to be under gravimetric control for at least 90 percent of its cycle time for the accuracy expectation to be valid. Remember that the time in volumetric mode is defined as refill time plus scale stabilization time.
Weigh Hopper Selection
We are now ready to proceed with the actual sizing of the loss-in-weight weigh hopper and scale. This issue confronting the user is this: suppose the required feed rate is 600 lb/hr. The feeder can be supplied with a 1000-lb scale and hopper, and be refilled once an hour, or it can be supplied with 30-lb scale and hopper that permits 20-lb refills every two minutes. Which unit would be more accurate?
Unfortunately, there is no universally correct answer to this question, as there are several factors working in opposite directions. In a refill situation with a large scale the problems are twofold: (1) the larger scale reduces the scale sensitivity, which has a detrimental effect on control; and (2) some scales have an inherent friction component in the level fulcrum that limits the weight change to which the scale will respond. As the scale get larger, this weight change increases again and that has an adverse effect on control. In a refill situation with a small scale and hopper the problems of an infrequent refill are overcome, but raises others: (1) for high-displacement scales, time in volumetric control is significantly increased; and (2) mechanical limitations of the refill device become significant. For example, in a case with a 30-refill-per-hour and 5-second scale stabilization, the allowable refill time is only seven seconds. In this case, the refill device must be fast acting.
In summary, the following items are offered as points of consideration for process engineers when sizing loss-in-weight feeders:
Scale stabilization time. This varies among manufacturers and is dependent on the scale design and the type of load cell used. The manufacturer should quote scale stabilization time to the client and be prepared to demonstrate it in their test laboratory. This is accomplished by performing an actual refill or by adding and removing a test weight and observing the time required for the scale to return to its original reading.
Refill mechanism. If a refill mechanism exists, the manufacturer should be able to provide information on its capacity. When combined with the scale stabilization time, the capacity may limit the possible refill frequency. If a refill mechanism does not exist, the manufacturer should know what capacity is required.
Headroom. If a feeder needs to be fitted into a tight spot, then a small hopper unit that provides frequent refills may be the answer.
Material characteristics. If the material is difficult to handle, with a tendency to pack under pressure of re-agglomerate after a few minutes in storage, then a small hopper unit may again be the answer. In this case, small hoppers tend to improve material handling and provide better gravimetric control.
Process requirements. Some processes require second-to-second control and cannot tolerate being in volumetric mode for minutes at a time while a larger feeder is refilling. Although spending time in volumetric mode is inevitable, it may be desirable to spread it out over a large number of cycles-for instance, 20 times at 10 seconds each rather than once for three minutes-to minimize the impact of inaccurate feeding on the process.
Andy Kovats is a sales manager at Brabender Technologie Inc., Toronto, ON, Canada. He holds a BS in chemical engineering from Queen’s University, Kingston, ON and an MBA from York University, Toronto, ON. Kovats is a registered professional engineer in the province of Ontario. For more information, call 905-670-2933, email [email protected], or visit www.brabenderti.com.
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