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In the hundred-plus years since bulk solids feeders were first used in continuous processing, dozens of design approaches have been developed, yet only four emerged as mainstream methodologies for controlling flow rate. The screw/auger, belt, rotary valve and vibratory feeders together account for more than 90 percent of all feeders in use today, whether in volumetric or gravimetric form. Each approach has its own unique set of advantages and limitations.
Increasingly demanding performance requirements in hi-tech sectors of the plastics, food, pharmaceutical and other industries have brought these traditional feeding technologies under critical scrutiny. Concerns such as second-to-second repeatability, full range linearity, simplicity and cleanability have risen in importance in recent years. In an effort to address these and other concerns, a new design approach to feeding has been developed. Known originally as Posimetric feeding, this patented technology was first applied in the late 1970s to feed oil shale and coal into large-scale crushing equipment. It has now been adapted for process industry application, and has been introduced as the Bulk Solids Pump (BSP) by the K-Tron Feeder Group of Pitman, NJ. The new feeder does not use the usual screws/augers, belts, pockets or vibratory trays to convey the material. It employs innovative positive-displacement action to feed pelletized, granular, flaked or other free-flowing materials with consistently high accuracy, uniform discharge and gentle handling in a true linear volumetric fashion. The BSP employs one or more specially designed vertical rotating spools which create feeding ducts. Incoming material is captured in the duct and rotated to discharge. While within the duct, the material ''locks up'' and acts as a solid mass. Approaching discharge, interparticle forces relax and discharge occurs by cascade at the material''s natural angle of repose. The smoothness of material flow from the BSP is evident to even the untrained eye compared to the pulsing flow from a rotary valve or screw feeder. This smooth, even flow results in significantly improved accuracy levels, especially at short sample durations.
As illustrated in Figure 1, the BSP principle is best explained as consisting of four zones: consolidation, rotation, relaxation and discharge. In Zone 1 material enters the feeder and consolidates as particles settle and come into contact with one another and the sides of the duct. At the end of Zone 1 the material is fully constrained by the duct, and interparticle and surface contact forces produce the lock-up condition. In Zone 2 the material rotates as a solid mass. As the material moves past the 6 o''clock position Zone 3 marks the approach to unconstrained discharge where interparticle forces fall, the material relaxes and regains its natural flowability. Discharge occurs in Zone 4 where duct rotation causes the material to cascade from the feeder.
Repeatability, a statistical measure of the variability of feeder discharge at a given setpoint, is traditionally based on 30 one-minute samples and is expressed in terms of +X percent of set rate at a 2 Sigma or 95.5 percent confidence level. However, many of today''s critical processes require repeatability measurements not only throughout the feeder''s full operating range, but also at sample timescales reflective of the process itself... often only a few seconds.
To document the BSP''s performance and compare it with that of a single screw feeder, extensive, carefully controlled tests were conducted at K-Tron''s testing facility. Three relatively free-flowing materials (plastic pellets, wax granules, and citric acid granules) were fed on both a model BSP-125 Bulk Solids Pump and single screw model K2MVS60. Automated data collection of catch sample weights enabled sample durations as short as one second to be reliably obtained.
Figure 2 plots repeatability error vs sample duration (1, 5, 10 and 15 seconds) and shows the average repeatability for each of the materials tested. In each case, the BSP''s repeatability error was a fraction of that measured for the screw feeder. The exceptional uniformity of the BSP''s discharge contrasts sharply with the single screw''s discharge pulsation effect, as clearly revealed in the one-second sample duration results. Note that even though the BSP tests resulted in repeatability errors of almost exclusively less than 1 percent, all tests were conducted with the BSP acting as a speed-controlled volumetric feeder, not as a gravimetric feeder where performance would be further improved.
Linearity is a measure of a feeder''s ability to deliver an average discharge rate directly proportional to feeder speed over the feeder''s full operating range. In dozens of developmental tests involving comparison of linearity performance on identical materials over identical ranges of rates, the BSP approach averaged a linearity error of +0.1 percent over its 100:1 turndown range versus the screw/auger''s average error of +2.0 percent over the same turndown.
The BSP''s open, obstructionless design provides little opportunity for material build-up anywhere in the feeding zone. With quick and easy access to the feeding zone, no pockets or screws and only one moving part, the BSP feeder is cleaned in seconds, making it ideal for applications with frequent material changes.
For more information on the BSP, contact K-Tron Feeder Group at 856-589-0500, email firstname.lastname@example.org.