By Sharon Nowak
The volatile state of today's global economy is forcing dry powder processors to evaluate more-efficient and more-cost-effective technical solutions for their processes. Key objectives such as optimizing margins by minimizing downtime due to maintenance, cleaning and product changeover, utilizing higher accuracy dosing methods to optimize yields and minimize ingredient costs, and conversion of high-cost batch processes to continuous ones are all being investigated. The selection of the proper bulk solids feeder technology can be a critical step toward achieving these objectives while maximizing process efficiency and product quality.
Table I: Material Characteristics
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Bulk solids feeders utilized in dry powder processing include weigh belt feeders, vibratory feeders, rotary valves, and screw feeders (both twin and single screw). In many of these technologies, there are options for both gravimetric (via loss-in-weight) or volumetric feed. Gravimetric feeders with loss-in-weight technology directly measure and control the process variable of mass flow; volumetric feeders control flow by discharging constant volumes of material per specified time interval.
Bulk Solids Material Characteristics Defined
To choose the best feeder for the application it is essential to evaluate the appropriate material characteristics, as shown in Table I. The characteristics of the material to be handled will often narrow the feeder technology selection.
The properties of these characteristics aid in the classification of most materials into one of four categories: floodable, free flowing, difficult to flow, and cohesive or sticky. With a simple manual test, one can make a preliminary determination as to which category a material is classified. Take a fistful of material in your hand and squeeze it into a ball. If the material squirts out and doesn't form a ball, it is likely to be floodable. If the material does not escape the hand when squeezed, nor does it form a ball when compacted, it can be considered free flowing. If the material is stringy (like fibrous materials), it is considered difficult to flow. Finally, if the material makes a ball when squeezed and retains its shape when released, the material can be categorized as cohesive.
Measuring Feeder Performance
To fully define feeder accuracy, it is necessary to address three separate and distinct areas of feeder performance: repeatability, linearity, and stability. Repeatability reports how consistent the feeder's discharge rate is at a given operating point; linearity assesses how accurately the feeder discharges at the requested average rate over its full operating range; and stability gauges performance drift over time.
Repeatability is the performance statistic most familiar to feeder users. It quantifies the short-term level of consistency of discharge rate. Repeatability is of importance to quality assurance because it measures the expected variability of the discharge stream, and hence of the product itself. The repeatability measurement is made by taking a series of carefully timed consecutive catch samples from the discharge stream, weighing them, and then calculating the + standard deviation of sample weights; those are expressed as a percentage of the mean value of the samples taken.
Dry powder feeder technologies
It is important to note that the repeatability statistic reveals nothing at all about whether the feeder is delivering, on average, the targeted rate. Repeatability only measures variability of flow rate. Rather, it is the linearity statistic that reports how well the feeder delivers the desired average rate throughout the feeder's operating range. Perfect linearity is represented by a straight-line correspondence between the setpoint and the actual average feed rate throughout the feeder's specified turndown range. To perform a linearity measurement, typically 10 consecutive catch samples are obtained and weighed at each of the following flow rates: 5, 25, 50, 75, and 100% of full scale. The smallest tested flow rate should be at the feeder's maximum turndown.
A perfectly performing feeder is worth little if it can't maintain its performance over the long haul. Many factors can potentially contribute to performance drift, such as feeder type, control and weigh system stability, the handling characteristics and variability of the material, the feeder's mechanical systems, maintenance, and the operating environment itself. Drift is detected by calibration checks, and is typically remedied by a simple weight span adjustment. The user will ultimately determine the appropriate frequency of calibration checks based on operational experience.
For accuracy requirements in the 1–5% range, volumetric feeders will usually suffice, while gravimetric feeders are used for performance in the ¼–1% range.
Feeder Technologies Defined
Quick clean design rotary airlock for ease in access during cleaning
Rotary airlocks are typically used for high-volume applications and in cases in which the accuracy ranges of a volumetric device are acceptable. The delivery of product is done by a series of pockets, which can be either shallow or deep, that deliver a certain volume of product in a set period of time. However, due to the nature of the shallow or deep pocket, they are not ideally suited for cohesive materials, because these materials will often pack in the pockets and cause the feed to be erratic. In addition, they can also experience a pulsating discharge. This makes the flow from them inconsistent.
In the case of a vibratory feeder, the material is metered by means of vibration. A set amount of material can be delivered by adjusting the amplitude of the vibration. These devices can be provided in both gravimetric and volumetric options, and are often well suited for free-flowing materials, as well as materials with high aspect ratios. They are generally not well suited for the metered feed of cohesive materials because the vibration effect may cause the material to clump in the delivery tray. That, in turn, may cause delivering the product inconsistently to the process below.
Weigh Belt Feeders
Single and twin screw feeders with various screw configurations
Weigh belt feeders are extremely gentle on the product, can be used for very high feed rates, and can oftentimes be extended in use as a conveyor. Weigh belt feeders are also often chosen due to their low headroom requirement, thus making them ideal for cases in which multiple feed devices may be needed for a single process. Weigh belt feeders operate by continuously weighing a moving bed of material on its short conveyor and controlling belt speed, resulting in the desired flow rate at discharge. Weigh belt feeders can achieve high rates while remaining compact, simply through a combination of manipulating material bed geometry and operating at higher belt speeds.
Single- or twin-screw feeders represent the most versatile feed technology for most bulk solids. Given the material characteristics, the suitable screw configuration is made for the required feed rate. In the case of a continuous operation, the feeder hopper is sized based on the refill requirements of the feeder and the space available. Due to high turndown ratio and flexibility of design, twin-screw feeders can be provided to feed accurately at rates as low as 20 g/hr.
Volumetric versus Gravimetric Feeding
Volumetric feeders operate by delivering a certain volume of material per unit time. Flow-rate changes are accomplished by altering speed. In the case of a screw feeder, three factors affect volumetric screw feeder accuracy: the consistency of delivered volume per screw revolution; the accuracy of screw speed control; and material density variability.
Open frame weigh belt feeder with easy belt removal feature
Typically, volumetric feeders are open-loop devices that cannot detect or adjust to variations in the material's density. Due to the open-loop concept, head load variations and material buildup on the feed device change the volume-per-revolution relationship, throwing off calibration without any outward sign. Gravimetric feeders automatically detect and adjust to these conditions. In cases of screw feeding of cohesive materials, it is possible in volumetric mode to have relatively no material discharging while the screws are running, such as in cases of bridge building or packing in the hopper. Similarly, flood-through can also remain undetected since the feeder has no way of knowing the out-of-control condition. Since the feed rate in a volumetric feeder is purely a function of speed, the feeder and the process below have no way of detecting this upset condition. Oftentimes, even the use of level sensors in the feed hopper may not alert the process of this upset in a timely fashion. Most gravimetric feeders can automatically detect and alarm to these conditions.
The Loss-in-Weight Principle
The most popular type of gravimetric feeder used is the loss-in-weight feeder. Loss-in-weight feeders directly measure and control the process variable of flow rate and can fully contain the material within the confines of the feeder. Loss-in-weight feeders are typically either mounted on weigh scales or suspended from load cells.
Gravimetric/loss-in-weight operating principle
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A loss-in-weight feeder consists of a hopper and feeder that are isolated from the process so the entire system can be continuously weighed. As the feeder discharges material, system weight declines. 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, or mass flow rate. As feeding proceeds, the measured weight is continually compared with the setpoint line's target weight. Any difference between the two values triggers a change in feeder speed. For example, if an overfeed condition occurs due to an abrupt increase in material density, sensed weight falls below the desired (setpoint) weight, triggering a reduction in screw speed to return to the setpoint value. Additionally, since the integrated error associated with the overfeed is known, screw speed may be further reduced to immediately and precisely compensate for the overfeed condition. The opposite occurs with an underfeed condition.
As outlined above, there are many choices and critical components of feeder selection. It is important to fully understand both the material characteristics and the influences of the overall process and subsequent process environment that the feeder will become a part of. By conveying these details to the feeder manufacturer, the essential requirements will be defined and the most efficient and accurate design will be incorporated into the process.
Sharon Nowak serves as global business development manager for the food and pharmaceutical industries for the K-Tron Process Group (Pitman, NJ). Nowak works closely with the R&D and engineering departments to identify new applications and focus on the specific needs of these industries. Nowak has a 20-year background in the process equipment industry for food and pharmaceuticals, as well as a degree in chemical engineering from Rutgers University. For more information on K-Tron, visit www.ktron.com.