Hard Rules
There are few hard and fast rules when determining whether a WBF Feeder or LIWF Feeder is better for your process requirements. However, in some cases, the nature of an application may cause a feeder technology to be eliminated from consideration.
An example of “Hard Rules” may include:
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Hard Rule
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Should Use:
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Zero delay between the material measurement point and the discharge point
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Loss-In-Weight
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Feeding or batching a liquid or uncontrollably floodable material
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Loss-In-Weight
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Traceable and certifiable gravimetric material totals in a continuous system.
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Weigh Belt Feeder
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Periods of time when the feeder in volumetric mode is not acceptable
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Weigh Belt Feeder
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Remember the hard rules leave little room for discussion. Each of these rules will normally force a user to choose a particular technology.
Tendencies
Rather than the hard rules discussed above, more commonly found are tendencies towards choosing a Weigh Belt Feeder (WBF) or Loss-In-Weight (LIWF) Feeder for your process requirements. These tendencies are the root of many myths concerning feeders and their applications, but do have some basis in fact. Some Tendencies Include:
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Tendencies
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Lean Towards:
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Continuous feeding at very high feedrates
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Weigh Belt Feeder
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Weigh out batching of large amounts of material
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Weigh Belt Feeder
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Continuous feeding at very low feedrates
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Loss-In-Weight Feeder
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Weigh out batching of very small amounts of material
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Loss-In-Weight Feeder
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Weigh out batching in extremely short periods of time
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Loss-In-Weight Feeder
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High turndown (ratio of minimum to maximum feedrate) required
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Weigh Belt Feeder
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Pressurization of the feeding system required
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Loss-In-Weight Feeder
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Overlap
Because there are more tendencies than hard rules associated with choosing a LIWF or WBF solution, there are large areas of operational overlap between these types of feeders. In the overlap ranges, the accuracy and repeatability of both types of feeders are normally equivalent. Therefore, if your application falls into the overlap region, considerations other than performance come into play.
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Commercial Considerations
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Pros and Cons
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Space occupied by the feeder
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See Diagram A for an example of the physical envelope overlay of a WBF Feeder and a LIWF Feeder. In this example, the application overlaps both feeders, but the WBF Feeder can cover an operational range more than double that of the LIWF
Feeder.
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Investment in the feeder
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In general, an integrated LIWF Feeder will require a larger investment than a WBF Feeder. This becomes even more substantial at higher feedrates.
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Components required to supply material to the feeder
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A WBF Feeder will require that material always be present at the infeed. The rate at which material needs to be supplied to a WBF Feeder only needs to be slightly higher than the discharge rate of the WBF Feeder. For proper operation of a LIWF Feeder, the refill cycle requires that material be refilled into the hopper in a very short time. For this reason an industry standard of 1000% of the discharge rate of the LIWF Feeder is needed for proper refill. In addition, a control device such as a gate is required to provide refill control.
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Complexity of control algorithm
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In a weigh belt feeder a standard PID Algorithm is typically utilized. This algorithm is well known and performs well without complex tuning. In a LIWF Feeder, there are at least three algorithms that are used. A control algorithm for the deviation from setpoint, an algorithm used during refill, and under disturbance, and an algorithm to develop the actual feedrate measured by the weighing device.
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Sensitivity of the feeder to outside influences and disturbances
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While the performance of both types of feeders can be affected by disturbances such as vibration, the LIWF Feeder is much more sensitive to a disturbance than a weigh belt feeder. The physical installation of a WBF Feeder is also very simple when compared to the flexible connections and wiring routing needed to avoid transfer of disturbances to the weighing system. See Example 1 for an example of how a disturbance affects each feeder.
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Electrical Requirements
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In most cases, the motor and drive of a LIWF Feeder will be much larger than an equivalent belt feeder. In addition, if a mechanical agitator is required for the LIWF Feeder, a second motor and drive will be required.
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Flexibility in changing process rating of the feeder.
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Changing a material or the feedrate range of a LIWF Feeder is generally more difficult than changing a WBF Feeder. The hopper size and refill cycle often interfere with raising or lowering the operational parameters. Within limitations, often only a few drive components are needed to affect an appreciable change in throughput on a WBF Feeder.
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Diagram A Explanation
- The LIWF feeder shown has a twenty cubic foot material storage
- The Belt Feeder shown is enclosed and has a 12-inch belt width
- The belt feeder shown has at least four times the feedrate capacity range than the LIWF Feeder
- A refill control device would also be required between the LIWF Feeder and the material supply
- Belt Feeder normally requires a support stand, while the LIWF Feeder normally has its own support
Example 1 – Effects of a Disturbance
Example 1 demonstrates the effect of a disturbance on both a LIWF and WBF Feeder. The following parameters are used to generate the example:
- Feedrate of 20.00 Lbs/Minute
- Belt Speed of 5.000 Ft/Minute
- Belt Load of 4.00 Lbs/Foot
- Disturbance of .1 Lbs with a 1-second duration
Both the WBF Feeder and LIWF Feeder disturbance examples are shown in their simplest case.
The belt load is an integration of all material weight across a span, so the amount of belt travel, which occurs during the disturbance, affects the actual load variation. Since the belt is moving at 5.000 feet per minute and the disturbance lasts for one second, the belt load disturbance is very minimal. Complex control algorithms and the physical installation for LIWF can make the situation worse, but rarely better when compared to the sensitivity of a belt feeder.
Checklist
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Leans Towards
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Choice Factor
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LIWF
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Belt
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Batching Small amounts of material
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✔
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Batching Large amounts of material
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✔
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Continuous feeding at high feedrate
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✔
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Continuous feeding a low feedrate
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✔
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Pressurization of feeder required
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✔
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Continuous or batch feeding of Liquids
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✔
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Feeding an uncontrollably floodable non-liquid material
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✔
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Limited Rate of material supply to feeder
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✔
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Feedrate requirements could increase by an order of magnitude
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✔
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Summary
Using a hammer to saw a board or a screwdriver to pound a nail is not using the right tool for the right job. It is just as important to use the correct feeder for the correct process. When applied correctly, a Loss-in-Weight Feeder and a Weigh Belt Feeder can achieve the same accuracy and repeatability. While there are applications, which may require a LIWF Feeder, it is more common that either technology can be used. Given the commercial and process considerations outlined above, it makes sense to use a WBF Feeder wherever possible. Coupled with the release in recent years of sanitary and low-maintenance WBF Feeders, it makes the choice even easier than before.
