Vibratory feeders for powders.
A tire manufacturer needs to feed 30 pounds per minute of minus 80-mesh carbon black powder at an even rate of 35 pounds per minute, plus or minus two pounds, into a larger batch. There can be no product degradation and the equipment must be easy to clean.
A specialty glass company needs to add a fine powder color additive at 40 pounds per minute. There can be no cross-contamination from one batch to the next.
In these two examples, a new generation of hybrid feeder may offer the best solution to the feeding and metering challenges, which are certain to develop.
Operation and advantages
Electromagnetic vibratory feeders are unique in that they have no belts, bearings, rotating or sliding parts. They are virtually maintenance free. It is common for an electromagnetic feeder to have been in service for 15 to 20 years without requiring service.
Electromagnatic feeders having trays with bolted covers or tube trays that are totally enclosed can be easy to clean. There are no belts that material gets trapped under and no screws that can degrade material or leave a small amount in the bottom of the tube. There is also no metal to metal contact, which can cause metal contamination, so they are considered very good where cross contamination is a problem.
Some powders do not feed well in traditional electromagnetic feeders, particularly if the material is less than 40 to 50 mesh in size and/or less than 50 pounds per cubic foot. The reason is that an electromagnetic feeder produces approximately 11 to 12 Gs of force at 3,600 cycles and .060-inch amplitude. With light, fine material there is not enough mass to cause the particles to return to the tray before it starts its next cycle. The particles of powder actually become suspended in the air and the material fluidizes and takes on the properties of a liquid. When this happens, the material will flow over the sides of the tray or may become very shallow and only feed at about 1/8 to 1/4-inch deep in a tray. These conditions cause erratic feeding and low capacity feed rates.
By decreasing the frequency, fluidization can be eliminated or reduced. By increasing the amplitude or "throw," the feed rate can be increased.
The challenge is in providing a stable control that will convert the line frequency to a lower frequency. With today's electronics, this can be achieved and the result is a feeder with the advantages previously only available in a mechanical feeder, i.e., high-amplitude and low-frequency, along with the advantages of an electromagnetic feeder such as quick stopping, no rotating or sliding parts, no belts and compact design.
A new generation of electromagnetic feeders is coming to market that will revolutionize how fine powders are fed. Two of the manufacturers of electromagnetic feeders are already building this type of unit.
These units can be used to feed powdery-type materials at controlled rates in pharmaceutical and chemical plants, the food industry, glass making, tire manufacturing, steel production and any other industry that has a need to feed, meter, blend or convey minus 50 mesh materials.
How it works
The feeder is similar to other electromagnetic feeders, except that it is designed to operate at 30 cycles instead of 60 cycles. This is accomplished by intermeshing an electromagnet with a rare earth permanent magnet to keep heat down and maintain a compact size. The tuning springs that "match" the tray to the drive unit are increased in length to increase the amplitude to approximately 1/4-inch.
The electronic controls are the heart of the feeder's operation because control must be very precise. It cannot vary more than 1/2 of a cycle or erratic action from the feeder could occur.
What is done is to remove every second cycle to change the frequency from 60 cycles to 30 cycles, and for foreign operation from 50 cycles to 25 cycles.
The difference in feeding characteristics between a standard electromagnetic feeder and a "high deflection" electromagnetic feeder handling various common powders has been demonstrated.
First, flour that is approximately 40 pounds per cubic foot and 100 mesh was tested in a standard electromagnetic feeder with a small 1/3 cubic foot hopper. At very low amplitude, the flour only moved at approximately 10 to 15 feet per minute. As it progressed down the tray, it fed very shallow, only about 1/8 of an inch. As the amplitude was increased, the material started to fluidize and actually fushed out of the hopper and over the sides of the tray.
Second, the same material was placed into a small hopper over a high deflection feeder. As the amplitude began to increase, the material fed in a uniform pattern and maintained its flow depth. When the amplitude was increased all the way to maximum, the material still acted as a solid instead of a fluid.
Next, cocoa powder that weighs approximately 23 pounds per cubic foot and is 100 mesh was tested. It was tested on a standard electromagnetic feeder where it was shown to feed at only about 10 feet per minute. Again, as the amplitude was increased, the material started to fluidize and became uncontrollable.
Finally, the same material was fed in a high deflection feeder. Again, it was shown that the same material fed faster, 30 to 40 feet per minute, and as the amplitude was increased, the material did not fluidize.
The difference that frequency and amplitude can have when trying to feed fine, powdery-type product has been demonstrated. These materials can successfully be fed in vibratory-type equipment if the proper equipment is used.
An alternative to screw feeders, belt feeders and mechanical feeders has been illustrated for handling fine products.
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|Author:||Mitchell, James D.|
|Date:||Mar 1, 2001|
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