Additional airflow is usually required when pneumatically transferring the milled material to a filter receiver. This is the most common method of handling the milled material, but it is also feasible to discharge the milled material directly into a large bin with a bin vent sized for the induced airflow (thus, the mill discharges directly into this hopper equipped with filtration). The amount of additional airflow provided for pneumatic transfer is in the range of 20% of the overall airflow (the airflow through the mill and the amount added for conveying). Conveying velocities should be kept at a minimum of 5,000 fpm when friability is not a factor. Higher velocities may be needed for some materials. The conveying distance and routing must be kept at a minimum, with as few turns as is reasonable. Some materials are highly friable and may require semi-dense phase vacuum conveying or a similar method.
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Directly discharging material from a mill into a small plenum above a screw conveyor, even when equipped with adequate filtration, is not recommended. Typically, the air velocities inside the plenum (extension above the screw conveyor) are still adequate to carry dust to the filter unit instead of allowing the material to remain as part of the screw conveyor feed. This results in overloading of the filtration system. This also represents a significant combustible dust hazard that must be considered when handling combustible milled products.
The controls for this portion of the milling operations can vary, but proper monitoring and control of these operations is critical. Typical parameters include the temperature of the airflow into and out of the mill, the differential pressure into and out of the mill, the mill bearing temperature and vibrations, the amperage of the rotary feeder (with VFD), amperage of the mill drive motor, and the amperage of the fan package drive motor used to provide the induced airflow through the mill and for pneumatic transfer.
Materials that are temperature sensitive may require cooled and humidity-controlled airflow to prevent screen blinding and material degradation.
Proper screen sizing is critical to hammermill performance. Have a supplier test the material to determine what screen size will be best to produce the desired material results. Testing is also beneficial for determining the proper induced airflows, operating temperatures, and more. For existing operations, experimentation may be necessary to determine the optimal screen sizing.
Improperly sized and designed filter receivers result in poor and inconsistent mill performance and poor product quality. In addition, the increased maintenance often results in unacceptable operational costs and machine downtime.
Do not consider the filtration device as a simple dust collector. True dust collectors are designed to operate at grain loads far below the loads that come from a milling process. Filter receivers are designed for high material loads and have cylindrical filter units with tangential inlet sections designed to remove, mainly by cyclonic flow, the vast majority of the milled material before it reaches the filters. The cylindrical shape is also better suited for collected material discharge and for handling food products with stringent cleaning requirements.
The filter receiver is often one of the more costly components of a hammermill system, so there may be a desire to cut costs here. This is usually a major mistake. The amount of filter area in relation to the volume of airflow is highly critical to successful milling. There is no such thing as too much filter area, but air-to-filter media ratios should be no more than 5:1 for bag filters and 1.5:1 for cartridge filters. Some materials will require even lower ratios due to their material characteristics, such as cohesiveness, adhesiveness, particle size, and bulk density. The air-to-filter media ratios are also the air velocities through the filter — an air-to-filter media ratio of 5:1 is an air velocity of 5 fpm.
Also, the rising air velocity in the filter unit (the velocity between the filters) must be kept as low as is reasonably possible (due to typical physical limitations). This velocity is based on the material characteristics and testing. Velocities must usually be below 200 fpm and may need to be significantly lower for some materials. Excessive velocities will cause material to remain airborne around the filters and lead to premature filter blinding and problems with the collected material discharge system.
Controls include the differential pressure across the filters, the differential pressure to the fan inlet, monitoring of the material level inside the unit (do not use the filter receiver as a storage unit), and performance of the collected material discharge system.
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Proper fan selection, along with proper filter receiver selection, represent the typical problem with many hammermill operations. The fan must be sized for the full range of milling system requirements. The fan must be located on the clean side of the filter receiver. Always get a fan performance curve from the vendor before purchase and ensure that the curve fits the system requirements. Do not use a VFD for operation, as extensive experience shows that this will lead to future operational problems. However, do include an inlet radial damper (manual or automatic) which can be used to vary the performance curve without wasting energy.
Jack Osborn ([email protected]) is senior project engineer at Airdusco Engineering and Design Services and a member of Processing’s editorial advisory board. He has more than 46 years of experience in dust collection systems, centralized vacuum cleaning systems, pneumatic conveying systems and all types of bulk handling systems and is a participating member of all six NFPA combustible dust committees (61, 484, 652, 654, 664, and Correlating).
Airdusco Engineering and Design Services
www.airdusco.com
Mills are used for milling solids. This milling fulfils two purposes: First, it enlarges the particle surface which increases the speed of dissolution. Second, it evens out differences between particle sizes in order to ensure that mixing can be homogeneous.
Due to their versatility, sieving machine mills play a crucial role in pharmaceutical chemical and food production. They can be integrated into complex production lines, for example for filling/emptying or transfer operations at process machines, or for filling and decanting containers (IBCs).
Arguments for use of mills:
In particular in pharmaceutical clean rooms where there is often not much space, the small footprint of these systems is a great benefit. Hammer mills can also be equipped with an integrated lifting column so that they can be integrated into almost any conceivable process scenario.
Another feature that increases the efficiency of the processes is the mobility of the machines. For use or for cleaning, they can be moved or even disassembled quickly and easily.
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