Weighing with pneumatic scales.
If high weighing accuracy is to be achieved on any scale system, it requires an even and controlled volumetric feed for each component fed to the weigh hopper. Pneumatic scale systems are no different. Each material has a dedicated conveying line fed by rotary feeders especially designed and selected to ensure a very even product flow is achieved. Variable speed drives are then used to adjust the product flow according to the requirements of the scales. Coarse and fine dosing rates are used to achieve a repeatable and guaranteed dynamic accuracy, usually in the range of 0.2-0.3% of maximum scale value, for both pressure and vacuum systems. Like conventional systems, the accuracy depends on the number of components and the scale time available in the process cycle. In some circumstances, scale times may be so short, component quantities so many and/or accuracy requirements so high, that more than one scale has to be used. Alternatively, the use of a holding bin below the scale can effectively extend the weighing cycle by removing waiting time, allowing the next weigh cycle to start independent of product demand in the process. Today, it is possible to install systems with a distance of >100m between storage and scale, with batch weights of 200 kg and cycle times of 90 seconds.
Scaling with pressure and vacuum systems
In a pneumatic scale system with pressure conveying, a blower or compressor is located at the beginning of the system near the material charging point. The positive pressure differential between the rotary feeder and the scale provides the motive force for material transfer. Pressure conveying is recommended for longer conveying distances and higher transfer rates because the differential pressure, and thus the motive force, is not limited. Another reason for applying pressure systems is where one charging point serves multiple scales, so it makes sense to have a single air source.
It can also be necessary due to the specific characteristics of the material being conveyed. For example, Ti[O.sub.2], calcium carbonate and carbon black can only be conveyed reliably over long distances by pressure.
In vacuum conveying systems, the blower is located near the scale end of the system, so they are particularly well suited to plant arrangements where there are multiple charging areas feeding a single scale. Like pressure systems, the motive force comes from the differential pressure available, but with vacuum systems this has a practical limitation of about 75% vacuum or 0.25 BarA. Most operate at approximately 50% vacuum (50.7 kPa or 15" mercury) so they are usually applied to lower conveying rates and shorter distances where the conveying energy requirements are fewer. This is reflected in the product: Gas ratio is an important measurement in pneumatic conveying systems that refers to the mass of product compared to the mass of gas used to convey it. This can also be expressed as a loading density (O = kg product/kg gas). The O for a vacuum system is typically in the 2-10 range. Low O can mean longer conveying times, so vacuum systems arc easier to apply where longer cycles times are acceptable.
Another reason to use vacuum systems is when the plant must be kept as dust-free as possible. Since all product-loaded parts of the system, especially conveying lines, are under suction, any leakage flow is into the system, giving a much cleaner working environment. This gives an added benefit in applications involving IBCs (containers or big bags), because it can simplify plant design. The IBCs can be positioned on steel supports with simple gravity seals at the hopper/bin connection, because the vacuum created in the rotary valve and storage bins reliably prevents dust egress at the connection points. Product loss is reduced, and plant hygiene is improved.
The vacuum seen at the rotary valve has the added benefit of increasing fill factors, because the product is pulled in rather than relying on gravity. This can reduce rotary valve sizes, but more importantly, it improves reliability when handling ultra light materials (e.g., fumed silica).
It is also worth mentioning that the use of rotary feeders for dosing both pressure and vacuum scales can offer advantages over the more traditional rotary screws and vibratory feeders, because they effectively prevent flushing. This loss of control can be a problem with materials that require aeration to begin flowing, but act almost like water when they are fluidized (e.g., lime).
Special importance of the filter
With a pneumatically charged scale, the filter system is especially important and must be designed carefully to avoid loss of accuracy. The filter area and material must be selected in such a way that the conveying pressure (positive or negative) does not influence the scale and can be cleaned effectively. The filter cleaning cycle must be synchronized with scale discharge by the control system. Since the filter is included in the tare weight of the scale, any material stuck to the filter is included by it. The filter must therefore be completely cleaned of residual dust for the discharge cycle to be complete.
Of course, cleaning the filter is of little use if the weigh hopper itself does not discharge fully, so the design of the hopper must also reflect the material characteristics. Hoppers are often lined with a fluidization membrane that is active during the discharge cycle. Other designs include flexible hoppers coupled with massaging devices to promote material flow. Downstream of the weigh hopper, the use of flexible materials can also provide sufficient mechanical movement to prevent product sticking in chutes, even without forced vibration.
Product characteristics are as important in the selection of conveying pipes as they are in the rest of the system. Some products have a tendency to stick to the inner wall of the pipe, and if overlooked, this can result in a gradual deterioration in performance and extend cycle times. The use of rubber pipelines can overcome this by allowing the pipe to be expanded periodically, cracking off the residue prior to a purge cycle.
Simple and flexible plant design
Conventional/vertical weighing systems are limited by the number of storage bins that can be placed around the scale hopper and the space available to connect the dosing units. Installations are also often made without future expansion in mind, making system changes very difficult and expensive. By separating the scale and the dosing system, the pneumatic/horizontal weighing arrangement gives a plant almost unlimited flexibility for future material additions.
Since the charging system can be at ground level, the logistics of storage and handling become simplified, operations safer and building heights can be reduced. The latter can be a significant advantage on new buildings where height restrictions may be in place for planning permission.
There are limitations to pneumatically charged scales, some of which are common to the conventional vertical systems, and must be considered in any application.
Material characteristics must be considered in both vertical and horizontal arrangements, and non-free-flowing products must be evaluated carefully to determine what discharge aids may be required, or whether other arrangements may be more suitable.
Large differences between the smallest and the largest batch weights may limit the use of pneumatic scales because of the tare weight of the filter. This will also limit their application on extremely small dosing requirements, which sometimes leads to minor ingredients being handled in a separate (manual or automated) scale. The need for very high dosing accuracies (<0.05%) and very short cycle times (<90 seconds) can also create practical limitations.
On balance, compared to conventional vertical scales charged by screws or vibratory feeders, etc., the benefits of pneumatic weighing and dosing systems far outweigh the limitations. The advantages include flexibility in positioning of storage, feeding and weighing equipment; efficient management of long distances between storage and scale; cleaner and safer operations; ease of future expansion or operational changes: lower building heights/easier to satisfy planning restrictions; and lower investment costs for multiple component scales.
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|Title Annotation:||Process Machinery|
|Author:||Tittensor, Christian D.|
|Date:||Jul 1, 2003|
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