Weighing with pneumatic scales
It is not uncommon for existing buildings to be too low for conventional layouts for charging systems. Activities, such as day bins, dosing devices and scales for gravity flow arrangement all require height, and this can mean high costs.
David Scott of Zeppelin Systems UK explains why pneumatic weighing and dosing systems can provide a more cost effective solution for even difficult materials.
For accuracy to be achieved on any scale system, it needs a controlled volumetric feed for each component fed to the weigh hopper – this is no different for pneumatic scale systems.
In a pneumatic scale system, each material has a dedicated conveying line that is fed by a dosing device with turndown facility. You can then adjust the product flow according to the requirements of the scale using variable speed drives. Coarse and fine dosing rates are used in order to achieve a repeatable and guaranteed dynamic accuracy, usually in a range of 0.2-0.3% of maximum scale value.
Of course, like conventional systems, the accuracy depends on the number of components and the weighing time available in the process cycle. Sometimes weigh times may be too short, component quantities too many or the accuracy requirements are too high so that more than one scale has to be used. In this situation, the use of a holding bin below the scale can effectively extend the weighing cycle by removing ‘waiting time’, which allows the next weigh cycle to start independent of product demand in the process.
It is now possible to install systems that provide a distance of >100m between storage and scale, with batch weights of 200 kg and cycle times of 90 seconds.
In a pneumatic scale system, the pressure conveying (a blower or compressor) is located at the beginning of the system near to the material charging point – having a positive pressure differential between the feeder and the scale offers a motive force for material transfer.
When there is a longer conveying distance and a higher transfer rate, pressure conveying is recommended, as the motive force is not limited.
Applying pressure systems in plant engineering
Another reason to applying a pressure system is when one charging point serves multiples scales – in this case it makes sense to have a single air source.
In a vacuum conveying system, the blower is located near the scale end of the system. This is common in plant arrangements where there are multiple charging areas feeding a single scale which is more common.
As with pressure systems, the motive force comes from the differential pressure which in this case is limited. Most operate at approximately 50% vacuum so they are usually applied at a lower conveying rate and shorter distances so conveying energy requirements are minimal.
Using vacuum systems in plant engineering
Another reason for using vacuums systems in when the plant must be kept as dust free as possible. As all product-loaded parts of the system are under suction, it provides a much cleaner working environment. This also gives an added benefit in application involving IBCs (containers or big bags) because it can simplify plant design.
By positioning IBCs on steel supports with simple gravity seals at the hopper/bin connection, a vacuum is created in the rotary valve and storage bins, preventing dust egress at the connection points. The means product loss is reduced and plant hygiene is improved.
Increasing Fill Factors
The vacuum seen at the dosing device (rotary feeder or screw) has the added benefit of increasing fill factors as the product is pulled in and does not rely solely on gravity. In some cases, this can even reduce the size of the dosing unit but more importantly it can improve reliability when handling ultra light materials (e.g. fume silica).
With a pneumatically charged scale, the filter system is especially important. It must be designed carefully to avoid loss of accuracy.
The filter area and material must be selected to ensure the conveying pressure (positive or negative) does not influence the scale, and can also be cleaned effectively. The filter cleaning cycle must be synchronised 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 and therefore the filter must be completely cleaned of residual dust for the discharge cycle to be completed efficiently.
However, the cleanliness of the filter is of little use if the weigh hopper itself does not discharge fully, so it is vital that the design of the hopper must reflect the material characteristics. Hoppers are often equipped with fluidisation or a massaging non-stick rubber liner that is “active” during the discharge cycle.
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.
Simple and flexible plant design
Conventional gravity weighing systems can be limited according to the number of storage bins placed around the scale hopper and the space available to successfully connect the dosing units.
Installations are also often made without future expansion in mind, which can make system changes very difficult and expensive. By separating the scale and the dosing system, the pneumatic 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 can become simplified, operations made safer and building heights reduced. The latter can be a significant advantage on new buildings where height restrictions may be in place for planning permission.
Of course there are some limitations to pneumatically charged scales and these must be considered in any application.
Material characteristics must be considered in both arrangements, and non-free flowing products must be evaluated carefully to determine what discharge aids may be more suitable.
Very large differences between the smallest and the largest batch weights (i.e. scale range) 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.
However such limitations are rarely seen, and pneumatic scales remain a very useful technology.
On balance, compared to conventional gravity dosing and weighing, the benefits of pneumatic scales far outweigh the limitations. The advantages include:
- Flexibility in positioning of storage, feeding and weighing equipment
- Efficient management of distances between storage and scale
- Cleaner and safer operations
- Ease of future expansion or operational changes
- Lower building heights / easier to satisfy planning restrictions
- Lower investment costs for multiple component scales