The moisture measurement of bulk materials is an essential part of process control in many sectors, such as the stone and earth, glass and food industries. Higher quality requirements, increased plant productivity and more efficient production processes require precise knowledge of the process parameter ”bulk material moisture”. Especially in an economically difficult environment, the process optimisations listed below can lead to a competitive advantage.
Positioning of the measuring sensors
Moisture measurement sensors are used in a wide variety of places depending on the system concept. Firstly, in gravimetric dosing and weighing systems, the sensors mentioned above should ensure exact net dosing of the additives. The process control system can now take the known water content into account and compensate for it during dosing and weighing. Depending on the dosing system concept, the sensors are installed on belts, in the silo or at the silo outlet. You would think that this would be sufficient to ensure precise control of the process sequence - but this is not the case for various reasons. As daily practice shows, there is no uniform moisture distribution within silos. This is also the reason for using systems that ensure continuous silo discharge. In addition, it has been shown that only a percentage cross-section of the individual components is recorded - it is assumed to be around 30 per cent. The reason for this is that bulk materials that do not come into direct contact with the probe are not measured. Other reasons can be, for example, a high dosing speed of the individual components or large quantities of bulk material during weighing. A large number of components in separate bulk solids containers can also speak against a sole measurement in the silos for process control. Moisture measurement in the mixer itself should therefore be considered.
Moisture measurement in mixers
In mixing systems with a fixed mixing trough and rotating mixing tools, it is advisable to install the sensor in the floor area. In exceptional cases, the lower area of the mixer wall may also be suitable. In mixers with a rotating mixing trough, the sensor is often integrated into a separate rod probe fitted with Widia armouring. In some mixer types, the sensors can also be accommodated in the bottom wall scraper. However, constant cleaning of the measuring surface must be ensured for both versions. On the one hand, this can be achieved by the mixing material itself - in practice, cleaning is relatively unproblematic, especially with coarse bulk materials with grain sizes over eight millimetres. For fine products, very small quantities or mixtures with strongly adhering additives, however, cleaning is often only possible with a separate high-pressure cleaning unit. However, this is generally rejected by plant operators, as the cleaning unit causes additional costs and maintenance, as well as introducing unnecessary water into the mixer for the process. In addition, there are also processes where sufficient cleaning is no longer possible even with a high-pressure system. In mixer types with a fixed trough and rotating mixing stars, cleaning of the probes is ensured via an optimally adjusted mixer blade. This was the approach taken to create similar conditions in a rotating trough mixer. To do this, a sensor unit had to be developed for extremely tight spaces. A wide variety of attachments, such as gearboxes, gear rims and protective grids and plates, are fitted directly to both the rotating outer side and the rotating trough base, allowing only a few millimetres of protrusion.
New system development
The following requirements had to be considered and implemented in the new system development:
- Compact dimensions for sensor and evaluation unit; z Low power consumption for long running times of the wireless moisture probe before battery replacement or recharging;
- Use of standard rechargeable batteries;
- z simple installation and integration into new and existing systems.
The new development described above was installed and tested at a well-known concrete manufacturer in south-west Germany. The requirement was for precise moisture measurement in an intensive mixer with a rotating trough, in which a wide variety of products for civil engineering are produced. Here, mixtures are produced for various points of use, which can differ greatly in terms of consistency, quantity and formulation. A very close timing of the successive mixes is also essential in order to be able to supply the various call-off points with the desired quantity of concrete without waiting times.
FL Mobimic-Slim Line radio moisture measuring system
These requirements were met by using a FL Mobimic-Slim Line wireless moisture measurement system in an Eirich DE22 mixer. The microwave probe was installed at a suitable location in the mixing trough, separate from the associated evaluation electronics. This was located on the rotating outer trough wall. This has the advantage that, on the one hand, lower maintenance costs are incurred in the event of wear and, on the other hand, setting parameters are retained when the probe is replaced. Special attention was paid to the good accessibility of the battery and the longest possible operating time per charge. For this purpose, readily available solutions for operating times of several production days were found. The evaluation electronics transmit the measurement data via Bluetooth. Even metal covers or protective plates are no obstacle to the flawless transmission of measured values. The receiver unit converts the digital Bluetooth signal into an analogue output signal, which is sent directly to a water dosing device or a process controller for exact water addition calculation.
Fast installation and commissioning
Installation and commissioning were carried out within a few hours as part of the scheduled maintenance work on the mixing plant. After two weeks in operation, a positive summary of the investment could already be drawn:
- The measuring probe installed in the mixer floor remains free of any build-up throughout the entire production day (up to 15 hours) and does not need to be cleaned manually or automatically.
- In addition, the wear on the measuring probe is likely to be much less than with the rod probe solution, as the measuring probe rotates with the mixing trough so that the mechanical shear forces that cause wear are significantly lower.
- The risk of destruction through manual or mechanical cleaning or through foreign bodies, such as very coarse grains, is also minimised.
- A further positive finding was made with regard to very small mixing batches. Until now, sufficient mixer filling was a prerequisite for sufficient flow around the measuring probe in the rod or the base wall scraper. Initial tests have shown that very small batches no longer have to be produced with a fixed amount of water, but can be produced in fully automatic mode.
No increase in mixing time
Fears about an increase in mixing time proved to be unfounded in retrospect. Instead, a shorter net daily production time was achieved compared to installation in the bar or base wall scraper. A wide range of possible applications in other industrial sectors or production processes can be derived from the use case described. Basically, it should be noted that reliable moisture monitoring in rotating drums or moving bulk material containers is made possible in the first place by wireless moisture measurement technology. Convincing economic advantages can be achieved in drying, homogenising, pelletising, conveying bulk materials or in coating processes. The development by Franz Ludwig has simplified mixing processes in bulk solids processing and improved production processes.
