Easy to handle and suitable for hazardous areas

The author: Thomas Ramme Sales Manager, Volkmann Vakuum Technik

Vacuum is one principle commonly utilised for conveying bulk and powdered materials. While there are many overly complicated, large-scale mechanical designs for creating a vacuum, the functional principle of the Multijector – a multiple-stage compressed air driven ejector – is easy and effective. The driving gas (p+) enters the pump’s casing and flows through the first stage of the Laval nozzles. As the compressed air enters the first ejector nozzle, it is highly accelerated due to the decreasing diameter in the subsonic section. When the air stream reaches the smallest diameter of this nozzle, it is accelerated to sonic velocities. As the stream of compressed air leaves the first nozzle, it is captured by another, larger nozzle. This second ejector nozzle performs a similar task to the previous, smaller one; however, it also increases the total amount of air in the stream. The next three or four nozzle stages utilise the existing high-energy air flow to multiply the suction capacity of the pump without any additional energy. The final outcome of this process is an incredible increase in velocity as well as an immense pressure drop that creates the vacuum necessary for plug-flow conveying. This use of the physical principle allows a level of efficiency that was considered to be unachievable before the advent of the multiple-stage design.

However, vacuum pumps with Multijector technology have several other beneficial characteristics. Multijectors have no moving parts, which means no lubrication is necessary du-ring maintenance, and they generate no heat while in operation, making mechanical breakdowns impossible. As a result, pumps featuring this novel technology can achieve a life span far longer than that of conventional pumps or blowers.

Nevertheless, Multijector technology is not static. With the help of complex, computerised flow simulations and long-term empirical optimisations, nozzle enhancements that reduce compressed air consumption and increase suction capacity are possible. Such innovations have also led to a much improved silencing system. At the same time, the use of new materials to build the pumps and the deliberate avoidance of plastic parts guarantee a long life even if the Multijector is operated under rough plant conditions.

Inhomogeneous or problematic materials can be transported using a vacuum conveyor. Thanks to the modular design of the Multijector conveyors, all of these items can now be effortlessly conveyed with the same machine without fear of cross contamination. This is possible because all parts in contact with the product to be moved can be easily dismounted and cleaned without any special tools. The resilient materials of the filter unit are also able to withstand wet cleaning under high pressure or in an ultrasonic bath. This good cleanability is especially desirable for applications in the pharmaceutical and food industries.

Vacuum conveying is, by definition, completely dust-free, since any dust clouds formed by the process are sucked in as well. It is different from other forms of conveying in that it uses negative pressure, which means that material flow cannot leave the transport hose or pipe. Various adapter systems can also be built to provide an air-tight seal at the discharge, so that contamination of the environment by the material is ruled out. Even extremely fine dusts like toner powder or palladium ash can therefore be conveyed.

When the stainless-steel modular vacuum conveyor was first developed, particular importance was attached to preserving electrical conductivity. The numerous realisable combinations provide an optimal solution that is a reliable option for hazardous areas. All the individual modules are assembled and joined in such a way that only one central earthing connection is required.

Unlike so-called bolted conveyor models, these lightweight modular designs are easy to disassemble and clean as well as much more flexible. For example, it is possible to attach powder locks to the conveyor’s discharge module to achieve a safe connection to a reactor while maintaining a closed system. The filter housing is also electrically conductive and integrated into the conveyor body. The modular concept allows the use of different filter materials and types, including conductive materials.

Suction hoses are either stainless steel or dissipative and FDA-rated. They contain a conductive wire throughout for vacuum stability that also absorbs and earths any static electricity which builds up in the line, eliminating the potential hazard.

Multijector vacuum conveyors can be fully integrated with many other powder handling solutions. A special material feeding elbow can be directly attached to a big bag unloading system. If smaller batch sizes are called for, an automatic bin unloading system is recommended. This system can be used to unload drums, sacks, octabins or other small recepta-cles without any undesirable manual labour. A suction lance is simply attached above the bin and submerged in the material to be conveyed; the material is then transported pneumatically, e. g., by a standard vacuum conveyor.

If the product needs to be weighed, this stand-ard conveyor can be converted to a Conweigh system. This combination of conveying and weighing is achieved by mounting the sepa-rator in a force-compensating frame. The incoming product is then gravimetrically measured by the load cells. In addition to optimising residues and dilute-phase control, this intelligent control system can store up to 256 different powder types, meaning that even the most complex recipes are conveyable. Alternatively, the Conweigh system is also able to communicate with a master process control system. Further adaptations and combinations are available for applications in hazardous areas. In order to match the application to the process optimally, conveying tests should be performed either on the manufacturer’s test bench or directly at the prospective buyer’s production facility.

In addition to the familiar batch conveying principle, it is now also possible to convey powders, bulks and solids continuously using Multijector technology. This is facilitated by a combination of two stainless steel separator containers. In practice, this design can reach extraordinary conveying heights and distances. If a particular production process requires a constant supply of product, the Multijector conveyor can be easily equipped with a continuous discharge module.

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