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Efficient vacuum

Pick-up, conveying and feeding of bulk materials
Efficient vacuum

Total cost of ownership (TCO) and overall equipment effectiveness (OEE) are now-adays more important for industrial items than a supposedly low purchase price. This is particularly true where the transfer of powders and bulk materials is concerned. Vacuum conveyors, with their modular, stainless steel design and Multijector technology, are especially durable and cost effective owing to their simple mounting and freedom from maintenance.

The author: Thomas Ramme Sales Manager, Volkmann

Different processes often require multiple powders or pellets to be conveyed using the same equipment. To avoid cross-contamination, it is therefore desirable to have a simple type of construction that allows easy manual disassembly and cleaning. A modular, stainless steel design permits fast product changes while fulfilling the most stringent hygienic demands of the pharmaceutical and food industries. This design also enables vacuum conveyor configurations to be individually tailored to specific conveying tasks; for instance, successful conveying can depend crucially on suction modules with special suction inlets or integrated cyclones. The stainless steel arrangement additionally results in a robust (up to 90 mbar absolute vacuum can be achieved), lightweight and fully mobile plant. In most cases, the small footprint means the bulk material receiver can be mounted directly above the plant vessel or equipment to be loaded.
If the bulk material receiver is combined with stationary or mobile hoists, there is scope for even more applications. It therefore comes as no surprise that these pneumatic powder transfer systems are increasingly replacing conventional, mechanical conveying methods like screws, augers, lifts, belts or bucket conveyors. The optimum configuration of each vacuum conveyor is often determined by means of practical, one-to-one scale trials. Modular, stainless steel systems are very easy to handle and integrate, so that these trials can be performed either in the manufacturer’s laboratory or directly on site at the process. The bulk material receiver is fitted with various powder process valves as well as optional fluidisation and discharge aids to allow even bridging or sticky powders to be fed into the process.
Vacuum pumps for conveying
The driving force for the powder transfer is pure vacuum. The right choice of pumping principle and pump size is therefore just as important for a working conveyor as an optimal bulk material receiver configuration. The actual pump selected in each case depends on the application. The vacuum pump can be either electric or air driven. The heart of the vacuum conveyor is often a multiple-stage Multijector pump. Criteria such as maintenance-free operation, a small footprint and low weight tend to be as vital to users as easy adjustment and control. Generally speaking, compressed air is not recognised as a cheap source of energy; however, specially designed multiple-stage Venturis offer effective energy conversion and ensure economical performance. Multiple-stage (Multijector) ejectors are five times more efficient than their conventional, single-stage counterparts and because they operate cyclically and non-continuously, their running costs are often comparable to those of electric pumps, which are forced to run continuously by design.
Several other advantages are apparent when the ejector is installed in a production plant. The Multijector has no revolving parts and therefore needs no lubrication or mainte-nance, and there is absolutely no heat generation. On the contrary, a cooling effect is measurable as the compressed air expands.
A broad spectrum of powders and bulk materials can be conveyed because the multiple-jet pumps create large volumes of suction air flow (e.g. for dilute phase conveying) and, if required, are able to produce enormous vac-uums of up to -910 mbar (e.g. for dense phase and plug flow conveying). The performance characteristic (air flow at vacuum level) is closer to that of an electric positive displacement pump than to a conventional, single-stage ejector.
If a Multijector vacuum pump is combined with the bulk material receiver as described above, the whole conveying system works pneumatically, allowing operation in hazardous areas without any problems. This is the only kind of vacuum conveying system that is fully Atex certified for all relevant powder and gas Ex zones. Even an inerting system is available.
Multijectors build up and break the vacuum rapidly. This principle harmonises with intermittent conveying, because the bulk material receiver is filled and discharged in cycles. The pump consumes no energy during the discharging cycle. By adjusting the compressed air supply pressure, the conveying step can be controlled according to the application. Even with air flows of between 15 and 1200 m³N/h, the noise level is always less than it would be with an equivalent electric pump due to the lack of moving mechanical parts. The wide range of available pump sizes, allied to the different bulk material receivers, enables energy consumption to be optimised in relation to conveying capacity. Before making the final selection, potential users should seek advice from an experienced manufacturer.
One of the smallest conveyors in the portfolio, a VS 200, is able to feed 540 kg of sugar per hour into a mixer at a height of 4 m, for example. The conveyor itself has an overall height of 450 mm, a diameter of 210 mm and a weight of only 9 kg. When space is limited, the small size of these powder transfer systems plays an important role.
The volume of the separator-container and choice of vacuum pump determine the main applications for vacuum conveyors. Assuming an average atmospheric pressure of approximately 1013 hPa, a vacuum of -910 hPa can theoretically lift a closed water column about 9 m vertically. However, powders and bulk materials are always conveyed with a certain amount of feeding air and are therefore capable of conveying to much greater heights. More than 40 m of vertical conveying height has already been accomplished, depending on the characteristics of the material to be conveyed. The actual throughputs and capacities are strongly dependent on the properties of the powders and bulk materials. The bulk density, adherent or bridging characteristics, particle size, surface shape, humidity and fat content are crucial parameters for specific conveying tasks along with the design of the pick-up point, the feeding air supply and of course the total conveying distance, height and number of bends. A variance in the throughput rate of up to 1000 kg/h is not unusual from product to another, even with the same kind of conveyor.
The majority of transport tasks for pneumatic vacuum conveyors involve capacities of between 10 and 6000 kg/h; in extreme cases, up to 16 t/h can occasionally be achieved. The distances vary from just a few metres to as much as 100 m horizontal and 40 m vertical.
In all vacuum conveying systems, the solids (or liquids) are drawn into the bulk material receiver and collected before being fed into the process. Special filters are used in the receiver to ensure dust-tight operation.
Moreover, the physical principle of vacuum helps to avoid contamination of the environment. Leakage in a positive pressure conveying system will inevitably lead to dust emissions, whereas with a vacuum system additional atmospheric air is drawn in. In order to improve cleanliness in its production plant, a manufacturer of black carbon and soot therefore upgraded its existing (old-fashioned) positive pressure system to vacuum conveying.
Vacuum is the preferred technology for harmful powders or potent pharmaceuticals because the enclosed system maintains a high containment level. Special PPC vacuum conveyors are already used in production areas where an OEB (occupational exposure band) of 4 with OELs (occupational exposure limits) from 1 to 10 µg/m³ is monitored.
Fine filtration of the conveying air is the next step after vacuum transport through the conveying line. First of all, the material velocity is significantly reduced by the increased diameter of the receiver. The majority of the powder is thus collected just above the discharging valve. Tangential suction modules with internal cyclones are then used to collect the fines. Residual fines are retained in the receiver on the filter unit, directly below the Multijector vacuum pump.
The filter systems need to work virtually maintenance-free and be easy to clean in case of product changes; at the same time, they must be capable of filtering superfine dusts like toner powder (with particle sizes as small as 0.2 µm) and fulfil hygienic regulations in the pharmaceutical and food industries.
Various procedures are used for cleaning on the fly. Intermittent conveying, for instance, permits effective filter cleaning by means of reverse air blasts. Gradual filter blockage is consequently avoided. In food and pharmaceutical applications, solids filters are generally manufactured from polymers or stainless steel. They guarantee wear-resistant operation and permit wet cleaning with warm water or steam if the product has to be changed. Depending on the application, filter lifetimes of two years or more are not uncommon. If the standard filter cleaning device is not sufficient for exceptionally adherent materials, it is possible to vibrate the filter unit and the bulk material receiver. Once again, the choice of a modular filter designs facilitates an optimal solution for each individual application.
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