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Conveying systems in line with energy saving trend

Minimum wear, segregation-free and energy efficient
Conveying systems in line with energy saving trend

A broad range of different systems is available for pneumatic conveying but the trend is clearly towards dense-phase conveying systems. The arguments are compelling: they not only operate with minimal wear and no segregation but also save energy. Two examples of this technology are the VacuumPlus and Multiair dense-phase conveying systems from AZO.

The author: Christian Leist Product manager, Pneumatic conveying systems, AZO

The VacuumPlus conveying system operates at velocities lower than 4 m/s and offers the combined advantages of vacuum and pressure conveying. Whenever the task is to convey frag-ile, heat-sensitive bulk materials, this innovative handling technology is the obvious so-lution. It is recommended for use with average throughputs and conveying distances, the current maximum being 100 m. Thanks to the low velocities that VacuumPlus conveying affords, it is possible to achieve a dense-phase conveying system with very low segregation. Another advantage: handling is very gentle, so there is little wear on the conveying pipe and little attrition of the conveyed materials. With vacuum conveying systems, the design of the product intake is likewise simple, even with several product feeding points. In addition, this system is very efficient as a result of its low air and energy consumption, and it can also be operated using conditioned conveying air.
We generally distinguish between two types with this materials handling technology: batch (discontinuous) operation on the one hand and continuous operation on the other. The choice of conveying system depends on the processes upstream or downstream of the conveying step. For instance, if batch operation is the downstream process, discontinuous feeding obviously makes more sense; if the downstream process is continuous operation, VacuumPlus conveying should be designed continuously as well.
Secondary air reduces friction
The AZO Multiair conveying system likewise works at velocities lower than 4 m/s but in the overpressure range: the discharge pressure is between 1 and 4 bar overpressure. The optimum discharge pressure setting depends on the product and throughput. AZO Multiair is especially well suited for abrasive products, for example PVC Dryblend and glass fibre reinforced granulates, as well as for gentle, low-segregation, pneumatic transport of foodstuffs like cereals, instant products or milk powder. Owing to the low conveying velocity, both the products and the equipment are handled with particular care. This system is predestined for high throughputs and for bridging long conveying distances as a result of the extremely diverse discharge pressure rates. Since the controls are adjusted to match the product and secondary air is injected selectively, operation of the system is also very energy efficient.
This optimised pressure vessel conveying system retains the fluid state in the bulk material by injecting secondary air. This reduces the friction coefficients between the pipe and the product considerably. The standardised proportional valves used to inject the air are controlled centrally. It is possible to carry out improvements for specific products at any time. Special non-return valves located between the conveying line and the injector valves ensure total system safety and reliability. These mechanisms facilitate operation with high loads at low velocities without the conveying line becoming clogged. Wear on the plant and the products is substantially less than with dilute-phase pneumatic conveying. The TCO (total cost of ownership) decreases as a result of the lower compressed air consumption, reduced load on the filtration area and smaller pipe cross-sections.
Tests unavoidable
It is absolutely essential to assess the properties of primary bulk materials in order to make optimum use of conveying systems. These properties include the particle distribution, surface texture, particle shape and bulk and material densities, which can be established using appropriate laboratory equipment. The moisture content and absorption (hygroscopy) of bulk materials are also important. Secondary properties are derived from the primary properties, e.g. caking, pressure resistance, internal and wall friction, cohesion, adhesion and fluid energy, compressibility, angle of repose, fluidisation and lastly permeability. The compacted bulk density, compressibility and fluidisation are measured and analysed using specific test methods. One particularly crucial property of bulk materials is permeability, i.e. the permeability to air of the bulk solids being conveyed. It is possible to carry out these studies in a well-equipped laboratory as a precondition of ideal preliminary considerations and ultimately good handling results. Ultra-modern visualisation systems, which show handling with individual parameters extremely clearly, then serve as a basis for design-ing the plant and for engineering and improving the handling systems. However, the complexity of the issues involved necessitates extensive experience.
Increased focus on energy efficiency
A customer was looking to increase the energy efficiency of existing pneumatic conveying systems, which had been running for years without any problems. At the AZO research centre for raw materials handling, a modular, pneumatic conveying system was installed to allow various conveying distances to be tested with different outputs and different pressure generators using the manufacturer’s original prod-uct. The customer’s material feeding process was subsequently scrutinised in engineering phase 1 and essential improvements were undertaken to the plant in phase 2. Trials were conducted at the AZO test centre over four different conveying distances. The conveying capacity over the longest distance (200 m) amounted to 6000 kg/h. This capacity was thereafter applied to the shorter conveying distances and different pump combinations used or the settings changed. The conveying capacity, time, weight, power consumption, discharge pressure, quantity of air and differential pressure at the filter were recorded. The result was impressive: by using various pump combinations with frequency converters, relocating the main and additional components, optimising the conveying distances and improving the location of the clean air lines, it was possible to achieve energy savings of between 50 and 70 %. The additional costs for analysing the actual situation, refitting the plant and optimising the controls would be recovered within 15 months assuming a planned production time of 4000 operating hours per year. If this time is increased to 6000 h/a, the ROI takes just 10 months. This very transparent example illustrates how existing pneumatic conveying systems can be optimised so that they save energy sustainably.
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