Thursday, December 19, 2019
Of all the pneumatic conveying trends for 2020, most notable are the expansion of applications to new materials. In fact, the list of bulk materials that can now be conveyed pneumatically is as diverse as the pneumatic conveying technologies available. New systems with greater throughput and gentler, more energy-efficient processes are available in the marketplace. As a result, it is not always easy to know which pneumatic conveying process is best suited to a specific bulk material. Read on to find out about the latest developments in this exciting area of engineering.
Pneumatic conveying trends that will expand their application
The requirements for transporting bulk materials within processing plants have evolved well beyond the simple systems that involved transporting from a single feeding point to a single delivery point. Today, bulk material often needs to be conveyed from many feeding points to several many points. A combination of vacuum and pressure conveying systems is ideal for this type of situation. The type of pneumatic conveying process used depends on the bulk material (product) and production processes. It is widely accepted today that there are basically four different types:
There are also various configurations of these, each utilising either vacuum or pressure to transport bulk material from feeding point to delivery point.
In principle it can be said that vacuum pneumatic conveying systems make greatest sense if a bulk material (product) needs to be transferred from many feeding points to a single delivery point. Product entry into the system is particularly simple, with no accumulation of dust, because there is no excess pressure. The blower, which is always situated at the end of the system, generates the partial vacuum required to suck the conveyed product through the pipelines. This effectively prevents heating of the conveying air.
Pressure conveying systems are best employed if the bulk material has to be transferred over long distances from one feeding point to several delivery points. Since the product is conveyed against air pressure, locks or pressure vessels are essential. Product delivery, however, is much simpler. In this case, the blower is always installed at the beginning of the system, where it generates the pressure required to blow the conveyed product through the pipelines. If sensitive bulk materials are conveyed, the conveying air has to be partially cooled.
Simple feeding systems are the classic method of pneumatic conveying for feeding dryers, filling and packaging machines, mills and injection mould machines. This also includes silos, tanks and containers. Bulk materials can be fed from virtually any kind of packaging, including sacks, barrels, containers and silos. From the bulk material feeding point a partial vacuum draws in the product, so it is transported to a receiver via the conveying pipe. From there, the bulk material is separated as required. Between each conveying run, the filter is automatically cleaned by means of compressed air pulses. All systems are ready for connection, fitted with the latest electronic control systems capable of signalling when there is insufficient product at a feeding point.
If the bulk material has to be transported to several processing machines rates, centralised feeding with only one common partial vacuum generator is recommended. All receivers are connected to this generator on the clean air side. An electronic control system scans their respective demand, one at a time, and switches the appropriate receiver to suction mode by means of an angle valve. As soon as the 'full' signalling device in the receiver responds, the conveyor stops and the receiver is emptied. If the receiver is fitted with a primary filter only, or very fine dust is involved, a secondary filter can be employed upstream of the partial vacuum generator.
Of all the pneumatic conveying methods, vacuum pneumatic dense-phase conveying is the gentlest. It is suitable for batch or continuous feeding of processing machines with materials that like to stick together like:
At a feeding point (sack, barrel, container or silo) the product is drawn into the pipe and to the receiver with timed air vacuum impulses. At the receiver, the product is gently conveyed and separated using the air stream. It is then deposited into the processing machine. The filter automatically cleans itself. A separate level indicator on the machine hopper functions as a demand signaller.
A new trend is now in evidence. In addition to dense-phase conveying, a self-optimising function enables secondary air to be forced, on demand, into the pneumatic partial vacuum system by means of activators. It is possible, therefore, to lower the conveying velocity while simultaneously increasing the throughput. This is commonly predicted to be the technology of the future, and in the meantime it has been proven in various systems where bulk materials need to be moved over long distances gently, economically and at a high speed.
Any powdery or granular bulk materials such as flour, semolina, sugar, salt, PVC powder or pellets are typical examples of conveyed products. The bulk material is fed by means of a rotary feeder (drop-through valve with pick-up pan or blow-through valve). It is important to ensure adequate valve venting. The pressure required for conveying is generated by a blower. The product is continuously transported and distributed via pipe gates to the respective silos, separators or storage tanks. Suitable filters must be provided at the product delivery points.
The products typically handled by these conveyors include all granular or powdery bulk materials that cannot be conveyed via locks because too much abrasion would occur. This is the case with highly abrasive material like salt or cement and is most relevant at very high conveying rates.
The product is fed into the pressure vessel from the top, for example from a scale, tank, silo or sack feeding station. It is then sealed tight and the required conveying pressure is applied. A rotary piston blower or air taken from the system can generate this pressure. The compressed air can be distributed to the product and bypass is controlled and monitored according to the pressure by an electronic, self-optimising control unit. Only when an empty state is signalled can a new filling cycle be started. In many cases, emptying is monitored according to weight.
Dense-phase conveying also takes place here. The conveying pipe has an air cushion forced into it between each product plug. This is by using a special conveying-air feed device called a Laval nozzle. The product is distributed via pipe gates to separators that are designed for pressure conveying systems and equipped with correspondingly large filters.
Once again a new trend is in evidence. Secondary air is introduced into the conveying pipe via activators, according to a pressure-dependent control principle, allowing transporting of bulk materials especially gently and economically, at very low conveying velocities and over very long distances.
There is an exciting future ahead for pneumatic conveying systems.
New product feeding technologies are regularly developed; for example, suction nozzles, suction hoppers, rotary valves and vibrating dosing screws. The same applies to the receivers where the product is collected. In the meantime, receivers are available with a pendulum flap, conical valve, butterfly valve or rotary valve for continuous operation. On the product feeding side, receivers with a butterfly valve, rotary airlock or – in case of direct silo filling – corresponding pipe bottoms and dust filters remain the optimum solution.
Ring blowers, vacuum pumps and rotary piston blowers are used to generate partial vacuum in vacuum pneumatic conveying systems.
In pressure conveying systems, the required pressure is generated by ring blowers, rotary piston blowers and helical piston compressors. New production technologies enable drop-through valves, blow-through valves and high-pressure airlocks to be manufactured ever more precisely with the aim of reducing air leakage. The vessels in modern pressure-vessel conveying systems are smaller than ever, because filling and discharging can be designed much more easily with the help of intelligent control technologies.
Systems made from new materials will permit more abrasive products to be conveyed reliably over a long period of time, while innovative filter materials allow continuous pneumatic conveying of very fine dusts without any reduction in filter capacity.
All in all, there is a lot to look forward to.
For more than 30 years Pneuvay has been keeping up-to-date with pneumatic conveying trends in Australia and across the world. We have much experience in a broad range of industries, designing and installing pneumatic conveying systems that keep:
If you have any questions in relation to a pneumatic conveying system, feel free to call us on 1300721458 or send us a message via our Pneuvay website contact page or Pneuvay Facebook page.
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