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Pneumatic conveying

The sandstorm in the desert or the tornado that covers the house roof are natural occurrences of pneumatic transport. Technically, however, it is used in a variety of applications. Pneumatic transport takes place either in suction or pressure mode. The basic operations of a suction or pressure system are identical:

  • Feeding of the bulk material into the conveying line

  • Transport through the conveying line due to a negative pressure difference

  • Separation of the bulk material from the conveying gas at the receiving point (exceptions are, for example, direct reactor or burner feeds in pressure conveying, such as coal firing in the power plant or cement industry)

  • Pressure generation (overpressure with pressure conveyance, negative pressure with suction conveyance).

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Figure: Pressure system (left) / suction system (right) with: A - storage silo, B - filter, C - lock, D - receiving silo, E - overpressure generator, F - vacuum generator, G - conveying line diverters

 

The bulk material is introduced into the conveying line during pressure operation by means of rotary valvepressure vesselscrew pumpinjector (nozzle conveyor), flap systems or with the exclusively vertical transport means airlift. At the receiving point, the bulk material is separated from the transport gas for further processing. This is done using cyclones and filters. 

 

The generation of compressed air depends directly on the conveying system. While pressure vessel systems are equipped with compressors, blowers (up to 1.0 bar(g)) and compressors (up to 2.0 bar(g)) supply screw pumps and rotary valves. Flaps and injectors are almost exclusively combined with blowers. 

 

Whether a pneumatic conveying system is used and a mechanical one is preferred depends on many criteria. The decision in favor of pneumatic conveying is usually made when the conveying route is either very long or the route between the starting point and receiving point is characterized by bends and differences in height.

You have problems with your pneumatic transport or are planning a new conveying system and need support - please contact us.

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Feeding devices for the pneumatic conveying line

A pneumatic conveying is usually made up of the following system areas:

  • Compressed air generator (fan, blower, compressor, compressed air station)

  • Air line between pressure generator and feeding device

  • Feeding device

  • Conveying line

  • Receving point (silo, burner, reactor)

The majority of technical problems arise at or through the feeding device. Compressed air generation rarely has problems, but it is often incorrectly dimensioned or dimensioned in a very unfavorable way in terms of energy. The air transportline is negligible. Pressure losses due to too many deflections are usually not relevant in the air pipe. The conveying line is often in need of optimization. Inclined conveying lines, incorrect or unfavorably positioned pipe bends lead to unnecessary additional pressure loss or even trigger system blockages.

The infeed of the bulk material into the pressurized conveying line is carried out by the so-called feeding device. This is to be selected according to the system conditions, the bulk material, the Capex and the Opex. Every feeding device has its right to exist.

 

You have problems with your pneumatic transport or are planning a new conveying system and need support - so please contact us.

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Injector 

The injector works on the principle of the water jet pump. This sucks in the bulk material that falls vertically from above into the inlet area and transports it into the downstream Laval nozzle. There the air/solid stream expands. This converts the kinetic energy back into pressure. 

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The advantages of the injector are the small space requirement, no rotating components in the bulk material flow and the self-regulation of the conveying capacity. Due to the lack of components, bulk material temperatures of up to 900 °C can be conveyed.

Significant disadvantage is the high energy requirement. Injectors convert only a small amount of energy (< 25%) into conveying pressure. The bulk of the energy is lost for the bulk material acceleration as far as the gas acceleration. Due to the operating principle, the conveying capacity is limited to 5 - 10 t/h at max. conveying distances of 200 m.

Main problems with jet conveyors are:

  • performance problems due to incorrect dimensioning of the conveying line,

  • performance problems due to incorrect positioning of the nozzle ,

  • performance problems due to incorrect dimensioning of the jet nozzle,

  • blockage in the conveying line.

 

You have problems with your pneumatic transport or are planning a new conveying system and need support - so please contact us.

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Pressure Vessel 

The pressure vessel system is also a feeding device with the task of feeding bulk material into the pressurized conveying line. The process takes place in the 4 typical steps:

  1. Filling the pressure vessel

  2. Pressurization of the pressure vessel

  3. Conveying / sending the bulk material

  4. Blow the line clear / vent the vessel

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The process already shows the disadvantage of the pressure vessel system. The bulk material is not conveyed continuously. During steps 1, 2 and 4 there is no bulk transport. This can be achieved by a so-called double-decker pressure vessel construction. Here 2 pressure vessels are positioned one above the other. The upper vessel runs through the 4 cycles mentioned above, while the lower vessel operates continuously and is fed by the upper vessel.

Other disadvantages of a pressure vessel system are the high overall height with medium to large (> 50 m³/h) flow rates, the control technology effort (regulation of various flaps, pressure and level sensors), the recurring tests according to PED and additional units, such as compressed air dryer and cooler, since the pressure dew point is undershot due to the high delivery pressure (3 - 20 bar(g)) and condensate is formed from the conveying air.

 

The advantages are that the highest delivery pressures can be achieved with the pressure vessel. This means that the longest conveying distances can be overcome with pressure vessel systems. Due to the high pressure level, the lowest conveying gas speeds can be achieved, which is gentle on the product and reduces wear on the conveying line.

  • Typical problems with pressure vessel systems are:

  • Performance problems due to incorrect dimensioning of the conveying line,

  • Performance problems due to incorrect structural and process design of the pressure vessel system,

  • blockage of the conveying line,

  • high compressed air consumption,

  • strong vibrations of the conveying line,

  • increased wear.

 

You have problems with your pneumatic transport or are planning a new conveying system and need support - so please contact us.

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Screw pump

The screw pump is a feeding device which is mainly used in the minerals industry. In 1918, Alonzo G. Kinyon from Wisconsin invented the screw pump principle, the Fuller-Kinyon pump, which was patented in 1925. It was originally intended to replace the pressure vessels used to convey coal dust in order to eliminate disadvantages such as height or leaks. The system, which works according to the press screw principle, quickly found its way into all bulk material processing industries.

The bulk material falls into the pump surge bin. There it is picked up by the high-speed conveyor screw (speeds 700 - 1500 rpm) and transported to the outlet. On the way there, the first bulk material compaction takes place. The goal is to build a plug that seals the pressure of the conveying line. At the end of the screw conveyor, the so-called end wing area, the bulk material is further compacted. Depending on the manufacturer, this is supported by an additional flap. The compressed bulk material then falls into the outlet area, where it is caught by the air flow, which is greatly accelerated by means of a nozzle, and transported into the conveying line.

 

Advantages of the screw pump are:

  • robust construction,

  • low overall height even with high flow rates,

  • large conveying capacities > 200 t/h possible,

  • no recurring pressure tests according to PED,

  • Depending on the bulk material, pressures of up to 2.5 bar(g) can be achieved.

Disadvantages are:

  • High energy requirement due to the nozzle and the drive of the compression screw,

  • increased wear when driving at low loads,

  • not suitable for granules,

  • not suitable for sensitive bulk goods (keyword: grain destruction).

 

Typical problems with screw pumps are:

  • performance problems due to incorrect dimensioning of the conveying line,

  • performance problems due to incorrect dimensioning of the conveying air nozzle,

  • drops in performance with fluctuating bulk material properties,

  • drops in performance when using grinding aids in the upstream process,

  • blockage of the conveying line due to incorrect dimensioning of the conveying line or incorrect operation of the screw pump and its peripherals

  • high compressed air energy consumption when driving at low loads,

  • vibration of the screw pump,

  • high energy demand,

  • Increased wear of the screw pump when driving at low loads.

If you have problems with your screw pump or would like a mechanical or energetic assessment, please contact us.

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Rotary valve

The rotary valve is, together with the nozzle conveyor, the oldest continuously working infeed element of pneumatic conveying. The lock consists primarily of the housing and the rotor. The pressure difference at the lock results in a so-called leakage air flow  into the inlet. This must be discharged in such a way that the incoming bulk material is not impeded.

If the conveying line is mounted directly on the valve without a blower shoe, this is known as a blow-through valve. In this case, the conveying air flows directly through the rotor. The advantage of this variant is that adhering bulk material particles are blown out of the rotor by the conveying gas.

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The disadvantage is that the rotor has no side discs, which reduces the pressure seal considerably. 

 

The advantages of a rotary valve are:

  • Large flow rate with small size,

  • Low energy consumption,

  • Flow rate independent of material fluctuations,

  • it can be used as a dosing device at the same time,

  • can be used for fine and coarse bulk goods.

 

The disadvantage of the rotary valve is:

  • Leakage gas quantity (which is pressure-dependent)

 

Typical problems with rotary valves are:

  • Performance problems due to incorrect dimensioning of the conveying line,

  • Performance problems due to incorrect dimensioning of the rotary valve,

  • Performance decrease with increasing service life,

  • Wear and tear,

  • leaks in the bearings,

  • formation of dust.

 

You have problems with your pneumatic transport or are planning a new conveying system and need support - so please contact us.

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Airlift

The airlift was originally intended for pure vertical transport. Its main fields of application are the cement industry, the power plant industry and the aluminum industry. It is used when large bulk material flows (> 50 t/h) have to be transported over large height differences (> 50 m). Since the requirement for such a system is a very high level of reliability, the airlift does not contain any moving parts.

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The bulk material falls into the cylindrical container at the airlift head. A bulk material column forms in the airlift. It has the task of sealing the pressure of the conveying line. The weight of the column of bulk material now opposes the compressive force of the conveying pressure. There is an air nozzle on the aerated bottom of the airlift. In it, the conveying air is accelerated similarly to the injector conveyor. On the way into the conveyor pipe, it picks up bulk material and transports it through the conveying line. Airlift performance is determined by 2 factors:

  1. Due to the inflow mass flow: The airlift can only transport as much bulk material as is fed to it.

  2. By the weight of the bulk solid column: which in turn is determined by the bulk density and the fill level. Only as much bulk material can be transported as can be sealed in equilibrium by the weight force.

The advantages of the airlift are the large flow rates, highest conveying heights, highest availability, no moving parts and a simple structural design.

The disadvantages are der higher energy consumption as well as die large height.

You have problems with your pneumatic transport or are planning a new conveying system and need support - so please contact us.

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