Pneumatic Conveyor Working Principle | Pneumatic Conveyor Advantage and Disadvantage | Powder Transfer System

What is Pneumatic Conveying

The word pneumatic means air, wind or gas etc. The word conveyor means equipment and machinery employed for transporting bulk material on a mass scale. So pneumatic conveying is the process of transporting bulk material on a mass scale. Pneumatic conveying is the process of transporting materials with the help of flowing stream of air or gas. Obviously, the air or gas must flow through a pipe or conduit. In fact, it is also obvious that if a stream of air passes at some velocity through a pipe, and if a powder like material is introduced in the pipe line, it must be carried by the air stream to a distant place.

Pneumatic Conveyors Application:

All materials cannot be transported by pneumatic conveyors. Those materials which are sticky or moist, cannot be carried away by air stream in a pipeline. On the other hand, those materials which are powder-like, free flowing, dry and not friable, are ideally suitable for pneumatic conveying. With the advancement of industries mass scale transportation of such materials are often necessary for different industrial and common applications. For example, cement, flour, powdered milk, powdered ingredients for medicines, explosive powder for ordinance factories, to mention only a few are basically powder-like, free flowing, dry and non sticky substances which are to be transported in a concealed condition in order to avoid contamination and hazards. So, for mass-scale handling of such materials pneumatic conveyors are essential, irrespective of cost factor.

Advantages of Pneumatic Conveying System:

In modern transport technology for handling bulk materials pneumatic conveying is gaining greater popularity day by day despite some disadvantages. The following advantages which a pneumatic handling system enjoys often makes its choice inevitable to achieve economical transportation in the plant.

• A simple technology which enable us to transport various products in a closed system without polluting the environment and being unaffected by the environment and weather changes
• It enjoys flexibility of installation and routing. Pipelines can easily be installed and routed along walls and ceiling to avoid obstruction without major structural modification. This is true for new buildings as well as existing building. Thus, material can also be transported vertically, horizontally or along inclined path
• Materials can be picked up from different points in a plant and can be delivered to a single point or many points in the same plant or in a different plant
• Low maintenance cost and lesser manpower for operation
• Multiple use of the same system can be achieved. The same pipelines can be used to transport variety products
• The system control is easy and can be converted to partial or fully automatic in operation
• It offers maximum security in transportation. Hence valuable bulk materials can be transported
• It offers less danger to operators and is safe for the environment
• It can be integrated into processes and hence great economy can result

Disadvantages and Limitations:

Pneumatic conveyor systems usually suffer from the following a disadvantages and limitations. While selecting a pneumatic conveyor to serve a particular purpose, the designer should give due considerations to the following points:

• Pneumatic conveyors usually involve relatively high energy consumption per ton of material conveyed. The specific energy consumption for a horizontal pipeline is of the order of 1-10 KW-h/ ton-km
• The principal limitation on the use of pneumatic conveyor is imposed by the properties of the material to be conveyed. All materials cannot be and should not be transported by pneumatic means. Before designing the system, the designer has to ascertain whether the material is powder-like, free flowing, explosive or non explosive, abrasive or non abrasive, toxic or non-toxic and so on. Usually, friable materials are not transported by pneumatic conveyors. Thus, a limited number of materials may be transported by this method
• Wear and abrasion are very high in pipeline used for pneumatic conveyors
• Conveying distance for a pneumatic conveyor is limited. Without booster stations, vacuum or low pressure pneumatic conveyors are workable up to 500m in length and for high pressure systems up to 2 to 3 km
• Pneumatic conveyor design requires a high degree of skill. As the flow mechanism in a two phase flow is really complex, mathematical modelling is in most of the cases impractical and the design of the system is based on empirical correlations which must be confirmed by experimental data. Therefore, even the most experienced designer cannot guarantee that the performance of the system will strictly be in accordance with the design specification.

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Types of Pneumatic Conveyor | Vacuum and Positive Pressure Pneumatic Conveying Systems | Dense Phase Pneumatic Conveying System

Classification of Pneumatic Conveying Systems

Pneumatic conveying systems can be classified on different basis of consideration. These basis are listed below:

• On average particle concentration (modes)
• On air pressure (Types of systems)
• On air supply arrangement
• On solid feeder type

Of these, the first two are very important and often influence the choice of the specific design for a given material, loading and delivery condition and the distance to be covered.

Classification based on Average Particle Concentration – Modes of Conveying:

Depending on the mass flow ratio, defined as the ratio of mass of particles conveyed to the mass of fluid used to convey, pneumatic conveying system may be classified into two modes; namely

1. Dilute phase
2. Dense phase

If the mass flow ratio is low, the system is said to operate in dilute phase. Whereas, if the mass flow ratio is high, the system is said to operate in dense phase. If the mode of operation of a system is in dilute phase, the probable range of mass flow ratio is 0 – 15. The dense phase system operates at mass flow ratio over 15.

In a dilute phase system the material is carried through the pipeline by a large volume of air having high velocity but relatively low pressure. The stream of air or gas carries the material in suspension in the pipeline as discrete particles owing to lift and drag forces acting on each particles. The distribution of particles over the cross section of the pipe is fairly uniform. In order to keep the particles in suspension, the air must possess a minimum velocity, called the pick up velocity. The pick up velocity for a particular material depends on many factors, like, the shape, size, specific weight of material and inclination of the pipeline.

If the velocity of air is gradually lowered down below what is required to keep the particles in suspension, the particles gradually settle down and form dunes at the bottom of the pipeline all along the length of the pipe. In this condition the material is about to choke the pipeline and if the pressure is increased these dunes and plugs of materials may move along the pipeline causing a dense phase flow. In a dense phase flow usually the velocity of fluid is much lower than the minimum velocity required for a dilute  phase flow. The distribution of particles over the cross section of the pipe is non uniform. At the lower part of the pipe there is slow moving dunes or plugs of material and the upper part of the cross section of the pipe is filled with certain proportion of finer particles in suspension in a state of dilute phase.

The maximum mass flow ratio achievable for a dense phase flow depends on many factors like the nature of materials and air velocity. It is usually greater than 30

Classification based on Air Pressure:

Based on air pressure, Pneumatic conveying systems may be classified as follows:

• Low pressure systems, in which the operating air pressure is about 1 atmosphere (760 mm Hg). This type of systems may be further sub classified into
• Positive pressure system
• Negative pressure system
• Combined positive – negative pressure system
• Medium pressure system
• High pressure system

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