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

History of Pneumatic Conveyors | Pneumatic Capsule Transport

Historical Development:

The basic Principles of Pneumatic conveying stated by Great Alexander in 100 BC itself. In pre historical age Romans used Water supply pipelines and sewage disposable systems. Chinese conveyed Natural gas through Bamboo’s. These are the evidence for transporting physical objects in Pneumatic tubes.

The first industrial application of pneumatic conveying was probably in the form of capsule transportation system in which materials are enclosed in cylindrical or spherical capsules of diameter only slightly less than that of the pipe line and then use the gas or liquid to propel the capsules from one end of the pipe line to the other. The first pneumatic capsule system was built and demonstrated in England in 1820’s by John Vallance. Considerable work on pneumatic capsule transport was undertaken by Pneumatic Despatch Company who laid an experimental tube, about 400 meters in length, along the bank of the river THAMES in London. Various similar tunnels and capsule systems were constructed in England to carry letters and parcels. Attempts were made in London and New York around 1860’s to use pneumatic capsule systems to carry passengers, but these attempts proved unsuccessful due to practical difficulties with human load. In 1864, Pneumatic railway line was built in Crustal palace to move a carriage, which had been fitted with a sealing diaphragm.

In 1847, Peugeot plant in France used pneumatic conveying plant for exhaust dust from number of grind stones with the help of an exhaust fan

It was in 1866 that a demonstration was arranged by B.E. Startevant to show that solid particles can also be conveyed directly by a stream of air through a pipeline. The first experimental type of pneumatic conveyors were fan driven vacuum systems employed to transport food grains and sawdust. From the early years of 20th century high pressure air was employed in pneumatic conveying. In mid 1920’s the technique of fluidisation discovered and since then the technology of pneumatic conveying has grown enormously since 1970’s because of its suitability for modern industrial processes and economies of bulk handling methods. During the first World War, the development of pneumatic conveying was influenced by the high demand for foods, labour scarceness and risks of explosion. Since the pneumatic conveying systems were seen as the answer for those situations, so a huge evolution of pneumatic transport was achieved during that time period. In the post war period, pneumatic conveying systems were used for more industrial related materials like coal and cement. Beginning of theoretical approaches, invention of blowers, introduction of batch conveying blow tanks etc., were among the highlighted milestones of the evolution of pneumatic transport systems during the era.

Now-a-days, pneumatic handling of solids is common place in industries like pharmaceutical, cement, food, chemical, glass, plastic and mining etc. Some industries have transport objects for the distance of more than 40 km, material flow rate of few hundred tons per hour and solid loading ratio of more than 500 also possible.

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