User:Miko1uk/Pneumatic conveying

Pneumatic conveying is the process of transporting material, usually particulates such as powder, granules, grains, dust etc. through a duct by means of an air flow.

Application, advantages and disadvantages
Pneumatic conveying is used chiefly as a means for moving material within the confines of a process plant, as the conveying distances achievable are limited.

The advantages are principally two-fold. First, a pneumatic conveyor can traverse changes in direction (by means of a bend in the pipeline) without the need for transfer points such as would be needed for mechanical conveyors (belt, chain, screw conveyors etc). Secondly, the material is separated from the surroundings by the wall of the pipe, so the material cannot escape into the environment (an advantage when conveying hazardous materials such as toxic chemicals or dusty powders) and foreign objects from the environment cannot enter the material being transported, (a great advantage when conveying materials such as foodstuffs, pharmaceuticals, cements etc. which would be damaged by contamination or moisture from the environment).

Against these advantages must be set the disadvantage of high energy cost for operation, typically up to thirty times as high as belt conveyors and three or four times as high as screw or chain conveyors. Additionally, the impact of the conveyed particles against the inside of the pipe in bends means that there may be wear of the bend (if the particles are harder than the material of construction of the bend) or breakage of the particles.

History
The earliest pneumatic conveying systems on record were designed by Frederick Eliot Duckham, Engineer of the London Docks, in the late 18th Century. His first system blew mud dredged from the docks to a disposal point at Mudchute on the Isle of Dogs, where the Docklands Light Railwaystation of the same name recalls the practice. This was a blow tank system, large by the standards of today, blowing the mud a distance of X along cast iron pipes with a diameter of y. His second system, designed to extract grain from the holds of lighters, was installed at the Millwall Docks in 1892 as described by EG Phillips, and a contemporary engraving shows it to be recognisable as similar to a modern vacuum pneumatic ship unloader.

Design
Design of pneumatic conveyors is a specialised business, as there are many critical factors affecting choice of pipe size, air flow required, pressure drop, and selection of the correct ancillary components eg. feeder, air mover, air/powder separator. A detailed description of the latest design practice is described in a paper by Bradley. This employs a test rig located at The Wolfson Centre for Bulk Solids Handling Technology at the University of Greenwich in the UK, a leading research institute dedicated to the technology of bulk solids handling.

Conveying Characteristics
The critical starting point of design is to obtain information on how the material to be conveyed behaves in a pipeline when blown in an air stream, in particular the minimum air velocity required for reliable conveying of the material, and the relationship between the air velocity, solids concentration and pressure drop. This relationship can only be obtained by taking a sample of the material for which the system is to be designed, and conveying it in a test pipeline fitted with instrumentation to record these variables. The resulting data is known as the "conveying characteristics" of the material.

System design
Once the conveying characteristics are available it is possible to undertake the design of a ocnveying system to satisfy the demands of the specification. In many instances, there is a choice between various system designs, all of which may be technically suitable but there may be subtle differences between the way in which they work which may make one approach more suitable than another.