Dense & Dilute Phase Conveying

Explaining Dense & Dilute Phase

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Many people don't understand the difference between dense and dilute phase conveying.  The difference between dense phase conveying and dilute phase conveying is that in case of dense phase conveying SOLID TO AIR RATIO is high ( above 12 ) (i.e) 1 kg of air or conveying medium is used to convey more amount of solids. Normally the three phases are represented as follows.

  1. Dilute Phase = solid to air ratio up to 6

  2. Medium Phase =solid to air ratio from 7 to 13

  3. Dense Phase = solid to air ratio beyond 14

Since less amount of air or any other gas media is used for conveying more amount of solid, the compressor motor rating will be less in case of dense phase conveying resulting in less specific power consumptions (i.e) kwh/ton of material conveyed.

In dense phase conveying, due to less amount of air is used, the conveying velocity is less compare to lean phase conveying, but no case velocity of conveying media should not be less than terminal settling velocity of bigger size particle in the material.

The flow of material in dense phase conveying is not continuous, it is plug flow, the material move in pockets.

All the material is not suitable for dense phase conveying. Some material which is normally conveyed by lean phase, medium phase pneumatic conveying system, can not be conveyed by dense phase pneumatic conveying.  It is purely dependent on the fluidisation behavior of the material.

In case of dense phase conveying pressure drop is more, hence you require pressure vessel associated with gas tight valves etc for pumping material.
Hence the investment cost is more in case of dense phase pneumatic conveying, but operating cost is less due to less power consumption and less wear of pipe lines bends etc.

The power consumption of dense phase pneumatic conveying system for any material is like inverted " U ". The compressor motor rating will decrease as we go on increase the SOLID TO AIR RATIO, but beyond certain point the motor rating of compressor will start increase as we increase the solid to air ratio, because the increase in the pressure drop across the conveying line. So SOLID TO AIR RATIO should be at this optimum value only.

Normally before selecting any kind of high solid to air ratio for particular material in the view of designing DENSE PHASE PNEUMATIC CONVEYING SYSTEM, those materials should be tested in pilot plant for it's fluidisation behavior for such high solid to air ratio or there should be proven record / earlier reference for such kind of densphase system for that material.