Pneumatic transport - Dilute Phase - Lean Phase
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Types of pneumatic transport
Dilute Phase transport
Dense Phase transport
Conveying speed / velocity
Air volumetric and mass flowrate
Pipe Equivalent Length
Solids particle velocity in pipe
Pipe Diameter or Bore
Selecting dilute or dense phase
1. PRESSURE - Lean Phase
The air mover in pressure lean phase is most of the cases a Blower or compressor. The most common type is to use a Roots Blower which combines good reliability, high air flow and can reach enough pressure for most of the industrial needs. This Blower will allow to reach max 1 bar g.
Another possibility is to use compressed air for some specific cases using a venturi to pick-up the powder. This solution is not very economical, therefore it should be kept for very specific cases and low throughput.
In order to introduce the powder to the conveying pipe, an Airlock rotary Valve should be used. This equipment will allow a rough dosing of the powder to the pipe, which is necessary to avoid blockages but will have as main function to isolate the area of high pressure (the pipe to the area of low pressure (the hopper introducing the powder. This avoids to have the air going in the product and preventing its flow.
The receiver is most of the time equipped with an Airlock rotary Valve also in order to allow a separation of the air and the product. In some cases, it is possible to avoid such star valve if the receiver is big enough and the situation is safe (discharge directly to a silo.
Figure 1 : Pressure lean phase typical process arrangement
2. VACUUM - Lean Phase
In vacuum lean phase, the air mover is basically the same than in pressure lean phase, except that it is positionned at the end of the line thus sucking the air and creating a vacuum. The vacuum created is around -500 mbar g at compressor suction.
At product pick up, there is the need of having an air inlet to allow the air sucked to flow in the pipe. On the product side, no Airlock rotary Valve is necessary here since the air will go in the direction of the product and will therefore not prevent it to flow. A system to control the product intake is however necessary (screw, simple valve) in order to avoid that a surge in product flow block the pipe.
At reception, the best practice is to position a cyclone with an Airlock rotary Valve. The design must take into account that the air flow from the downstream equipment will this time flow against the product. A proper degassing must be considered.
Typical instrumentation should include the following : pressure sensor at compressor suction, differential pressure drop through filter, level sensor at reception
Figure 2 : Vacuum lean phase typical process arrangement
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