Pneumatic Transport Phase
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# Types of pneumatic transport
# Conveying phases
# Dilute Phase transport
# Dense Phase transport
# Air mover
# Roots Blower
# Roots Blower performance curve
# After Cooler
# Airlock Rotary Valve
# Product inlet / Injector
# Choking velocity
# Conveying speed / velocity
# Air volumetric and mass flowrate
# Pipe Equivalent Length
# Solids particle velocity in pipe
# Solids Breakage
#Pipe Diameter or Bore
# Selecting dilute or dense phase
1. Geldart Classification
Be careful, not all the solids can be transported with any technology. Actually, the adequacy of a particular technology will depend on the fluidization properties of the solid to be transported. Before designing a system, the class of the solid must be established. Such class is determined thanks to a classification proposed by Geldart in 1973.
It is a very shortcut method to classify powders according to their Geldart nature in order to determine if they can be transported dense phase. However, with this basic approach, the following can be considered.
Class A : powders adapted to dense phase conveying, they have a very high and lasting aeration. They can be transported at high concentration but do not form naturally some plugs, an active system is therefore require to create them.
Class B : sand-like powders, may be difficult to transfer in dense phase
Class C : these cohesive powders, may be difficult to transport in dense phase
Class D : these powders can be transported in dense phase at concentration in between class B and class A
Figure 1 : Geldart classification
Following Geldart classification is a 1st indication, with group B and C being the less suitable for dense phase conveying. However subsequent works have shown that among group B and C, some powders are able to transfer in dense phase, provided they have the right permeability and / or air rentention. The air permeability allows indeed to fluidize the product and therefore change its rheology, while the air retention allows to keep the fluidized state a long time which can be interesting in a conveying pipe. As an indication, high permeability should allow to convey in plugs while a good air retention would allow a dune flow.
2. The different solids / air phases in a pneumatic transport line
It is important to understand how the concepts of lean phase and dense phase translate on the way the product is flowing in the pipe. Basically, depending on the velocity of the transporting gas, 5 flow regimes can be identified.
Table 1 : Conveying phases
It must be noted that the particles circulate at a lower speed than the gas. Depending on the flow regime, the particle average speed can be 0.4 to 0.8 times the gas velocity.
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