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Transporting fine powders

Q: With a given pneumatic conveying system (6-in diameter), transport of 11 ton/h of powder (150 micron D50) is possible. Material that is finer (125 micron D50) transports much more difficult. It seems that as the powder settles somewhere along the first part of the transport line and is then (due to pressure increase) blown as a package through the line, the pressure spikes and causes the safety valve of the blower to go off. This causes the line to block and transport capacity is therefore limited to 2/3 of normal transport rate. (Velocity in the line is high enough. Mass to air ratio -3.5.) What is the powder property that relates to this behavior? What can we to increase transport capacity of this "fine" powder?

A: I can comment on the material property involved, i.e., permeability. The finer powder has a much lower permeability. If it settles in any place, it will consolidate to a cohesive solid.

The potential solutions include:

• be sure that the hopper and line charger are feeding a uniform rate and density*
• be sure that the line charger is not leaking air back into the feed hopper
• carefully eliminate any locations in the system where the velocity drops below the settlement velocity
• minimize the length and eliminate any lines that taper down in elevation

*The finer material may not be suitable for the feed hopper. If the material is forming a rat hole due to an improper hopper shape, periodic flushing can cause a swing in bulk density of up to 30%, which will unbalance the line.

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