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Increasing capacity in pneumatic conveying systems
Q: To increase our pressure conveying system's capacity, we increased the blower speed and airflow, but we found that our capacity was reduced instead. What did we do wrong?
Pneumatic Conveying Consultants LLC, says: You're obviously conveying in dilute phase (stream flow). When you increase airflow in dilute phase, either the pressure increases or the capacity decreases. Although it seems wrong, you may have been able to increase the capacity by decreasing the amount of air in the system (slowing the blower).
Increasing the airflow increases the material and air velocity in the conveying line. The increased velocity has a higher friction factor and thus a higher pressure requirement. To get the maximum capacity out of a given conveying line diameter and length at a given operating pressure, reduce the air volume in small steps. Each reduction in air volume will result in a reduced operating pressure (or increased capacity) until the saltation velocity is reached. Reducing the airflow below this point will reverse the action, causing the pressure to rise or the capacity to drop.
Remember, although there are tables for design pick-up velocities for many materials, these are safe velocities that will work with any line configuration. Your particular material and conveying line may be able to operate below these standard pick-up velocities, but you can only know for sure by adjusting the airflow in your installation until you discover the airflow point where you have maximum capacity.
Paul Solt is principal consultant at Pneumatic Conveying Consultants LLC. He will be presenting two conference sessions at PBE’s 2012 Southeast Conference & Exhibition, to be held March 13-15 at the Cobb Galleria Centre in Atlanta, Georgia.
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