Choking in a pneumatic conveying installation occurs when:

1)The local air wall velocity becomes too low in comparison to the local suspension velocity.

2)The capacity / pressure drop gradient becomes too low in the capacity – pressure/vacuum curve.

These 2 conditions can be investigated by the calculation of:

1)The Zenz curve

2)The capacity – pressure/vacuum curve.

The Calculated Zenz curve shows the minimum airflow, whereby the pneumatic conveying is still free from sedimentation.

Sedimentation always starts at the beginning of a pipe section with a certain diameter.

That is the reason that a diameter step in a pipeline must be at a point, where the local air pressure/velocity ratio has decreased sufficiently to keep the material in suspension.

The calculated capacity – pressure/vacuum curve shows the capacity / pressure drop gradient at a high pressure drop.

In this region, a slight fluctuation of the Solid Loading Ratio increases the pressure drop above the set maximum pressure drop or causes a too low air velocity at the beginning of a pipe section with a certain diameter.

For safe and stable operation, both conditions must be calculated, to be certain that the pneumatic conveying installation will operate trouble free.

The operating conditions are then chosen at 80% to 90% capacity from the choking zone.

Therefore, it is essential that both curves are calculated, and that a simple one capacity-pressure drop design is not enough and even risky. (Or over-designed) ■