Dear Amit,

The calculated saltation velocity is used as a reference value to determine (or choose) the mean gas velocity for the pneumatic conveying installation.

In order to be certain that there will be no sedimentation, the gas velocity (gas volume / pipe cross section area) is taken as a factor times the CALCULATED saltation velocity.

This practical factor, which is depending on the product properties for pneumatic conveying, eliminates your (true and existing) concern.

therefore there are also tables, giving gas velocities for various products, bypassing the need for calculating the saltation velocity.

Another approach is to determine the floating velocity of the mean particle size in atmospheric air and choose the atmospheric gas velocity as a factor times this floating velocity. (basically the same method).

This factor ranges from 2.5 for fine particles (cement, flyash) to 5 for coarse particles (soy beans, cereals)

Whether saltation occurs, depends on the size of the particles and the thickness of the boundary layer of the flow in the pipeline.

If the velocity in the boundary layer of the flow is lower than the local floating velocity of the particle and the thickness of the boundary layer is more than the size of the particle, then sediment builds up, until the gas velocity in the remaining pipe cross section causes enough velocity to prevent additional sedimentation.

As these methods are product related, the normal average size distribution of these products is also considered in the figures.

Then the particle size fractions also determine the K-factor (product resistance factor or friction multiplier for solids conveyed).

If a product has a wide particle size distribution, the factor K is different from the K-factor for a narrow particle size distribution.

Considering pneumatic conveying as a system that works by transferring impulse from the gas to the particles, then the smaller particles acquire higher velocities than the bigger particles.

When the particles have different velocities, the number of inter particle collisions will increase and the particles transfer impulse among themselves.

The smaller particles transfer impulse to the bigger particles, keeping the bigger particles in suspension.

In theory, this is all true, but in the end an installation is designed on educated decisions for gas velocity and K-factor (a formula in itself as a function of loading ratio and turbulence)

and based on experience (good and bad)

Your question is related to the main principle of pneumatic conveying (How to keep particles moving in a gas flow), but is in practice only good for theory building.

This also makes pneumatic conveying an interesting challenge.

best regards

teus ■

Dear Amit

you are welcome

teus ■

saltation velocity and mean particle size .

mean particle size 50 percent or your particles are smaller and 50 % of your particles are latger .

so strictly speacking 50 percent of your particles will not be saltated but will sit on the bottom ogf your pipe until the flow channell is constricted such as to make them jump .

when and how they will jump I don not want to elaborate , but they can certainly do damage . It would be better if they just sit down there forever .

beter follow our friends advice use a factor or simply use a smaller diameter pre acceleratiion section . This is more important in hot pneumatic conveying .

marco ■