PHASEBOOK Part 2: Breaking down Dilute Phase

Experience is the key in efficient and well performing pneumatic conveying systems

Bulkagram 7 Oct 19

Hello Again!

The earlier bulkagram 5 (dated June 15, see link below), dealt with basic definitions of Dense and Dilute Phase conveying.

Today, let’s understand the crucial elements that need to be considered when designing a dilute phase system:

A. Conveying Air Velocity and Pressure Drop: Dilute phase is suspension material flow. Material particles get suspended in the air or gas stream at a particular velocity called the saltation velocity. The saltation velocity varies for different bulk materials and is dependent on a variety of factors including bulk density, particle size and shape etc. Generally conveying air velocities in dilute phase range from about 20 to 30 m/sec. Remember we are talking about air velocity. These values are all conveying line, inlet air velocity values. As per fundamentals, air is compressible, so as the material is conveyed along the length of the pipeline, pressure will decrease and the volumetric flow rate increases.

Again, getting back to fundamentals: (Boyle’s law): P1V1 =P2V2

Considering pressure at 1 bar gauge at the material feed point, and discharge at atmospheric pressure, there will be a doubling of flow rate and hence velocity in the pipeline. (So if conveying line inlet air velocity was 20m/s at the start of the pipeline, it would be approximately 40m/s at the outlet.

But guess what, (and this is where experience counts): These velocity values are air velocities. Once the material particles are introduced into the airstream the velocity of the particles will be lower than the airstream velocity by about 20 to 40% depending again on bulk density and particle size/shape.

B. Particle Velocity: Dilute Phase conveying uses drag force which means the particles move slower than the air. How much slower is very difficult to predict even with the most sophisticated mathematical modelling.

Sometimes customers call in to ask for the ‘formula’ for particle velocity in a pipeline. Since no such formula exists we rely on our vast database from which we have culled data and created emprical relationships that give us a band of particle velocities for a given set of air velocities.

In a horizontal pipeline, velocity of particles will typically be about 70-80% of that of air. The velocity of the particles divided by the velocity of air is termed ‘slip ratio’ which in this case is 0.7-0.8. Again, this value will depend on particle size, shape and density and can vary across a wide range.

Experience is a major factor in arriving at the correct design of pneumatic conveying systems. By documenting experience and actual field data of operating systems vendors like us who have been in business for over 30 years are able to develop spreadsheets that lead us closer to the actual performance in a pneumatic conveying pipeline.

C. Pressure Drop due to friction and work : Pressure drop due to friction and work occurs at feed points and at bends. In traversing the bend, the particle will also impact the bend and be retarded. The slip velocity at exit from a bend will be lower than at inlet, so the particle has to be reaccelerated. By having the correct pressures in the conveying line, pressure drop due to reacceleration becomes part of the total pressure drop in the system and the air movers must be sized accordingly. Spreadsheets with actual field data aid enormously in correct numbers being generated.

In summary: though dilute phase conveying appears to be a simple job of feeding bulk powder into a pipeline with a steady gas flow above saltation velocities, the pressure drop and particle behaviour during transport are complicated physics. It is still a black box and the box becomes grey and whiter with experience in conveying a wide variety of bulk powder in a long list of installations. Experience is the key in efficient and well performing pneumatic conveying systems either dilute or dense phase.

We will be delving deeper and deeper so stay tuned!

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