Two basic modes of flow are possible for materials in pipelines: dilute phase or suspension flow, and dense phase or nonsuspension flow. For dense-phase conveying there are two possibilities, dependent upon material properties. One is sliding bed flow, for materials that have good air retention, and the other is plug flow, for materials with good permeability.
|Dr. David Mills|
Virtually anything can be pneumatically conveyed, provided it can be fed into a pipeline. But to maintain a particle in suspension in the conveying air, a certain minimum value of air velocity must be maintained, otherwise the particles are likely to drop out of suspension and ultimately lead to blockage of the pipeline. For flow in horizontal pipelines, this is referred to as saltation, and for vertically upward flow it is called choking. It would generally be recommended that a safety margin of about 20% be used for actual conveying line inlet air velocity.
The velocity of the air necessary to convey a material in suspension flow depends mainly on the particle properties of size, shape, and density. For fine powders the minimum conveying air velocity required will be in the region of 2000–2400 ft/min; 2000–2200 ft/min for cement; and 2200–2400 ft/min for fly ash. The slightly lower value for cement can be attributed to the difference in particle shape. Cement is manufactured in a grinding process to produce a fibrous particle having a relatively large surface area, whereas fly ash is generated in a combustion process that results in an essentially spherical particle. If velocities are preferred in m/s, dividing the above figures by 200 will give a reasonably accurate conversion.
For fine granular materials the value will be in the range of 2600–3000 ft/min. For granular sugar, however, it is about 3200 ft/min, mainly because the material is produced to a narrow particle size. For materials having a wide particle size distribution the minimum conveying air velocity will be slightly lower. For larger and higher density particles, higher velocities will be required. The minimum conveying velocity for flow vertically up is slightly lower than that for horizontal conveying, but as most pipelines incorporate elements of both horizontal and vertical sections it is not often that this fact can be taken advantage of.
For materials that are capable of being conveyed in dense phase, much lower conveying air velocities can be employed. For fine powders that have good air retention, properties conveying air velocities can be as low as 600 ft/min. Such low velocities, however, can only be achieved at high values of solids loading ratio, and high values of solids loading ratio can only be achieved with a high conveying line pressure gradient.
Solids loading ratio is defined as the ratio of the mass flow rate of the material conveyed to the mass flow rate of the air used to convey the material and is evaluated as a dimensionless number. For dilute-phase conveying, values are typically up to about 15, although they can be higher if the pressure gradient is high, as with a short pipeline or a high pressure drop. For the dense-phase flow of materials, such as fine fly ash and cement, values can be in excess of 100, and again higher values can be achieved with a short pipeline or a high pressure drop.
For the dense-phase flow of pelletized materials conveying line inlet air, velocities can also be as low as 600 ft/min. Solids loading ratios, however, are rarely greater than 30, since much of the conveying air actually permeates through the plugs of material as they are conveyed.
It should be noted that all references here are to pick-up velocity, for as air is compressible the conveying air velocity will gradually increase in value along the length of the pipeline. As a consequence, the lowest value of velocity will always be at the start, but this is only for single bore pipelines. If the pipeline is stepped to a larger bore part way along its length, to take up the expansion, this will no longer apply. But that is another story.
David Mills has a degree in mechanical engineering and undertook a PhD program on pneumatic conveying in 1973 and has been working in this area ever since, first at the University of Greenwich in London, then as Professor of Bulk Solids Handling at Glasgow Caledonian University in Scotland. Since 1997 he has been operating as an independent consultant.