Introduction to pneumatic conveying and the guide:Free air velocity,Minimum conveying air velocity and Conveying line inlet air velocity.

Free air velocity

This is the superficial velocity of the air when evaluated at free air conditions.

Minimum conveying air velocity

The minimum conveying air velocity is the lowest superficial air velocity that can be used to convey a material.

Note: In dilute phase flow this is the lowest air velocity that can be achieved with- out saltation or choking occurring. The value of the minimum conveying air velocity in dense phase flow is significantly influenced by the solids loading ratio of the con- veyed material, in the case of materials having good air retention properties.

Conveying line inlet air velocity

This is the superficial air velocity at the point where the material is fed into the pipeline.

Note: In a single bore pipeline this will be the lowest air velocity in the conveying line and so it must be greater than the minimum conveying air velocity required to ensure successful conveying of a material. This is variously referred to as the pick-up or entrainment velocity. In a vacuum conveying system it is approximately equal to the free air velocity.

Conveying line exit air velocity

This is the superficial air velocity at the end of a conveying line where the material is discharged into the receiving vessel.

Note: In a single bore pipeline this will be the highest air velocity in the conveying line. In a positive pressure conveying system it is approximately equal to the free air velocity.


Saltation is the process of deposition of solid particles along a horizontal pipeline.

Note: This phenomenon occurs in dilute phase flow when the air velocity falls below the minimum conveying value. The saltation velocity is the minimum velocity at which a dilute phase system will operate and is equivalent to the minimum conveying air velocity.


Choking occurs in vertically upward flow and is the process that commences when solid particles near the pipe wall begin to flow downwards. As the process continues the pipeline eventually becomes blocked or chokes.

Note: Choking in vertical transport is somewhat analogous to saltation in horizontal transport, for both phenomena represent the onset of saturation conditions in dilute phase flow.

Acceleration length

This is the length of pipeline required for particles to reach their terminal velocity.

Note: When material is fed into a pipeline the particles are essentially at zero velocity and so have to be accelerated to their terminal value. A similar situation occurs fol- lowing bends since a degree of retardation is likely to occur in the flow around a bend.

Null point

The null point in a system is the position where the pressure is equal to the ambient pressure.

Note: This is generally used in relation to closed loop systems and identifies a natural point of access to the system for monitoring or conditioning.

Specific humidity

Specific humidity, w, is the ratio of the mass of water vapour to the mass of air in a given volume of the mixture.

Relative humidity

Relative humidity, cp, is the ratio of the partial pressure of the air, at a given tempera- ture, to the partial pressure of the air when saturated, at the same temperature.

Note: Whereas specific humidity gives an indication of the amount of water vapour that is actually contained in air, relative humidity gives an indication of how much more water vapour the air is capable of supporting before it becomes fully saturated. Its value is usually expressed as a percentage.

Stoichiometric value

The dust cloud concentration at which the quantity of air available exactly matches that necessary for combustion of a material.

Pulsating flow

Pulsating flow is continuous alternating high and low rates of flow.

Note: Pulsating solids flow in a pipeline can be caused by pulsating material flow from the feeding device, such as rotary valves, or by pulsating conveying air flow from an air mover, such as a positive displacement blower. Pulsating air flow is a result of continuous alternating high and low air compression by the air mover due to the man- ner in which the machine operates. Pulsating air flow in the conveying line can be reduced by the use of an air receiver.

Stepped pipeline

A continuous pipeline in which the diameter of the conveying pipe changes, generally to a larger bore, at points along its length. The purpose is to accommodate the change in volumetric flow rate of the conveying air as the pressure changes, without the vel- ocity falling below the minimum value of conveying air velocity at any point. This is sometimes referred to as a telescoped pipeline.

Air retention

The ability of a bulk material to retain air in the interstitial spaces between particles for a period of time. Very fine materials such as cement can exhibit this property, and when first poured into a container the material can behave almost like a liquid.


This is a measure of the ease with which air will pass through a bed of bulk particu- late material when a pressure difference is applied. Pelletized materials generally have very good permeability for there is little resistance to the flow of air through the inter- stitial passages. Materials that have a very wide particle size distribution generally have very poor permeability. If a pipeline blockage occurs with such a material a small plug of the material is often capable of holding an upstream pressure of 5 bar for a period of several minutes.


Hardness can be defined as the resistance of a material to an applied pressure or force.

Mohs’ scale

The Mohs’ scale of hardness is based on the ability of each material to scratch ones that come before it on the scale. Each material is allocated a number, 1 for the least hard material through to 10 for the hardest material. These are talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, feldspar 6, quartz 7, topaz 8, corundum 9 and diamond 10.

Brinell hardness

The Brinell hardness number is a number proportional to the load or test force of a hard steel ball to the calculated curved area of the indentation formed. The ball diameter is 1, 2.5, 5 or 10 mm.

Vickers hardness

Vickers hardness is a ratio of the load expressed as kilograms force, of a square base diamond pyramid shaped indenter, to the sloping area of the indentation formed. Very small indenters are used to measure the harness of small particles.


A temporary continuous changing rate of flow caused by non-steady state flow condi- tions, such as starting up and shutting down conveying systems, particularly where blow tanks are employed.

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