Conveyors are vital systems in industrial facilities serving various industries, including food processing, automotive, agricultural, and mining industries. Conveyor systems come in several types with different functions and capabilities, for example, belts, powered roller, bucket, ball transfer and pneumatic conveyors. The pneumatic type remains one of the most in-demand conveyor systems today.

Pneumatic conveying describes the transportation of materials (granules, powders and pellets) through a pipe using compressed air or another gas. A pneumatic conveying system relies on a combination of pressure differential and airflow from an air mover, such as a compressor, centrifugal fan and blower to transport bulk materials.

But as simple as these systems might seem to operate, there are several things that engineers must understand when specifying pneumatic conveying systems.

This article presents answers to three frequently asked questions about pneumatic conveying systems.

What are the types of pneumatic conveying?

Pneumatic conveying systems can be grouped into three categories:

• Dilute phase pneumatic conveying
• Dense phase pneumatic conveying
• Semi-dense phase pneumatic conveying

1. Dilute phase pneumatic conveying

In dilute phase pneumatic systems, the material to be conveyed is metered into a moving air stream. This material is suspended in the conveying air stream as it is blown (or sucked) through the pipeline at speeds ranging between 10 m/s and 35 m/s.

A dilute phase pneumatic conveying system is suitable for a wide variety of materials, including fine powder and granular materials. However, engineers must give adequate consideration to abrasive and fragile materials. This is because there will be contact between the material being conveyed and the pipeline, especially at the bends. As a result, fragile materials will tend to disintegrate, while abrasive materials might wear the pipelines under dilute phase conveying systems.

Engineers looking to transport fine powder must be able to achieve velocities ranging from 10 m/s to 12 m/s. For fine granular materials, the velocity must range between 10 m/s and 16 m/s. Velocities higher than 16m/s are ideal for larger particles or materials with a higher density.

2. Dense phase pneumatic conveying

Dense phase pneumatic systems differ from dilute phase systems in the air velocity used to transport the materials and the type of material that can be transported.

Unlike the dilute phase systems, the dense phase pneumatic conveying system uses low air velocity (usually less than 7 m/s) to transport dense concentrations of bulk solids. These bulk solids are usually not suspended in the conveying air stream, and the low conveying air velocity means there is less wear and tear on the conveying system and material being conveyed.

Dense phase pneumatic conveying systems are ideal for low- and high-rate transfer of abrasive, friable and slightly hygroscopic materials; its low-velocity nature prevents materials from disintegrating.

3. Semi-dense phase pneumatic conveying

The velocity of the transported material in the semi-dense phase typically ranges from 7 m/s to 10 m/s, which is higher than the dense phase and lower than the dilute phase. The semi-dense phase conveying system is ideal for transporting products in an aeratable state. They are also ideal for the medium-rate transfer of abrasive and fragile materials.

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What is solids loading ratio?

The solids loading ratio is a parameter used to determine the concentration of the material being conveyed in the air. It is expressed mathematically below:

pneumaticonveyor_eq1

Where:

f = solids loading ratio

mp = product mass flow rate (ton/h)

ma = air mass flow rate (kg/s)

Dilute phase systems usually have a solids loading ratio ranging between 0 and 15, whereas dense phase systems have a solids ratio greater than 15.

What are some of the things to consider when specifying a pneumatic conveyor system?

Important factors to consider when specifying pneumatic conveying systems include the bulk density of the material to be conveyed, conveying distance, type of conveyor and cost.

1. Bulk density

The bulk density of a material is a measure of its weight per unit volume, as shown in the equation below.

pneumaticonveyor_eq2

The bulk density of most of the materials transported pneumatically ranges between 0.1 kg/dm³ and 1 kg/dm³. Materials with low bulk density (in the range of 0.4 kg/dm³ to 0.88 kg/dm³) are usually easier to convey, while heavier materials demand more power and larger vacuum receivers.

2. Conveying distance

As a rule, engineers must determine the required length of piping needed and whether the pneumatic conveying system will conveniently handle the distance and direction that the bulk material will travel.

3. Vacuum or pressure pneumatic conveyor

A vacuum system conveyor essentially sucks materials through the pipeline via negative pressure, whereas pressure pneumatic conveyors apply positive pressure to the material to push it through the pipeline.

The vacuum pneumatic conveyor is ideal for pressure-sensitive non-abrasive materials, while pressure pneumatic conveyors are ideal for moving abrasive materials.

4. Dilute phase, dense phase or semi-dense pneumatic conveying system

Dense phase pneumatic conveyors are ideal for very fragile materials since their low-velocity nature prevents the materials from disintegrating. They can also be used for low- or high-rate transfer of friable, abrasive, and hygroscopic materials.

In contrast, dilute phase conveyors are better suited for non-abrasive and non-fragile materials with low density, for example, cornstarch. Semi-dense pneumatic conveying systems are ideal for products in an aeratable state such as cement.

The dilute phase systems are usually the least expensive to create of these three pneumatic conveying systems.

Conclusion

Pneumatic conveyors are important systems in many industrial facilities today as they are helping businesses meet their bulk material transportation requirements.

While this article presents helpful information about pneumatic conveyors, there are still many factors engineers must consider when specifying pneumatic conveyors. It is recommended that engineers reach out to pneumatic conveyor manufacturers to discuss their application requirements.