How do material properties influence the selection of a pneumatic conveying system?

Material properties form the basis for selecting a pneumatic conveying system. Depending on the specific material properties, we must choose the appropriate system type and components in order to achieve the desired conveying results. Here are some considerations for system selection based on typical material properties.

1. Adhesiveness and stickiness. Adhesive materials may stick to or block discharge hoppers, feeders and conveying pipes. Therefore, in the case of rotary vane feeders, blow-through rotary feeders should be the preferred choice.

2. Flammability and explosiveness. When conveying flammable or explosive materials such as plastics, chemicals, metal powders and pulverised coal, safety measures such as explosion-proof valves and automatic fire-extinguishing systems should be employed.

3. Moisture content. If the proportion of fine particles smaller than 50 μm in moist material is less than 10 per cent, most such materials can be conveyed using conventional pneumatic conveying systems. However, if the moisture content of the material is high and fine moist particles adhere to the inner walls of bends, causing pipeline blockages, a purged rotary vane feeder should be selected. If the material is not excessively damp, heating the conveying air can help alleviate adhesion and blockage issues.

4. Static electricity. The build-up of static charge in the material can cause adhesion and affect flowability; this can be resolved by online air humidification. In dense-phase conveying, as the volume of air used is relatively small, the cost of humidification is low.

5. Abrasiveness. To minimise wear on conveying pipes and components, lower conveying speeds should be adopted when handling abrasive materials. In dilute-phase systems, feeders with moving parts should be avoided, and the service life of the pipework should be extended through measures such as the use of short-radius bends (R/D = 2–3), cast iron T-joints with one end closed, and ceramic-steel composite pipes manufactured using self-propagating high-temperature synthesis (SHS) technology.

6. Fragility. During conveyance, damage to most materials occurs in feeders such as elbow pipes or screw pumps. Therefore, when designing a system, the use of elbow pipes should be minimised and feeders such as screw pumps—which are prone to breaking brittle materials—should be avoided.

7. Granularity. Top-discharge hopper pumps and standard rotary vane feeders are not suitable for conveying granular materials. The latter can shear granular materials, whereas an offset rotary vane feeder can prevent this phenomenon.

8. Hygroscopicity. Problems associated with hygroscopic materials can be avoided by conveying them using dry air. Freezing methods or desiccants can be used to keep the material dry. In some cases, where moisture adsorption is minimal, the material can also be conveyed in a dense phase using undried air.

9. Low melting point. When high-speed particles with a low melting point (softening temperature of 150 °C) strike the inner walls of pipes and bends, localised melting may occur. For most materials with a low melting point, this phenomenon can be eliminated by conveying at low speeds.

10. Particle size. Micron- or sub-micron-sized fine powders may coat the inner walls of pipes during conveyance, thereby reducing the pipe’s cross-sectional area and lowering the conveying capacity. This problem is typically resolved by using a hopper pump and incorporating a device within the pipeline to periodically vibrate the flexible hose.

11. Gas permeability and retention capacity. Dilute-phase conveying is characterised by low pressure, high velocity and uniform distribution of the material across the cross-section of the conveying pipeline; consequently, the conveying process is essentially determined by the properties of individual particles affecting the surrounding gas flow. Dense-phase conveying, by contrast, is characterised by high pressure, low velocity and strictly separated two-phase flow; the material being conveyed flows primarily in a束状 form at the bottom of the pipeline, with occasional dune-like formations, irregular agglomerations or plug-like flow filling the pipeline cross-section. This conveying process is influenced by the overall flow properties of the material rather than the characteristics of individual particles. Consequently, the material’s gas permeability and air retention capacity have a greater impact on dense-phase systems than on dilute-phase systems.

When the material has a high air retention capacity (i.e. poor gas permeability), a relatively small volume of air is sufficient to fluidise the material and reduce the internal friction angle. Once the air flow ceases, this flow process can continue for a certain period; during this phase, the internal friction angle is typically less than the wall friction angle. In dense-phase conveying systems, agglomerations and plugs formed by such materials are easily broken up, and the material flows at the bottom of the pipeline essentially in the form of a fluidised bundle. In dense-phase plug transport, the critical plug length for such materials is relatively short. Furthermore, for coarser materials with higher air permeability (i.e. lower air retention capacity), the permissible critical plug length may exceed the pipe diameter; the stability of the material plug is directly proportional to the ratio of the plug length to the pipe diameter. Such materials are prone to agglomeration and plug formation during dense-phase transport, which can easily lead to blockages; their use in dense-phase systems should therefore be minimised or avoided altogether.