Pneumatic Conveying

A pneumatic conveying line is a coordinated process system. The air mover cannot be selected independently from the feeder, pipe, receiver, filter, and discharge equipment. A restriction or poor control choice in any one part can limit capacity, damage product, or cause unstable operation.

How pneumatic conveying works

Air or another gas creates a pressure difference along a pipe. Material enters at a controlled rate, accelerates into the moving gas, travels through the route, and separates from the gas at the destination. The receiver retains the solids while the conveying gas passes through filtration and leaves the system or returns through a closed loop.

The transport behavior is a gas and solids interaction. Particle size, density, shape, cohesion, moisture, permeability, and air retention influence how the bulk solid moves. Pipe diameter, bends, vertical lift, pressure, and gas velocity act on the same flow. This is why a material name and desired capacity do not provide enough information for design.

Main conveying regimes

The regimes differ in solids concentration and gas velocity. AIChE's practical comparison of dilute and dense phase conveying and a recent review in Particuology examine the trade-offs summarized below.

Dilute phase conveying

Dilute phase uses enough gas velocity to keep most particles suspended. It is the most broadly applicable regime and often supports continuous operation. Higher velocity can increase wear and product attrition, so the route and material must be assessed together.

Dense phase conveying

Dense phase moves a greater concentration of solids at lower velocity. The flow can appear as strands, dunes, slugs, plugs, or a fluidized mass. Dense phase can protect fragile material and reduce wear, but not every powder forms a stable dense regime.

Intermediate and unstable flow

Operating between established regimes can produce saltation, deposits, pressure fluctuation, and intermittent movement. The lowest pressure condition is not necessarily the most reliable one. A design needs a stable operating window that covers startup, normal duty, turndown, and shutdown.

Pressure and vacuum arrangements

A pressure system places the air mover before the material pickup and pushes toward the destination. It can suit longer routes and several destinations. A vacuum system places the air mover after the receiver and draws material from one or more pickup points. Inward leakage can support dust containment at the source.

Pressure and vacuum do not define the conveying phase. Both dense and dilute systems can use a suitable pressure or vacuum arrangement. The choice depends on source access, destination pressure, distance, filtration, leakage direction, discharge method, and maintenance access.

Core system components

Material properties that matter

• Particle size distribution and fine content.
• Particle and bulk density.
• Particle shape and surface condition.
• Cohesion, permeability, and air retention.
• Moisture and hygroscopic behavior.
• Abrasion and attrition sensitivity.
• Temperature and chemical compatibility.
• Combustibility and electrostatic behavior.

Geldart classification can support early screening of fluidization behavior, but research reviewing 50 years of the classification confirms that its boundaries do not replace conveying trials. Powders near a boundary can respond differently because of fine content, permeability, or deaeration.

Benefits and limits

A closed pipeline can provide flexible routing, dust containment, automation, and fewer exposed moving parts than some mechanical alternatives. It can move material vertically and around existing plant structure. The same flexibility can hide poor design because the pipe appears simple while solids behavior remains complex.

Potential problems include insufficient capacity, plugging, buildup, filter restriction, particle degradation, segregation, wear, excessive energy demand, and electrostatic charging. These risks are managed through material data, appropriate testing, sound geometry, stable feeding, instrumentation, and operating procedures.

Selection sequence

1. Define the material range and collect representative samples.
2. Map the complete route and all operating cases.
3. Set capacity and product quality acceptance criteria.
4. Define hygiene, containment, and safety requirements.
5. Screen dense, dilute, pressure, and vacuum concepts.
6. Test uncertain materials and scale the selected regime.
7. Design feeding, pipework, receiving, filtration, and controls together.
8. Commission against the measurable operating envelope described below.

Turn the route into a pressure budget

The available pressure difference is consumed by gas flow through straight pipe, bends and fittings; acceleration and elevation of solids; the feed interface; and the receiver and filter. These contributions change with solids rate and gas density. A design review should show where pressure is expected to be used and which measurement will reveal a deviation. A single pressure value at the air mover cannot identify a blocked pickup, restricted filter or unstable section of pipe.

Route geometry should be based on an installed path, not a straight-line distance. Include vertical lifts, flexible connections, diverters, expansions and the spacing between disturbances. Provide supports for pipe weight and dynamic loads, access to likely deposition points and replaceable protection where wear is expected.

Design normal and abnormal sequences together

Startup establishes gas flow before uncontrolled solids entry. Shutdown stops feed, clears the line only as far as the material and process allow, and leaves receivers and filters in a known state. Loss of power, a full destination, high filter differential pressure or a failed valve each need a defined response. The safe response may be different from the fastest way to restore production.

Recovery depends on what remains in the pipe. A cohesive deposit may not move when full air is restored, while a fragile product can be damaged by repeated high-velocity clearing. Instrumentation should help operators distinguish insufficient gas, excessive feed, receiver restriction and material buildup before they intervene.

Commission the operating envelope

Acceptance should cover minimum, normal and maximum rate with representative material. Record gas flow where available, pressure at useful locations, feeder output, filter differential pressure, receiver level behavior and product condition. Include a planned stop and restart, a change of destination where relevant, and the difficult material condition identified during design.

Retain the pressure and cycle-time signatures as the healthy baseline. They allow later maintenance teams to separate gradual filter restriction, increasing leakage, pipe buildup and a changed product from an undersized air mover. Review the design when the route, material supplier, production rate, filter media or control sequence changes.

Safety and combustible dust

The pipeline can disperse combustible material and connect several vessels. A hazard assessment should address ignition, electrostatic discharge, frictional heating, fire propagation, explosion pressure, and transfer between connected equipment.

HSE guidance on dust explosions identifies pressure monitoring, adequate air removal at receivers, and level detection as important safeguards; the OSHA technical manual chapter on combustible dusts covers the wider hazard assessment. The final protection concept depends on the dust properties, equipment, process, and jurisdiction.