Dilute Phase Conveying

Dilute phase is the most common pneumatic conveying regime because it can transport many powders, granules, and pellets in continuous operation. Its apparent simplicity should not hide the central design problem. Gas velocity must remain high enough to prevent unstable deposition while staying low enough to control attrition, wear, energy demand, and filter load.

Particle suspension and transport

Most particles travel suspended in the gas, although concentration is not perfectly uniform. Gravity and particle inertia can create a denser region near the bottom of horizontal pipe. Bends, vertical sections, and changes in diameter alter particle velocity and distribution.

The gas moves faster than the solids. Particles require distance to accelerate after entering the pipe and after a major disturbance. Poor pickup geometry or a bend placed too close to the feed point can create fallback, impact, and blockage.

Pressure and vacuum dilute phase

In a pressure arrangement, a blower or compressor supplies gas before material enters the line. A rotary airlock often meters solids across the pressure boundary. Pressure conveying can move material toward one or more destinations and can suit longer routes.

In a vacuum arrangement, a receiver and filter sit before the vacuum source. Material can be collected from several pickup points. Inward leakage can improve source containment, but the receiver, filtration, and discharge device must operate at the required vacuum.

Materials and applications

Dilute phase accepts a broad material range, including powders that do not form stable dense flow. It is often selected for robust ingredients, pellets, and granular solids when flexible routing and continuous transfer matter.

Fragile particles can break through repeated impact. Abrasive particles can wear bends, pipe, and valves. Mixtures can segregate when components respond differently to acceleration and suspension. These effects must be evaluated using the actual product and route.

Velocity and pipe diameter

The minimum stable conveying condition changes along the route because gas density and solids behavior change. Selecting one velocity without considering pressure and temperature can be misleading. A design normally checks the full line and the expected range of feed rates.

An oversized pipe can reduce velocity and allow deposits. An undersized pipe can raise velocity, pressure loss, wear, and product damage. Stepped pipe diameters can be considered where gas expansion would otherwise create excessive velocity, but every transition requires engineering review.

Feeding and air leakage

The feed device should deliver a stable solids rate that matches the available gas flow. Rotary airlock leakage varies with pressure difference, clearances, speed, material, and wear. Leakage contributes to the air balance and can disturb material entering from the hopper.

Vacuum pickup points also need controlled solids admission. An unrestricted flood of material can choke the line. A screw, valve, or engineered pickup geometry can regulate entry and provide the air needed for acceleration.

Wear and attrition control

Particle impact is concentrated at bends and other direction changes. Reducing unnecessary bends, selecting suitable bend geometry, limiting velocity, and using appropriate materials can improve life. Replaceable wear sections and inspection points can reduce maintenance risk.

Product attrition should be measured rather than judged only by appearance. Useful checks include particle size distribution, dust generation, bulk density, and product function before and after conveying. A small increase in fines can matter greatly in food, pharmaceutical, plastics, or formulated products.

Receiver and filter performance

The receiver must separate solids and release conveying gas at peak duty. Inadequate filtration area or cleaning can increase differential pressure, reduce capacity, and carry material toward the air mover. Filter media must suit the particle size, temperature, chemistry, hygiene, and hazard.

Level protection prevents overfilling, while differential pressure helps identify filter restriction. The discharge valve or feeder must remove solids without upsetting receiver pressure or the next process.

Energy and control

Dilute phase generally moves a larger gas volume at a lower pressure difference than dense phase. Energy depends on actual gas flow, pressure, air mover efficiency, leakage, and operating time. Excess air can waste energy and accelerate wear.

Variable speed control can match gas flow to duty, but the minimum setting must preserve stable transport. Pressure, gas flow, feeder rate, filter differential pressure, and motor load can support safe optimization.

Common failure modes

• Feed rate exceeds the transport capacity of the gas flow.
• Air leakage reduces useful flow or disturbs pickup.
• Filter restriction raises system pressure and lowers capacity.
• Moisture or changed particle distribution promotes buildup.
• Pipe diameter or route creates low velocity regions.
• Excess velocity causes wear, fines, noise, or segregation.

Safety considerations

Dilute phase transport can maintain a dispersed dust cloud inside the pipe. Combustibility, electrostatic charging, ignition sources, isolation, explosion protection, and fire propagation require assessment. Receiving vessels need adequate air removal and overfill protection. Safe shutdown should consider whether material remains suspended, deposited, or trapped in connected equipment.

Sources and further reading

• AIChE, Dilute or Dense Phase Pneumatic Conveying?
• Particuology, Review of dilute and dense phase pneumatic conveying
• Advanced Powder Technology, Review of particle concentration in horizontal dilute flow
• Pneumatic Conveying Design Guide
• Health and Safety Executive, Prevention of dust explosions in the food industry

• Define the powder or bulk solid and its relevant physical and safety data.
• State the required transfer rate, operating schedule and acceptable residual material.
• Map horizontal distance, vertical lift, bends, pickup points and destinations.
• Define product degradation, wear, contamination and containment limits.
• Record available utilities, filtration, controls, cleaning and maintenance access.