Dilute-Phase Conveying System

Overview

A dilute-phase conveying system moves bulk powder by suspending the particles in a fast stream of air, so the product travels through the pipe like dust carried on a strong wind. It is the most common form of pneumatic conveying because it is simple, tolerant, and cheap to build. The system pairs a high-volume blower package with a rotary feeder that drops material into the line, a pipeline of pipe and sweep bends, and a filter receiver that separates the product from the air at the destination.

The defining feature is the low solids loading ratio. Only a small mass of powder rides in a large mass of air, typically a few kilograms of solids per kilogram of air. That keeps the pipe from plugging but means the air does most of the work, so power consumption per tonne is higher than in dense-phase systems. Dilute phase suits free-flowing materials that are not badly abrasive or fragile, since the high velocity that keeps particles airborne also drives wear and breakage.

Keeping particles suspended

The whole design hangs on staying above the saltation velocity, the air speed below which particles start dropping out and settling along the bottom of the pipe. Once they settle, they pile up and eventually block the line. The conveying velocity is held in the 18 to 35 m/s band to stay safely above this point across the full length of the route. Because air expands as pressure falls along the pipe, the velocity rises toward the destination; the pipe is sometimes stepped up to a larger bore partway along to bring the velocity back down and limit wear at the far end.

Blower package

The blower package is built on a Roots blower, where two lobed rotors turn inside a casing and push a fixed volume of air per revolution, kept in time by a gear pair. A Roots machine delivers the high volume and low pressure that dilute phase needs. A three-phase motor drives it through a drive belt, and the airflow passes an inlet filter and reactive silencers to keep dirt out and noise down. A relief valve protects the blower if a line blockage spikes the pressure.

Feeding the line

Powder enters through the rotary feeder, a drop-through airlock under the supply hopper. Its open-end rotor fills with powder at the top, carries it around, and dumps it into the airstream at the bottom while sealing against the line pressure. A blow-through adapter sends part of the conveying air directly across the rotor outlet so each pocket is swept clean and the powder is picked straight into the flow. A gearmotor sets the rotor speed, which sets the feed rate, and the rotor runs on ball bearings with oil seals to keep product out of the bearings.

Pipeline

The pipeline is mostly straight conveying pipe joined by compression couplings, with flexible hose sections where the route needs to flex. The long sweep bends use a wide radius so particles change direction gradually rather than slamming into a tight elbow, which limits both wear and the dust generated by impact. Bends are still the highest-wear part of any dilute system and are treated as wear items.

Separation and filtration

At the destination the product enters a filter receiver, a vessel that combines the separator and the dust filter. The air slows on entry and most of the powder falls into the cone, while the remaining fines collect on filter bags held open by support cages. The bags are cleaned on the run by reverse-pulse valves that fire bursts of compressed air to knock the dust cake off, dropping it down to a discharge airlock that lets product out without losing the air seal. Clean air vents from the top.

Control and instrumentation

The control system runs the cycle from a PLC with an HMI touch panel. The instrumentation watches line pressure at several taps and air flow through a mass flow sensor, so the PLC can trim the blower through a variable frequency drive and keep the velocity above saltation while wasting as little air as possible. Rising differential pressure across the line warns of a building blockage before it stops the system.