Pneumatic Conveying System

Overview

A pneumatic conveying system moves dry bulk material such as powder or granules through a sealed pipeline by entraining it in a stream of air. Instead of belts, chains, or screws, the only moving thing in the line is the air and the product it carries, which makes the route flexible: the pipe can climb walls, cross roads, and turn corners that a mechanical conveyor cannot follow. The system has an air mover to generate flow, a feed station to introduce material, a conveying line of pipe and bends, a receiving vessel to drop the product out, and a filtration unit to clean the spent air before it is vented.

Plants use pneumatic conveying because it is dust-tight, needs little floor space, and routes around obstacles. Cement works, plastics plants, food factories, and chemical sites all rely on it. The trade-off is that air is an inefficient carrier compared with a belt, so the power per tonne is higher and abrasive or fragile materials need careful handling.

Dilute phase and dense phase

The two operating regimes differ in how much air carries how much product. In dilute phase, a high air velocity of 18 to 30 m/s keeps the particles fully suspended, like dust in a wind. It tolerates almost any free-flowing material and runs at low pressure from a Roots blower, but the high velocity grinds abrasive powders and shatters fragile ones. In dense phase, the line runs slowly at 3 to 8 m/s and pushes the material along as moving slugs or a sliding bed at higher pressure, typically from a compressor. Dense phase is gentler and uses less air per tonne, but it needs a pressure vessel feeder and works best with materials that hold a slug together. The system here is built around dilute phase with a blower, the most common configuration.

Air mover

The air mover package is built around a Roots blower, a positive-displacement machine in which two tri-lobe rotors mesh inside a casing and trap air between the lobes and the wall. A timing gear pair keeps the rotors synchronized so they never touch. The blower delivers a steady airflow against the line resistance and is driven by a three-phase motor through a drive belt. Because the rotors clatter, the package carries an inlet silencer and a discharge silencer, plus a pressure relief valve to protect against a blocked line.

Feeding and the rotary valve

Getting powder into a pressurized airstream without leaking air is the job of the rotary airlock valve at the bottom of the feed hopper. A vaned rotor turns inside a close-fitting housing; each pocket fills with powder at the top, rotates around, and empties into the line at the bottom, while the vanes maintain a running seal against the pressure difference. A gearmotor turns the rotor at a controlled speed, which sets the feed rate. A bin vibrator on the hopper stops the powder from bridging, and level probes tell the controller when the hopper is full or empty.

Pipeline and routing

The conveying line is mostly straight pipe joined by quick-release couplings, but the long-radius bends take the abuse. At every elbow the abrasive product slams into the outer wall, so bends are cast or hardened to resist erosion and are the most replaced parts in the system. Where one source feeds several destinations, a diverter valve swings an internal tunnel to route the flow down a chosen branch under a pneumatic actuator.

Separation and filtration

At the destination the air-product mixture enters a cyclone separator, which spins the flow so the heavier particles are thrown outward and fall into the receiver hopper, discharging through a butterfly valve. The air still carries fine dust, so it passes through the filtration unit, where pleated cartridges catch particles down to a micron. The cartridges are cleaned on the run by pulse-jet valves that fire bursts of compressed air to shake the dust cake off into a collection hopper. The whole sequence is run by the control panel, where a PLC reads line pressure and adjusts the blower through a variable frequency drive to keep the conveying velocity in the right band.

Materials and wear

The material being conveyed sets most of the design choices. Abrasive powders such as cement, sand, and alumina drive hardened bends and chrome-lined valves, and they push designers toward dense phase to cut velocity. Fragile granules such as plastic pellets and sugar crystals break into fines at high speed, so they too favor lower velocity or a stepped pipe bore. Sticky or hygroscopic powders need dried air and smooth product paths to avoid build-up. Many fine organic and metal powders are also combustible, so a system handling flour, sugar, or aluminium dust is built to ATEX rules with explosion isolation valves, vent panels, and earthing throughout. Matching the conveying mode, the air velocity, the pipe material, and the bend type to the specific powder is what separates a system that runs for years from one that plugs, wears out, or fails its dust-explosion assessment.