Pneumatic conveyor for powders and granules

01

Pick dilute or dense phase for the material

Writing in Processing magazine, Ray Cocco and Shrikant Dhodapkar explain that dilute-phase systems are designed to run at gas velocities slightly above the saltation velocity, where particles would otherwise drop out of the flow, with superficial velocities over 4,000 ft/min and solids loading under about 15 mass of solids per mass of gas and particles fully suspended. Dense-phase systems run below saltation at much higher solids concentration, around 200–600 ft/min at pick-up and not more than 1,600–2,000 ft/min at the end of the line. They note that a long dilute-phase line can run at more than 3 bar, so pressure and loading ratio do not by themselves define the mode. [2]

David Mills describes two dense-phase modes, sliding bed flow for materials with good air retention and plug flow for materials with good permeability, and notes dense-phase solids loading ratios can exceed 100 for fine fly ash and cement but are rarely above 30 for pelletised materials, because air permeates through the plugs. [1]

A Powder & Bulk Solids article on dense phase argues that velocity, not loading ratio or pressure, is what distinguishes the modes, that "Many dilute phase conveying systems operate at pressure differentials well above 1 bar(g) [15 psig]", that dense-phase slugs can exceed 20 pipe diameters in length and travel at 6–10 m/s, so pipe supports must take the impact at bends, and that above 1 bar(g) [15 psig] "much of the equipment will fall under the design purview of the American Society of Mechanical Engineer’s (ASME) Codes & Standards". [3] Editorial note: ask the supplier to test your material, since its air retention or permeability decides which dense-phase mode, if any, will work.

Give the supplier the material (name, particle size and distribution, bulk density, moisture, friability, abrasiveness, temperature, combustibility), conveying rate, distance with horizontal and vertical runs and bends, source and destination equipment, available compressed air or blower, number of destinations, cleaning and product-change needs, and hazardous-area classification.

02

Size the air flow, pick-up velocity and pressure

Mills recommends a safety margin of about 20% on the conveying-line inlet air velocity and gives minimum dilute-phase pick-up velocities of about 2,000–2,400 ft/min for fine powders (2,000–2,200 for cement, 2,200–2,400 for fly ash), 2,600–3,000 ft/min for fine granular materials and about 3,200 ft/min for granular sugar, with higher values for larger or denser particles; dividing ft/min by 200 gives an approximate m/s value. He notes that because air expands as pressure falls, velocity increases along a single-bore pipeline, so the lowest velocity is at the pick-up point unless the line is stepped to a larger bore. [1]

A Powder & Bulk Solids item on Coperion K-Tron's dense-phase pressure vessel describes units using air above 15 psig and low air volume to move powders and granules through the pipeline at low velocity, suited to premixed, high-bulk-density, abrasive, friable or segregating materials, in purged (clear-line) or full-line systems. [4]

Editorial calculation: a 100 mm bore at 12 m/s pick-up carries 12 × π/4 × 0.1² ≈ 0.094 m³/s (about 340 m³/h) of air at line conditions; at an assumed free-air density of 1.2 kg/m³ that is roughly 400 kg/h of air, so a solids loading ratio of 15 would mean about 6 t/h of solids. Higher line pressure raises air density and changes these figures; ask the supplier for its air-mass and pressure-drop calculation. [1] [2]

03

Powder conveying systems sold with filters, silos and building-material plant

Listings include pneumatic conveying systems for powders sold with filters and dust collection equipment, and pneumatic conveyors offered with silos and conveyors for building-material plants; ask each for the flow mode, design velocity, air requirement and a material test.

04

Air mover, feeder or blow tank, pipeline and bends, receiver and filter, and controls

In dense phase the feeder and the pipe supports go together: Coperion K-Tron's pressure vessel uses air above 15 psig to push material at low velocity, and the slugs it forms hit the bends hard enough that supports must take the impact. [4] [3]

05

Conveying rate and distance, pick-up velocity, loading ratio, pressure and pipe bore for a conveyor quotation

Pick-up velocity carries the margin: Mills gives about 10–12 m/s for fine powders in dilute phase and recommends about 20% on top of the minimum. [1]

06

Send the material, route and air data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Material: [name; size; bulk density; moisture; abrasiveness; friability; combustibility]
Rate: [t/h; batch or continuous]
Route: [horizontal; vertical; bends; destinations]
Mode: [dilute or dense; vacuum or pressure]
Air: [available supply; quality]
Vessels: [pressure; code; materials]
Test: [material test results; pressure drop calculation]

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