Pneumatic Conveying System Design: How to Size Pipeline, Bends, and Air Supply for Your Application

Why Does Pneumatic Conveying System Design Matter So Much?

A pneumatic conveying system that is undersized for its application will block repeatedly, reduce plant throughput, and require constant attention from maintenance teams. A system that is oversized wastes energy and can cause particle degradation from excessive velocity. The design must be matched precisely to the material being conveyed, the required tonnage, the pipeline geometry, and the physical layout of the plant.

Unlike mechanical conveying systems, which are relatively forgiving of modest over or undersizing, a pneumatic conveying system operates within defined velocity windows. Below the minimum transport velocity, material settles out of suspension and blocks the pipe. Above the optimum velocity, energy consumption rises steeply and material attrition increases. Getting the design right is not optional.

Step 1: Define the Material Properties

The starting point for any pneumatic conveying system design is a complete understanding of the material to be conveyed. The following properties must be known before any engineering calculation begins:

• **Bulk density** in kilograms per cubic metre, both loose poured and aerated, since both values affect conveying behaviour differently
• **Particle size and size distribution** as the minimum and maximum particle diameter, and the d50 and d90 values from a particle size analysis
• **Particle shape**, whether spherical, angular, fibrous, or irregular, as this affects pick up velocity and bend wear significantly
• **Abrasiveness** rated on the Mohs scale or by standard abrasion tests, since abrasive materials require hardened or lined pipeline and bends
• **Hygroscopicity** and moisture content, as wet or moisture absorbing materials can cake on pipe walls and block the system
• **Friability** and fragility, since delicate particles require dense phase mode or carefully controlled velocity in dilute phase
• **Explosivity and flammability**, as combustible powders require earthed pipelines, explosion venting, and in some cases nitrogen blanketing

Step 2: Determine Conveying Mode and Minimum Transport Velocity

Based on the material properties established above, the conveying mode is selected. For robust free flowing materials, dilute phase conveying is appropriate. The minimum conveying velocity for dilute phase transport depends on particle size and density. A commonly used reference value is:

Minimum transport velocity = K x square root of (particle density x particle diameter)

Step 3: Select Pipeline Bore

The pipeline bore is calculated from the required volumetric air flow and the design conveying velocity using the continuity equation:

Pipe cross sectional area = Volumetric air flow rate divided by conveying velocity

From this area, the internal diameter is calculated and the nearest standard pipe bore is selected. The volumetric air flow required depends on both the air velocity and the mass flow rate of material. For dilute phase systems, the solid loading ratio (mass flow of material divided by mass flow of air) is typically between 3 and 15.

Standard pipe bore selection is also influenced by the nature of the material. Fine powders with tendency to compact or cake should be conveyed in slightly larger bore pipes to allow bypass air flow around any incipient block. Abrasive materials should be conveyed in Schedule 40 or heavier pipe to provide a safe wear allowance.

Step 4: Calculate System Pressure Drop

Total pressure drop across the pneumatic conveying system is the sum of:

• Pressure drop to accelerate material from rest to conveying velocity at the pickup point
• Friction pressure drop in straight horizontal pipe runs
• Pressure drop through each vertical section, both upward and downward
• Pressure drop through each bend
• Pressure drop through the separation equipment at the destination
• Additional safety margin, typically 15 to 20 percent

The total pressure drop determines the operating pressure at the blower or compressor discharge, which in turn determines the machine selection. For dilute phase systems, pressure drops of 0.3 to 0.8 bar are typical. For dense phase systems, 1.5 to 4.5 bar is the normal range.

Step 5: Design the Bends Correctly

Bends are the most wear prone component in any pneumatic conveying system and the most common location for pipeline failures. They also impose the highest local pressure drop. The following principles govern bend design:

• **Long radius bends** with a radius to diameter ratio of 6 or greater dramatically reduce wear compared to short radius elbows. Acme recommends minimum R/D of 8 for mildly abrasive materials and 12 or greater for highly abrasive materials
• **Blind tee bends** use a closed pocket at the corner where material accumulates and forms its own cushion against the wall. They are very effective for abrasive materials but impose slightly higher pressure drops than smooth radius bends
• **Ceramic lined bends** or hardened steel bends with wear resistant alloy inserts should be specified for materials above a Mohs hardness of 4
• **Bend spacing** matters as much as bend design. Material re accelerates to full velocity after each bend over a distance of 10 to 20 pipe diameters. Bends placed too close together leave material partially decelerated entering the next bend, which sharply increases wear
• **Vertical to horizontal transitions** at the bottom of downcomer sections are particularly susceptible to wear and should always use long radius bends with wear protection

Step 6: Specify the Air Source

The air source is selected on the basis of the calculated volumetric flow in cubic metres per hour at the operating pressure. Acme Air Equipments manufactures two types of motive air equipment for pneumatic conveying systems:

Twin lobe and tri lobe roots blowers are the standard choice for dilute phase systems requiring pressures up to 1.0 bar gauge. Acme’s blower range covers flows from 100 to 18,000 cubic metres per hour. The positive displacement characteristic of a roots blower means it delivers constant flow regardless of modest pressure variation, which gives good conveying stability.

Rotary compressors in the twin lobe and tri lobe configuration cover the range from 1.0 to 3.0 bar gauge and are used for higher pressure dilute phase systems and for dense phase conveying at moderate pressures. Acme’s rotary compressor range is available in both air cooled and water cooled versions.

Step 7: Size the Separator and Filter

The separator at the destination must handle the full air and material flow without significant material carryover into the exhaust air. For most applications, a pulse jet bag filter is used, with the filter area calculated using an air to cloth ratio appropriate for the material. For coarse granular materials, air to cloth ratios of 2 to 4 metres per minute are standard. For fine powders below 10 microns, ratios of 0.8 to 1.5 metres per minute are used to avoid premature filter blinding.

The compressed air supply for the pulse jet cleaning mechanism must also be specified. Pulse cleaning pressure of 5.5 to 7 bar is standard. Acme supplies complete filter systems including the pulse valves, solenoid panel, and compressed air header as part of its turnkey pneumatic conveying system packages.

Frequently Asked Questions