Rules of Thumb: Pneumatic Conveying

Stephen Hall discusses the golden rules for design

GRANULAR and powdered solids are conveyed in gas streams throughout the chemical industry. This article discusses the advantages and challenges of pneumatic conveying systems and provides some general rules of thumb for the design engineer.

Keep two concepts in mind when considering the use of pneumatic conveying. First, the system pressure decreases along the length of the conveying pipe. This means that the specific volume of the gas increases as does its velocity. These factors limit the conveyance length. Second, there is a minimum velocity needed to carry the solids. This velocity depends on the ratio of gas to solid, the gas and solid densities, particle size and shape, and the pitch of the pipe.

Context

Pneumatic conveyors are composed of four main parts:

• Prime mover. Compressors, fans, blowers, and vacuum pumps are used to move the gas stream. The engineer must determine the gas flow rate and pressure (positive or negative) required to convey the process stream.
• Feed. Solids are introduced into the gas stream by vacuum or through specially designed valves, pumps, or blow vessels. The solids must be accelerated rapidly to the conveying velocity, which causes a large pressure drop.
• Conveying piping. The pipe sizing and design (eg slopes, bends) must be suitable for the characteristics of the solids and the intended conveying rate(s).
• Disengagement. Solids are removed from the gas stream with cyclone separators or fabric filters. This part of the system includes capturing the solid (into a bin, for example), and disposition of the gas stream (as an emission or recycled for reuse).

Pneumatic conveyors are classified as dilute phase or dense phase.

In dilute-phase systems (Figure 1), the conveyed solid particles are uniformly suspended in the carrier gas. They operate at relatively high velocity to prevent the particles from settling onto the bottom of a horizontal run, or choking flow in a vertical section. The majority of pneumatic conveyors operate in the dilute phase. Characteristics of dilute-phase systems include:

• High velocity (700–1,000 m/min, or 2,400 to 3,200 ft/min for fine powders or granules).
• Very high attrition as the particles collide with each other or the pipe wall.
• Pressure typically <100 kPa (15 psig).
• Low cost.
• Larger pipe size.
• Low loadings (mass ratio solid to gas up to 15).

Dense-phase systems convey the particles at lower velocity, which is insufficient to carry the solids uniformly. Instead, they flow as full-bore slugs or plugs, or as moving-bed waves like sand dunes blown by the wind. Free-flowing fine particles and granules, with high gas permeability (such as plastic pellets) or retention (such as cement), are candidates for dense-phase conveying; tests are typically conducted (by vendors) to ensure the technology is applicable to the specific system being considered. Characteristics of dense phase systems include:

• Low velocity (200 m/min, 600 ft/min typical).
• Low attrition.
• High pressure (typically 100–600 kPa).
• Comparatively high cost.
• Small pipe size.
• High loadings (mass ratio solid to gas 100 or more).