Dense Phase Explained

Dense Phase Systems

Dense phase is characterized by high positive pressure (<15psi) conveying at low velocity of 2-15 ft/sec. The most common positive pressure dense phase pneumatic conveying regimes are:

Continuous Dense Phase

High pressure – low velocity
Material is conveyed below the saltation velocity in a moving bed flow pattern (rolling or plowing).
Velocities are 10-25 ft/sec at the feed end, and 40-70 ft/sec at the terminal end.

Continuous dense phase is used to handling powders that may be easily fluidized and are able to retain air.

Discontinuous Dense Phase

High pressure – low velocity
Material is conveyed below the saltation velocity in a slug-type flow pattern, i.e., a slug or series of slugs separated by air gaps are conveyed along the pipe. The slugs completely fill the full cross section of the pipe.
Velocities at the feed end are typically 3-15 ft/sec and 10-30 ft/sec at the discharge end.

Provided the material is suitable for discontinuous flow regime, this is the best regime for most applications in which power economy, pipe erosion and material degradation issues are important.

Solid Flow Dense Phase

High pressure – low velocity system
Material is conveyed well below the saltation velocity with the pipe almost completely full of material. The material is virtually extruded through the pipeline. The conveying velocity is 2-5 ft/sec throughout the system.
There is virtually no agitation or turbulence within the pipeline with this flow regime.

Solid dense phase is best suited for fragile materials with attractive operating economy and very low pipe erosion.

Below table provides a comparison of the four regimes.
Flow RegimeAverage Velocity M/SecondAir-to-Material Ratio
AirMaterial
Dilute Phase28-3520-33Over 100:1
Continuous Dense Phase7.5 to 22.55 to 1520 to 100:1
Discontinuous Dense Phase2.5 to 12.51.0 to 7.5Below 30:1
Solid Flow Dense Phase2.0 to 5.00.5 to 2.5Below 20:1

In the above table, air to material ratio measures system efficiency. It is the ratio of weight (lbs/kg) of material conveyed per weight (lbs/kg) of conveying gas used.

Due to low conveying velocity, dense phase systems are often chosen for applications where the product is friable, abrasive or smears such as sand, glass, and coke.

Most applications will benefit from low velocity, if not due to reduced wear, then due to reduced air consumption. As a general rule, the best economies are achieved by using the lowest possible conveying velocity.

Limitations:
Can be sensitive to variation in the material specification