Historically, pneumatic conveying systems have been separated into two basic categories: dense phase and dilute phase systems.
While there is no one specific operating criterion that clearly differentiates the two, several design considerations and operating parameters are generally used to classify a pneumatic conveying system as one type or the other.
In this blog post, we’ll provide a basic overview of both system types and explain how they affect pneumatic conveying of solids, including operating advantages, solids flow patterns, and conveying distances.
Solids Flow Patterns
Dilute Phase Systems
Almost any material can be conveyed in dilute phase fashion. It is simply a matter of accelerating the conveying gas stream to a velocity where all the material particles are picked up, suspended, and kept in suspension as they move through the conveying line. This gas velocity is termed the “saltation velocity” of the material, and it is determined primarily by the size, shape, and density of the solid particles.
Dilute phase systems typically have a gas stream velocity of 4,000–6,000 ft/min. and a solids loading ratio in the gas stream of no more than 15 lb of solid per lb of conveying gas. If the velocity of the gas stream falls below the material’s saltation velocity, some particles will begin to drop out of suspension, but will likely still be swept along the bottom of the conveying line due to friction with the gas stream and the particles that remain suspended in it. This condition is termed dilute phase “strand flow” and represents the beginning of the transition from dilute to dense phase conveying.
Dilute Phase
Gas velocity v = 4,000 – 6,000 ft/min
Solids Velocity c = 3,000 – 5,000 ft/min
Dilute Phase – Strand Flow
Gas Velocity v = 3,000 – 5,000 ft/min
Solids Velocity c = 2,000 – 4,000 ft/min
Dense Phase Systems
Dense phase transfer generally occurs in one of two basic forms, plug flow or moving bed (dune) flow. During plug flow, solids fill the conveying line and move through it as individual plugs separated by pockets of air. During moving bed (dune) flow, material is carried along in individual waves that sweep over a bed of particles moving along the bottom of the conveying line.
Which form of dense phase transfer occurs depends primarily on specific properties of the solid, namely its ability to be fluidized (i.e., its level of gas permeability and air retention), its particle size-distribution and its particle shape.
Dense Phase – Plug Flow
Gas Velocity v = 600 – 1,000 ft/min
Solids Velocity c = 20 – 200 ft/min
Dense Phase – Dune Flow
Gas Velocity v = 800 – 2,500 ft/min
Solids Velocity c = 40 – 400 ft/min
Dense phase systems typically have a maximum gas stream velocity of 2,500 ft/min. and solids loading ratio of 20–150 lb of solids per lb of conveying gas.
Conveying Distance Considerations
Dilute Phase Systems
The primary factor that determines the distance a pneumatic conveying system can transfer a specific material is the pressure drop required to convey it from its initial pick-up point in the conveying line to its discharge point.
Since pressure drop is directly related to conveying line length and solids loading ratio, most long-distance conveying is done using dilute phase systems, and the conveying gas pressure is typically between 15 and 100 psig.
One limiting factor is that the gas pressure drops as it moves along the conveying line. This reduction in pressure allows the gas to expand, increasing its velocity according to Q = VA. If the conveying gas flow (Q) remains constant, and the line has a fixed area (A - “diameter”), the increasing gas velocity results in an increasing pressure drop that is proportional to the velocity squared (V2). For this reason, it is often necessary to increase the conveying line diameter in stages along its path to maintain a relatively constant velocity and pressure drop in long-distance transfer applications.
It is worth noting that the conveying line length must be calculated using the “equivalent length” of elbows and vertical rises because these will have a significantly higher relative length (up to 40-60 times) than that of a single foot of horizontal line length.
Dense Phase Systems
While most dilute phase systems operate using positive pressure gas, dense phase systems can operate under either vacuum or positive pressure, although they typically use lower positive pressures than dilute phase systems.
Materials with a relatively large particle size and a narrow particle size distribution typically have good air permeability characteristics and convey well in dense phase plug flow. Conversely, powdered materials with small particle sizes, such as flour or fly ash, have good air-retention properties and will typically move along the conveying line as a fluidized mass in moving bed fashion.
A quick and simple test for evaluating a material’s air-retention or air-permeability characteristics is to shake a closed sample jar to aerate the material, remove the lid, and drop a small ball bearing into the sample.
If the ball bearing falls through the sample to the bottom of the jar, the material demonstrates good air retention and it will likely transfer well in dense phase moving bed flow. If the ball bearing does not penetrate the material very far, the material likely has good air permeability and may convey well in dense phase plug flow. However, only material transfer testing can reliably confirm whether it can be pneumatically conveyed using a dense phase system.
The following video demonstrates dense phase transfer during a test performed for a client at our test center:
Pneumatic Conveying of Solids FAQ
What is pneumatic conveying of solids?
Pneumatic conveying of solids is the process of transporting dry bulk materials through enclosed pipelines using air or another gas.
What is the difference between dense and dilute phase conveying?
Dilute phase conveying suspends material in a high-velocity gas stream, while dense phase conveying moves material at lower velocities in plugs or dunes. The right method depends on the material's properties, conveying distance, and application requirements.
Which materials are best suited for dense phase conveying?
Materials with suitable particle size, permeability, and air-retention characteristics often perform well in dense phase systems. Because every material behaves differently, transfer testing is the most reliable way to determine whether dense phase conveying is the right solution.
What factors affect pneumatic conveying system selection?
Selecting a pneumatic conveying system depends on material characteristics, conveying distance, capacity requirements, product fragility, pressure drop, operating costs, and maintenance considerations.
Can pneumatic conveying systems handle long conveying distances?
Yes. Pneumatic conveying systems can transfer materials over long distances when they are properly designed. Pipeline layout, pressure drop, conveying velocity, gas supply requirements, and material characteristics all influence overall system performance.
Choosing the Right Pneumatic Conveying System
The capabilities of pneumatic conveying systems are extremely diverse, as almost any dry solid can be conveyed long distances at relatively high flow rates in either dilute or dense phase fashion. While the compressibility of the conveying gas will present some practical limitations, application requirements, power consumption, capital costs, material characteristics, and conveying distance will often determine whether a dilute or dense phase system is right for a specific application.
Choosing the right approach to pneumatic conveying of solids requires balancing technical performance with operating and economic considerations. To learn more about dilute phase and dense phase methods of transfer and how they compare to other conveying methods, download our Comparison Guide on Pneumatic Conveying vs. Mechanical Conveying.
Need help evaluating the right system for your material and process requirements? Contact our team to discuss your application.
