Dilute phase pneumatic conveying operates at gas velocities commonly in the 700-1,000 m/min band, with many industrial pressure systems exceeding 5,000 ft/min (~1,525 m/min), while dense phase pushes material at 100 kPa or higher in compact slugs or plugs at velocities well below the saltation point [S3][S2].
Medium phase conveying, sometimes called strand phase or mixed phase, occupies the hydrodynamic band between these two extremes: higher solids loading than dilute, higher velocity than dense, and a flow regime that the engineering literature flags as inherently unstable near the saltation point [S1][S4]. That instability is the central reason picking among the three regimes is mostly a material-handling decision, not a throughput decision. For a broader primer on the carrier-gas mechanics that drive all three regimes, see the pneumatic conveying basics overview.
How the three regimes are actually defined
Pneumatic conveying classification hinges on the solids-to-gas mass ratio (Ms/Mg) and the gas velocity relative to saltation. In a horizontal pipe at constant solids feed, raising gas velocity walks the system from packed bed through moving slug, unstable dune, and finally homogeneous flow, while raising solids concentration at constant gas velocity does the opposite [S1].
Dilute phase is the homogeneous-suspension end: particles move at nearly gas velocity, the line is fully entrained, and gas velocities often exceed 5,000 ft/min in practice, which is why dilute designs can burn half of their total system power just pushing air down an empty pipe [S2]. Dense phase is the other end: material is pushed or extruded by the gas pressure, with the bulk moving as waves, plugs, or strands inside the pipe at 100 kPa or higher line pressure [S3][S9]. Medium phase falls between, and Dynamic Air's guidance is explicit that medium phase requires careful gas-velocity control at the material pickup to avoid unstable flow [S4].
Velocity, pressure, and concentration: the numerical bands
Dilute phase air streams typically run 700-1,000 m/min for many common bulk solids, with no fixed upper limit beyond the 5,000 ft/min figure routinely cited for long runs, and the system pressure is usually low (a blower-driven vacuum or modest positive pressure is enough) [S3][S2].
Dense phase lines run at much lower velocities, often under 10-15 m/s in plug-flow service, with line pressures above 100 kPa (gauge) and, in long-distance or high-rate designs, substantially higher [S3][S9]. Magnum Systems frames this as the key tradeoff: high-pressure capability lets dense phase move higher mass rates in the same line size because the lower velocity keeps the solids loading up [S9]. Medium phase sits in the middle on both axes, higher velocity and lower pressure than dense, lower velocity and higher pressure than dilute [S4].
Material fit: which regime suits which bulk solid
Dense phase is the right call for abrasive materials (sand, gravel, glass batch, refractory batch), because a dense slug keeps most particles off the pipe wall; for fragile or friable products (coffee beans, granulated sugar, dry laundry detergent, breakfast cereal) where particle degradation matters; for moisture-sensitive or oxidizable powders where dry air or nitrogen blanketing is required; and for explosive or flammable dusts where inert-gas blanketing is a process safety control [S4].
Dilute phase is the right call for non-abrasive, lighter materials such as plastics, grains, and flour; for short conveying distances and lower capacities where the per-metre gas-power penalty is small; and for budget-sensitive builds where the rotary airlock plus blower architecture is the lowest-cost path [S4][S2]. Medium phase is the right call when the material can tolerate moderate velocities but you want better pipe life and lower particle degradation than a dilute line delivers, which is why it tends to show up as a tuned compromise rather than a default specification [S4][S1].
Comparison matrix across the four decision criteria that matter most
Across the four decision criteria that drive most specifier choices, the three regimes line up as follows. On gas velocity, dilute phase is highest (commonly 700-1,000 m/min, often 1,500+ m/min on long runs), medium phase is intermediate, and dense phase is lowest (slugs below saltation) [S3][S4]. On line pressure, dilute phase is blower-class low pressure, medium phase is moderate, and dense phase is high pressure at 100 kPa and above [S3][S4]. On distance capability, dilute phase is best for short runs and loses efficiency fast as distance grows because roughly half the system power is consumed just moving gas through the empty pipe, while dense phase is built for long distances and high rates in the same line size [S2][S9]. On material suitability, dilute phase suits non-abrasive light particles, medium phase suits moderate-fragility products, and dense phase suits abrasive, friable, moisture-sensitive, and oxidizable bulk solids [S4][S3].
The single best gate for picking a regime is material behavior: if the product degrades, the pipe wears fast, or you need inert blanketing, dense phase wins; if the product is light, non-abrasive, and the run is short, dilute phase wins; if neither extreme fits, medium phase is the tuned middle that demands tighter velocity control [S4][S1].
Failure modes and constraints that change the decision
Operating at or near the saltation point, which is the transition between dense and dilute flow, is the most common failure mode flagged in the engineering literature, because saltation is unstable and any small drop in gas velocity drops particles out of suspension [S1]. Medium phase sits closer to that instability than dilute phase does, which is why the same source warns that medium or strand phase requires careful control of gas velocity at the material pickup point [S1].
Dilute phase systems lose efficiency as conveying rate or distance increases, and the high gas velocities cause more wear and particle damage on abrasive or fragile products, so specifying a dilute line for a long run of an abrasive powder is a known under-perform [S2]. Dense phase systems require more operator know-how, designs are held more closely than dilute designs, and the high-pressure hardware raises both capex and maintenance burden, which is the practical reason dense phase is not the default for every application [S3]. Nitrogen blanketing, used for oxidation- or explosion-sensitive duty, adds another constraint because dense phase systems are the natural fit for inert-gas operation and dilute phase can usually only blanketed effectively with extra sealing [S2][S4].
Real plant applications by industry
In plastics, food, and chemicals plants, dilute phase dominates short-distance pellet, grain, and flour transfers because the rotary airlock plus blower build is cheap, the products tolerate suspension, and the runs are short [S2][S4]. In cement, minerals, and refractory batch plants, dense phase dominates the long-distance, high-rate, abrasive side of the duty because the slug keeps particles off the pipe wall and the high pressure carries the line over the distance [S4][S9].
Pharmaceutical and food-grade fragile products, including coffee beans, granulated sugar, dry detergent, and cereal, are a dense phase application set because the dense slug limits how many particles actually contact the pipe wall and degrade [S4]. For comparison-style spec work on adjacent equipment decisions in a plant, the Kammprofile vs spiral wound gasket construction comparison applies similar matrix thinking to a different component class. Process engineers who frame these choices around bulk-material behavior, rather than throughput alone, end up with a regime match that survives the first six months of operation without saltation-related upsets [S1][S3].
Selection checklist: dense, medium, or dilute
Use dense phase when the bulk solid is abrasive, fragile, moisture-sensitive, oxidizable, or potentially explosive, when the conveying distance is long, when multiple discharge points are needed, and when an inert-gas or cool-gas conveying stream is available or required [S4]. Use dilute phase when the material is non-abrasive and tolerates suspension, when the conveying distance is short and the capacity is low, and when the lowest initial capex drives the decision [S4][S2].
Use medium phase when the product can accept moderate velocities but the specifier wants a step up from dilute in pipe life and particle integrity, accepting that medium phase requires tighter pickup-point velocity control to stay out of the saltation instability band [S1][S4]. Two trackable signals to watch on the next review: whether your plant's existing compressed-air or nitrogen supply can deliver the 100 kPa-plus line pressure that dense phase needs without bottlenecking other consumers, and whether the material's degradation limit under dilute-phase velocity has been measured rather than assumed, because that single number is usually the deciding spec between medium and dense phase [S3][S4].
For the relevant spec sheets and selection criteria, see conveying, and medium voltage vfd.