Medium-phase pneumatic conveying sits between dilute and dense in the solid-to-air ratio range 7–13, and that middle position translates directly into middle-of-the-pack specific energy consumption (kWh per ton moved) for most bulk solids [S2]. Dilute-phase systems, which operate at solid-to-air ratios below 6 with conveying velocities of 15–30 m/s and line pressures below about 1 bar (15 psi) [S1], typically demand the most compressor or blower power per ton because the vast majority of the gas stream moves air rather than product.
The key decision factor is that dense-phase conveying can carry 20 to 400 times more material per unit of conveying gas than dilute-phase, which directly lowers both energy demand and pipe wear on the same duty [S3]. That ratio is why pneumatic conveying selection is usually framed as an energy-per-ton trade, not just a capital-cost trade, especially for plants handling more than a few tonnes per hour.
Solid-to-Air Ratio Bands: Defining the Three Phases
Dynequip's widely cited industry convention sets dilute phase at a solid-to-air ratio up to 6, medium phase at 7 to 13, and dense phase at 14 and above [S2]. Jenike & Johanson state the dilute-phase cutoff at solids loadings well below 15 kg of solids per kg of air, which is consistent with the same boundary [S1]. In practice, the medium-phase band from 7 to 13 is where many real bulk-solids lines actually sit, because operators push solid loadings up from the dilute-phase default to reduce air-handling cost without committing to the pressure-vessel and plug-flow hardware that dense-phase requires [S2].
Because the medium-phase band is defined by ratio rather than by velocity, the same line can move from medium into dense simply by throttling feed or raising supply pressure; conversely, a medium-phase design that is pushed too hard on pressure drop will start to slug, plug, or stall, and motor power will rise sharply instead of falling [S2]. This is the "inverted-U" motor-rating curve Dynequip describes for dense-phase, and it applies at the medium-phase/dense-phase transition as well: the optimum solid-to-air ratio is a real engineering point, not a slogan.
Velocity and Pressure Windows per Phase
Dilute-phase pressure systems run below about 15 psi (typically 4 to 8 psi, or 0.3 to 0.55 bar(g)), and dense-phase pressure systems run above 15 psi to provide the higher pressure differential needed to push plugs of material through the line [S6]. Indpro reports dilute-phase pressures of 0.1 to 1 bar(g), which is consistent with the same 1 bar ceiling for vacuum dilute systems and the 15-psi positive-pressure dilute upper limit from Nol-Tec [S9][S6].
Independent bulk-solids engineering sources cite dilute-phase transport speeds of 15 to 30 m/s, with dense-phase inlet speeds of approximately 3 to 10 m/s, and medium-phase velocities falling in the transitional band where material starts to lose full suspension and begins to travel as a moving bed or partial plug [S5]. Jenike & Johanson reinforce the dilute-phase 15–30 m/s figure and the sub-1 bar static pressure range, and add the operational caveat that velocity must stay above the terminal settling velocity of the largest particle in the distribution to keep the line from saltating or dropping out [S1].
Specific Energy per Ton: Where Medium-Phase Loses to Dense
Because dense-phase conveying moves 20 to 400 times more material per unit of conveying gas than dilute-phase, the compressor or blower motor rating for a dense-phase system at the same tonnage is markedly lower, and the specific power consumption (kWh/ton) drops accordingly [S3][S2]. Jenike & Johanson illustrate the underlying cost driver: 1 kg of air at standard conditions occupies roughly 800 L, so a dilute-phase line running at 5 kg of solids per kg of air must move about 160 L of air to deliver 1 kg of product, which is mostly why dilute-phase specific energy is high and why filter, blower, and receiver sizing all scale up with throughput on dilute systems [S1].
For medium-phase systems in the 7–13 solid-to-air band, the specific energy per ton is materially better than a 4–6 ratio dilute-phase design, often by a factor of roughly 1.5 to 2 on the same duty, but still well above a true dense-phase system running at ratios above 14 with the same material and distance [S2]. Toltec and the AZO comparison both frame the ordering as dense-phase < medium-phase < dilute-phase on energy and wear, with the gap between medium and dense widening as the line gets longer, the material gets heavier, or the conveying distance includes vertical lifts where pressure drop compounds [S8][S3].
Material, Distance, and Abrasion: When to Pick Which Phase
Dilute-phase suspension flow is the most universally applicable mode and the cheapest to install, but its high specific energy and high particle kinetic energy make it a poor choice for abrasive or friable products at any meaningful tonnage, and Jenike & Johanson flag particle breakage and pipe-erosion risk on abrasive duties as a hard limit, not a soft preference [S1]. Dense-phase plug or slug flow is the right answer for fragile food powders, friable chemicals, blended or segregation-sensitive products, and long runs where a dense-phase system can use smaller pipe diameters to flex through tight plant layouts with lower installation cost [S3].
Medium-phase is the right answer when a material cannot be fluidized cleanly enough for stable plug flow (which dense-phase demands) but the operator still wants better energy per ton than dilute-phase can deliver, and for runs that are too short, too low-pressure, or too variable in feed to justify pressure-vessel and rotary-valve dense-phase hardware [S2]. A useful self-check: if a pilot test on the actual product shows a clean, stable fluidization curve, the line is a dense-phase candidate; if the fluidization curve is marginal or material is cohesive, plan for medium-phase with a properly sized blower and reserve the option to convert later; if the material is free-flowing and the tonnage is low, stay with dilute-phase and accept the energy penalty in exchange for simpler equipment [S2][S5].
Comparison Table: Dilute vs Medium vs Dense on Energy and Hardware
On a head-to-head basis across the four criteria that drive energy-per-ton and total installed cost, dilute-phase wins on capital and breadth of applicability, dense-phase wins on energy and wear, and medium-phase is the middle option on both. The table below lines the three modes up against the same duty (same material, same tonnage, same distance) so a process engineer can read across and pick the column that matches the binding constraint [S1][S2][S3][S6]:
Solid-to-air ratio: dilute below 6, medium 7 to 13, dense 14 and above [S2]. Typical line pressure: dilute below about 1 bar(g) / 15 psi and often 4 to 8 psi on positive-pressure units, dense above 15 psi with higher pressure drop [S6][S9]. Conveying velocity: dilute 15 to 30 m/s, medium transitional, dense 3 to 10 m/s [S1][S5]. Relative specific energy per ton at the same tonnage: dense lowest, medium roughly 1.5 to 2 times dense, dilute 2 to 3 times medium on the same duty (qualitative ordering, not a single number) [S2][S3].
Failure Modes and Selection Watch-Outs
On dilute-phase lines, the two recurring failure modes are particle attrition from 15–30 m/s impacts at bends and tee-entries, and line saltation or settling when feed air fluctuates below the minimum conveying velocity, both of which scale with tonnage and roughly explain why the energy-per-ton penalty for dilute-phase gets worse, not better, as plants try to push more tonnes through a line sized for suspension flow [S1]. On medium-phase lines, the binding failure mode is loss of suspension: as solid-to-air ratio climbs past about 13, the line starts to slug and the compressor motor-rating curve bends upward, which is the "inverted-U" pattern Dynequip documents for dense-phase and which applies at the medium/dense transition as well [S2].
On dense-phase lines, the recurring failure mode is plug collapse, line pluggage, or unstable fluidization for materials that do not fluidize cleanly, and for that reason dense-phase should not be specified without pilot-scale fluidization data on the actual product, or without a proven reference installation on the same material, in line with the standard industry practice Dynequip describes [S2]. AZO's vacuum dense-phase work, which carries dense-phase slug flow under about 1 bar of vacuum differential, is specifically aimed at extending the dense-phase envelope to applications where positive-pressure vessels were previously the only option, and the published case data shows energy per ton improving against the dilute-phase vacuum baseline by roughly the same 2 to 3× factor that separates medium and dense on pressure systems [S3].
Procurement and Sourcing Standards
There is no single ISO or ASME standard that fixes the dilute/medium/dense solid-to-air ratio bands, and the 7–13 medium-phase and 14-plus dense-phase boundaries used across the industry are conventions reported by equipment vendors and bulk-solids engineering firms, not code-defined limits [S2]. The reliable procurement practice is to require vendor solid-to-air ratio and specific power (kWh/ton) data on the actual material at the actual tonnage, ideally from a pilot test, and to verify the line pressure window against the duty: dilute-phase positive-pressure units below 15 psi (typically 4 to 8 psi) and dense-phase units above 15 psi with the higher pressure drop the duty implies [S6].
For plants that run on the medium-phase band because the material will not fluidize cleanly enough for stable plug flow, a useful procurement signal is to size the blower or compressor at the 7 to 13 solid-to-air ratio with margin for the inverted-U motor-rating curve at the dense-phase end of the band, and to keep the conveying velocity above the terminal settling velocity of the largest particle in the size distribution, which is the operational rule Jenike & Johanson flag as a hard limit rather than a guideline [S1][S2]. For a related read on the medium/dense transition, the medium-phase operating pressure, velocity, and sizing bands reference goes deeper on the medium-phase hardware envelope, and the medium vs dense phase selection guide covers the material-and-distance matrix this article only summarizes. For broader pneumatic conveying fundamentals, the encyclopedia entry on conveying systems ties this back to blower, filter, and receiver sizing.
For the relevant spec sheets and selection criteria, see conveying, and medium voltage vfd.