Why conveying air starts with the material, not the machine
Many enquiries arrive as “we need a compressor for 10 tonnes per hour of powder”. That figure alone cannot size anything. Two powders with the same bulk density can need very different air volumes and pressures, because pickup behaviour, cohesion, abrasiveness and fragility decide how the material must be moved through the pipe.
A conveying line is a system: feeder, pipeline, bends, diverters, receiver, filter and the air source all interact. Getting the material description right is the cheapest way to avoid a line that plugs, wears through its bends or degrades the product.
Record these material properties before anyone talks about air:
- Product name and grade, with any supplier handling notes.
- Bulk density (loose and, if known, tapped) and, separately, particle or true density. They are different quantities and must not be swapped.
- Particle-size distribution, not just a nominal size, plus fines content, lumps, flakes or fibres.
- Moisture range in normal and adverse conditions, especially in the monsoon.
- Stickiness, flowability and any tendency to bridge in hoppers.
- Abrasiveness (wear history on existing bends is valuable evidence).
- Friability: will breakage or fines generation affect product quality?
- Temperature of the material and any heat sensitivity.
- Contamination sensitivity: food, pharma or chemical products may need oil-free and dried air.
Dilute phase or dense phase: which regime suits the duty?
The conveying regime is the most important early decision because it sets the order of magnitude of pressure and air volume. In dilute phase, particles are suspended in a high-velocity airstream. In dense phase, the material moves as slugs or a moving bed at much lower velocity with higher pressure.
| Factor | Dilute phase | Dense phase |
|---|---|---|
| Material movement | Suspended in the airstream | Slugs, plugs or a moving bed |
| Air velocity | High | Low |
| Typical air source | Positive-displacement (Roots) or screw blower; vacuum pump for suction systems | Compressor, usually a screw compressor with a dryer |
| Typical pressure class | Below about 1 bar gauge (about 15 psig) for pressure systems | Usually several bar, commonly up to about 6–7 bar (87–101.5 psig) at the source |
| Wear and degradation | Higher on abrasive or fragile material | Lower, because velocity is low |
| Capital cost | Simpler pipework and feeders | Pressure vessels (transporters) and more controls |
| Suitable materials | Free-flowing, non-abrasive, non-fragile | Abrasive, fragile, mixed or dense materials such as cement, fly ash and sand |
Some materials suit both regimes, and some suit neither without test work. Fly ash handling in thermal power plants, cement transfer, plastic granules, flour, sugar, soda ash and pharmaceutical powders all have well-known regime preferences, but the material on your site and your route decide the final answer. Where there is genuine doubt, a conveying test or a design by an experienced conveying engineer is the correct step. More detail is on our dilute phase conveying air supply and dense phase conveying air supply pages.
How to state throughput as a duty envelope
A single tonnes-per-hour figure hides the conditions that actually size the air system. State the duty as an envelope:
- Normal solids rate, peak solids rate and any planned future increase.
- Batch or continuous operation, with batch size and cycle time for batch systems.
- Hours per day and days per week, so that duty and standby can be planned.
- Start-up and line-clearing conditions: a line restarted full of material needs more than steady running.
- Recovery after a stoppage or blockage, and how often that happens today.
Line clearing and restart after a trip are frequent causes of “the compressor is too small” complaints, when the real issue is a sequence that leaves the pipe charged with material. Write these cases down; they are design conditions, not exceptions.
Pipeline route, bends, feeder and receiver
The route drawing matters as much as the material sheet. Pressure loss rises with horizontal length, vertical lift, number and type of bends, diverters and pipe bore changes. Send an isometric or a marked-up layout showing:
- Total horizontal and vertical lengths, with the vertical sections located (a lift near the end behaves differently from one near the feeder).
- Number of bends, their radius or type, and any diverter valves.
- Pipe bore, material and any stepped sections.
- Feed device: rotary valve, screw feeder, venturi, blow tank or transporter vessel.
- Destination: silo, day bin, hopper or process vessel, with the receiving filter and its venting arrangement.
Rotary valves leak air back to the feed hopper, and that leakage must be allowed for in the air supply. Blow tanks and transporters need a defined pressurisation and conveying air split. Receiver filters must handle the conveying gas volume plus displaced air from the receiver, or the silo will pressurise.
Pressure and flow must be stated together
An air source is never sized on pressure alone or flow alone. A conveying duty is a point: a required air volume at a defined pressure, measured on a defined basis.
Three points cause most confusion:
- Flow reference basis. Blower and compressor flows are quoted as free air delivery at inlet conditions. Conveying calculations often work in actual volume at line pressure. Air expands as pressure falls along the pipe, so velocity rises from feeder to receiver. Always say whether a flow is free air, normal or actual.
- Velocity is a result, not a target to copy. Minimum conveying velocity depends on the material and regime. Pickup velocity at the feeder and the velocity at the receiver end are both checked. A velocity figure borrowed from another material or plant is not a design basis.
- Solids loading ratio. The mass of solids per mass of air is a useful check on regime, but it is an output of the design, not an input you can choose freely.
The pressure budget at the air source must cover the conveying pressure drop, the feeder, the filter and the receiver back-pressure, plus air-line losses from the compressor or blower to the injection point. For dense phase, also allow for the dryer and filters between compressor and transporter.
Choosing the air source: blower, compressor or vacuum
| Air source | Where it fits | Points to check |
|---|---|---|
| Roots (twin- or tri-lobe) blower | Dilute-phase pressure systems at low pressure | Pressure ceiling, relief valve, inlet filter, silencers, discharge temperature |
| Screw blower | Larger dilute-phase systems needing better efficiency at low pressure | Turndown, control method, oil-free design for sensitive products |
| Vacuum pump or exhauster | Suction systems collecting from several points to one receiver | Vacuum level, filter protection of the pump, leakage into the line |
| Oil-injected screw compressor with dryer and filters | Dense-phase transporters and blow tanks | Pressure dew point, oil carry-over, receiver size for transporter cycles |
| Oil-free screw compressor | Dense phase for food, pharma and chemical products | Class of air needed, dryer type, total package cost |
Moisture is a frequent cause of plugging in dense phase. A refrigerated dryer giving a typical pressure dew point around +3 °C suits many duties. Where the line or receiver runs cold, or the material is hygroscopic, a desiccant dryer at a typical −20 °C or −40 °C pressure dew point may be needed. Indian summer intake temperatures of 40–45 °C also raise the moisture load on aftercoolers and dryers, so allow for them. See end-use air preparation for the treatment side.
Energy matters on a line that runs every shift. For positive-pressure compressors, each extra bar of discharge pressure costs about 7% more power, so do not set the compressor higher than the conveying budget needs.
Troubleshooting an existing conveying line
When an existing line blocks, slows or wears, do not start by buying a larger compressor. More air at the same pressure can raise velocity and wear without curing the cause. Work through the system in order:
- Record actual air pressure at the source and at the feeder during conveying, line clearing and restart.
- Check the delivered air volume of the blower or compressor against its rating, including inlet filter condition, belt slip, valve wear and unloading.
- Check rotary valve clearances and air leakage back to the hopper.
- Look for moisture: drain points, dryer operation and pressure dew point.
- Inspect bends and diverters for wear, build-up or misalignment.
- Review material changes: new grade, higher moisture, different supplier.
- Review the sequence: purge timing, transporter fill levels, interlocks.
- Check the receiver filter differential pressure and venting.
A capacity increase or route change on an existing line should be checked as a new design point. Longer routes and extra bends raise the pressure needed; higher solids rates may push a dilute line into an unstable region. The air source may be adequate, or it may need more pressure rather than more flow.
Temporary or rental air for conveying
Rental air is useful during a compressor overhaul, a commissioning trial, a plant shutdown or a capacity test. For dense-phase transporters, a rental screw compressor should come with an aftercooler, a dryer and filtration suited to the product, and the connection point, hose sizes and pressure setting must be agreed in advance. A diesel portable without a dryer is rarely acceptable for fine powders. We can brief this through our rental engineering service.
How Reddy Compressors helps
We are an independent multi-brand compressed-air business based at Tarnaka, Secunderabad, serving industry since 2002. For conveying systems we work on the air-supply side: selecting, supplying and servicing the blowers, screw compressors, dryers, filters and receivers that feed your conveying line. We do not design the conveying pipeline itself; where that is needed, the material and route data above go to your conveying specialist, and we match the air system to their duty point.
- Sales of new and used/refurbished compressors and blowers (Roots, screw, centrifugal and side-channel) across makes such as ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.
- Rental compressors from our own fleet, 50–2,000 cfm and 2–30 bar, diesel and electric, for trials, shutdowns and breakdown cover.
- Service, AMC, overhaul and spare parts for blowers, compressors and dryers, with over 20 service engineers and technicians covering Telangana and Andhra Pradesh.
- Air audits measuring pressure, flow, power and dew point on existing lines.
- Supply of equipment and spares across India.
What to send us
- Material name, bulk density, particle size, moisture and any handling notes.
- Normal and peak solids rate, batch or continuous, hours per day.
- Route layout with horizontal and vertical lengths, bends and pipe bore.
- Feeder type and receiver/filter arrangement.
- Duty point from your conveying designer, if available (air volume, pressure, basis).
- Existing air source make, model, nameplate photo and current problems.
- Air quality needs: oil-free, dew point, food or pharma contact.
Frequently asked questions
How do I calculate the air needed for pneumatic conveying?
Air demand comes from the conveying design, not a rule of thumb. The designer uses the material’s minimum conveying velocity, pipe bore, route length, bends and solids rate to find the air volume and pressure drop. That result, stated as free air delivery at a defined pressure, is what the blower or compressor must supply. Without material and route data, any air figure is a guess and should not be used to buy equipment.
What pressure is used for dense phase conveying?
Dense-phase systems typically run at several bar, with the compressor commonly set up to about 6–7 bar (87–101.5 psig) at the source and the transporter operating below that. The exact pressure depends on route length, material and solids rate. Because each extra bar costs about 7% more compressor power, the setting should match the conveying pressure budget plus dryer, filter and pipe losses, rather than the highest pressure available.
Can I use a Roots blower instead of a compressor for conveying?
For dilute-phase pressure systems working below about 1 bar gauge, a Roots or screw blower is usually the right air source and is more efficient than a compressor at that pressure. Dense-phase systems need higher pressure, which is beyond a blower’s range, so they use a screw compressor with a dryer. The conveying design decides the regime, and the regime decides which machine fits.
Why does my pneumatic conveying line keep blocking?
Common causes are insufficient air volume at the feeder because of a worn blower or leaking rotary valve, moisture in the air or material, a change in material grade, worn or poorly placed bends, and restart sequences that leave the line full. Check delivered air, pressure during conveying, dew point and the purge sequence before adding capacity. A larger compressor can raise velocity and wear without curing the blockage.
Do I need a dryer for pneumatic conveying air?
Dense-phase systems nearly always need dried air, because condensed water makes powders stick and plug. A refrigerated dryer with a typical pressure dew point around +3 °C suits many duties; hygroscopic materials or cold lines may need a desiccant dryer. Dilute-phase blower systems run at low pressure with warm air and often need no dryer, but sensitive products may still require filtration and oil-free air.
Can I hire a compressor for a conveying trial or shutdown?
Yes. Rental screw compressors and blowers are often used for trials, overhauls and breakdowns. For powders, the hire package should include an aftercooler, a dryer and suitable filters, and the hose sizes, connection point and pressure setting should be agreed before delivery. Share the duty point, product sensitivity and run hours so the machine and treatment can be matched to the line.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.