Why conveying air cannot be sized from cfm alone
Pneumatic conveying moves bulk solids, such as cement, fly ash, flour, sugar, plastic granules or chemicals, through a pipeline using air. The air supply needed depends on far more than line length and pipe diameter. Material behaviour, how much solids each kilogram of air carries, the conveying regime, the pick-up velocity, vertical lifts, the number of bends, the feeder type, the pressure at the destination and the filter all interact strongly.
Because of that, this calculator is deliberately an enquiry tool rather than a full design program. Its job is to collect a technically usable brief, calculate a few simple supporting quantities, and show what information is still missing before a supplier or a specialist conveying designer can commit to a solution. Final conveying system design should come from the conveying system designer, using test data where the material is unusual.
Dilute phase and dense phase in brief
| Feature | Dilute phase | Dense phase |
|---|---|---|
| How material moves | Suspended in a fast air stream | Slugs, plugs or a moving bed at low velocity |
| Solids loading ratio | Low, typically below about 15 | High, often well above that |
| Air velocity | High, typically 15–25 m/s at pick-up depending on material | Low, typically a few metres per second |
| Usual air source | Roots, screw or centrifugal blower, or vacuum pump | Compressor, usually several bar |
| Typical pressure | Up to about 1 bar(g) (about 15 psig) for blower systems | Commonly 2–6 bar(g) (about 30–90 psig), sometimes higher |
| Wear and product damage | Higher with abrasive or fragile materials | Lower, gentler on product |
| Air quality concern | Oil-free air for food, pharma and chemicals | Clean, dry, often oil-free air; moisture causes plugging |
More detail is on our dilute phase conveying air supply and dense phase conveying air supply pages.
What the enquiry calculator records
| Group | Field | Why it matters |
|---|---|---|
| Material | Product name, grade and composition | Flow behaviour and contamination risk |
| Material | Bulk density and particle density | Volume of material and required loading |
| Material | Particle size distribution | Pick-up velocity, settling and wear |
| Material | Moisture, stickiness, temperature | Plugging, build-up and need for dry air |
| Material | Abrasiveness and friability | Pipe wear and product breakage |
| Process | Required solids throughput, kg/h or t/h | Main design duty |
| Process | Batch or continuous, hours per day | Peak versus average air demand |
| Route | Horizontal length, vertical lift, number of bends | Pressure loss of the line |
| Route | Pipe size and material, if existing | Velocity check and wear |
| Equipment | Feeder type (rotary valve, blow tank, screw) | Air leakage and pressure limits |
| Destination | Silo, hopper or process; filter type | Back-pressure and venting |
| Air | Required quality, existing air supply | Oil-free need, dryer, use of plant air |
Supporting calculations the tool performs
- Solids mass flow. Convert throughput to kg/s or kg/h on a consistent basis.
- Air mass flow from a chosen loading ratio. Solids loading ratio (SLR) = solids mass flow ÷ air mass flow. For a trial SLR, air mass flow = solids mass flow ÷ SLR.
- Free air volume. Free air flow = air mass flow ÷ density of free air (about 1.2 kg/m³ at typical reference conditions). Convert to m³/min and cfm.
- Actual volume and velocity in the line. At conveying pressure p (absolute), actual flow ≈ free air flow × (atmospheric pressure ÷ p), corrected for temperature. Velocity = actual flow ÷ pipe area.
- Velocity profile along the line. As pressure falls towards the destination, air expands and velocity rises. The tool shows velocity at pick-up and at the end of the line.
- Data completeness. List each missing field with its effect on confidence, so the enquiry can be completed before quotation; the live calculator covers the numeric items above.
Worked example: a dilute-phase cement transfer
The figures below are illustrative assumptions to show the method, not a design.
- Throughput: 5 t/h of cement, continuous.
- Trial solids loading ratio: 10.
- Air mass flow: 5,000 ÷ 10 = 500 kg/h.
- Free air flow: 500 ÷ 1.2 ≈ 417 m³/h, which is about 6.9 m³/min or about 245 cfm.
- Conveying pressure at pick-up: about 1.0 bar(g), so roughly 2.0 bar absolute.
- Actual flow at pick-up: about 417 × (1.013 ÷ 2.013) ≈ 210 m³/h, or about 0.058 m³/s.
| Pipe internal diameter | Velocity at pick-up | Comment |
|---|---|---|
| 100 mm | about 7.4 m/s | Too low for dilute phase; material would settle |
| 80 mm | about 11.6 m/s | Still likely low for cement in suspension |
| 68 mm | about 16 m/s | In the typical dilute-phase range; check end velocity |
The example shows why pipe size, pressure and air flow must be considered together. The same 245 cfm can be too little in one pipe and right in another. At the destination, where the pressure is close to atmospheric, the velocity in the 68 mm line roughly doubles, which increases wear on bends. A conveying designer would refine the loading ratio and pressure from material data and line losses; the enquiry brief ensures they have that data.
Choosing the air source
- Roots (lobe) blowers: common for dilute-phase pressure systems up to about 1 bar(g); simple and robust. See Roots blowers.
- Screw blowers: similar pressure range with better efficiency at many duty points.
- Vacuum pumps and blowers: for suction systems collecting from several points into one receiver.
- Oil-free compressors: for dense-phase systems at several bar, or where air must meet ISO 8573-1 Class 0 or a similarly strict oil class for food and pharma.
- Plant air: sometimes used for small dense-phase systems, provided there is spare capacity and the dryer and pressure are suitable.
Moisture is a frequent cause of trouble. Moisture-sensitive materials such as cement, sugar or certain chemicals can pick up moisture and build up in the line; a dryer and aftercooler are often needed, particularly during the monsoon.
Pressure loss along a conveying line
The air source must overcome the full resistance of the system, not just the pipe. A practical enquiry therefore lists each contributor separately so the designer can check it:
- Acceleration at pick-up: energy to bring the material from rest to conveying velocity, highest at the feed point and after each bend.
- Horizontal friction: air and solids friction along straight runs, rising with loading ratio and velocity.
- Vertical lift: the weight of the material column in rising sections.
- Bends: each bend slows the material, which then re-accelerates; many bends close together add up quickly.
- Feeder and filter: rotary valve leakage, blow-tank vent losses and the filter resistance at the destination.
Blower or compressor discharge pressure is the sum of these losses plus a working margin. If the line is later extended or throughput raised, the same sum must be rechecked, because a Roots blower working near its pressure limit runs hot and loses volumetric efficiency.
Common conveying problems the brief helps diagnose
- Line blockages after an increase in throughput, because air flow was not increased.
- Excessive bend wear from too high a velocity at the end of the line.
- Product degradation of fragile materials in dilute phase.
- Air leakage through worn rotary valves reducing air available for conveying.
- Blower overheating when line back-pressure rises above design.
- Filter blinding at the destination raising back-pressure.
How Reddy Compressors helps
Since 2002 Reddy Compressors has supplied and serviced compressed-air and blower equipment for industry. We sell new and used/refurbished blowers (Roots, screw, centrifugal and side-channel) and compressors, rent them, service them and supply their spares, working independently across brands and not as an authorised distributor.
- Help in completing a conveying air enquiry so it can be quoted correctly.
- Supply of blowers and oil-free or oil-injected compressors to the duty set by you or your conveying designer.
- Rental blowers and compressors for trials, temporary conveying or breakdown cover.
- Service, overhaul and spare parts for existing blowers and compressors.
- Dryers, aftercoolers and filtration where moisture or oil is a risk.
Our office is at Tarnaka, Secunderabad. We serve all of Telangana and Andhra Pradesh and supply equipment, rentals and spares across India.
What to send us
- Material name, bulk density, particle size and moisture, or a data sheet.
- Required throughput and whether conveying is batch or continuous.
- Route sketch with horizontal length, vertical lift and number of bends.
- Pipe size, feeder type and destination details for an existing system.
- Existing blower or compressor nameplate photo and the problem you see.
- Air-quality requirement, especially for food, pharma or chemical products.
Frequently asked questions
How much air is needed for pneumatic conveying?
It depends mainly on the solids throughput and the solids loading ratio the material can carry, together with the pipe size and conveying pressure. A useful first estimate is air mass flow equals solids mass flow divided by a trial loading ratio, converted to free air. That estimate must then be checked against pick-up velocity and line pressure loss, ideally by a conveying designer using material data.
Should I use a blower or a compressor for pneumatic conveying?
Dilute-phase systems working up to about 1 bar(g) (about 15 psig) usually use Roots or screw blowers, which are efficient at low pressure. Dense-phase systems working at several bar normally need a compressor, usually oil-free for sensitive products. Vacuum systems use vacuum pumps or blowers. The choice follows from the conveying mode, which follows from the material and the route.
What is the difference between dilute phase and dense phase conveying?
In dilute phase, material is suspended in a fast air stream at a low solids-to-air ratio, which suits many free-flowing materials but causes more wear and breakage. In dense phase, material moves as slugs or a moving bed at low velocity with a high solids-to-air ratio, using higher pressure but less air. Dense phase is gentler on fragile and abrasive products.
Why does my pneumatic conveying line keep blocking?
Common causes are too little air for the current throughput, velocity falling below the material’s pick-up value, moist air or moist material causing build-up, air leaking through worn rotary valves, or a rise in back-pressure from a blinded filter at the destination. Logging blower discharge pressure and checking air flow during a blockage usually points to the cause quickly.
Do I need oil-free air for conveying food or pharma products?
Where the air contacts the product, oil-free air is strongly preferred, and many food and pharmaceutical users specify an ISO 8573-1 oil class appropriate to their product risk. Oil-free blowers and compressors remove the oil source, but drying and filtration may still be needed to control moisture and particles. Your quality team’s specification should set the required class.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Pneumatic conveying air estimate
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Solids throughput (t/h): 5
- Conveying mode: Dilute phase (blower, suspended flow)
- Trial solids loading ratio (kg solids per kg air): 10
- Line pressure at pick-up (bar g; negative for vacuum systems): 1
- Line pressure at the destination (bar g; negative for vacuum systems): 0
- Air temperature in the line (°C): 20
- Pipe internal diameter (mm): 68
- Target pick-up velocity (m/s): 16
Result
- Solids mass flow: 5,000 kg/h (1.389 kg/s)
- Air mass flow: 500 kg/h
- Free air: 416.7 m³/h = 6.94 m³/min = 245 cfm
- Actual flow at pick-up: 209.7 m³/h (0.0583 m³/s)
- Velocity at pick-up: 16.04 m/s
- Velocity at the destination: 31.87 m/s (1.99 × pick-up)
- Bore giving the target pick-up velocity: 68.1 mm internal diameter
- Pick-up velocity check: Within the typical 15–25 m/s dilute-phase range typical 15–25 m/s, material-dependent
Method. Air mass flow = solids mass flow ÷ solids loading ratio (SLR). Free air = air mass flow ÷ free-air density, taken as 1.2 kg/m³ (air at about 1.01325 bar a and 20 °C). Actual flow at a line pressure p = free air × 1.01325 ÷ (p + 1.01325) × (T + 273.15) ÷ 293.15 (free air taken at 20 °C). Velocity = actual flow ÷ pipe area. The bore for a target pick-up velocity is √(4 × actual flow ÷ (π × velocity)). Bend, lift, feeder and filter losses are not calculated: the pressures are your inputs. For a vacuum (suction) system enter negative gauge pressures, pick-up nearest atmospheric. The velocity flags (15–25 m/s dilute pick-up, 30 m/s at the destination, 10 m/s dense phase) are screening values used by this tool, not material data.
What to take from it. The same free-air flow can be too little in one pipe and right in another. Air velocity rises as pressure falls towards the destination, so check both ends.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.