Pneumatic Systems
Compressed-air supply, air preparation, cylinder air use, valve flow, tubing, vacuum and sizing.
The compressed-air chain
A compressor (piston, screw or scroll) raises the air to 7 to 10 bar; an aftercooler and dryer remove the heat and the water; a receiver smooths the demand; and pipes carry it to the machines, where each cell has a filter-regulator-lubricator group (FRL), valves and cylinders. The thermodynamics of the compression is in Ideal Gases and Processes; the pipe flow is in Compressed Air and Gas Flow.
Air is measured as free air (FAD: free air delivered at the inlet conditions, liters per second or m³/min) or normal liters (ANR: 20 °C, 1.013 bar), not at the working pressure. 1 m³ of 6 bar gauge air is 6.92 m³ of free air, so every consumption figure must say which basis it uses.
Air preparation
| Item | What it does | Typical specification |
|---|---|---|
| Dryer | removes water vapor (condensate corrodes and washes out lubricant) | refrigerated: pressure dew point +3 °C; desiccant: −40 °C for outdoor or critical use |
| Filter | removes dirt, water and oil | 5 µm general; 1 µm and 0.01 µm coalescing for spraying and instruments |
| Regulator | sets a constant working pressure | set 0.5 to 1 bar below the supply; most tools 6 bar |
| Lubricator | adds a mist of oil | not needed for modern lube-free components; never add oil to lube-free seals |
| Soft-start and exhaust valves | bring up pressure slowly; exhaust it safely | safety: a dump valve that vents the machine |
Quality is classified by ISO 8573-1 (particles, water and oil); a typical machine air is class 1.4.1 to 1.4.2 or 2.4.2. Pressure drops in the preparation are 0.1 to 0.3 bar each: check them when a cylinder is slow.
Cylinders: force and air use
The force is the same relation as in hydraulics, F = p A, but with a lower pressure and with a real efficiency of 80 to 90 percent for friction. The air use is the volume of the cylinder at the working pressure, converted to free air:
Vfree = (Acap + Aann) × stroke × (p + 1.013) ÷ 1.013
Worked example: a 63 mm cylinder
Worked example: a 63/18 mm cylinder, 200 mm stroke, 60 cycles a minute at 6 bar
- Piston area = 31.2 cm²; theoretical push = 6 bar × 31.2 cm² ÷ 10 = 1 870 N; at 85 percent efficiency 1 590 N (retract 1 460 N).
- Air per double stroke = 8.28 L free air; at 60 cycles a minute the consumption is 497 L/min = 8.3 L/s.
- Add 20 percent for leakage (typical): 596 L/min, so a compressor of 0.60 m³/min FAD is needed for this one cylinder.
- At a typical specific power of 6.3 kW per m³/min the compressor draws 3.8 kW: 15 023 kWh a year over 4 000 hours, or about 1 803 at 0.12 per kWh, for one cylinder moving a few kilograms. Compressed air is expensive.
- The pipe from the receiver: the compressed-air volume flow is 497 ÷ 6.92 = 72 L/min, so for a velocity of 15 m/s the tube needs an inside diameter of about 10.1 mm; use 12 mm or larger tube, or the cylinder will starve.
Valve flow and speed
A valve’s capacity is given as a flow factor C (L/s·bar), Cv (US gal/min per psi½) or Kv (m³/h per bar½), together with the critical pressure ratio b (about 0.3 to 0.5). Above the ratio the flow is choked at the sonic limit and proportional to the upstream absolute pressure (see Compressed Air and Gas Flow): Qfree = C p₁ (ANR L/s with p₁ in bar absolute).
Worked example: how fast will it go?
- A valve with C = 2.0 L/s·bar at 7.0 bar absolute passes 14.0 L/s = 842 L/min of free air.
- In the cylinder the air is at about the supply pressure, so it fills 842 ÷ 6.92 = 122 L/min of cylinder volume, a piston speed of 0.00203 ÷ 0.00312 = 0.65 m/s as an upper estimate.
- Real speeds are lower: the pressure in the cylinder drops as it fills, the exhaust restricts, and the load takes pressure. Use the manufacturer’s sizing tool or a measured test for a speed-critical job, and the Cylinder Force and Speed Calculator.
- Speed is set with one-way flow controls on the exhaust (meter-out): this keeps the cylinder pressurized on both sides so the motion is smooth and stable.
What compressed air costs
It takes about 6.3 kW of electricity to make 1 m³/min of 7 bar air, and only a tenth or less of the electrical energy ends up as useful work, so compressed air is the most expensive energy on the shop floor. The other source of waste is leakage: a 2 mm hole at 6 bar passes about 5.0 L/s, costing about 907 a year at 4 000 hours, and a plant with 100 such leaks wastes a whole compressor. Typical systems lose 20 to 30 percent to leaks. Cures: fix leaks (listen with an ultrasonic detector), turn compressed air off at night, lower the pressure (every 1 bar lower saves about 7 percent of the power), use blowers or electric actuators where they fit, and size the receiver for the peaks: a 1 000 L receiver between 8 and 6 bar delivers 1.97 m³ of free air.
Vacuum
Suction cups and grippers hold a part with the atmosphere pushing on it: F = Δp A, where Δp is the vacuum below the atmosphere. A 50 mm cup at 60 kPa vacuum holds 60 kPa × 19.6 cm² = 118 N. With a safety factor of 4 for a vertical lift (friction and acceleration) it holds about 3.0 kg. Vacuum is made with a venturi ejector (simple, uses compressed air) or a vacuum pump (efficient for continuous use). Seal condition, surface porosity and speed all reduce the holding force.
Related
- Previous: Fluids, Filtration and Reservoirs. Next: Designing a Fluid Power System.
- Compressed Air Consumption Calculator and Cylinder Force and Speed Calculator.
- Pneumatic and Hydraulic Actuators.
- Hydraulics and Pneumatics: the whole guide.
FAQ
What is FAD?
Free air delivered: the flow of a compressor expressed as air at inlet conditions. Always compare compressors and loads on the same basis.
Why does a cylinder stall at low pressure?
The force falls with pressure, the seal friction does not, so below a threshold the cylinder cannot move the load. Check the regulator and the pressure at the cylinder, not at the receiver.
Should I use lubricated air?
Not for modern components: they are greased for life. Once a lubricator has been used, the grease is gone and oil must be kept in.
Is pneumatic actuation always the cheapest?
Purchase price is low and the equipment is simple, but running costs are high. Electric actuators win on energy for long duty cycles.
References
- Esposito, A., Fluid Power with Applications, Pearson (pneumatic systems); Beater, P., Pneumatic Drives, Springer.
- ISO 8573-1 (compressed air quality), ISO 4414 (pneumatic safety requirements) and ISO 6358 (flow-rate characteristics of components); CAGI, Compressed Air and Gas Handbook.
- US DOE, Improving Compressed Air System Performance: A Sourcebook for Industry; SMC, Festo and Parker pneumatics catalogs.
Last reviewed: October 2026. Costs, leak rates and specific power are typical.