Compressed air is treated as free because it is not metered at the point of use. It is, in fact, the most expensive common utility: overall efficiency from electrical input to mechanical work at the actuator is typically 10 to 15 per cent. Every design decision that wastes air is multiplied by that factor at the meter.

Sizing a cylinder

Theoretical force (extend)  = P x (pi/4) x D^2
Theoretical force (retract) = P x (pi/4) x (D^2 - d^2)

  P = working pressure, D = bore (mm), d = rod diameter (mm)
  Force in newtons when P is in N/mm^2   (1 bar = 0.1 N/mm^2)

Apply a load ratio: use no more than 70 per cent of theoretical force for a static clamping application, and no more than 50 per cent for a moving load that must accelerate. Sizing to 95 per cent of theoretical force produces a cylinder that stalls the first time the supply pressure dips.

Worked example: a 50 mm bore cylinder at 6 bar. Area = 1,963 mm2. Force = 0.6 N/mm2 x 1,963 = 1,178 N theoretical. At a 50 per cent load ratio, design for about 590 N of actual load.

Air consumption, and why it is the number that matters

Consumption per cycle (litres FAD)
  = (Extend area + Retract area) x Stroke x (P_abs / 1.013) / 10^6

For 50 mm bore, 20 mm rod, 300 mm stroke, 6 bar gauge (7.013 bar abs):
  Extend area  = 1963 mm^2
  Retract area = 1649 mm^2
  Swept volume = (1963 + 1649) x 300 = 1,083,600 mm^3 = 1.084 litres
  FAD per cycle = 1.084 x (7.013 / 1.013) = 7.5 litres

At 10 cycles per minute over two shifts, that single cylinder consumes about 7,200 litres per hour, or 115 cubic metres per shift. At a typical specific power of 0.11 kWh per cubic metre of compressed air, that is roughly 12.7 kWh per shift — for one cylinder.

Valve sizing: Cv and flow

An undersized valve is the most common cause of a cylinder that is slower than the specification. Required flow follows from the swept volume and the required stroke time; then choose a valve whose Cv or nominal flow rating exceeds it with margin. Remember that the fittings and tubing are part of the restriction — a correctly sized valve behind 6 mm tubing on a 50 mm cylinder is still a slow circuit.

The four cheap wins

  1. Fix leaks. A 3 mm hole at 6 bar leaks roughly 11 litres per second — about 4 kW of compressor power, continuously. Ultrasonic leak detection pays back in weeks in most plants.
  2. Lower the pressure. Every 1 bar of reduction cuts compressor energy by about 7 per cent. Most plants run 7 bar because someone once needed it for a machine that has since been removed.
  3. Regulate at the actuator. Retract strokes usually do no work. Regulating the retract side to 3 bar on a dual-pressure circuit cuts consumption by 25 to 30 per cent on that cylinder.
  4. Stop using air for the wrong jobs. Open-tube blow-off, air-driven mixing and air motors used continuously are almost always cheaper as electric or as engineered nozzles. Engineered blow-off nozzles alone cut consumption 50 to 70 per cent against an open tube.

Air quality classes

ISO 8573-1 specifies air quality as three digits for particles, water and oil. Matching the class to the application avoids both failures and waste: a general workshop needs roughly 7:4:4, machine tools and cylinders 6:4:3 or better, and instrumentation or food contact considerably tighter. Specifying instrument-grade air across an entire plant because one machine needs it is a permanent and unnecessary drying cost.

A weekend audit

Shut all production, leave the compressor running, and measure the load and unload cycle. The air being consumed with no production running is your leak rate. In plants that have never done this, 20 to 30 per cent of total compressed air output is normal, and it is the cheapest energy project available.