Pneumatic Systems

Pneumatic systems transmit force with compressible air drawn from the atmosphere. Pascal's Law, Bernoulli's principle and conservation of energy all apply, as they do in hydraulics.

• Absolute scales start at absolute zero. Temperature: Kelvin (from °C) and Rankine (from °F).
• 0 K = 0 °R = −273 °C = −459 °F.
• Absolute pressure starts at 0 psia (a perfect vacuum). At sea level (15.5 °C / 60 °F) the atmosphere is ≈ 1 bar = 14.7 psia, and 1 psi = 2.04″ Hg.
• Vacuum = pressure below the surroundings. A perfect vacuum = 0 psia, so the most a vacuum can give is 1 bar (14.7 psi) of pressure difference.
• A vacuum cup grips because the atmosphere outside pushes harder than the thin air inside it, so any leak at its lip costs grip. If one cup lets go while the generator's gauge reads normal, the fault is that cup or its line: replace a cup whose lip is worn, cut or hardened, and check its tubing and fittings.
• Barometer: sea-level atmosphere supports a 760 mm / 29.92″ Hg mercury column; pressure drops ~1″ Hg per 1000 ft of altitude.

Vacuum pumps remove air rather than compress it. Rough maximum ratings:

A venturi vacuum generator (ejector) has no moving parts: plant air speeds up through a nozzle, its pressure falls (Bernoulli), and the suction port beside the jet draws air out of the cups. It uses air the whole time it runs and needs clean, dry, oil-free air. With nothing inside to wear, a slow loss of vacuum (supply pressure right, lines tight) means a clogged exhaust silencer or suction filter: clean or replace them.

Milky oil in an oil-sealed vane vacuum pump means water: vapour drawn from the process condenses into the oil as the pump compresses it. Wet oil can't seal the vanes, so the vacuum falls off. Change the oil, then run with the gas ballast open: it lets a little air into the compression stage so the vapour leaves with the exhaust instead of condensing.

Free air carries dirt, moisture and varying heat, so treat it before use: filter → cool → remove moisture / oil → store. Full chain: intake filter → intercooler → aftercooler → receiver → dryer → filter → regulator → lubricator.

• At the compressor end, the intake filter cleans the air, the coolers drop out most of its water and the receiver stores it.
• Dew point = the temperature at which water condenses. Water traps (float-operated, manual or auto drain) go wherever air may fall below its dew point — after the intercooler and aftercooler, and at the dryer, which drains its own condensate.
• The dryer then lowers the dew point, and the FRL at each point of use filters, regulates and lubricates, in that order.

• Two grades: instrument air (clean, dry, oil-free) and plant air (clean, low moisture / oil).
• Air dryers lower the dew point to as low as −40 °C.
• A refrigerated dryer sheds the heat it takes out of the air through its condenser. If the fins of an air-cooled condenser mat with dust, the dew point climbs on hot days and water gets past: lock the dryer out, vacuum or brush the fins clean, and check its drain.

Air filters

poly / metal bowl with a deflector plate that swirls out heavy particles and moisture, plus a baffle quiet zone. Coalescing filters remove solids to 0.3 µm plus liquid oil / water droplets and aerosols — not vapour; removing vapour is the dryer's job.

Filter bowls

a float drain can gum shut with oily sludge; water then rises past the baffle and the air carries it to the tools. Drain the bowl by hand, then clean or replace the drain. Clean a clear polycarbonate bowl with mild soap and water only: solvents (acetone, gasoline, brake cleaner) craze it, and a crazed bowl can burst.

Pressure regulators

reduce line pressure (piston or diaphragm + poppet; gauge on the downstream side).

Regulator creeping up

a downstream gauge that climbs to line pressure while no air is being used means the poppet isn't sealing (dirt on the seat, or a worn seat or O-ring). Clean or rebuild the regulator, and check the filter ahead of it.

Turning the regulator up can't make up what a dirty filter loses: a regulator only reduces the pressure that reaches it. Judge a filter element by the drop across it with air flowing (at no flow the gauges agree); a clean one drops only a few psi. Change it at about 10 psi, or yearly.

Air lubricators

meter oil into the air feeding tools and cylinders; an oil-sensitive device (e.g. a paint sprayer) takes its air from a branch ahead of the lubricator. Fog (larger droplets — short / straight runs) vs mist (fine — long runs). Venturi + capillary; only a few drips per minute.

A lubricator feeds oil only while air flows through it (the flow past its venturi lifts the oil), so check and set the drip with the machine running, not standing idle. Fill it with the maker's light air-line oil: motor oil or a heavier oil gums the valves.

• Installation: slope lines to a drain (2.5–5 cm per 3 m).
• Take air-drops from the top of the main via a tee that rises then drops, so moisture flows past to a water leg at the end.
• Circuits read like hydraulic ones (an outlined triangle = pneumatic flow), starting from a pneumatic pressure source.
• Common circuits: two-hand / safety (both DCVs must shift — protects hands), dual-exhaust speed control, quick-exhaust (impact tools), and multiple remote positions via pilot + shuttle valves.
• Meter-out speed control: set an air cylinder's speed by metering the air leaving it, with flow controls at the cylinder ports or in the valve's exhausts; the air held back on the far side steadies the piston, while metering the supply lets the springy air make it lurch. Cushions only soften the last part of the stroke, so a cylinder that bangs into both ends needs meter-out, not more cushion.

• Find leaks by sound.
• Leak load test: with all production stopped, leaks are the only demand, so leakage % = load time × 100 ÷ (load time + unload time). Under 10 % is a well-kept system; 20–30 % is typical of a neglected one.
• Valves freeze in cold weather because air expanding through a pressure drop cools.
• Lines that run outdoors also freeze when the air's dew point is above the outdoor temperature: water condenses in the cold pipe and ices the valves. A refrigerated dryer can't get the dew point below about +3 °C (any colder and it would ice up), so dry the air for an outdoor line to about −40 °C with a desiccant or membrane dryer.
• Prevent freezing with an infra-red lamp, anti-freeze in the lubricator, alcohol in the lines, a heated air supply, or a warm room.