Compressed air sizing

Tools — add the tools you use at the same time from the library (impact wrenches, die grinders, DA and orbital sanders, HVLP, LVLP and conventional spray guns, nailers, blow gun, tyre inflator, plasma cutter, sandblasters…) with how many run at once. Each has its typical SCFM at 90 psi and a duty — the share of the time it actually runs while you’re using it. The average demand adds a leak allowance and a margin; the peak is everything running continuously.

Compressor and tank — enter the label figures: delivery in SCFM at 90 and 40 psi, tank size, cut-in and cut-out pressures, the rated duty cycle and the motor’s horsepower. With your tools running, the tool gives the off time (the tank feeding the tools), the on time (the pump refilling while they draw), starts per hour, the share of time the motor runs against its rating, the time to fill from empty, and the tank that would hold the worst case to six starts an hour. If the pump can’t keep up, it says how long you have before the pressure falls below what the tools need.

Piping — the run to the furthest outlet: material, length, flow (your demand unless you give one), starting pressure, elbows, tees and valves, and whether it is a loop. Every size of that material is listed with its velocity and pressure drop by Darcy–Weisbach (Colebrook friction at the air’s density in the pipe) and by the Atlas Copco empirical formula, and the smallest that stays within the drop you accept is marked. Below that are the layout rules: slope to a drain, take drops off the top, drain legs, metal for the first run from the hot compressor, and never PVC.

Water — intake temperature and humidity, line pressure and the tank temperature give the water the air drops per hour and per day, the pressure dew point, and the dew point once the air expands back to atmosphere.

Electrical — the motor’s full-load current from the NEC tables (or P ÷ V·η·pf for UK/EU), the wire it needs (125 %), the largest breaker the NEC allows (250 %) and the starting surge. Add to shop power puts the compressor in the shop power planner to check it against the rest of the circuit.

The units — SCFM, psi, gallons and feet, or l/min, bar, litres and metres — are shared with the other workshop tools.

Tank times are isothermal (Boyle’s law): the free air stored between two pressures is V_tank × ΔP ÷ P_atm, the off time that divided by the demand, and the on time that divided by delivery minus demand. Starts per hour peak when the demand is half the delivery. Pipe friction is Darcy–Weisbach with the Colebrook friction factor; fittings are equivalent lengths in pipe diameters. Water is psychrometrics: the saturation pressure by Buck (1981), humidity ratio 0.622·p_v/(p − p_v), and the difference between the intake air’s moisture and what saturated air holds at line pressure and tank temperature. The tests check the pump-down against T = V·ΔP / (C·P_atm), the pipe drop against a hand calculation and the Atlas Copco formula, and the water against the psychrometric chart.

For liquids, see the fluid flow and pipe pressure drop calculator; for the dust collector, the dust collection planner.

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