Pick an air compressor by the pressure you need and the power that will turn it: a bellows gives a gentle blast for a forge, a trompe turns falling water into air at roughly 1 bar per 10 m of fall, and a piston compressor is the only one that reaches tool pressure. A piston machine delivering 235.6 L/min at 7 bar gauge needs at least 1.13 kW at the shaft, so a 785 W water wheel cannot drive it.
Part of Pneumatic Systems
Compressors turn mechanical work into stored air pressure. The three buildable types differ most in the pressure they can reach and the power they demand.
Three Compressors, Three Pressure Ranges
| Type | Pressure it can give | What it needs | Best use |
|---|---|---|---|
| Bellows | A fraction of a bar | A person’s arm, a cam or a wheel | Forge or furnace blast |
| Trompe | Roughly 1 bar per 10 m of fall | Falling water, nothing else | Furnace blast, ventilation, low-pressure air |
| Single-stage piston | Up to 8 bar (carrier figure) | A shaft turning at hundreds of RPM | Tools, spray, clamps |
| Two-stage piston with intercooler | Up to 15 bar (carrier figure) | The same, with more power and a cooler | Higher pressure, better efficiency |
Pressure and flow are separate limits. A trompe or bellows may move plenty of air at a pressure too low to run a drill, and a piston compressor may make tool pressure at a flow the drill uses up in seconds.
Match the Machine to the Power
Power is what turns every compressor, and the arithmetic comes first, in the order of Gas Laws in Practice:
- Free-air flow Q in m³/s, pressure ratio r = absolute outlet / absolute inlet pressure
- Minimum shaft power (isothermal limit) = 101,325 x Q x ln r
- Realistic ideal power (adiabatic) = 3.5 x 101,325 x Q x (r^0.2857 - 1)
For the carrier’s reference piston compressor, 235.6 L/min (0.003927 m³/s) at 7 bar gauge (r = 8) gives 827 W and 1,130 W (1.13 kW) respectively, before friction and valve losses.
Power available from the corpus’s own sources:
- One person cranking a chain pump: 50-75 W (Chain Pump)
- A 20 L/s flow falling 5 m through an 80%-efficient overshot wheel: 785 W (Water Wheel Types)
Dividing the wheel’s 785 W by the 1,130 W of the reference machine gives 0.69, so a wheel that size supports about 164 L/min of free air at 7 bar gauge at most. Anything bigger than that wants a bigger wheel or a smaller compressor. The corpus gives no figure for mechanical losses, so treat 164 L/min as a ceiling.
Choosing by Job
- A forge or furnace that wants a steady, low-pressure blast: a bellows or a trompe. Do not compress further than you need to, since every extra bar costs shaft power.
- A shop with air tools: a piston compressor, single-stage for up to 8 bar and two-stage for higher pressure or lower running cost.
- A site with a stream and no power: a trompe for low-pressure air, with the limits in Water-Powered Compressor (Trompe).
Common Mistakes
- Choosing by displacement alone. The compressor’s swept volume is not its delivery; leakage, clearance and valve lag cut it, so measure the real output by timing the filling of a known tank.
- Sizing the wheel to the average flow of a stream. Design for the dry-season flow, since the compressor will stall when the stream drops.
- Compressing to tool pressure for a job that needs a gentle blast. It wastes power and adds a pressure vessel to guard.
Safety
A compressor is a machine that stores energy and cannot be stopped from where you stand
A piston compressor turns a flywheel, a belt and a crank, and the air it makes is held in a vessel that can burst. Put the stop where the operator can reach it without crossing the belt, learn how long the flywheel takes to coast to a halt by timing it, and fit a relief valve between the compressor and any closed valve so the compressor can never pump against a shut line. See Piston Compressor and Safety Devices.
What’s Next
- Learn next: Bellows — the simplest compressor, with nothing stored
- Combine with: Air Storage and Treatment — what the compressor feeds