A trompe compresses air with falling water and no moving parts, but the pressure it reaches is capped by the height of the fall: about 1 bar for 10 m, which is 98.1 kPa. That is enough for a forge or furnace blast and far too little for air tools, which want about 6 bar and so roughly 61 m of fall.
Part of Pneumatic Systems
A trompe is a water-driven air pump: water falling down a pipe drags air in, carries it to a sealed chamber, and releases it there under the pressure of the water column.
The Fall Sets the Pressure
Pressure from a column of water is density x gravity x height. For a 10 m fall:
- 1,000 kg/m³ x 9.81 m/s² x 10 m = 98,100 Pa = 98.1 kPa, just under 1 bar
The corpus’s head-calculation page uses the same 9,810 Pa per metre and the same 10.2 m per bar (Head Calculation). So the air in the chamber cannot be at more than about 1 bar gauge for each 10 m of fall, and real units reach less. A tool at 6 bar gauge would need 6 x 10.2 = 61 m of fall. The carrier’s tip to build a trompe wherever there is a 5 m fall is right for a blast, but a 5 m trompe (about 0.5 bar) will not run a drill or hammer.
How the Trompe Works
- Water falls down a vertical pipe, 3-10 m tall per the carrier.
- Small holes near the top let the falling water draw air in.
- At the bottom the pipe enters a sealed chamber; the water strikes a plate or slows in the chamber and the air separates.
- Water leaves through a low outlet while air collects above it under pressure.
- An air pipe at the top of the chamber carries the compressed air away.
What You Can Expect
The water power available is density x gravity x flow x height. Take 5 L/s falling 10 m: 1,000 x 9.81 x 0.005 x 10 = 490 W. The most air that could be compressed to 1 bar gauge with 490 W, if every watt went into the air, is 490 / 70,233 J per m³ = 0.00698 m³/s, about 419 L/min, using the isothermal work per cubic metre of free air at a pressure ratio of 2 (101,325 x ln 2). Real output is a fraction of that. No efficiency figure for a trompe is sourced here, so measure yours by filling a known volume and timing it.
The air is not dry. The carrier calls trompe air moisture-free. It leaves the chamber saturated with water vapour, and spray carries droplets. Because it is held at higher pressure, it holds less water per cubic metre of free air than ambient air does: air saturated at 10 °C at 2 bar absolute, once expanded to atmospheric pressure, would be at only about half saturation. It is cool and fairly dry once it expands, but the chamber, the pipe and the tank still need a drain, as in Moisture Removal.
Where It Fits
- Forge or furnace blast: a good fit. Steady, cool air with no electricity or fuel, with a valve or regulator to lower the pressure.
- Ventilation or low-pressure air for a workshop: a good fit.
- Air tools or spray equipment: not without a fall of tens of metres.
Common Mistakes
- Expecting tool pressure from a few metres of fall. The pressure cannot exceed the fall.
- Letting the chamber flood or drain. If the water level in the chamber rises to the air outlet it carries water into the line, and if it falls to the water outlet air escapes down the tailrace.
- Treating the air as dry. It is saturated at the chamber; drain it.
Safety
Falling water and a sealed chamber
A 3-10 m vertical pipe of falling water and a sealed chamber hold a lot of weight and some pressure. Keep the pipe supported and fenced, stay off a wet ladder or platform beside it, and fit a relief valve on the chamber. Even 1 bar gauge in a 100 L tank holds 1 x 10⁵ Pa x 0.1 m³ = 10,000 J, comparable to the 4,000 J the corpus treats as dangerous in a tire (Tire Basics), so do not stand over or lean on a pressurised vessel, and do not store the air in a vessel of unknown strength.
What’s Next
- Learn next: Air Storage and Treatment — draining and holding the air
- Combine with: Bellows — what a trompe usually replaces