In Paris they were used for a time to compress air to drive the city's first electricity generation scheme, and in the Alps they were used in France and Switzerland to provide compressed air for early alpine tunnels.
Trompes can be enormous. At Canadian Hydro Developers' Ragged Chute facility in New Liskeard, Ontario, water falls down a shaft 351 feet (107 m) deep and 9 ft (2.7 m) across to generate compressed air for mining equipment and ventilation.
Trompes are very simple devices. A vertical pipe or shaft goes down to a separation chamber, a pipe coming away from that chamber allows the water to exit at a lower level, and another pipe coming from the chamber allows the compressed air to exit as needed.
Water rushing down the vertical pipe falls through a constriction. The constriction produces a lower pressure because of the venturi effect, and an external port allows air to be sucked in. The air forms bubbles in the pipe. As the bubbles go down the pipe they are pressurized proportionally to the hydraulic head, which is the height of the column of water in the pipe. The compressed air rises to the top of the separation chamber. The separation chamber has a compressed-air takeoff pipe, and the compressed air can be used as a power source.
The energy of the falling water entrains the air into the water, but that is not the energy that pressurizes the air, as is often incorrectly claimed. That energy is solely a derivative of the hydraulic head.
Large trompes were often situated at high waterfalls so that plenty of power was available. The Ragged Chute plant on the Montreal River near the town of Cobalt, Ontario, is a trompe and tourist attraction. It is now owned by Canadian Hydro and exists beside a modern hydroelectric plant.
Compressed air from a trompe is at the temperature of the water, and its partial pressure of water vapor is that of the dewpoint of the water's temperature. If the water is cool, the compressed air can be made very dry by passing it through pipes that are warmer than the water. Often, ordinary outside air can warm the pipes enough to produce very dry, cool compressed air.