For a more basic descriptions of trompes, please read our trompe overview.
The trompe–a sixteenth-century technology–uses falling water to collect pressurized air. But exactly how does a trompe work? Though simple to make, trompes use complicated physics. Below, we have compiled a basic explanation on the science of trompes and cited some of the numerical data scattered across the internet.
(Please note that the following is based solely on books, videos, and articles created by external sources. We have not yet conducted tests to verify their accuracy. )
How a Trompe Works
Let’s start with the physics of the air-intake, which works via the Venturi effect. When water first enters the trompe, it falls into a constricted pipe, reducing the pressure and creating suction. Air then rushes downward through the air-intake tubes to fill the vacancy.
The diameter of the air-intake tubes dictates the size of the air bubbles that enter the water. Bruce Leavitt found that thinner air-intake tubes make the trompe a more efficient aerator. A large trompe in Michigan used 11,800 tubes, 3/8-inches in diameters, on each of its three aerating cones.
The height of the water-intake pipe affects the pressure inside the air reservoir. Though hard numbers are hard to come, trompe discussions across the internet tend to use 14.5 psi : 10 m or 15 psi : 32 ft to estimate pressure output.
Historical (and more recent) records can also give us a good estimate on the air pressure created by a trompe. According to Bill Mollison’s video, the ratio of the water intake’s height to the air pressure inside the reservoir is about 25 ft : 14 psi (.56 psi/ft). According to Bruce Leavitt’s presentation, the trompe in Ragged Chutes created 128 psi with a 345-foot drop (.37 psi/ft). The trompe in Victoria Mine created 117 psi off of a 342-foot drop (.34 psi/ft).
Charles H Taylor’s hydraulic air compressor (a version of the trompe), he recorded that the air pressure int he reservoir increased 1 psi per every 2ft 3.5 inches of water displaced. He also noted that the air pressure was proportional to the height of the water column in the return pipe.
Next, let’s consider the quality of the captured air.
The air in the trompe undergoes isothermal compression, meaning the compression does not affect the captured air’s temperature. Normally when a gas is compressed, it heats up. However, in a trompe, the water encompassing the air bubbles removes the excess heat. This allows the compressed air to be stored in a smaller space and prevents overheating any machinery powered by the compressed air.
The pressurized air comes out cool (at the same temperature as the water) and dehumidified.
Once the water flows into the outtake pipe, pressure from the air reservoir pushes the water upward. According to Geoff Lawton, the returning water can be lifted to 85% of its original height. When the compacted version of a trompe is used, Mollison described the returning water as going back up in a whirl.
Trompes can easily self-regulate. If pressure inside the air chamber becomes too great, it will push the water level below the mouth of the outtake pipe. The air then escapes through the outtake pipe as bubbles, spewing out a geyser of water, steam, or ice in the discharge. A trompe in Michigan was recorded to have shot discharge up to 700 feet high.
The water intake pipe’s diameter needs to be smaller than that of the outtake pipe.
Trompes can provide ventilation, power pneumatic tools, aerate water, and much more. Though not much data is available on trompe performance, here are some numbers we have found through preliminary research. (We will update this article as more data is found.)
Geoff Lawton noted that trompes with different purposes need different sizes. If the trompe will aerate a fish pond, for example, the water needs about 3 feet of head. For collecting compressed air, the trompe needs a 100-foot-plus drop.
In Bill Mollison’s lectures, he talked about air-powered cars and how compressed air collected from trompes could run them. He said that a 2-cubit-foot tank of compressed air could drive a car for 200 miles. If a intensifier raised the pressure to about 1,000 psi, he said the car could travel 300 miles.
Though we are unsure how Bill Mollison’s calculations compare to modern automobile standards, we can use statistics from Motor Development International (MDI)–an emerging air-powered car company–as a gauge. MDI claims their Compressed Air Technology (CAT) car can run on 4,300 psi and travel 125 miles on a full tank. They say their city version can reach 70 mph. They also have hybrid models that use both gasoline and pressurized air.
This article will be updated in the future. If you find anything that we failed to include, feel free to let us know.