Harness Hydro Power with a Trompe

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ILLUSTRATION: MOTHER EARTH NEWS STAFF
This diagram shows the inner workings of a trompe and "blow-off" pipe.

The trompe. Its use dates back to the beginning of the Iron
Age, and yet — like many good ideas involving the
manufacture of power — the trompe concept has been all
but forgotten in the recent stampede to mine, refine, and
consume readily exploitable supplies of fossil fuels.

For the homesteader or farmer with a small waterfall or a
good-sized stream on his property, the trompe is a natural.
It offers a virtually inexhaustible supply of free
compressed air … cool, dry air that can be
used to operate a forge, drive machinery, or aircondition a
house or barn in hot weather.

What exactly is a trompe? Very simply, a trompe
(sometimes spelled trombe) is a device that uses
the energy of falling water to pressurize air. This
pressurization is achieved by means of a standpipe or shaft
down which a column of water is allowed to fall. As it
drops, the water draws air through small inclined orifices
(see the accompanying diagram) and carries it to a
submerged plenum or reservoir, where the air separates from
the water and is held under pressure. (The
water — meanwhile — continues to flow to an exit
pipe, the end of which is high enough to balance the
pressure in the reservoir.) The pressurized air can then be
drawn off through a tuyere — or escape nozzle — to
be used as needed.

Many large-scale trompes — or hydraulic air compression
plants — were built at the turn of the century to
supply mines with fresh air. One of the biggest of
these — and probably the last one still in use — is
the Ragged Chutes plant on the Montreal River near the town
of Cobalt, in northern Ontario’s silver mining country.

At Ragged Chutes, water falls down a shaft 351 feet deep
and nine feet across to generate the compressed air that
supplies the area’s mines. The Ontario Hydro Electric
Commission engineers who operate the plant are said to view
the giant trompe with some disdain, since — except for
a simple water-flow control — it has no moving parts,
relies on no computers, makes no noise, and doesn’t pollute
the environment. So far, however, the mining companies have
successfully resisted attempts to have the plant replaced
with something more “modern.”

One remarkable feature of trompes (the Ragged Chutes plant
included) is that the air that comes out of the
system is actually cooler and drier than the air that goes
in. The air comes out cooler because the cold
water flowing through the trompe absorbs the heat that’s
usually generated by the compression of air. It comes out
drier because the atmospheric moisture held in the
air bubbles that flow through the system condenses — so
to speak — on the bubbles’ walls (since those walls
are, after all, colder than the air they’re surrounding).
The result: Compressed air that’s the same temperature
as the cool water it just left and drier than it was
when it entered the trompe. Free air conditioning!

The Ragged Chutes plant employs a battery of 72
14-inch-diameter pipes to mix the air and water as they
fall into the inflow shaft, but the builder of a
small-scale trompe should be able to achieve the same
result with only one or two such tubes. Here are a couple
of things to remember if you decide to jury-rig your own
hydraulic air compressor:

  1. If you want to be able to switch the air off but leave
    water running through the device, build a “blow-off” pipe
    (see diagram), the lower end of which is located slightly
    below the normal water level in the reservoir. This way,
    when you block the airflow through the discharge line, the
    air pressure in the reservoir will increase to the point
    where the water level drops and surplus air can escape
    through the blowoff pipe. (At Ragged Chutes, the blowoff
    sends a mixture of water and air 100 feet into the sky.)
  2. When you first start up your trompe, don’t let any air
    escape through the discharge line until the pressure has
    had a chance to build. Otherwise you’ll end up with a
    plenum full of water and no compressed air.

The trompe: a mighty old idea … and a mighty good one,
too.