This is the Ragged Chute hydropower plant on the Montreal River in eastern Ontario,
Canada. It doesn’t look like much from an aerial photo, but that’s because the most
interesting parts of this facility are underground: two massive vertical shafts and large tunnel
connecting the two. Before it was converted to generate electricity, Ragged Chute was
one of the world’s only water-powered compressed air plants. Starting around 1910, this plant
sold compressed air to be used in the silver mines around Cobalt, Ontario. The way this
ingenious facility harnessed the power of water to generate compressed air with no moving
parts is fascinating and its use is seeing a small revival in modern days. Hey I’m
Grady and this is Practical Engineering. On today’s episode we’re talking about the trompe.
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Compressed air is an excellent way to store and transport energy. It’s not quite as
convenient as electricity for homes and businesses, which is why you don’t see air lines strung
on poles throughout cities, but in certain situations it makes a lot of sense. This is
particularly true in mines, where a variety of tools and equipment need a consistent and
safe source of power. But it’s not just pneumatic tools; Pretty much every step of
the mining process - including exploration, blasting, ventilation, smelting, and refining
- makes use of compressed air as a source of power. It’s reliable, simple, easy to
transport, and often safer than the other options because it doesn’t have the risk
of sparks or explosions that come with electricity or diesel.
We normally get compressed air from... a compressor, a device that does exactly what you’d expect:
uses a mechanism to take outside air and squish it into a tank. But, air compressors had a
major disadvantage to the mining professionals working in the early 20th century: they didn’t
exist (at least not ones that were commercially available). Also, a compressor is just an
energy converter. It takes one type of energy (usually rotational kinetic energy from a
diesel or electric motor) and converts into potential energy stored in pressurized air.
You still need a source of power. So, to be able to operate a mine using compressed air
back in the day would have required both maintaining a separate source of power and a complicated
and custom piece of machinery just to keep the tools and equipment running.
You can imagine how valuable it would be to be able to take advantage of a natural source
of power - falling water - and avoid the need for complicated machinery and moving parts.
That’s exactly what a trompe provided, and I’ve built a miniature version of one so
I can show you how it works. And of course it’s made of clear pipe so we can see exactly
what’s going on inside. The first step is the water supply. Just like hydroelectric
facilities, the amount of hydraulic energy you can convert to compressed air is based
on both the height and flow rate available. In my case, I’m using a garden hose, but
most trompes built for mines or forges took advantage of small streams or rivers.
As the water enters the first vertical shaft it passes by a series of air inlets. Because
of the water’s velocity as it travels down the shaft, the pressure at these inlets goes
below atmospheric. So, the trompe “sucks” air from outside into this vertical shaft
to join the water. The turbulence and surface tension of the flowing water entrains these
bubbles of air and carries them to the bottom of the shaft. This type of interaction between
flowing water and air is fairly complicated to characterize, and there are lots of situations
in engineering where air-water interaction can cause major problems like in spillways,
control gates, and pipelines. But, in a trompe, this is absolutely essential.
Once the air-water mixture reaches the bottom of the shaft, it enters a horizontal chamber.
The purpose of this chamber is to separate the air and water. The turbulence and velocity
are reduced, allowing the entrained bubbles to rise upwards. This air gets trapped in
the collection system while the water continues out the other side of the chamber and upwards
into the second vertical shaft. The purpose of this shaft is to give the water a way out
while leaving the air behind. The height of this shaft also determines the pressure of
the trapped air. I have a video on this topic if you want more detail, but the summary is
that the pressure in a body of fluid doesn’t depend on the volume, just the depth. So a
simple riser like my second pipe here is enough to hold pressure on the air in the collection
system, compressing it just like a mechanical compressor would. It’s pretty satisfying
to see it work. I could watch this all day.
Once enough air has collected in the system, I can open the valve to use it. I should say
that this is a scale demonstration, so it doesn’t do anything of significant value
unless you have a really tiny nail gun or air drill.
It's pretty satisfying to see it work. I could watch this all day.
One of the benefits of a trompe over a more traditional air compressor is related to temperature.
In technical terms, a compressor uses an adiabatic process where a trompe compresses air isothermally.
But there’s no need to get caught up in the vocabulary. If you’re familiar with
the behavior of gases, you know that (all other things staying the same) if you compress
a gas, it gets hot. And the hotter the air, the more moisture it can hold. If you’re
familiar with air tools, or just corrosion in general, you know that moisture is one
of a tool’s worst enemies. In a trompe, however, that heat of compression gets absorbed
by the water. So you end up with a much cooler and dryer source of compressed air, which
by the way, is the definition of conditioning air, something I pay dearly for here in San
Antonio, and I’m sure those miners in Canada appreciated as well.
I’m definitely not going to be powering any of my shop tools with my little demonstration
here, and it wouldn’t be a very efficient way to do it, even if I could. If you’ve
got grid power available, it makes sense to use a compressor designed to take advantage
of that. But sometimes you don’t. A trompe can be useful in off-grid aquaponics and hydroponic
systems that need aeration of the water. And, in fact, the design of my demonstration here
came from the late Bruce Leavitt, a mining engineer who pioneered the use of small trompes
for aeration and treatment of mining water in remote locations without access to electricity.
I love to see examples of ancient technology finding new uses in our modern world. Especially
in an age where renewable sources of energy are at the top of our minds, the trompe is
a really cool way to harvest the power of water for beneficial use.
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