Name: Why Are Cooling Towers Shaped Like That?
Uploaded: 2025-07-07T14:57:38.000Z
Duration: 19 min 48 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

This is not smoke, and this isn't a smokestack, at least not the kind we normally think of.

Adverbs of degree

It serves a totally different purpose at a power plant than smokestacks, whose job is moving combustion products high into the air, allowing them to disperse away from populated spaces.

Maybe you already knew that, or at least suspected it, after all, you saw the title of this video.

Plus, this kind of tower is commonly associated with nuclear plants that don't combust anything at all to create the heat that drives their generators.

The largest class of power plants, called thermal power stations, use steam turbines, or turbines, depending on how you say it.

But once that steam makes it through the turbine, it needs to be condensed back into liquid water.

You spend all those resources heating the water up, and then you spend even more resources to cool it back down.

And a power plant isn't much good if all the electricity it generates gets used up just trying to cool that steam back down.

So engineers have come up with some pretty creative ways to cool huge amounts of water, like millions of gallons or tens of thousands of cubic meters per hour, and do it relatively efficiently.

Not all cooling towers look like this, but there are some really clever reasons for that iconic shape we all recognize, and I'm going to build one in the garage to show you how they work.

I'm Grady, and this is Practical Engineering.  Power plants could just vent steam into the atmosphere, but generally, they don't do that.

For one, it wouldn't be good for the environment.

The heat, moisture, and noise would affect wildlife and weather.

The feedwater for a boiler is often carefully treated to avoid corrosion and mineral buildup in the machinery.

It's expensive water, so it doesn't make sense to set it free.

And for three, it would waste a lot of energy.

It's generally less expensive to cool the steam down just enough to condense it back into liquid water so it can be reused as feedwater.

I talked a little bit about how power plants actually consume a lot of energy in a previous video.

It's a net positive, of course, but any energy you spend on all the industrial processes required to produce electricity at scale is energy not being sent out to the grid.

One simple option is to use a cooler stream of water that already exists, like a river, lake, or sea.

And in fact, there are a lot of power plants near me that do exactly that.

This plant draws water from the lake on the south side, sends it through condensers, and then releases it into a channel where it flows to the north side of the lake.

The slow circulation gives that water time to cool down before it reaches the plant again.

But it's not feasible to have a lake or river for cooling water at every thermal power plant, and there are a lot of environmental impacts with the heat and intakes that require careful consideration.

So instead, lots of power plants use cooling towers.

You might be familiar with the various machines humans have devised for cooling stuff down.

You might even be enjoying the comfort of such a device right this very minute.

But like I mentioned, the simplest way to cool something is to simply let natural physical processes do the work.

After all, the temperature is usually less than boiling outside, so the heat from steam will naturally transfer to the ambient air if you let it.

I designed a cooling tower in my garage so I can show you exactly how this works.

This is made from laser-cut strips of acrylic with a carefully selected shape.

And when I carefully tape these carefully sized strips together, I get a nice, somewhat transparent cooling tower.

This is a model of a natural draft tower.

It's not the most common type out there, but it is one of the simplest and also the most iconic and recognizable, so it's perfect for this demonstration.

You may have noticed the holes I drilled in the bottom of each acrylic strip.

This tower needs a way for air to get inside at the bottom.

If you look closely at the real thing, you'll see something similar.

They aren't continuous all the way down, but actually open around the bottom.

Steam from turbines doesn't go to the cooling tower directly.

Instead, there's a separate stream of water, aptly called the cooling water.

The steam is condensed into liquid water in the condenser that's cooled by the cooling water, which then flows between the condenser and the tower.

I'm simulating that here with a bucket of hot water and a beer brewing pump.

That hot water gets pumped to sprayers inside the tower.

If you know a little bit about thermodynamics, you know that we can only get this water as cool as the ambient air temperature.

Heat naturally flows from hot to cold, so you can't get any more heat transfer once the water reaches the outside temperature.

But if you know a little bit more about thermodynamics, you know there's a trick that can improve the performance of a system like this.

And this layer of material below the sprayers is the key.

I'm just using a dehumidifier pad, but in an actual cooling tower, the fill is usually a layer of plastic carefully designed to maximize the surface area of the water in the system.

It does this by forcing the water to either splash into tiny drops or form thin sheets as it falls downward.

The goal is to expose as much surface area of the hot water as possible to the air flowing through the tower.

The pros call this counterflow, and it's the trick to the whole process.

Actually, you can use crossflow as well, but let's jump over that rabbit hole for now.

You might think the outside air has just one temperature, but to cooling professionals it has two.

One we call the dry bulb temperature is what you normally encounter.

That's what shows up in the weather report.

If you soak the end of a thermometer and pass it through the air, that water will evaporate.

The drier the air, the more easily water evaporates.

This is why it feels so much hotter when it's also humid outside.

It takes energy, called latent heat, to convert water from a liquid to a gas, and that energy is absorbed from the liquid water, cooling it down.

So as long as the ambient air isn't already saturated, 100% relative humidity, you can actually cool water below the dry bulb temperature using evaporation.

And the lower the humidity of the air, the more evaporation can take place, so the bigger the difference in wet and dry bulb temperatures.

Nature figured out evaporative cooling millions of years ago.

But using it at this scale is really impressive.

And it's not the only innovation in a natural draft cooling tower.

For that, I need to show you a cool graph.

You could spend an entire college course learning about this stuff, and there are probably a few HVAC professionals groaning at the screen right now.

But I just want to use it to explain a few important things about the physical and thermal properties of air.

First, as the temperature of air goes up, its capacity to hold water goes up too.

Kind of like hot tea can dissolve more sugar than cold tea, hot air can hold more water than cold air.

Next, as air temperature goes up, its density goes down.

Confusingly, the psychrometric chart actually shows specific volume, which is the inverse of air density.

So as you move up in temperature, these lines slope downward.

But maybe less intuitively, it's also true for humidity.

If you hold the temperature constant and just increase the amount of water in the air, its density goes down.

Water molecules actually weigh less than the nitrogen or oxygen molecules in the air, so humid air is more buoyant than dry air.

And this is the second key to a cooling tower, convection.

The hot water transfers its heat to the air.

The warm air becomes buoyant, flowing upward in the tower and drawing fresh air in through the intakes.

But some of that hot water is also evaporated, removing more heat from the water and making the air even more buoyant.

The process both cools the water down and creates a natural draft up through the tower, drawing in even more fresh, drier air as it does.

Ignoring the pumps and minor control features, there are no moving parts.

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Why Are Cooling Towers Shaped Like That?

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