Name: How Not to Set Your Pizza on Fire: Crash Course Engineering #15
Uploaded: 2020-03-30T03:34:33.000Z
Duration: 10 min 39 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...

In almost everything we do, we're trying to heat something up, cool something down, or just trying to maintain a temperature.

Relative clauses (Restrictive vs. nonrestrictive)

As an engineer, you'll often need to find that Goldilocks temperature, the one that's “just right” for your devices and designs.

But once you figure that out, how do you achieve it?

Well, you'll need some equipment, and to learn how to use it.

More specifically, you'll need to know about heat exchangers, and how they can affect heat transfer.

Just make sure to watch out for those three bears.

So far in this course, we've learned a good deal about heat transfer and the different ways heat moves throughout our world.

We've also talked a bit about the devices that help move heat energy, like refrigerators and heat pumps,

and how you can slow down the transfer of heat with layers of insulation.

But that's just the beginning of the ways you can affect heat transfer!

There are lots of different types of equipment you can use to transfer heat between two things.

They're called heat exchangers, because they exchange heat.

But don't let the simplicity of the name fool you.

They show up as radiators in cars, where they transfer heat energy away from the engine so it doesn't...overheat.

You'll also find them in military equipment and power supplies.

You can even find them in medical devices.

Well, X-rays actually produce a large amount of heat, so they need heat exchangers to draw that heat away and keep it from damaging the equipment.

Even when you create something amazing – something that can literally see the bones under your skin – you still have to account for its byproducts.

Engineers can't just make a good meal; we have to clean up the kitchen, too.

Without them, there's all kinds of stuff we wouldn't be able to do.

And the type of heat exchanger you use is even more important,

because it's not always as simple as heating something up or cooling it down in any way that you can.

For example, let's say you want to heat up your leftovers from last night.

Technically, you could do that by setting your pizza on fire, but unless you'd like your crust extra crispy, that seems a bit extreme.

A much better choice would be a microwave.

Or, say your tea is a little too hot to drink and you want to cool it down.

You could blast it with a firehose of cold water, but that would likely ruin your tea, and everything else around it.

It would probably be better to just wait a bit – maybe put your tea in a colder room or leave it in the refrigerator.

In engineering, you need tools and methods that are more precise.

Surgeons have their scalpels; we have heat exchangers.

The first, and most basic example of a heat exchanger is a concentric tube.

Here, one pipe or tube is placed inside another one, with a colder fluid moving through the center tube, and a warmer fluid moving through the outer tube.

This fluid might be a liquid, or it could be a gas.

A common place you'd find concentric tube heat exchangers is inside air conditioners.

With concentric tubes, and in most heat exchangers you'll encounter, it's important to note that the two fluids are sealed off from each other and never mix.

But as the fluids move down their separate tubes, energy transfers from the hotter outer fluid to the colder inner fluid through the wall of the inner tube.

In some concentric tubes, the fluids will flow in the same direction – what's known as parallel flow –

and other times they'll move in opposite directions, which is called counterflow.

Whichever way the fluid flows, You'll probably want to know just how good the heat transfer is.

That's the point of a heat exchanger after all.

There are two main equations for heat transfer that you can use to figure this out.

The first looks at each fluid individually, and defines heat transfer – represented with the letter Q – as the product of three of the chosen fluid's properties:

its mass flow rate, m, or how fast it's moving;

its heat capacity, c, or how much heat you need to raise the fluid's temperature;

and its change in temperature, ΔT, after it passes through the heat exchanger.

This equation tells you that no matter what, if there's a greater change in the fluid's temperature, there was more heat transfer.

It also tells you that there's more heat transfer if it's a type of fluid that just generally needs more heat to raise its temperature.

Finally, it says that it takes more heat transfer to accomplish a given temperature change in a fluid that's moving really fast.

If each particle of fluid isn't staying in the heat exchanger for very long, you need more heat transfer to raise its temperature quickly, before it leaves.

Let's say you have a heat exchanger with the colder fluid in the inner tube moving at a high flow rate and the warmer fluid in the outer tube moving slowly.

Even if there's plenty of heat being transferred, you might not get a major temperature increase in the inner tube since it has such a high flow rate.

Meanwhile, all that heat being transferred to the outer tube will cause a significant temperature change, since it's moving so slowly.

The whole point of a heat exchanger is to accomplish that significant temperature change.

Now, the other equation for heat transfer also describes it in terms of three properties, but it takes both fluids into account:

The first property is the heat transfer coefficient, U, which is a measure of how easily heat is transferred between the fluids through whatever is separating them;

second, there's the area, A, over which the heat transfers;

and third, there's the temperature difference, ΔT, but this time between the two fluids.

The heat transfer coefficient is actually the inverse of the thermal resistance we discussed last time,

so the larger the value for U, the less resistance there is, allowing for more heat transfer.

This equation also tells you there's more heat transferred when there's a greater area of contact between the two fluids.

And no matter what the heat transfer coefficient is or how much contact there is between the two fluids,

a greater temperature change will always involve more heat.

You can use these two different ways of defining heat transfer to change your operating conditions as necessary and get the heat transfer you need.

In the design of the heat exchanger, you can affect the heat transfer through the heat transfer coefficient and the area of contact between the fluids.

And while the heat exchanger is up and running, you can affect its heat transfer by the temperature differences between the fluids and their mass flow rates.

But all of this leads to some inherent problems with the simple concentric tube heat exchanger.

If the temperature difference between the fluids is the driving force,

then the heat exchanger will need to have an appropriate area and U value to achieve a reasonable amount of heat transfer.

There are two ways to increase that heat transfer: increase the value of U, or increase the value of the area.

You could increase the heat transfer coefficient by using more conductive pipes or making them thinner, but at a certain point you'll hit a physical limit.

Which leaves you with only one real way to increase the heat transfer: increase the area of contact between the fluids.

For a concentric tube design, the only way to increase the area is either by the pipe's radius or length, which isn't too practical.

For one thing, the heat exchanger will take up more space.

And it's going to increase not only the cost of the building materials, but the operating cost of any pumps pushing the fluid through the device as well.

If we only used concentric tubes in our designs, we'd need more space under the hoods of our cars and our X-ray machines would be even bigger and clunkier.

So it's worth looking at some other heat exchanger designs too.

Take finned tubes, for example, which you'll often find in industrial applications like power plants, industrial dryers, and in the air conditioning units of large buildings.

In these designs, fins are added to a tube to increase its surface area, which enhances its rate of heat transfer at the same time.

There are two main types of finned tube designs.

With axial fin structures, fins run along the tube lengthwise.

They're best suited for devices where fluid flow outside of the tube is slower and more viscous, like oil, but you still want it to distribute a greater amount of energy.

With radial fin structures, on the other hand, discs are added to the tube and spaced out from each other, usually in regular intervals.

This type of finned design is best suited for a faster-moving fluid like air to flow around the tube.

Another heat exchanger worth looking at is the plate heat exchanger, which uses metal plates to transfer heat between fluids.

With these, the warmer fluid flows through one port and the colder fluid flows through another, typically in counterflow.

Both fluids are restricted by seals so they can only follow a certain path, kind of snaking their way through the exchanger.

The fluid between each set of plates alternates, with the plates providing a large surface area for a high rate of heat transfer.

So, plate heat exchangers would be a little better than concentric tubes for something like an X-ray machine, since it produces a lot of heat you'd want to get rid of.

Now, both finned tubes and plate heat exchangers are usually a step up from concentric tubes,