How Not to Set Your Pizza on Fire - Crash Course Engineering #15

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Language: en

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Temperature matters.
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In almost everything we do, we’re trying
to heat something up, cool something down,
or just trying to maintain a temperature.
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As an engineer, you’ll often need to find
that Goldilocks temperature, the one that’s
“just right” for your devices and designs.
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But once you figure that out, how do you achieve it?
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Well, you’ll need some equipment, and to
learn how to use it.
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More specifically, you’ll need to know about
heat exchangers, and how they can affect
heat transfer.
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Just make sure to watch out for those three
bears.
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[Theme Music]
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So far in this course, we’ve learned a good
deal about heat transfer and the different ways
heat moves throughout our world.
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We’ve also talked a bit about the devices
that help move heat energy, like refrigerators
and heat pumps,
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and how you can slow down the transfer of
heat with layers of insulation.
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But that’s just the beginning of the ways
you can affect heat transfer!
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There are lots of different types of equipment
you can use to transfer heat between two things.
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They’re called heat exchangers, because
they exchange heat.
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But don’t let the simplicity of the name
fool you.
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Heat exchangers are everywhere.
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They show up as radiators in cars, where they
transfer heat energy away from the engine so
it doesn’t...overheat.
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You’ll also find them in military equipment
and power supplies.
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You can even find them in medical devices.
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Have you ever had an X-ray?
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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.
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Even when you create something amazing – something
that can literally see the bones under your skin – you
still have to account for its byproducts.
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Engineers can’t just make a good meal;
we have to clean up the kitchen, too.
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So heat exchangers are pretty important.
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Without them, there’s all kinds of stuff
we wouldn’t be able to do.
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And the type of heat exchanger you use is
even more important,
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because it’s not always as simple as heating
something up or cooling it down in any way
that you can.
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There’s a lot more to consider.
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For example, let’s say you want to heat
up your leftovers from last night.
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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.
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A much better choice would be a microwave.
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Maybe an oven.
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Or, say your tea is a little too hot to drink
and you want to cool it down.
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You could blast it with a firehose of cold
water, but that would likely ruin your tea,
and everything else around it.
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It would probably be better to just wait a
bit – maybe put your tea in a colder room
or leave it in the refrigerator.
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In engineering, you need tools and methods
that are more precise.
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Surgeons have their scalpels; we have heat
exchangers.
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So let’s look at the ones we’ve got!
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The first, and most basic example of a heat
exchanger is a concentric tube.
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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.
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This fluid might be a liquid, or it could
be a gas.
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A common place you’d find concentric tube
heat exchangers is inside air conditioners.
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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.
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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.
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That’s the heat transfer.
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In some concentric tubes, the fluids will flow in
the same direction – what’s known as parallel flow –
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and other times they’ll move in opposite
directions, which is called counterflow.
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Whichever way the fluid flows, You’ll probably
want to know just how good the heat transfer is.
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That’s the point of a heat exchanger after
all.
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There are two main equations for heat transfer
that you can use to figure this out.
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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:
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its mass flow rate, m, or how fast it’s
moving;
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its heat capacity, c, or how much heat you
need to raise the fluid’s temperature;
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and its change in temperature, ΔT, after
it passes through the heat exchanger.
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This equation tells you that no matter what,
if there’s a greater change in the fluid’s
temperature, there was more heat transfer.
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Which, obviously.
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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.
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Finally, it says that it takes more heat transfer
to accomplish a given temperature change in
a fluid that’s moving really fast.
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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.
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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.
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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.
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Meanwhile, all that heat being transferred to the
outer tube will cause a significant temperature
change, since it’s moving so slowly.
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Which is what we want!
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The whole point of a heat exchanger is to
accomplish that significant temperature change.
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Now, the other equation for heat transfer
also describes it in terms of three properties,
but it takes both fluids into account:
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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;
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second, there’s the area, A, over which
the heat transfers;
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and third, there’s the temperature difference,
ΔT, but this time between the two fluids.
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The heat transfer coefficient is actually the inverse
of the thermal resistance we discussed last time,
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so the larger the value for U, the less resistance
there is, allowing for more heat transfer.
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This equation also tells you there’s more
heat transferred when there’s a greater area
of contact between the two fluids.
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And no matter what the heat transfer coefficient
is or how much contact there is between the two fluids,
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a greater temperature change will always
involve more heat.
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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.
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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.
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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.
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But all of this leads to some inherent problems
with the simple concentric tube heat exchanger.
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If the temperature difference between the
fluids is the driving force,
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then the heat exchanger will need to have an
appropriate area and U value to achieve a reasonable
amount of heat transfer.
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There are two ways to increase that heat
transfer: increase the value of U, or increase
the value of the area.
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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.
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Which leaves you with only one real way to
increase the heat transfer: increase the area
of contact between the fluids.
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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.
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For one thing, the heat exchanger will take
up more space.
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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.
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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.
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So it’s worth looking at some other heat
exchanger designs too.
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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.
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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.
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There are two main types of finned tube designs.
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With axial fin structures, fins run along
the tube lengthwise.
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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.
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With radial fin structures, on the other hand,
discs are added to the tube and spaced out from
each other, usually in regular intervals.
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This type of finned design is best suited
for a faster-moving fluid like air to flow
around the tube.
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Another heat exchanger worth looking at is
the plate heat exchanger, which uses metal
plates to transfer heat between fluids.
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With these, the warmer fluid flows through
one port and the colder fluid flows through
another, typically in counterflow.
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Both fluids are restricted by seals so they
can only follow a certain path, kind of snaking
their way through the exchanger.
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The fluid between each set of plates alternates,
with the plates providing a large surface area
for a high rate of heat transfer.
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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.
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Now, both finned tubes and plate heat exchangers
are usually a step up from concentric tubes,
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but one of the most common heat exchangers is
the shell-and-tube design.
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You can find them practically anywhere, from
large oil refineries, to engines and transmissions,
and even in swimming pools.
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Like its name implies, a shell-and-tube heat
exchanger is made up of a larger shell with
a bundle of smaller tubes inside it.
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One fluid, usually the colder one, moves through
this series of tubes while another fluid flows
outside of them and through the shell.
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There would be large pockets of stagnant
shell-side fluid in the corners of the shell if
this design was left as-is, though.
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So you can put baffles, which are obstructing
vanes or panels, inside the shell to drive the
shell-side fluid through in a maze-like pattern.
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Baffles not only help to increase the overall
average heat transfer through the system by
directing the flow of the fluid,
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but also by increasing the shell-side velocity
and promoting turbulence.
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So, between concentric tubes, finned tubes, plates,
and shell-and-tube designs, you’ve got plenty of
options when you need to transfer heat.
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Which, among other things, means there’s
no need to set any pizza on fire.
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That would just be a travesty.
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Today we learned all about the different types
of heat exchangers and how they can be used
to transfer heat.
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We started off with concentric tubes , and
the two main equations that can help us define
heat transfer in heat exchangers.
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Then we flowed on over to finned tubes and
found the differences between axial or radial fins.
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Finally, we covered plate heat exchangers
and studied the most common heat
exchanger design: shell-and-tube.
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I’ll see you next time, when we’ll continue
on our journey and learn all about mass transfer.
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Crash Course Engineering is produced in association
with PBS Digital Studios.
00:10:05.840 --> 00:10:13.160
You can head over to their channel to check
out a playlist of their latest amazing shows, like
America from Scratch, Hot Mess, and Eons.
00:10:13.160 --> 00:10:20.420
Crash Course is a Complexly production and this
episode was filmed in the Doctor Cheryl C. Kinney
Studio with the help of these wonderful people.
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And our amazing graphics team is Thought Cafe.
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