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How Not to Set Your Pizza on Fire - Crash Course Engineering #15
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00:00:03.100 --> 00:00:04.260 Temperature matters. 00:00:04.260 --> 00:00:10.160 In almost everything we do, we’re trying to heat something up, cool something down, or just trying to maintain a temperature. 00:00:10.160 --> 00:00:16.140 As an engineer, you’ll often need to find that Goldilocks temperature, the one that’s “just right” for your devices and designs. 00:00:16.140 --> 00:00:18.720 But once you figure that out, how do you achieve it? 00:00:18.720 --> 00:00:21.800 Well, you’ll need some equipment, and to learn how to use it. 00:00:21.800 --> 00:00:27.240 More specifically, you’ll need to know about heat exchangers, and how they can affect heat transfer. 00:00:27.240 --> 00:00:30.020 Just make sure to watch out for those three bears. 00:00:30.020 --> 00:00:40.220 [Theme Music] 00:00:40.220 --> 00:00:46.000 So far in this course, we’ve learned a good deal about heat transfer and the different ways heat moves throughout our world. 00:00:46.000 --> 00:00:51.620 We’ve also talked a bit about the devices that help move heat energy, like refrigerators and heat pumps, 00:00:51.620 --> 00:00:54.680 and how you can slow down the transfer of heat with layers of insulation. 00:00:54.680 --> 00:00:57.920 But that’s just the beginning of the ways you can affect heat transfer! 00:00:57.920 --> 00:01:02.440 There are lots of different types of equipment you can use to transfer heat between two things. 00:01:02.450 --> 00:01:06.200 They’re called heat exchangers, because they exchange heat. 00:01:06.200 --> 00:01:08.860 But don’t let the simplicity of the name fool you. 00:01:08.860 --> 00:01:10.700 Heat exchangers are everywhere. 00:01:10.700 --> 00:01:16.840 They show up as radiators in cars, where they transfer heat energy away from the engine so it doesn’t...overheat. 00:01:16.840 --> 00:01:20.119 You’ll also find them in military equipment and power supplies. 00:01:20.119 --> 00:01:22.020 You can even find them in medical devices. 00:01:22.020 --> 00:01:23.280 Have you ever had an X-ray? 00:01:23.280 --> 00:01:31.000 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. 00:01:31.000 --> 00:01:37.980 Even when you create something amazing – something that can literally see the bones under your skin – you still have to account for its byproducts. 00:01:37.980 --> 00:01:42.000 Engineers can’t just make a good meal; we have to clean up the kitchen, too. 00:01:42.000 --> 00:01:44.260 So heat exchangers are pretty important. 00:01:44.260 --> 00:01:47.080 Without them, there’s all kinds of stuff we wouldn’t be able to do. 00:01:47.080 --> 00:01:50.460 And the type of heat exchanger you use is even more important, 00:01:50.460 --> 00:01:55.320 because it’s not always as simple as heating something up or cooling it down in any way that you can. 00:01:55.320 --> 00:01:56.860 There’s a lot more to consider. 00:01:56.860 --> 00:02:00.880 For example, let’s say you want to heat up your leftovers from last night. 00:02:00.880 --> 00:02:09.140 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. 00:02:09.140 --> 00:02:11.360 A much better choice would be a microwave. 00:02:11.360 --> 00:02:12.340 Maybe an oven. 00:02:12.340 --> 00:02:16.060 Or, say your tea is a little too hot to drink and you want to cool it down. 00:02:16.060 --> 00:02:22.360 You could blast it with a firehose of cold water, but that would likely ruin your tea, and everything else around it. 00:02:22.360 --> 00:02:27.790 It would probably be better to just wait a bit – maybe put your tea in a colder room or leave it in the refrigerator. 00:02:27.790 --> 00:02:31.040 In engineering, you need tools and methods that are more precise. 00:02:31.040 --> 00:02:34.020 Surgeons have their scalpels; we have heat exchangers. 00:02:34.020 --> 00:02:35.620 So let’s look at the ones we’ve got! 00:02:35.620 --> 00:02:39.720 The first, and most basic example of a heat exchanger is a concentric tube. 00:02:39.720 --> 00:02:48.320 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. 00:02:48.320 --> 00:02:51.000 This fluid might be a liquid, or it could be a gas. 00:02:51.010 --> 00:02:55.870 A common place you’d find concentric tube heat exchangers is inside air conditioners. 00:02:55.870 --> 00:03:04.500 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. 00:03:04.500 --> 00:03:13.980 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. 00:03:13.980 --> 00:03:15.500 That’s the heat transfer. 00:03:15.500 --> 00:03:20.920 In some concentric tubes, the fluids will flow in the same direction – what’s known as parallel flow – 00:03:20.920 --> 00:03:25.060 and other times they’ll move in opposite directions, which is called counterflow. 00:03:25.060 --> 00:03:29.340 Whichever way the fluid flows, You’ll probably want to know just how good the heat transfer is. 00:03:29.340 --> 00:03:31.500 That’s the point of a heat exchanger after all. 00:03:31.500 --> 00:03:34.920 There are two main equations for heat transfer that you can use to figure this out. 00:03:34.920 --> 00:03:43.220 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: 00:03:43.220 --> 00:03:46.280 its mass flow rate, m, or how fast it’s moving; 00:03:46.280 --> 00:03:50.680 its heat capacity, c, or how much heat you need to raise the fluid’s temperature; 00:03:50.680 --> 00:03:54.960 and its change in temperature, ΔT, after it passes through the heat exchanger. 00:03:54.960 --> 00:04:01.140 This equation tells you that no matter what, if there’s a greater change in the fluid’s temperature, there was more heat transfer. 00:04:01.140 --> 00:04:02.540 Which, obviously. 00:04:02.540 --> 00:04:09.430 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. 00:04:09.430 --> 00:04:15.520 Finally, it says that it takes more heat transfer to accomplish a given temperature change in a fluid that’s moving really fast. 00:04:15.520 --> 00:04:23.320 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. 00:04:23.320 --> 00:04:31.220 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. 00:04:31.220 --> 00:04:38.400 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. 00:04:38.400 --> 00:04:45.380 Meanwhile, all that heat being transferred to the outer tube will cause a significant temperature change, since it’s moving so slowly. 00:04:45.380 --> 00:04:46.900 Which is what we want! 00:04:46.900 --> 00:04:50.880 The whole point of a heat exchanger is to accomplish that significant temperature change. 00:04:50.880 --> 00:04:57.910 Now, the other equation for heat transfer also describes it in terms of three properties, but it takes both fluids into account: 00:04:57.910 --> 00:05:06.460 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; 00:05:06.460 --> 00:05:10.120 second, there’s the area, A, over which the heat transfers; 00:05:10.120 --> 00:05:15.520 and third, there’s the temperature difference, ΔT, but this time between the two fluids. 00:05:15.520 --> 00:05:20.680 The heat transfer coefficient is actually the inverse of the thermal resistance we discussed last time, 00:05:20.680 --> 00:05:26.000 so the larger the value for U, the less resistance there is, allowing for more heat transfer. 00:05:26.000 --> 00:05:32.380 This equation also tells you there’s more heat transferred when there’s a greater area of contact between the two fluids. 00:05:32.380 --> 00:05:37.600 And no matter what the heat transfer coefficient is or how much contact there is between the two fluids, 00:05:37.600 --> 00:05:41.120 a greater temperature change will always involve more heat. 00:05:41.120 --> 00:05:48.460 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. 00:05:48.460 --> 00:05:55.860 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. 00:05:55.860 --> 00:06:03.290 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. 00:06:03.290 --> 00:06:08.000 But all of this leads to some inherent problems with the simple concentric tube heat exchanger. 00:06:08.000 --> 00:06:11.220 If the temperature difference between the fluids is the driving force, 00:06:11.220 --> 00:06:17.500 then the heat exchanger will need to have an appropriate area and U value to achieve a reasonable amount of heat transfer. 00:06:17.500 --> 00:06:23.560 There are two ways to increase that heat transfer: increase the value of U, or increase the value of the area. 00:06:23.560 --> 00:06:31.660 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. 00:06:31.660 --> 00:06:38.240 Which leaves you with only one real way to increase the heat transfer: increase the area of contact between the fluids. 00:06:38.240 --> 00:06:45.460 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. 00:06:45.460 --> 00:06:48.400 For one thing, the heat exchanger will take up more space. 00:06:48.400 --> 00:06:55.580 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. 00:06:55.580 --> 00:07:04.120 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. 00:07:04.120 --> 00:07:06.900 So it’s worth looking at some other heat exchanger designs too. 00:07:06.900 --> 00:07:16.140 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. 00:07:16.140 --> 00:07:22.700 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. 00:07:22.700 --> 00:07:25.080 There are two main types of finned tube designs. 00:07:25.080 --> 00:07:28.780 With axial fin structures, fins run along the tube lengthwise. 00:07:28.780 --> 00:07:38.000 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. 00:07:38.000 --> 00:07:45.400 With radial fin structures, on the other hand, discs are added to the tube and spaced out from each other, usually in regular intervals. 00:07:45.400 --> 00:07:50.600 This type of finned design is best suited for a faster-moving fluid like air to flow around the tube. 00:07:50.600 --> 00:07:57.680 Another heat exchanger worth looking at is the plate heat exchanger, which uses metal plates to transfer heat between fluids. 00:07:57.680 --> 00:08:05.020 With these, the warmer fluid flows through one port and the colder fluid flows through another, typically in counterflow. 00:08:05.020 --> 00:08:11.240 Both fluids are restricted by seals so they can only follow a certain path, kind of snaking their way through the exchanger. 00:08:11.240 --> 00:08:17.720 The fluid between each set of plates alternates, with the plates providing a large surface area for a high rate of heat transfer. 00:08:17.720 --> 00:08:25.940 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. 00:08:25.940 --> 00:08:31.420 Now, both finned tubes and plate heat exchangers are usually a step up from concentric tubes, 00:08:31.420 --> 00:08:34.760 but one of the most common heat exchangers is the shell-and-tube design. 00:08:34.760 --> 00:08:41.250 You can find them practically anywhere, from large oil refineries, to engines and transmissions, and even in swimming pools. 00:08:41.250 --> 00:08:48.180 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. 00:08:48.180 --> 00:08:55.740 One fluid, usually the colder one, moves through this series of tubes while another fluid flows outside of them and through the shell. 00:08:55.740 --> 00:09:02.260 There would be large pockets of stagnant shell-side fluid in the corners of the shell if this design was left as-is, though. 00:09:02.260 --> 00:09:10.020 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. 00:09:10.020 --> 00:09:16.260 Baffles not only help to increase the overall average heat transfer through the system by directing the flow of the fluid, 00:09:16.260 --> 00:09:19.900 but also by increasing the shell-side velocity and promoting turbulence. 00:09:19.900 --> 00:09:27.360 So, between concentric tubes, finned tubes, plates, and shell-and-tube designs, you’ve got plenty of options when you need to transfer heat. 00:09:27.360 --> 00:09:31.880 Which, among other things, means there’s no need to set any pizza on fire. 00:09:31.880 --> 00:09:33.480 That would just be a travesty. 00:09:33.480 --> 00:09:38.900 Today we learned all about the different types of heat exchangers and how they can be used to transfer heat. 00:09:38.900 --> 00:09:45.880 We started off with concentric tubes , and the two main equations that can help us define heat transfer in heat exchangers. 00:09:45.880 --> 00:09:51.200 Then we flowed on over to finned tubes and found the differences between axial or radial fins. 00:09:51.200 --> 00:09:56.720 Finally, we covered plate heat exchangers and studied the most common heat exchanger design: shell-and-tube. 00:09:56.720 --> 00:10:01.840 I’ll see you next time, when we’ll continue on our journey and learn all about mass transfer. 00:10:01.840 --> 00:10:05.840 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. 00:10:20.420 --> 00:10:23.220 And our amazing graphics team is Thought Cafe.
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