Heat Transfer – Fundamentals of Engineering Module #14

what is heat transfer

The inlet and outlet thermometers on a heat exchanger.

If you’re read anything at all on this site, you’ve probably noticed me mention heat transfer before. As I see it, heat transfer is life. Everything comes down somewhere to a heat transfer. It’s really neat if you think of life in terms of energy exchanges, it’s how things happen.

Yes, heating your home, how a car engine works, that heat transfer is easy to see and honestly, kind of boring Steve. The cool stuff that races across my brain is how the same heat transfer concepts can be found in normal things we do and probably don’t think about.

Every single step you take, throughout your life, every one, is an energy exchange. Each time your foot touches the ground, you and Earth are swapping something. Can you just stand ankle deep in a pile of snow? Why not? Because the Earth is like “heck no man, this stuff is cold. Give me your warmth!”

You yell across a room, audio energy hits the little hairs in my ears, they shake and wiggle, in just a way to convert that energy into something my brain can decode.

heat transfer examples

The inlet temp, 136F. This is the temp of the hot water on one side. Steam is on the other.

Here’s another way. Allegedly it is better for you to drink as ice cold of water as possible. Why? Because your body has to expend energy to heat up that water. I guess it’s not better for you but it will cause you to burn a tiny amount of calories to warm up that water so essentially you’re losing weight. Tiny, miniscule amounts we’re talking. Still though, it’s there.

For you to do anything, you must exchange energy. One of the ways to do this is through heat transfer which is what this post is about. Going to quickly go over what heat transfer is, three main methods of heat transfer (there might be more, I’m not a rocket scientist,) and perhaps how that 2nd Law of Thermodynamics that we just talked about ties in. First, let’s define heat transfer.


What is Heat Transfer?

Heat transfer is the movement or flow of heat from one material or substance to another.

heat transfer

The outlet temp. So 14F of heat exchange across?

Easy enough. Warm soda can sitting on a counter, the air surrounding it giving up its heat. Now we put the soda in an ice bath. The ice surrounding the can takes away the heat from the warm liquid. As the soda gets colder and colder, and closer to the temperature of the ice, the RATE of heat transfer slows down.

The greater the temperature difference the greater the rate of heat transfer. Hint, hint – the 2nd Law of Thermodynamics. Which we just talked about here.

Part of the reason is because as temperature drops, molecule activity drops. They are little atoms and stuff bouncing around like hot peppers in hot temperatures, as the temperature decreases, they become less active, less bouncing around. So they transfer less heat. How many hyperactive water molecules do you think there are in a sample of water that is almost freezing? Not much right?

Name a piece of equipment in the engineering spaces that is designed for heat transfer. A boiler. More generally, a heat exchanger.

What is a heat exchanger?

A heat exchanger is a device that allows a fluid or substance to exchange heat by being in close proximity to each other but not allowing them to actually touch and mix. The fluids are separated by piping or tubing walls. Just like the fire is on one side of the tubes in the boiler, and water is on the other.

The water becomes steam due to an exchange of heat. Brilliant. Does it just “do it” though? No, there are different ways heat transfers can occur. We’re gonna go back to grade school here and talk about those main 3 methods – conduction, convection, and radiation. First up is conduction.

Conduction

conduction

pic from vtaide.com

Conduction is a method of heat transfer that happens when heat is transferred from one molecule to the next, throughout the material.

I used to camp a lot more. I have this cast iron skillet. My Grandpa had a cast iron skillet for camping from way back, my Dad didn’t really camp but he had one. You’re supposed to oil them, smoke them off, cook everything in them, barely clean it. Adds flavor, it’s what you’re supposed to do.

Being as how I haven’t camped recently, I hadn’t used the skillet. I thought I’d get it out and use it to cook with inside the house. I turned on the stove burner and put the skillet down to heat up. I did it all gradual over maybe a half hour. Got the food out, poured it on, grabbed a wooden spatula.

Grabbed the cast iron frickin’ handle.

Mother of t-bone Sally. That was essentially eternal damnation of the hellfire to my hand. What happened?

Conduction happened.

The iron molecules were like, “okay mf’er, Try and burn us.” They slowly but surely passed that heat energy along, slowly but surely, molecule by molecule, from the bottom of the skillet, up the sides, down the handle… bastards. Never again iron will I trust you.

Some metals transfer heat better than others – copper is one. If heat energy is being transferred through conduction, there is no flow of actual material. This is not true though about the next popular method of heat transfer, a big invention of the 70’s, maybe 60’s, convection (the convection oven for all the good women.)

Convection

convection

pic from vtaide.com

Convection is a method of heat transfer that happens when because of the temperature difference the fluid circulates. It naturally circulates.

Like the air in your house. Heat rises, cool air falls. As the warm air from your furnace cools off, it starts to fall and collects at the bottom. This is why you should run a ceiling fan, not because it keeps you cool but because it adds to the natural circulation.

Most fans now have direction switches. In the summer, cooling, you’d want it blowing down. The cold air on the floor will be forced up the walls and then mixed with the cold air being discharged from your registers. A cold cycle down.

In the winter, have the fan drawing up. This way the cold air on the floor will be drawn up through the fan and mix with the warm air from the furnace. This will then fall down the walls and into the room. Thanks Steve. I know.

In a boiler, the water closest to the fire breathing tubes is of course the hottest. As it boils, it rises. Above it is another water molecule that passes by maybe picking up a little heat but it’s cooler – slightly. So it drops down a notch.

The warmer fluid, water, rises and the colder molecules of water drops because they are more dense. It creates a cycle of flow without anything pumping it.

Radiation

radiationRadiation was the one that one back in school. Mmm Hmmm. Radiation got it. Now I’ve been to a place that covered it. Still don’t get it. Ha.

Radiation is a method of heat xfer, you catch it, that happens due to the radiant energy being emitted from objects.

All object emit some amount of radiant energy. How much depends on the intensity of the heat. Radiant energy shows up as electromagnetic waves. These radiant energy waves move through the air and until they strike an object, do nothing.

If the object doesn’t let light through, heat transfer. It shows up now as thermal energy. Confused? Don’t be. If you have ever felt the warmth of the sun on your skin, you are feeling it due to radiation.

When you lift the lid to your bbq grill and feel that blast of heat – radiation. When you pit the lid down you feel it less.


Remember the 2nd Law, Not the Alamo

The 2nd Law of Thermodynamics applies to everything we talked about here, all three of these methods of heat transfer. Heat flows from warmer to cooler, and the greater the temperature difference between the two, the faster the rate at which it will be transferred.

Conclusion

Steve, you’re never quick. Oh believe me I know. I know.

This was a pretty in depth post actually. The main topic was heat transfer. Provided a definition of it and gave some examples of where you can see it in action in your everyday life. Also used a boiler to show how important the ideas of heat transfer and heat exchange are to the behind the scenes operation of a building or facility.

Conduction, convection, and radiation are three popular (only?) methods of heat transfer. Well, the only 3 we talked about here. Can you think of different examples of each kind that you’ve seen?

Again, we mentioned the 2nd law and how it’s rules apply to heat transfer. By the way the shorthand in engineering lingo for transfer is xfer. And for heat exchanger it is hx. You could write heat xfer. Comment? Donation? Want to contribute? Lemme know.