Thermodynamics – Fundamentals of Engineering Module #13

what is thermodynamics

Image from NASA

What the heck is thermodynamics anyways? How does the science of thermodynamics apply to our boilers? Right now, I don’t know and I’m not sure how the laws of thermodynamics apply. This is what we are going to try and learn about today. Sit back, grab your coffee, a pencil and some paper for notes, and let’s figure out thermodynamics.

What is Thermodynamics?

Thermodynamics is the science of thermal energy, or heat, and the processes by which it changes between different forms of energy.

More broadly, thermodynamics falls under the umbrella of physics. It studies heat and temperature and how they relate to energy and work. In building engineering, or life, thermodynamics in action can be seen everywhere and the science of it has countless applications.


Laws of Thermodynamics

law of conservation of energy

Energy in equals energy out. Always.

In earlier posts we talked about Newton’s Laws, Boyle’s Law, etc. Of course there are laws that govern thermodynamics, you didn’t think there wouldn’t be did you? If you are into science, engineering, or Einstein at all, we’re pretty certain you could probably recite the first law. Maybe not the second though. Both are vital to building engineering and the fundamentals of engineering.

First Law of Thermodynamics

Apparently it is also called the law of conservation of energy. Don’t quote me on that but that’s what my book says. The first law of thermodynamics states that:

  • energy can neither be created nor destroyed but can be altered in form.

See? I knew you knew it.

As an example, let’s use the burning or combustion of a fuel, any fuel, gas for your car, natural gas for your boiler, or oil for your residential burner, some crumpled newspaper on a campfire. When a fuel burns, energy is not created or destroyed. It only changes form. It changed from a form of chemical energy to thermal energy.

We wad up a section of newspaper and throw it on the fire. What happens? It catches fire, burns brightly, gets hot while burning, but what is really going on?

As it burns, oxygen, carbon, and hydrogen start doing stuff. They start reacting with each other as the fuel breaks apart, new oxygen may get sucked in even. It is during this process that the chemical energy of the elements is let go in the form of heat, or thermal energy, and even light.


Second Law of Thermodynamics

The second law isn’t as well known or as often quoted as the first but it is just as important. And it is
really important for you to remember this law as it is a good thumb rule that every building engineer should know. It says that:

  • heat will always flow between substances or materials from hotter to cooler.
second law of thermodynamics

Heat flows from warmer to cooler. Always.

This flow of heat happens naturally and we use it in certain systems to our advantage. Hotter stuff usually, like water or air, wants to rise naturally. The colder stuff wants to go down. Why? Probably because of the difference in densities.

Let’s put a 75F soda into the 42F fridge. The air immediately surrounding the soda can will get a little warm right on the edge of the aluminum. The fridge will then eventually make the air inside the fridge compartment cooler. The internals of the soda, the molecules and such, move around as they heat and cool, slow down, etc. The warm soda that was on the edge got cooled down so it mixes with some warm soda still left which takes its place against the edge of the can. The fridge cools that now, until everything is equal.

What if the soda was 38F? Would the fridge cool down to 38F or would the soda come up to 42F? It would come up to 42F because 1) the size difference and 2) because of the second law of thermodynamics, heat flows from warmer to cooler.

Thermodynamics and Boilers
thermodynamics

Thermodynamics came about trying to improve upon the steam engine.

What are the basics of how a boiler works? We’ve got water on one side of the boiler tubes and on the other, hot gases of combustion burning away, right?

Thermodynamics connects, relates, and tries to explain the events that happen as heat is generated by the fuel combustion and then how it generates steam inside the boiler because of the heat transfers.

How did Thermodynamics Originate?

Thermodynamics came about when man wanted to increase the efficiency and power output of early steam engines. The steam engine had been around for a bit and like humans do, they wanted to improve upon them.

Thermodynamics later expanded to the study of energy transfers in chemical processes. Originally it did not include the relationships between the energy exchanges of heat and work. From this evolved the study of chemical thermodynamics and the role of entropy in chemical reactions. Entropy is an entirely different and confusing topic ha ha.

Check out this Fundamentals of Thermodynamics book from Amazon.  Expensive!

Conclusion

Now you should be able to walk into any college classroom and give a lecture on thermodynamics.

Besides mentioning two very important laws regarding thermodynamics, heat flows from warmer to cooler objects and energy can not be created or destroyed only altered in form, we provided examples.

We also got into the concept of thermodynamics and how it applies to our every day life and in our jobs as building engineers. Just remember, thermodynamics is the science of thermal energy (heat) and how to best use the energy transformations.  If you think you’re still lacking on thermodynamic fundamentals, you can use the following link for more information (wiki entry,) or you can use this link to see how NASA explains thermodynamics.