You know what boiler steam is, and you know what it does. But do you know why and how? It all comes down to the science of thermodynamics, which is the set of simple rules that the universe follows when it comes to heat, pressure, and movement.
A CLOSER LOOK
To understand the physics behind boiler steam, we have to start with the understanding that everything around us is made up of atoms and molecules. We also have to understand that on the molecular level, everything is constantly in motion. Tiny, controlled motion, but it’s movement nonetheless. The other thing we have to understand is that the more heat you add to something, the more the molecules start to move around.
Think of it like a classroom full of children. Even if they’re sitting quietly at their desks, there’s still some minor fidgeting going on here and there. The more excited you get those children, perhaps by sending a parade past the window or bringing a live camel into the room, the more they’re going to move around.
Eventually, they’ll get so excited that they may start to run around the room. The more you let them run around the room, the more energy they’ll use up. Eventually (it may take a while) they will start to run out of energy, at which point they’ll start to slow down, and may even eventually return to their desks. Molecules are the same way; the more heat energy you add to them, the more they’re going to start moving around. The more energy they lose, the slower they go.
PHASE IT
Since we’re talking about boilers, the molecules we’re going to be discussing are water molecules. The thing about water is, it can change its state depending on how much heat energy it’s holding. When the water molecules are relatively cool, they’ll tend to move relatively slowly, and will clump together. This is when water is in its liquid state. If you start to add more heat to the water, though, the molecules will move faster and faster, vibrating and banging up against one another. If they get hot enough, they’ll be banging up against each other so much, they won’t clump together anymore. When they don’t clump together and start to fly apart, the water they comprise will change from a liquid to a gas, a process known as phase change.
When condensate changes into boiler steam, it phase-changes from liquid water to water vapor. When that water vapor gets hot enough that the molecules evenly distribute and can’t fly apart any longer, that’s when water vapor becomes steam.
PRESSURE TIME
Now that we’ve established what heat does to molecules, we need to talk about the other factor involved here, and that’s pressure. Pressure is a measure of how tightly those molecules are pressed together when they’re confined in a set space. Even when heat energy is trying to drive those molecules apart, pressure can force them to stay together. That’s why every temperature is dependent on a corresponding pressure, and every pressure is dependent on a corresponding temperature. Together, they form a concept known as “enthalpy”, which is the total thermodynamic potential of a set quantity of water at a specific temperature and pressure.
Here’s how they relate. If water molecules absorb a lot of heat, but they’re confined in a small space, they’ll still be moving around very rapidly. They just won’t have anywhere to go. Since they’re straining to get out, pushing against each other, the pressure in the container will go up. If you suddenly make the container larger, though, the heat energy won’t be as concentrated, and all those molecules will spread out. This will cause the temperature to go down, because there’s not as much heat energy concentrated in the small space. Conversely, though, if you shrunk the container instead of expanding it, you’d still have the same number of molecules and the same amount of heat, they’d just be pushed even tighter together. That means the pressure would go up, and the water molecules in the container would be hotter, because the heat energy is more concentrated.
The more concentrated the heat is in a given quantity of water, the more enthalpy it has. But, as we’ve just discussed, changing the temperature or the pressure of the water molecules will raise or lower its enthalpy. That’s also why a boiler operating at lower temperatures and higher pressures can have the same ability to get work done as a boiler that operates at higher temperatures and lower pressures. In other words, they have the same enthalpy.
BOILER STEAM WORKS
Now that we understand what happens to water molecules when you add or remove heat or pressure, we can get a better understanding of how steam does work. Steam is still used to move heat energy around because it’s actually one of the most efficient ways to do that. Even in a world of advanced battery technology and materials that seem to defy the laws of physics, steam remains the gold standard when it comes to moving heat and getting stuff done.
So if you have a piston that needs to be moved, you can load up the water molecules with enthalpy in the boiler, which will turn the water into steam, then you can move that steam to where the piston is. Once inside the piston, the steam (which is under pressure) will move the piston. As the piston moves, the cylinder in which it’s located will expand in size. Since the steam is now under a lower pressure, its heat will also go down, and it will start to cool. The enthalpy has now been converted from pure heat energy into mechanical work, moving the piston. In doing so, though, it’s lost its heat energy, which means the steam will start to condense into liquid water again. Once it does, that condensate is captured and rerouted back to the boiler to absorb some more heat, which it will then transport to the piston again and convert into mechanical work.
This is a really simple example of how boiler steam can turn heat into work, but it’s the basis for a lot of things around you. Most relevant are probably the steam turbines that generate the electricity you’re using to read this. By pumping high-pressure steam into a turbine, its pressure is used to move the turbine blades and generate power as the heat energy is converted into mechanical movement.
If you’d like an even deeper explanation of how all of this works, there’s no better place to learn about the power and potential of boiler steam than WARE’s Boiler University. There are online and in-person classes that can turn ordinary people into true boiler experts and highly skilled boiler operators. Of course, WARE can also help you keep your boiler running, or outfit you with a new or rental boiler. Whatever you need, we’re here. Just let us know how to help.