Water Tube Boilers and Other Types
Anyone can create steam with a little water and heat. You do it every time you heat water to make spaghetti or tea, but steam can actually serve a useful purpose, too. Of course, you can't power a train with your tea kettle, but industrial applications of the same general principle enable people to create steam in massive quantities so that they can power industrial equipment, move locomotives and even generate electricity! There are different types of boilers that can be used, each with distinct advantages and disadvantages, but if you know how they work from the inside out, you understand the secret of their incredible potential.
Fire and Water
Two of the most distinctly different types of boilers are fire tube and water tube boilers. In a fire tube boiler, burning fuel creates hot gas that travels through a series of tubes inside the boiler. The tubes actually course through a tank of water, and as the hot gas circulates in the tubes, heat passes through the walls of those tubes and heat the water. As the water heats, it evaporates into steam, which exits the boiler to provide power in whatever capacity it needs.
Compare this to a water tube boiler, which operates in a manner that is both remarkably similar and fundamentally different. Like a fire tube boiler, this model has a series of tubes and a fuel burner that creates hot gas. Instead of the gas circulating through the tubes, though, in this model, water circulates through the tubes. The hot gas fills the air outside the tubes, and heats the water from the outside in. Hot water rises to the tops of the tubes, where the steam collects and flows out of the boiler while cooler water circulates back down.
While water tube boilers first appeared in the 1700's, they did not become particularly popular for at least 100 years. This is because these types of boilers were, at the time, quite revolutionary—they were so advanced and complex that engineers struggled to perfect the design. They were not the only type of boiler being utilized during those early years, and they were far from being the easiest to construct. For those reasons, the industry greatly favored constructing other types rather than perfecting this model—once the kinks were finally worked out, though, this type of steam generator became much more popular.
Into the Superheater
Once the steam has been created, it isn't necessarily ready to power machinery—it may be treated further using a piece of equipment called a superheater. This can be installed in a steam boiler system to superheat the steam, a process which changes its molecular structure. When steam is superheated, its temperature is increased. This, combined with the pressurization inside the boiler, essentially dries out the steam—it can then lose some of its temperature without turning back into water.
Superheated steam isn't necessarily better than normal steam, though. For example, saturated steam that is still retaining moisture is preferable for heating. Superheated steam is, however, a very useful source of energy, specifically in engines and turbines. Engines and turbines have moving parts that can be damaged by the water droplets that may accompany saturated steam. The lack of moisture and higher levels of internal energy in superheated steam, then, make it perfect for applications like this—for example, it was often produced using mobile boilers in steam locomotives.
Other Boiler Types
While water tube boilers have been made for centuries, the industry continues to evolve and change as new types of boilers hit the market and new technological innovations appear. One example of this type of technology is the popularity of heat recovery steam generators. These types of boiler systems, also known as HRSG, heat water using waste heat instead of by burning fuel.
Waste heat is a natural by-product of energy production. For example, when your car engine gets hot or you burn your hand on a light bulb, that's because of waste heat. It's heat that gets created and has no discernible use, and in small volumes, there isn't much you could do with it anyway. But some energy production methods generate large quantities of waste heat. For example, when you see a factory smokestack or a nuclear power plant cooling tower pumping out massive billows of smoke and vapors, that's waste heat being directed away from the factory and into the atmosphere.
Waste heat isn't necessarily good for the atmosphere, though, which is why boiler manufacturers build HRSG-style boilers. This type of boiler kills two birds with one stone: It finds a use for the waste heat created by something else, and it generates heat for itself without burning fuel. It's an environmentally friendly win-win situation.