As you probably know, steam is awesome. But it’s also not as cut-and-dry as it appears to someone outside the industry. Steam actually comes in two forms, colloquially known as “wet steam” and “dry steam”. The difference between the two has to do with the amount of energy it’s carrying, and that’s where superheated steam comes into play.
Let’s go back to high school physics class for just a second. When you add heat to a quantity of water, you’re adding energy to it. If you put a thermometer in the water, you can see the energy level rising as the water gets warmer and warmer. On the molecular level, the water molecules are absorbing that energy, and they’re starting to vibrate. The more heat energy they absorb, the more they start banging around against each other. Eventually, they’ll absorb enough energy to break free from the molecules around them. That’s what’s called the boiling point (which is also a really cool YouTube channel), and that’s when water undergoes a phase change from a liquid to a gas. Under ideal conditions, water boils at 212°F. But what happens if you keep adding heat to it? You get superheated steam, a powerful tool that helps industry thrive and the lights stay on.
WET STEAM IS WET
Before we start talking about superheated steam, let’s talk about where it comes from: regular steam. The kind you can see coming out of factory stacks and rising out of saucepans on the stove. It’s also known as “wet steam” or “saturated steam”, because even though it’s in its vapor state, it’s still carrying tiny water droplets suspended in and around it. As an example, think about the vapor coming off a pot of spaghetti boiling on the stove. If you carefully (and quickly) pass your hand through the spaghetti steam, your hand will get damp from the water droplets that stick to your skin, even though those water droplets were suspended in the vapor.
Wet steam doesn’t need a lot of water in it to still be considered wet, though. Wet steam can still be invisible, it just has smaller droplets of water suspended in it. It’s because of those water droplets that you can’t use wet steam in any sort of turbine application like a power station or a giant factory flywheel. Even though those water droplets are very small, they still have mass. And when those turbine blades smack into them at high speed, those multiple tiny collisions will cause some damage. It may not look like much at first, but eventually, the blades will start to fracture and separate from slamming into all those water droplets over and over.
For those applications, you need to get all the excess water out of the steam, and the way to do that is by adding more heat. Which gets us superheated steam.
SUPERHEAT UNTIL DRY
Once you heat steam enough, all of the water molecules will pull apart, and you’ll have no condensate whatsoever hanging around. This is what’s called dry steam, also known as superheated steam. You can’t see it, but it’s there. And it’s fantastic at moving heat around. In fact, pure superheated steam is one of the best ways to move heat energy from one place to another.
Superheated steam is an ideal medium for heat transfer, because, well, it’s mostly energy held together by the occasional clump of oxygen and hydrogen atoms. This makes superheated steam the perfect medium to use in high-volume, high-heat heat exchangers, which is one of its main applications.
Superheated steam is also one of the most consistent ways to move heat, because once it reaches its superheated state, the temperature won’t fluctuate as long as the input temperature and pressure stay consistent. If there’s still water suspended in the steam, even a tiny amount, it can absorb some of the heat around it; get enough of that going on, and you’ll get a detectable variation in temperature. But superheated steam stays where you put it.
As mentioned above, superheated steam is used almost exclusively in power generation because it preserves the life of turbine blades by not beating them up. But you’ll also find it used to power steam-operated blowers and pumps. You’ll also see superheated steam in a lot of industrial drying applications, for two main reasons. First of all, it can transfer a lot of heat to the water molecules it’s trying to remove, which turns them to vapor very quickly. Second, it has a lot of room in any given volume to hold the water molecules it’s trying to remove. After all, you wouldn’t dry off with a wet towel.
WHERE TO WET IT
Wet steam has a lot of useful applications, of course. It’s useful in cleaning and sterilization, because the water droplets provide a lot of surface contact to transfer all that germ-killing heat energy. You’ll also find wet steam in a lot of low-temperature heat transfer operations, such as home heating. It’s also popular in humidification applications, and not just home humidifiers. Steam humidification is used in a lot of chemical and manufacturing processes to introduce moisture in tiny, gradual amounts to promote better mixing and dissolution.
HOW THE HEAT HAPPENS
To create superheated steam, you have to add more heat. That takes place through hot surface contact via extremely hot “superheater tubes”. As the wet steam leaves the boiler itself, it passes through the superheater tubes where it absorbs even more heat, causing all the remaining water droplets to turn into pure vapor. Then, you can add as much heat as your process will require, as long as you keep the superheated steam under the correct pressure. Many superheated steam processes can actually use steam that’s over 1000°F, though those kind of temperatures are used in only a select few applications.
If a work process only requires superheated steam, it will usually be delivered through a boiler with an integrated superheater. For some applications, though, a combination of regular and superheated steam is needed for a variety of processes. In cases like this, some of the steam is routed directly from the boiler to the wet-steam processes, while the rest of it is rerouted to pass through an external, gas-fired superheater to create the dry steam needed for those specific processes.
WARE HAS THE HEAT
WARE has a full fleet of superheating equipment standing by to help you with whatever you need, whether you’re looking for a temporary or emergency rental, or a new superheating boiler to replace your old one. Our superheating equipment can deliver up to 75,000 pounds per hour, rated for 750 psi and 750°F.
Whatever you need, whether it’s a rental or new boiler, or service and maintenance for your existing boiler, WARE is here to help. If you’d like to learn more about wet or dry steam, consider taking one of our many Boiler University classes, available online or in-person. Whatever you need, just let us know.