Steam Basics: Superheated steam
Superheated steam is the perfect match for applications like drying, cleaning or curing. Find out its benefits today and become a steam expert.
Superheated steam is another steam type like saturated or unsaturated steam. You get this type of steam by heating it beyond the boiling point. As a result, the steam temperature is higher (the density lower). This type is mainly used for power generation and in steam turbines.
Saturated steam is, in practice, never 100% dry. It usually contains 3% - 5% wetness. Which makes it a risk for lower heat transferring efficiency or even pipe corrosion. But there is a lack of moisture in superheated steam. And thus, it's the perfect match for applications like drying, cleaning or curing.
Superheated vs. saturated steam: what’s the difference?
When you heat saturated steam beyond the boiling point, you get superheated steam. During that process, you separate steam vapor from the remaining water droplets. So there are a couple of differences between superheated and saturated steam.
Pressure – temperature relationship
Unlike saturated steam, superheated steam does not have a direct relationship between pressure and temperature. This means superheated steam can exist at a wide range of temperatures. Typically, it's specified by pressure and a degree of superheat (= the degree above the saturated steam temperature).
Lowering the temperature
Compared to saturated steam, superheated steam also has a lower density. Lowering the temperature of saturated steam will revert it back to its liquid state including water droplets. Which is not the case for superheated steam.
Use in applications
Depending on the application, you’ll need a different steam type. This table gives you a quick overview of the preferred steam type for each application. But there are some nuances of course! You'll read more about them in this article.
Application |
Saturated Steam |
Superheated steam |
Heating |
V |
|
Sterilizing |
V |
X |
Drying |
|
V |
Stripping |
|
V |
Cleaning |
X |
V |
Sanitizing |
|
V |
Catalyzation (Steam reforming) |
|
V |
How do you create superheated steam?
You get superheated steam when saturated steam flows out of the drum of the boiler into a secondary heating area called a superheater. It’s the secondary heating that turns saturated steam into superheated steam. Superheated steam and water cannot coexist simply because the heat will evaporate it. Therefore, superheated steam must be created outside the main boiler drum.
Superheated steam is created in several phases
Advantages and disadvantages of superheated steam
We already mentioned that there is a lack of moisture in superheated steam. That's important for a couple of reasons:
- No moist increases the efficiency of the boiler.
- It reduces the chances for corrosion or the occurrence of water hammer.
Apart from that, superheated steam has some other benefits:
- High temperature at a normal pressure
- Because you can reach high temperatures without applying high pressure, the piping can be simple. High temperatures can be achieved regardless of the pressure vessel or boiler specifications.
- Extremely high thermal conductivity
- Superheated steam offers an extremely high thermal conductivity compared to hot air. Reason being that is has a high thermal capacity per unit volume.
- Low oxygen conditions
- Superheated steam is a gas with low oxygen levels. Which prevents oxidation of heated items and lowers the possibility of fires or explosions.
What is corrosion when generating steam?
When steam evaporates from the boiler drum, it leaves behind some impurities. And when steam is wet, the water droplets have these impurities dissolve in them. As a result, the impurities get carried into the steam network and equipment. That condensate can cause damage from erosion or carbonic corrosion.
What is water hammer?
Water hammer is the phenomenon when fluid in motion causes a pressure surge or wave. It's also known as hydraulic shock. In the case of steam: a lump of condensate travelling at high-speed that hits the piping. It can cause different problems from noise and vibration to pipe rupture or collapse. There are means to avoid or reduce these effects like using the right steam for each application. But also others like using blow-off valves among others.
There are also some disadvantages of superheated steam, certainly if you’d want to use it for heating:
- Low heat transfer coefficient
- This results in reduced productivity and a larger heat transfer surface area needed.
- Variable steam temperature, even at a constant pressure
- Superheated steam needs to maintain a high velocity. If it doesn’t, temperatures will drop as heat is lost.
- Sensible heat used to transfer heat
- Which means that when the temperature drops, it can have a negative impact on product quality.
- Temperatures may be extremely high
- You don’t need special piping because superheated steam operates at a normal pressure. But because the temperatures can get extremely high you may need stronger materials of construction. And that requires higher initial equipment outlay.
What is Heat transfer coefficient?
We use the heat transfer coefficient to calculate how well heat is transferred. Earlier we wrote: ‘A high heat transfer coefficient requires a small heat surface. Resulting in reduced initial equipment outlay.' Because more water has evaporated into saturated steam, the steam has also absorbed more latent heat. As a result, there is more heat contained in the same mass. And so, it has the ability to do more work.
Conclusion
- Superheated steam is steam that is heated beyond the boiling point.
- Superheated steam has a higher temperature and lower density.
- It’s mainly used for power generation and in steam turbines. Which makes it a perfect match for applications like drying, cleaning or curing. It can however not be used for sterilization.
- Superheated steam does not contain any moisture. This makes it a good approach for processes that do not allow any moisture. And additionally, it reduces the chances of corrosion or water hammering.
- There is no direct relationship between pressure and temperature. So unlike saturated steam, superheated steam can exist at a wide range of temperatures.
- The degree of superheat is the degree above the saturated steam temperature.
- Lowering the temperature of superheated steam will not result in reverting back to a liquid state like it does for saturated steam.
The 3 main benefits of superheated steam are:
- High temperatures at normal pressure: meaning you can use simple piping.
- Extremely high thermal conductivity compared to hot air because of the high capacity per unit volume.
- Low oxygen conditions: preventing oxidation and lowering the possibility of fires or explosions
