Superheated Steam Boilers
Even though most boilers today use saturated steam, there are certain times that superheated steam is preferred. A superheated steam boiler pushes past the saturated steam curve to generate steam with no moisture (or wetness) to it. The dry nature of superheated steam boilers is a great option for turbines as well as drying, cleaning, and curing applications. We’re excited to let you know that Powerhouse now offers superheated steam boilers that deliver high-quality steam efficiently and reliably.
What is Superheated Steam?
Let’s take a step back and discuss the science behind superheated steam. Most people are familiar with the three most common states of matter: solid, liquid and gas. If we use water as an example, then we can easily identify each of these states as ice, water and steam, respectively. When we take a closer look at steam, we can tell that there are differences in the types of steam based on pressure and temperature. There are two basic types of steam, which are known as saturated steam and superheated steam.
Saturated vs Superheated Steam
Steam is used prolifically across all industries because of its heat transfer characteristics. It is one of the most efficient and intrinsically safest methods of transferring energy over varying distances. Now that we know there are two basic types of steam, it’s important to understand how they are different and how they affect the boiler room.
Most boilers in the market today use saturated steam, which is created when the heat is applied to water and caused to boil. It is similar to a tea kettle whistling when it’s ready. The steam does not escape freely because the pressure is controlled for its intended use. This type of steam follows the saturated steam curve and has a direct relationship between temperature and pressure. Saturated steam is used for efficient and effective heat transfer across many industries.
Meanwhile, superheated steam is created when saturated steam flows out of the drum of the boiler and into a secondary heating area called a superheater. The secondary heating brings the saturated steam beyond the saturated steam curve and into the superheated steam area, which results in no moisture (or wetness) in the steam at all. This is the reason that superheated steam must be created outside the main boiler drum. Unlike saturated steam, superheated steam does not have a direct relationship between pressure and temperature so it is typically specified by pressure and a degree of superheat (° above saturated steam temperature). The lack of moisture in superheated steam makes it a perfect medium for turbines as well as drying, cleaning and curing.
What are the Advantages of Superheated Steam Boilers?
Superheated steam provides many advantages to a variety of processes and utilizations. When steam is heated past the saturated steam temperature, the lack of condensate is what makes it the preferred property for certain applications. It is used to rotate equipment such as turbines where the blades are highly susceptible to damage from water droplets and condensate. The dry nature of superheated steam is also the reason that it was preferred over saturated steam for use in the steam engines of locomotives.
For drying and curing applications, superheated steam has a higher thermal conductivity than air and a lower oxygen content which allows for efficient drying while preventing oxidation, fires, and explosions. Lastly, superheated steam has proved to be an effective alternative to chemical treatment in agricultural soils because the high heat energy assists in the deterioration of all organic material when the steam is injected into the soil.
What are the Disadvantages of Superheated Steam Boilers?
The disadvantages of superheated steam include lower heat transfer rates, the variable temperature at constant pressure, larger surface area requirements and higher temperatures than saturated steam. These are the reasons that superheated steam is typically not used in heat exchangers and other indirect heating equipment, especially when precise material temperature control is required.
Superheated Steam Boiler Considerations
There are special considerations and designs that must be accounted for in a system intended to use superheated steam. Operating pressure, degree of superheat, control of superheater output, the difference of start-up and running condensate conditions, high pressure and temperature ratings, and other design criteria play an important role in proper component selection.
While saturated boilers are able to control pressure and temperature simultaneously due to the properties of saturated steam, a superheated boiler typically must include a desuperheater for precise control of the final output temperature of the steam. The auxiliary boiler components (including safety devices, water treatment systems, feedwater pumps, and control valves) must be properly sized and selected to meet the more stringent requirements of the operation and control of the superheat boiler system.