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Power plants produce electrical energy for consumers to use, whether at home or for business, that’s obvious enough, but did you know that in order to produce that electrical energy they must first be supplied with heat energy? The heat energy that power plants crave comes from a fuel source, such as coal, oil, or natural gas, by way of a burning process. Once the heat energy is released from the coal through burning, it’s transported into a steam turbine by way of superheated steam, which is supplied to it by a piece of equipment named, appropriately enough, a superheater. So what is a superheater and how does it function? Take a look at the illustration below.
The superheater looks like a W. It’s actually a cascading array of bent steam pipe, situated above a source of open flames which are produced by the burning of a fuel source. A photo of an actual superheater is shown below.
So how many bends are in a superheater? Enough to fill the needs of the particular power plant it is supplying energy to. Since all power plants are designed differently, we’ll keep things in general terms. The many bends in the superheater’s pipes form a circuitous path for steam to flow as it follows a path from the boiler to the steam turbine. The superheater’s unique construction gives the steam flowing through it maximum exposure to heat. In other words, the bends increase the time it takes for the steam to flow through the superheater. The more bends that are present, the longer the steam will be exposed to the flame’s heat energy, and the longer that exposure, the more heat energy that is absorbed by the steam. Superheating routinely results in temperatures in excess of 1000°F. This superheated steam is laden with abundant heat energy which will keep the steam turbine spinning and the generator operating. The net result is millions of watts of electrical power. As we learned in a previous blog, the superheater is designed to provide the turbine with sensible heat energy to prevent steam from completely desuperheating, which would result in dangerous condensation inside the turbine. The newly added superheater is a major improvement to a power plant’s water-to-steam cycle, but there’s still plenty of waste and inefficiency in the system, which we’ll discuss next week.
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Posts Tagged ‘superheated steam’
Superheater Construction and Function
Sunday, September 15th, 2013
Desuperheating in the Steam Turbine
Monday, September 2nd, 2013|
Last time we learned that the addition of a superheater to the electric utility power plant steam cycle provides a ready supply of high temperature steam, laden with heat energy, to the turbine, which in turn powers the generator. But this isn’t its only job. One of the superheater’s most important functions is to regulate the ongoing process of desuperheating that takes place as the turbine consumes heat energy. To understand this, let’s see what takes place if the superheater were to be removed from its position between the boiler and turbine.
Figure 1
Without the superheater, the only available remaining source of sensible heat energy to the turbine would come from the meager amount present in phase C steam as shown in Figure 1. If you’ll recall from a past blog, the sensible heat energy contained in superheated steam is the best source of energy for a steam turbine, because it’s able to keep it operating most efficiently. As the turbine consumes the heat energy in phase C, starting at point 3 and continuing to point 2, the steam it’s consuming is in the process of desuperheating, as evidenced by the downward slope between the two points. Desuperheating is an engineering term which means that as sensible heat energy is removed from the steam due to its use by the turbine, there will be a resulting drop in steam temperature. And if this process were to continue without the compensatory function provided by the addition of a superheater to the steam cycle, the steam’s temperature would eventually return to mere boiling point, at point 2. This is an undesirable thing. With the steam’s temperature at boiling point, the only remaining source of heat energy to the turbine is the latent heat energy of phase B. This heat energy will lead to an undesirable circumstance for the operation of our power hungry turbine as we will see next week. ________________________________________ |
Superheating, Part 2
Sunday, August 25th, 2013|
Last time we added a piece of equipment called a superheater, positioned between the boiler and steam turbine, to our basic electric utility power plant steam and water cycle. Its addition enables a greater and more consistent supply of heat energy to the steam which powers the turbine. How much more? Let’s look at Figure 1 to get an idea.
Figure 1
You may have noticed that our illustration lacks numerical representation. That’s because power plants are designed differently, depending on fuels used and power output required. So unless we’re talking about a particular power plant, number values would be impractical. For example, I could specify a boiling point of 596°F at 1,500 pounds per square inch (PSI), and a superheater outlet temperature of 1,050°F at 1,200PSI, and I could make note of esoteric things like enthalpy (British Thermal Units per pound mass) values on the Heat Energy axis. But to facilitate our discussion we’ll keep things simple and focus on the general process. Figure 1 shows in phase D the additional heat energy being added to the steam, thanks to the superheater. This is significantly more than had been added by the boiler alone, as represented by phase C. The turbine consumes heat energy added in phases C and D and converts it into mechanical energy to drive the generator, resulting in electrical energy being provided to consumers in the most energy efficient way possible. But increasing power output and efficiency isn’t the superheater’s only job. The heat it adds during phase D ensures the turbine’s safe operation when it’s cranking at full capacity, as represented by the superheated steam zones of phases C and D. Next week we’ll discover how the superheater prevents a destructive process known as condensing from occurring inside the turbine. ________________________________________ |
Superheating, Part I
Monday, August 19th, 2013|
Last time we learned that our power plant boiler as presently designed doesn’t do a good job of producing ample amounts of superheated steam, the kind of steam that turbines need to spin and power generators. During the process of superheating the more heat energy that’s added to the steam in our boiler, the higher its temperature becomes. However, our boiler can only produce a limited amount of superheated steam as it stands now.
So how do we get more heat energy into the superheated steam? Take a look at the illustration below for the solution to the problem. You’ll note a prominent new addition to our illustration. It’s called a superheater. The superheater performs the function of raising the temperature of the steam produced in our boiler to the incredibly high temperatures required to run steam turbines and electrical generators down the line, as explained in my blog on steam turbines. The superheater adds more heat energy to the steam than the boiler can alone. In fact, the amount of heat energy in the superheated steam produced with our new design is proportional to the amount of electrical energy that power plant generators produce. Its addition to our setup will result in more energy supplied to the turbine, which in turn spins the generator. The result is more electricity for consumers to use and a more efficiently operating power plant. But inefficiency isn’t the only problem addressed by the superheater. We’ll see what else it can do next week. ________________________________________ |
Heat Energy Within the Power Plant— Water and Steam Cycle, Part 2
Wednesday, August 14th, 2013|
Last time we learned that electric utility power plants must have water treatment systems in place to remove contaminants from incoming feed water before it can be used. This clarified water is then fed to a boiler by the boiler feed pump as shown below.
As it stands this setup will work to provide electricity, however in this state it’s both inefficient and wasteful. We’ll see why in a minute. Boilers, as their name implies, do a great job of heating water to boiling point to produce steam. They do this by adding the heat energy produced by burning fuel, such as coal, to water, then steam. We learned in earlier blogs in this series that the energy used to heat water to boiling point temperature is known as sensible heat, whereas the heat energy used to produce steam is known as latent heat. The key distinction between these two phases is that during sensible heating there is a rise in temperature, during latent heating there is not. For a review on this, see this blog article. When water starts to heat inside the boiler, sensible heat energy is said to be added. This is represented by phase A of the graph below.
During A, heat energy will raise the temperature of the water to boiling point. As the water continues to boil in phase B, water is transforming into steam. During this phase latent heat energy is said to be added, and the temperature will remain at boiling point. In phase C something new takes place. The temperature rises beyond boiling point and only steam is present. This is known as superheated steam. For example, if the boiler pressure is at 1,500 pounds per square inch, steam becomes superheated at temperatures greater than 600°F. Unfortunately, boilers alone do a poor job of superheating steam, that is, continuing to raise the temperature of the steam present in phase C. This is evident by the fact that phase C is quite small in comparison to phases A and B before it. This inefficiency in producing ample amounts of superheated steam results in a small amount of useful energy being provided to the turbine down the line, which is bad, because steam turbines require exclusively superheated steam to run the generator. Next time we’ll see how to provide our steam turbine with more of what it needs to run the generator, more superheated steam. ___________________________________________
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