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.
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.
Archive for August, 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.
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.
Last time we learned that electric utility power plant boilers are vessels that are reinforced with thick steel and are closed off from the surrounding atmosphere so as to facilitate the building up of highly pressurized steam. This steam is laden with sensible heat energy, meaning it’s a useful energy, and it’s used to run steam turbines, which in turn drive electrical generators. The end result is power to consumers.
Let’s now revisit our basic electric utility boiler diagram to see how water and steam flow.
Water is fed into the boiler, heat is applied externally, and steam exits through a pipe leading to the steam turbine. You’ll notice that after the steam passes through the turbine, some of it is expelled into the surrounding atmosphere.
Since water is being continuously boiled off to produce steam, the boiler must be continuously replenished with a fresh supply. This is typically supplied by a nearby body of water, hence one reason that power plants are often situated on a lake or river.
Since water contains both minerals and organic matter, including algae, a treatment system to remove these contaminants must be added to the water’s inlet area before it can be used. This will keep operating parts such as the boiler and turbine free of damaging deposits.
The treatment system operates much like the water softener in your home, but on a larger scale. Lake water is drawn into the system by a make-up pump, so named because it makes up, or replenishes spent water with a fresh supply. The result is clean, mineral-free water that’s delivered to the boiler by a boiler feed pump, so named because its specific function is to feed water to the boiler.
Feeding water to the boiler on a continuous basis is no easy task because of the steam straining to break free, and boiler feed pumps are massively powerful devices built to accomplish this. They effectively force water into the boiler even as high internal pressures try to force the water out. This pressure is often greater than 1,500 pounds per square inch (PSI) in modern power plants.
So at this point we’ve discussed the fact that the boiler requires a continuous supply of fresh water, which is converted into high pressure steam, which is then sent on to spin a steam turbine. The turbine powers an electrical generator, resulting in usable energy.
If you’ve been reading along closely, you will have identified that as things stand now it’s a rather inefficient and wasteful system, a point which we’ll address in next week’s blog.