Posts Tagged ‘superheating’

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.

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      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.

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      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.

utility power plant expert

      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.

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      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.


Forms of Heat Energy – Latent

Monday, July 15th, 2013

      If you took high school chemistry, you learned that water is created when two gases, hydrogen and oxygen are combined.   You may have even been lucky enough to have a teacher who was able to perform this magical transformation live during class.

      Depending primarily on the amount of heat energy absorbed, water exists in one of the three states of matter, gas, liquid, or solid.   Its states also depend on surrounding atmospheric pressure, but more about that later.    For our discussion, the water will reside at the atmospheric pressure present at sea level, which is around 14.7 pounds per square inch.

      Last time we learned that the heat energy absorbed by water before it begins to boil inside our example tea kettle is known as sensible heat within the field of thermodynamics.   The more sensible heat that’s applied, the more the water temperature rises, but only up to a point.

      The boiling point of water is 212°F.    In fact this is the maximum temperature it will achieve, no matter how much heat energy is applied to it.   That’s because once this temperature is reached water begins to change its state of matter so that it becomes steam.   At this point the energy absorbed by the water is said to become the latent heat of vaporization, that is, the energy absorbed by the water becomes latent, or masked to the naked eye, because it is working behind the scenes to transform the water into steam.

      As the water in a tea kettle is transformed into steam, it expands and escapes through the spout, producing that familiar shrill whistle.   But what if we prevented the steam from dispersing into the environment and continued to add heat energy?   Ironically enough, under these conditions temperature would continue to rise, upwards of 1500°F, if the stove’s burner were powerful enough.   This process is known as superheating.   Now hold your hats on, because even more ironically, the heat added to this superheated steam is also said to be sensible heat.

      Confused?    Let’s take a look at the graph below to clear things up.

power plant engineering

      Sensible heat is heat energy that’s added to water, H2O, in its liquid state.   It’s also the term used to describe the heat energy added to steam that’s held within a captive environment, such as takes place during superheating.    On the other hand, the latent heat of vaporization, that is the heat energy that’s applied to water once it’s reached boiling point, does not lead to a further rise in temperature, as least as measured by a thermometer.

      Next time we’ll see how surrounding air pressure affects water’s transition from liquid to steam.