Posts Tagged ‘sensible heat’

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.

Coal Power Plant Engineering Expert Witness

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.


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.

power plant expert

      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.


Forms of Heat Energy – Sensible

Sunday, July 7th, 2013
      In our house the whistle of a tea kettle is heard throughout the day, no matter the temp outside.  So what produces that familiar high pitched sound?

sensible heat power plant boiler

      When a tea kettle filled with room temperature water, say about 70°F, is heating on the stove top, the heat energy from the burner flame will transfer to the water in the kettle and its temperature will steadily rise.  This heat energy that is absorbed by the water before it begins to boil is known as sensible heat in thermodynamics.  To read more about thermodynamics, click on this hyperlink to one of my previous blog articles on the topic.

      So, why is it called sensible heat? It’s so named because it seems to make sense.  The term was first used in the early 19th Century by some of the first engineers who were working on the development of boilers and steam engines to power factories and railways.  Simply stated, it’s sensible to assume that the more heat you add to the water in the kettle, the more its temperature will rise.

      So how high will the temperature rise?  Is there a point when it will cease to rise?  Good questions.  We’ll answer them next week, along with a discussion on another form of heat energy known as the latent heat of vaporization.