Posts Tagged ‘thermodynamics’

Thermodynamics In Mechanical Engineering, Part II, Power Cycles

Sunday, December 13th, 2009

     Last time we talked about some general concepts in an area of mechanical engineering known as thermodynamics.  In this week’s article we’ll narrow our focus a bit to look at a part of thermodynamics that deals with power cycles.

     One mammoth example of a power cycle can be found in a coal-fired power plant.  You can’t help but notice these plants with their massive buildings, mountains of coal, and tall smoke stacks.  They’ve been getting a lot of negative press lately and are a central focus of the debate on global warming, but most people have no idea what’s going on inside of them.  Let’s take a peek.

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Figure 1 – A Coal-Fired Power Plant

     A power plant has one basic function, to convert the chemical energy in coal into the electrical energy that we use in our modern lives, and it’s a power cycle that is at the heart of this conversion process.  The most basic power cycle in this instance would include a boiler, steam turbine, condenser, and a pump (see Figure 2 below).

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Figure 2 – A Basic Power Cycle 

     When the coal is burned in the power plant furnace, its chemical energy is turned into heat energy.  This heat energy and the boiler are enclosed by the furnace so the boiler can more efficiently absorb the heat energy to make steam.  A pipe carries the steam from the boiler to a steam turbine.  Nozzles in the steam turbine convert the heat energy of the steam into kinetic energy, making the steam pick up speed as it leaves the nozzles.  The fast moving steam transfers its kinetic energy to the turbine blades, causing the turbine to spin, much like a windmill (see Figure 3 below).

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Figure 3 – The Inner Workings of a Steam Turbine

     The spinning turbine is connected by a shaft to a generator.  The turbine works to spin the generator and thus produces electricity.  After the energy in the steam is used by the turbine, it goes to the condenser, whose job it is to convert the steam back into water.  To accomplish this, the condenser uses cold water, say from a nearby lake or river, to cool the steam down until it converts from a gas back to a liquid, that is, water.  This is why power plants are normally found adjacent to a body of water.  After things are cooled down, the pump gets to work, pushing the condensed water back into the boiler where it is once again turned into steam.  This power cycle keeps repeating itself as long as there is coal being burned in the furnace, the plant equipment is functioning properly, and electrical energy flows out of the power plant.

     Thermodynamics sets up an energy accounting system that enables mechanical engineers to design and analyze power cycles to make sure they are safe, reliable, efficient, and economical.   When all is said and done, a properly designed power cycle transfers as much heat energy as possible from the burning coal on one end of the cycle to meet the requirements for electrical power on the other end of the cycle.  As was mentioned in last week’s blog, nothing is 100% efficient.

     Next time we’ll learn about being cool.  No, I’m not going to talk about the latest cell phone gadget or who’s connected on Facebook.  We’ll be covering refrigeration cycles.

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Thermodynamics In Mechanical Engineering, Part I

Sunday, December 6th, 2009

     Last week we followed Dorothy through the forest and watched Scarecrow transform from a fire trap to a robust fire-retardant fiberglass composition with the help of materials science.  This week we’ll explore the magical world of thermodynamics, and nobody knows thermodynamics like the Great and Powerful Oz.  In fact, he’s a real “Wiz” at it! 

     But seriously, thermodynamics is one of those out-of-sight, out-of-mind things that we take for granted in our daily lives.  Without thermodynamics we wouldn’t have modern conveniences like electricity, air conditioning, or anything with a motor, like the cars we can’t seem to do without.  The world would essentially be in the Dark Ages again. 

     Often referred to as “thermo” among mechanical engineers, thermodynamics is the science that deals with heat and work in processes used in power plants, refrigeration compressors, and engines.  Thermo also deals with the properties of substances that absorb and release heat energy, things like water (steam), refrigerants, and fuels (coal, gasoline, natural gas, etc.).

     In thermodynamics there are basically two laws that must be obeyed.  The first law states that energy cannot be created or destroyed, it can only be transformed from one form into another.  An example of this principle at work would be when you gas up your car.  According to the first law of thermodynamics, the chemical energy that is released when gasoline is burned by the engine must add up to the work energy put out by the engine to move all its parts and accelerate the car.  The first law sets up an energy accounting system, so to speak.  This principle makes it possible to analyze and design engines, refrigeration equipment, etc. 

     The second law of thermodynamics states that it is impossible to build something that is 100% efficient.  So, going back to the car example above, the second law tells us that we must also account for things like the heat energy lost to the atmosphere from the hot engine parts and the fumes leaving through the exhaust pipe.  This heat energy essentially wastes gasoline and doesn’t do any useful work, but it is a real phenomenon which must be dealt with when doing engineering design work.

     Thermodynamics can be broken down into different subsets, including power cycle analysis, refrigeration cycle analysis, stoichiometry, and psychrometrics. We’ll begin exploring these next time.

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