Posts Tagged ‘steam turbine’

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.

Steam Turbine Engineering Expert

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.

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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.

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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.

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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.

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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.

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      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.

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Heat Energy Within the Power Plant – Water and Steam Cycle, Part 1

Monday, August 5th, 2013

      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.

Electric Utility Power Plant

      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.

electric utility power plant boiler and steam turbine

      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.

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Heat Energy Within the Power Plant—The Power Behind the Turbines

Monday, July 29th, 2013

      Last time we discovered that the boiling point of water varies.   It’s dependent upon the amount of pressure exerted on its surface, which varies due to a variety of reasons, including where it is in relation to sea level.   Before we see what happens under higher than atmospheric pressures, such as exist in an electric utility power plant boiler, let’s cover some basics.

      In the power plant, water is heated in a boiler specifically to produce steam, unlike our tea kettle where the primary purpose is to produce hot water.   The steam produced is used to spin turbine generators, which in turn generate electricity, as I explained in a previous blog on steam turbines.

      Unlike a tea kettle, which is open to the atmosphere on your kitchen stove, the boiler in a power plant is an enclosed, reinforced steel vessel.   See illustration below.

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      The reinforced steel boiler vessel is designed to withstand great internal pressure as temperatures rise within.   In addition to providing a safety feature, the enclosed space provides a sheltered environment for collecting steam so it can later be put to use spinning power generating turbines down the line.   In other words, surface water inside the boiler is closed off from the surrounding atmosphere, allowing its internal pressure to build for our specific purposes.

      As heat energy is added to water within the boiler, the water boils and steam bubbles break out from its surface, filling the empty space above the surface with pressurized steam.   This steam will try to expand here, but it can’t, because it’s being constrained by the reinforced steel vessel within which it is enclosed.   Instead, steam pressure builds up on the surface of the water inside the boiler until it is high enough to be released through an attached pipe which is connected to a nearby turbine.

      We’ll talk more about this pent-up energy and how it is put to use within the power plant in next week’s blog.

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Coal Power Plant Fundamentals – The Generator

Monday, March 7th, 2011
     When I was a kid I remember how cool it was to have a headlight on my bike.  Unlike the headlights that the other kids had, mine was not powered with flashlight batteries.  The power came from a little gadget with a small wheel that rode on the front tire.  As I pedaled along, the tire’s spinning caused the small wheel to spin, and voila, the headlight bulb came to life.  Little did I know that this gadget was a simple form of electrical generator, and of course I was oblivious to the fact that a similar device, albeit on a much larger scale, was being used at a nearby power plant to send electricity to my home.

     Over the last few weeks we learned how a coal fired power plant transforms chemical energy stored in coal into heat energy and then into mechanical energy which enables a steam turbine shaft to spin.  We’ll now turn our attention to the electrical generator.  It’s responsible for performing the last step in the energy conversion process, that is, it converts mechanical energy from the steam turbine into the desired end product, electrical energy for our use.  It represents the culmination in energy’s journey through the power plant, the process by which energy contained in a lump of coal is transformed into electricity.   

    To show how this final energy conversion process works, let’s look at Figure 1, a simplified illustration of an electrical generator.

Figure 1 – A Basic Electrical Generator

     You’ll note that the generator in our illustration has a shaft with a loop of wire attached to it.  When the shaft spins, so does the loop.  The shaft and wire loop are placed between the north (N) and south (S) poles of a horseshoe magnet.  It’s a permanent magnet, so it always has invisible lines of magnetic flux traveling between its two poles.  These magnetic lines of flux are the same type as the ones created by kids’ magnets, when they play with watching paperclips jump up to meet the magnet.  The properties of magnets are not completely understood, even to adults who work with them every day.  And what could be more mysterious than the fact that as the shaft and wire loop spin through the lines of magnetic flux in the generator, an electric current is produced in the wire loop.

     Now, this current that’s flowing through the spinning wire loop is of no use if we can’t channel it out of the generator.   The wire loop is spinning vigorously, so you can’t directly connect the ends of the loop to stationary wires.  A special treatment is required.  Each end of the loop is connected to a slip ring.  A part called a “brush” presses against each slip ring to make electrical contact.  The electrical current then flows from the loop through the spinning slip rings, through the brushes, and into the stationary wires.   So, if, for example, a light bulb is connected to the other end of the stationary wires, this completes an electric circuit through which current can flow.  The light bulb will glow as long as the generator shaft keeps spinning and the wire loop keeps passing through the magnetic lines of flux from the magnet.

     So we see that the key to the whole energy conversion process is to have movement between magnetic lines of flux and a loop of wire.  As long as this movement occurs, the electricity will flow.  This basic principle is the same in a coal fired power plant, but the electrical generator is far more complicated in construction and operation than shown here.  My Coal Power Plant Fundamentals seminar goes into far greater detail on this and other aspects of electricity generation, but what I have shared with you above will give you a basic understanding of how they operate.

     That concludes our journal with coal through the power plant.  This series of blogs has, you will remember, presented a simplified version of the complex material presented in my teaching seminars.  Next week we’ll branch off, taking a look at why electrical wires come in different thicknesses.   

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Coal Power Plant Fundamentals – The Steam Turbine

Sunday, February 20th, 2011

     When I was a kid I didn’t have video games or cable TV to help me occupy my time.  Back then parents tended to be frugal, and the few games I had were cheap to buy and simple in operation, like the plastic toy windmill I’d play with for hours on end.  All I had to do to make it spin was take a deep breath, pucker my lips together, fill my cheeks with breath, then blow hard into the windmill blades.  Its spin was fascinating to watch.  Little did I know that as an adult I would come to work with a much larger and complex version of it, in the form of a power plant’s steam turbine.

     You see, when you trap breath within bulging cheeks and then squeeze your cheek muscles together, you actually create a pressurized environment.  This air pressure buildup transfers energy from your mouth muscles into the trapped breath within your mouth, so that when you open your lips to release the breath through your puckered lips, the pressurized energy is converted into kinetic energy, a/k/a the energy of movement.  The breath molecules flow at high speed from your lips to the toy windmill’s blades, and as they come into contact with the blades their energy is transferred to them, causing the blades to move.  A similar process takes place in the coal power plant, where steam from a boiler takes the place of pressurized breath and a steam turbine takes the place of the toy windmill.

     If you recall from my previous article, the heat energy released by burning coal is transferred to water in the boiler, turning it to steam.   This steam leaves the boiler under great pressure, causing it to travel through pipe to the steam turbine, as shown in Figure 1.

Figure 1 – A Basic Steam Turbine and Generator In A Coal Fired Power Plant

     At its most basic level the inside of a steam turbine looks much like our toy windmill, of course on a much larger scale, and it is very appropriately called a “wheel.”  See Figure 2.  

Figure 2 – A Very Basic Steam Turbine Wheel

     The wheel is mounted on a shaft and has numerous blades.  It makes use of the pressurized steam that has made its way to it from the boiler.  This steam has ultimately passed through a nozzle in the turbine that is directed towards the blades on the wheel.  This is the point at which heat energy in the steam is converted into kinetic energy.  The steam shoots out of the nozzle at high speed, coming into contact with the blades and transferring energy to them, which causes the turbine shaft to spin.  The turbine shaft is connected to a generator, so the generator spins as well.  Finally, the spinning generator converts the mechanical energy from the turbine into electrical energy.

     In actuality, most coal power plant steam turbines have more than one wheel and there are many nozzles.  The blades are also more numerous and complex in shape in order to maximize the energy transfer from the steam to the wheels.  My Coal Power Plant Fundamentals seminar goes into far greater detail on this and other aspects of steam turbines, but what I have shared with you above will give you a basic understanding of how they operate. 

     So to sum it all up, the steam turbine’s job is to convert the heat energy of steam into mechanical energy capable of spinning the electrical generator.  Next time we’ll see how the generator works to complete the last step in the energy conversion process, that is, conversion of mechanical energy into electrical energy.

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Coal Power Plant Fundamentals – Combustion

Sunday, February 13th, 2011
     Ever have a small child threaten to hold his breath until he passes out and he actually managed to do it?  It’s not that unusual.  And if his body were prevented from acting in self preservation, that is, taking in breaths while he was unconscious, leading to his eventual awakening, he would die.  While the human body can survive about a month without eating and three days without water, under normal conditions it can survive only a matter of minutes without breathing.  Power plants, too, require oxygen to function, and this process is called combustion.

      Human lungs, along with the diaphragm which works to expand and release the lung cavities, enable our bodies to breathe in air, then expel the waste product, carbon dioxide.  Oxygen is needed to metabolize, that is burn, our food, enabling the food cells’ energy to be absorbed by our bodies and converted into energy to live.  Like us, coal power plants need to breathe in oxygen in order to convert coal’s latent energy into a usable form.

     Previously we learned how coal is fed to a coal mill where it is pulverized into a fine powder.  This powder is then sucked out of the mill by the exhauster and blown through a serpentine path of pipes leading to the burners on the furnace.  The burners will then act upon the coal, combining it with the oxygen in our atmosphere to create a chemical reaction capable of releasing coal’s energy in the form of heat.  All this activity looks to a bystander like a massive, sustained fire in the furnace.  See Figure l.   

Figure 1 – Coal Power Plant Combustion

     The boiler is contained within the furnace and is situated so it is exposed to fire from the combustion process.  Heat energy from the fire transfers into the water in the boiler, much like when you boil water for tea in a kettle on your stovetop.  If you’ve ever boiled water, you know that once it gets hot enough it will turn into steam, and the same for our furnace boiler.  The steam emitting from the boiler will cause a turbine-generator to spin, and the end result will be electricity for our use.  In the simple diagram of Figure 1,  waste products from the combustion process, like carbon dioxide, go up the smoke stack and are released into the atmosphere.  Incidentally, this is the same type of carbon dioxide that we exhale from our bodies when we breathe.  

     Please keep in mind that Figure 1 is a very simplified diagram.  In reality waste products leaving the furnace go through various pollution control devices where most pollutants are removed before they reach the smoke stack.  These details, and many more, are the type of information that would be covered during my training seminar, Coal Power Plant Fundamentals.    

     Next time we’ll learn how the heat energy in steam is converted into mechanical energy capable of spinning a turbine generator to make electricity.

 

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Coal Power Plant Fundamentals – Feeding The Furnace

Sunday, February 6th, 2011
     Today we’ll continue our discussion of coal’s journey through a power plant.  Keep in mind that the material presented in this series of blogs is meant to be a primer.  It is a simplification of what actually goes on.  My training seminars go into much more depth.

     Now imagine a five course meal spread out on the table before you. You load up your plate and pack a forkful of food into your mouth.  You instinctively chew, getting the digestive process underway and making it easier to swallow.  Power plants approach their consumption of coal in much the same way.

     Last time we talked about handling the coal and filling up silos for short term storage within the power plant building.  The coal silo is analogous to a dinner plate, and the furnace, which heats up the boiler water to make steam for the turbine, acts very much like a diner’s stomach.  As for the fork and your teeth, there are a couple of machines within power plants which mimic their behavior.  They’re called the coal feeder and coal mill.  The coal feeder does as its name implies, it systematically feeds a measured amount of coal to the coal mill.  The coal mill, also known as a pulverizer, then grinds the coal to make it easier for the furnace to burn it.

     Let’s take a look at Figure 1 below.  At the top of the configuration is the coal silo, which is fully open at the bottom.  Gravity draws the coal within the silo downward, facilitating the coal’s dropping through the opening into a chute, on its way to the coal feeder.  The coal from the silo spills into little buckets on a wheel within the feeder, and as the wheel turns, the coal spills out and falls down into another chute leading to the mill. 

Figure 1 – Feeding Coal To A Power Plant Furnace

     Now you could have the coal spill down a chute directly from the silo into the mill, bypassing the coal feeder entirely, but that’s really not a good idea.  Just think how difficult it would be to chew if you tried to stuff an entire plate of food into your mouth at once.  Just as your mouth requires to be fed in mouth-sized amounts, the coal mill must be fed coal in a size that it can handle.  It’s the job of the spinning wheel inside the coal feeder to keep coal flowing in measured amounts to the mill.  You see, the wheel is attached to a variable speed motor, and depending on how quickly the furnace needs to be fed, the wheel will either turn faster or slower.

     Once inside the mill, the coal is ground up before moving on to the furnace.  The coal mill contains massive steel parts capable of pulverizing chunks of coal into a fine black powder.  This pulverized coal is then propelled by means of an exhauster towards the burners.

     The exhauster sits next to the coal mill and both are often driven by the same electric motor.  The exhauster is  connected to the top of the mill by a pipe, and another pipe connects the exhauster to burners on the furnace.  The exhauster acts like a big vacuum cleaner, sucking coal powder out of the mill, then blowing it through pipes leading to the burners.  Finally, the powder ignites within the furnace, heating the water inside the boiler.

     Next time we’ll learn about the combustion process in the power plant furnace. 

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