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Try this for a tongue-twister: Coal fired electric utility power plant boiler… If you’ve been reading along with us for the last couple of weeks, you now have a pretty good idea of what these are and what they do. These boilers are contained within furnaces in coal fired power plants. The furnace’s job is to combine coal and air to create a combustion process. It is like a big, insulated enclosure that keeps the heat energy from the combustion process from escaping before it can be absorbed by the water and steam in the boiler tubes. The heat energy is then funneled to the steam turbine to spin an electrical generator, creating the energy which will eventually find its way into our homes and businesses. During the operation of the boiler, coal and air must be introduced into the furnace at carefully measured rates to maintain a proper fuel-to-air ratio which will enable the release of heat energy from the coal at a safe, controlled rate. Fuel-air ratio is the amount of coal entering the furnace divided by the amount of air entering the furnace. If this ratio isn’t precisely maintained, conditions may be right for an explosion to occur. Specifically, the ratio has to fall within an “explosive range.” Once within this range, all that is needed is an ignition source, such as hot ash, or even mere static electricity, and the result may be a furnace explosion. There are certain times at which furnace explosions are more likely to occur than others, such as when the boiler is being started, operated at less than full capacity, or shut down. When a furnace explodes, a pressure wave moves out from the center of the blast. This pressure wave will bear up against the sides of the furnace with great force, and if the pressure is high enough the sides of the furnace, which are made of heavy steel components, will actually bend and split open. Boiler tubes may even rupture, releasing high pressure steam and water into the power plant and furnace. At the very least, the boiler will be down for expensive repairs and no electricity can be produced by its turbine generator. This down time can last for many months and results in lost revenue to the energy producer. Aside from an explosive fuel-to-air ratio, there are other potential causes of furnace explosions. For example, poor coal quality can lead to incomplete combustion, or the flame going out completely, encouraging unburned coal particles to settle and accumulate in the furnace. The accumulation of coal can grow to the point where it forms an explosive mixture when combined with the right amount of air. So how can boiler explosions be prevented? The National Fire Protection Association (NFPA) looked into the problem and developed an industry standard. This standard is known as NFPA 85, Boiler and Combustion Systems Hazards Code. Its purpose is to contribute to operating safety and prevent uncontrolled fires, explosions, and implosions of coal fired boilers. NFPA 85 lays out guidelines to follow when designing, building, and operating boiler fuel handling systems, air handling systems, and combustion control systems. Following its guidelines will certainly significantly decrease the probability of explosions occurring. Another means of explosion prevention includes implementing a boiler operator training program. These enable attendees to better understand operating procedures and equip them with the knowledge to safely control the combustion process, particularly when a furnace explosion is most likely to occur. This training can be done with a combination of classroom instruction along with time on a simulator and may be followed up with hands-on training in the plant itself. Lastly, boiler explosions can be prevented by implementing an effective inspection and maintenance program to locate and repair or replace boiler components, averting the possibility of a potential disaster occurring. Things such as check lists can be used to ensure that nothing is missed. This is a strategy that all pilots must use before starting their planes, and it is now being used in hospitals as well to cut back on the rate of patient infection due to carelessness on the part of hospital staff. Hey, we’re all human, and humans are not perfect. But remember that an ounce of prevention is truly worth a pound of cure, and then some. A properly placed check on the list could mean lives will be saved. _____________________________________________
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Coal Fired Boiler Explosions
July 25th, 2010
Coal Power Plants, Far From Perfect
July 18th, 2010|
Did you know that even a perpetual motion machine will eventually come to a stop due to uncontrollable factors? Well, uncontrollable factors are at play in power plants, too. If you recall from our last article, heat rate is industry jargon for gauging how efficiently a coal-fired power plant is operating. We learned that heat rate can be affected by things like missing thermal insulation on pipes and equipment. Missing insulation is, of course, a thing that is under human control and easily corrected, but there are some things that affect heat rate that we just can’t do anything about. They’re called, appropriately enough, uncontrollable factors. Uncontrollable factors exist because anything devised and made by fallible humans who are beholden to the myriad laws of the universe cannot be 100 percent efficient. At their best utility coal fired power plants have an overall efficiency of between 30 and 40 percent. That means 60 to 70 percent of the energy available in the coal gets lost in the process of generating electricity. A terrible waste, right? And yet there’s nothing we can do to trim these losses until improvements in the present level of technology take place. Just as our ability to track microbes is dictated by the strength and accuracy of our magnifying equipment, so are we hampered by the tools we have at our disposal to deal with inefficiencies such as energy losses. So where does this energy get lost due to uncontrollable factors? The first and probably most obvious place to look is the smoke stack. Energy is also lost in three other ways: friction between equipment parts, auxiliary power consumption, and in a piece of equipment known as a condenser. Let’s look at each. In the most basic of terms, when coal is introduced into a power plant boiler it is combined with air and burned. This burning process releases heat energy, but it also forms gases that contain nitrogen and compounds like carbon monoxide and carbon dioxide. There’s also some water vapor formed by moisture in the coal and air. These gases and vapor absorb some of the heat energy released. To keep the combustion process going the gases and vapor must be removed from the boiler by powerful fans and sent up the smoke stack. Now, boilers are designed to absorb much of the heat energy from the gases and vapor that make their way to the stack, but they cannot possibly absorb it all. The result is that a significant amount of heat escapes up the smoke stack into the atmosphere along with the gases. Friction between parts is present everywhere in a power plant. It exists in the bearings on the shafts of motors, pumps, and steam turbines, slowing them down and hindering their operating capacity. Friction also exists where moving water and steam are present, impeding their ability to flow through piping systems. There is even friction working against the steam as it flows through parts in the turbine. Extra energy has to be expended to overcome this friction. This is energy that could be used to generate electricity. Now at some point in your life you’ve probably heard it said, “You need money to make money,” and this is very true. It takes a certain investment of resources to produce a profit-making enterprise. This investment principle holds true for the making of electricity, too. The bottom line is you need electricity to make electricity. Specifically, you have to use significant amounts of electricity to power machinery that is essential to move coal, air, combustion gases, and water through the process of making electricity in the power plant. This is called auxiliary power. It’s the electricity siphoned off by the various pieces of equipment in a power plant in its quest to generate electrical energy to be sold to customers. Another major factor at play in uncontrollable energy losses is in a piece of equipment integral to the very function of power plants: the condenser. It comes into play when water is boiled to make steam which then travels through the turbine, spinning its electrical generator and creating electric power. Unfortunately even the most efficient of steam turbines cannot use 100% of the heat energy coming at it from the steam. You see, after steam leaves the turbine, it’s turned back into water by a condenser so it can be sent back to the boiler to be turned into steam again. One of the reasons that this is done is so that the boiler does not have to be continuously filled with fresh, purified water. Water purification is necessary to keep minerals, seaweed, fish scales, and other nasty things from clogging up and damaging the boiler and steam turbine, and purified water is not as readily available as, say, lake water. The condenser acts as a heat exchanger that is hooked up to the steam turbine exhaust. It has tubes inside of it in which cold water flows, water which is drawn in from a nearby body of water, most often a river or lake. As steam blows across the outside of the cold water tubes in the condenser, it gives up its remaining heat energy and condenses into water again, then it is returned to the boiler to repeat its journey. The river water within the tubes of the condenser flows back into the river, carrying with it the heat energy removed from the steam. That wraps up our discussion about coal power plant efficiency. Next time we’ll discuss a new topic: coal fired power plant furnace explosions. _____________________________________________ |
Coal Power Plant Efficiency
July 11th, 2010|
Is there any price a man dying of thirst in the desert would not pay for a tall glass of cold water? What is the point at which Americans will decide they can do without heat, refrigerators, electric lights? My neighbor refuses to run the air conditioner, even when it’s 90 degrees and 90 percent humidity. They have obviously made the choice to sweat and be uncomfortable in their homes rather than pay high utility bills. Most of us are concerned with the environment, but when times are hard like they are now many of us become more concerned with our pocketbooks. Just as we need to make our financial ends meet, so do energy suppliers. Without a certain level of profit, their service to us will decline, and regular, dependable delivery of their precious commodities to us will suffer. If they were to go out of business, what then? Reading by candlelight may be romantic for a night or two, but nights on end? Let’s consider the energy provided by coal-fired power plants, for example. They’re in the electric utility business, and they provide us with the lion’s share of our energy. To keep a handle on operating costs, power plant engineers monitor how many British Thermal Units (BTUs) of heat energy are going into the power generation process versus how many kilowatt-hours of electricity are coming out. What’s a BTU and what does it matter to us? Well, it’s the amount of heat energy your kitchen stove uses to raise the temperature of one pint of water by one degree Fahrenheit. As for a kilowatt-hour, that’s a thousand watts of power produced over the space of an hour– enough to light ten 100 watt light bulbs. Now that we’ve explained the key term, we can explore the notion of heat rate, terminology very important to efficient power plant operation. Heat rate is simply the ratio of BTUs to kilowatt-hours. So what’s the importance of monitoring heat rate? For one thing, in order to get the most bang for your buck you want to keep the heat rate as low as possible. When the heat rate is high, you’re burning more coal than you have to because you’re wasting heat energy. This results in higher electricity costs to the consumer. This is exactly the situation at play when low sulfur coals are used as compared to the better burning coals of yester-year. So where does the wasted heat energy go if it isn’t being converted into electrical energy? For one thing, it can be lost through steam and water leaks in the power plant piping system. There are other losses too. Another way to lose heat energy is when thermal insulation is missing from pipes, causing heat to escape into the atmosphere. The opposite side of inefficiency is presented by the problem of too much heat energy building up, unable to be transferred to the steam. This is the result if ash is allowed to accumulate inside the boiler, acting as a thermal insulator. The heat has nowhere to go except up the smoke stack and into the atmosphere. Needless to say it’s important to keep heat rate as low as possible by keeping power plant equipment insulated and in good repair. But there are some things that affect heat rate that we just can’t do anything about, they’re known as “uncontrollable factors,” and we’ll learn about them next week. _____________________________________________
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Low Sulfur Coal – What Does It Cost?
July 4th, 2010|
Are you familiar with the adage, “Things are not always as they seem”? It’s probably come into play in your life at one time or another, like when you opted to buy the cheapest model of something, only to find out that its life span was two weeks before falling apart. Not such a bargain after all. Well, it’s kind of that way with low sulfur coal and its application in electric power production. All coals contain some sulfur, their content ranging from trace amounts to as high as 8%. This sulfur ends up as a byproduct of the combustion process, meaning it is released into the atmosphere when coal is burned. There it combines with moisture in the air to form sulfuric acid. If you will remember from last week’s blog, this is the stuff that forms acid rain, able to dissolve marble statues, corrode metal, and disrupt eco systems. In the process of generating electricity for homes and businesses, many utility power plants of the past burned coal with high sulfur content. This was the case through the middle of the 20th Century. This coal was brought into power plants by trains and river barges from nearby coal mines. In some cases power plants were actually built next to the mines, thereby eliminating shipping cost. It was effective and cheap. Then, in 1963, the Clean Air Act was signed into law, its purpose to improve, strengthen, and accelerate programs for the prevention of air pollution. By 1970 the Act had empowered the federal government to set and enforce national air quality standards for sources of air pollution, like coal burning power plants. Under the Clean Air Act, government was able to mandate to utilities that they reduce sulfur emissions or face court injunctions to shut them down. Caught between a rock and a hard place, utilities learned to comply, switching over to lower sulfur coals. But the story doesn’t end here. That lower sulfur created a whole host of new problems, for the power plant and their consumers. To begin with, low sulfur coals are scarce in areas of the country where electricity is needed most, like the densely populated eastern half of the country. It has to come from mines in the western states like Wyoming, and for a power plant located in Chicago, for example, this can get costly. A lot more costly than simply getting the coal, high sulfur content coal, that is, from nearby mines in southern Illinois. The result is higher transportation costs, and this cost is passed on to consumers. Another problem with low sulfur coals is that they tend to release less heat energy than higher sulfur coals when they are burned. That means that you have to burn more of it to generate the same amount of power. As a result, utilities ended up having to buy more coal, another cost that was passed on to the consumer. Yet another issue with the switch from high sulfur to low sulfur coals involved the reconfiguration of power plants that was made necessary. You see, when power plant boilers are designed, they have a particular type of coal in mind, and that originally was high sulfur coal. In addition, many power plants have been required to install equipment to scrub sulfur from the gases produced when the coal is burned. This scrubbing equipment is expensive to purchase, install, and operate. Pollution control equipment like this consumes power, but it does not facilitate the process of generating electricity. In addition to these costs, the switch to low sulfur coal causes many other problems that can raise the cost of operations and make the power plant less reliable. For example, some low sulfur coals have properties that tend to make ash stick to the surfaces inside of boilers, often leading to boilers overheating and springing leaks. If these leaks are bad enough, the boiler has to be shut down for cleaning and repair, and when this happens the electrical generating unit has to be taken off the utility grid. The net result is less power being available to meet consumer demand. We can thank the Clean Air Act for effectively reducing the amount of airborne pollutants, but we must acknowledge the cost to do so. Electric utilities are for-profit corporations, not charities, and someone has to pay for the increased coal consumption, higher transportation costs, equipment additions, and operating problems that are a result of the usage of low sulfur coal. That someone is the consumer. _____________________________________________
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Fossil Fuel, From Friend to Foe
June 27th, 2010|
Did you ever have someone you considered to be a great friend and then things suddenly went bad between you? One day you’re chums and then the magic fades, soon to disappear? Sound like some marriages you’ve heard about? Well, it wasn’t too long ago that coal was considered to be America’s affordable answer to our fuel needs. It was a friend of grand proportions, there when you needed it. It remains an abundant resource, so abundant in fact that according to the US Energy Information Administration (EIA) we are sitting on coal reserves so vast they can provide us with sufficient energy to get us through the next 250 years at current rates of consumption. It was for these reasons that electric utilities decided decades ago to use coal as the primary source of fuel to generate electricity, and as it stands now just over 50% of our electrical energy is generated by burning coal. So how did coal go from being friend to foe? Well, just as when you’ve known someone for awhile their “baggage” becomes more apparent, it eventually became apparent to Americans that burning coal comes with some nasty baggage of its own, known as byproducts. These unwelcome components of the burning/oxidation process were found in the plumes of smoke that billowed out of power plants’ smokestacks. So just what are these byproducts? Well, some of it is the same stuff that’s left over at the bottom of your barbecue grille after a cookout, and some of it comes with scientific names like sulfur dioxide (SO2), nitric oxide (NO), and nitrous oxide (N2O). Let’s look at these in more detail. Ash is the residue that’s left behind after coal is burned. Fly ash is a type of ash that is made up of some very light particles and it can get carried away by the hot gases coming off the fire in a power plant boiler. Some of those particles manage to leave the smokestack and enter the environment. Sulfur dioxide, or SO2, is formed when the sulfur in coal combines with oxygen in the air during burning. When the SO2 leaves the smokestack, it can combine with moisture in the atmosphere to form acid rain. Most of us know what acid rain is, but for those that don’t, acid rain does things like rust metal, dissolve marble monuments, and in general disrupt the balance of Earth’s eco systems. Nitric oxide, NO, and nitrous oxide, N2O, are chemical compounds composed of nitrogen and oxygen that fall into the group commonly referred to as NOx, pronounced “knocks.” NOx is formed when nitrogen and oxygen in the air combine at the high temperatures released when coal is burned inside power plant furnaces. NOx is bad because its compounds are key ingredients in the formation of both acid rain and smog. Over the last thirty years emissions of these byproducts have come under increasing scrutiny by federal and state regulators in their quest to curb them and their impact on our environment. As a result, electric utilities have had to comply with ever-tightening regulations. To comply, coals with lower sulfur content have been used, often brought in over very long distances from mines in the US and even foreign countries like Columbia. Utilities have also been installing expensive pollution control equipment in their coal fired power plants. But these changes make operations more expensive, eating into the utilities’ profits. Now we may not like the idea of utilities earning a profit, but this is a necessary reality to some extent in order to keep their business solvent. They’re not in it for the fun of it, after all. And I’m sure you guessed by now that the net result of the regulatory agencies’ mandates is that our electric bills just keep escalating. Now much of what lies behind the current unfavorable status of coal powered plants is that when operating on our native soil they have high visibility. We don’t like to be reminded of the negatives that accompany the production of energy. Put that same plant in another faraway country and the byproducts cease to be an issue. It’s happening over there after all, and we don’t have to be confronted with it. We neglect to remind ourselves that the earth’s atmosphere is for the most part a contained unit, and that means that what happens there is happening here, whether there happens to be on the other side of the globe or not. Next week we’ll continue our explorations into coal, examining the impact of the low sulfur variety on electric utility power generation. _____________________________________________
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CAD To The Rescue
June 20th, 2010|
Remember Mike, the dad on The Brady Bunch, forever hunched over his drafting table while Alice, the housekeeper, regales him with yet another tale of Brady kids gone rambunctiously wrong? Although he seemed very attached to his mechanical pencil and rolls of architectural drawings, he’d have been far better off with a computer and CAD software. CAD, or Computer Aided Design, makes the life of architects, engineers, and designers of various sorts a whole lot easier. It’s been around for a few decades now, and its applications just keep getting broader. Once familiar with the workings of this software, one can produce technical diagrams in record time, and mistakes are just a delete button away from being eradicated. In fact, it’s very much like a word processor for graphics, not words. Nowadays, for all intents and purposes, the drafting table, pencils and rulers are pretty much extinct from usage by architects and engineers, much like The Brady Bunch television show. Remember the days of typewriters and carbon paper? If you do, you just dated yourself, because these haven’t been in general usage for quite some time. Word processing software and computers in general have relegated these instruments to become dusty on the shelves of historical museums. Why laboriously copy an object by hand, over and over again, when you can cut and paste your way to duplication perfection using CAD? There are even special features within CAD that allow for replication of a drawing detail in an array, meaning it is automatically replicated a specified number of times while being spaced precise distances apart along a straight line or circle. Let’s look at a simple example of how CAD makes our life so much easier. Figure l shows a single gear tooth, drawn in CAD. Figure 2 shows the tooth automatically replicated ten times and displayed in a circular array, each tooth a precise space away from its nearest neighbor. Figure 3 shows all of the gear teeth connected together with lines to form a completed drawing. Before the advent of CAD, an engineer would have had to replicate each gear tooth by hand using physical tools like a compass, pencil, and scale ruler. Figure 1 – A Single Gear Tooth Drawn With CAD
Figure 2 – A 10 Tooth Circular Array Created With CAD
Figure 3 – A Completed CAD Drawing of a 10 Tooth Gear
Another useful feature of CAD is how it can be used to annotate drawings, that is, put notes, labels, and dimensions on them. This nifty feature has rendered precise penmanship obsolete. Gone are the days when student engineers labored by hand and pencil to produce precisely the lettering and numbering conventions that are acceptable within their discipline. Poor penmanship could be disastrous to both the engineer’s career and the final product due to the difficulty it would present in reading and proper interpretation. CAD produces a level of uniformity never before possible, but of course one still needs to know how to spell! If you look back through my blog articles, you’ll see many diagrams that I created with CAD software, thus rendering complicated technical concepts easier to understand. Illustrations are most often easier to understand by the general population than the written word, and as one of my past blogs was titled, A Picture is Worth a Thousand of Them, “them” being words, of course. I also routinely use CAD software in other aspects of my profession to create everything from electrical schematics to flow charts, PowerPoint presentations slides, and engineering expert reports. _____________________________________________
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Regenerative Brakes
June 13th, 2010|
Have you ever been stranded on a subway car? The lights flickered wildly, then the odd humming sounds all came to an abrupt stop, and you sat there exchanging uncomfortable glances with your fellow passengers? “Hey, get this thing going! I’ve got an appointment to make!” someone shouts at the driver, sequestered in his cab. The trouble is, he, like the rest of the passengers, is equally helpless in this situation. You see, streetcars, subway cars, and light rail cars are all forms of electric railway cars, and when their source of electricity goes out, so do they. They’re similar to the diesel-electric locomotive we looked at last week because they use electric traction motors for propulsion, meaning to move forward. But their difference lies in the fact that electric rail cars don’t carry their own source of power and are entirely reliant on an external source, an electrical substation. See Figure 1 below.
Figure 1 – Electric Railway Car Propulsion System This substation performs the task of taking the power provided by an electric utility power plant and converting it into a form of power that the electric rail car can use to operate its traction motors. The two are connected via a trolley wire and the two rails that the car runs on. The railway car has a spring-loaded arm called a pantograph on its roof that touches the trolley wire, allowing electrical current to flow into a speed control system housed under the car. This speed control system performs the task of varying the flow of electrical current to the traction motors, enabling the car to move, before it eventually exits the motor through its wheels, then back to the substation where it originated, thus completing an electrical circuit. Many newer electric railway cars couple a regenerative braking system with a mechanical one. Their operation is similar in nature to a dynamic braking system where the traction motors are turned into generators. The difference is that with regenerative braking systems the current from the traction motors is sent to the trolley wire through the pantograph as shown in Figure 2 below.
Figure 2 – An Electric Railway Car Using Regenerative Brakes This diagram shows the railcar generating electricity, but it may not be so obvious how its motion is made to slow down, after all, we see no resistor grids like we did in last week’s illustration of a dynamic braking system. So how does it stop? The trick here is that there are other cars running on the rail line at the same time which are using electrical current to move forward. So what does this have to do with stopping it you ask? Let’s take a look at Figure 3 for clarification. Figure 3 – How Regenerative Brakes Help Save Power In this illustration we see that as Car A goes downhill and the operator applies the brakes, the regenerative brake will be caused to start pumping electrical current into the trolley wire. Now, if Car B is on the same rail line going up the other side of the hill, it will need power to climb that hill, and it will need to draw that power from the trolley wire by way of its pantograph. But instead of drawing all its electrical current from the substation, Car B will first draw off the current produced by nearby Car A, and only then will it draw the remainder of its power requirements from the substation. During this braking process the kinetic energy in Car A is converted into electrical energy by its traction motors. Then Car B uses its own traction motors to convert the electrical energy drawn from Car A into mechanical energy, enabling it to climb the hill. Car B has effectively robbed Car A of its energy, so Car A slows down. As we discovered last week during our discussion of dynamic brakes, regenerative brakes become ineffective below a certain minimum speed. This is the reason that electric rail cars need mechanical brakes to complete the job of stopping. We see that the regenerative braking process is actually quite green. It allows for electrical energy that would normally be wasted as heat energy escaping into the atmosphere to be converted into useful energy, taking a significant chunk out of the demand for new energy off of substations. It also helps the electric railway to save money when it comes time to paying the electric bills. You may have noticed a copyright symbol and my name included in this week’s illustrations. That’s because it has recently come to my attention that many of my readers are unaware that I am the CAD (Computer Aided Design) artist who creates the illustrations featured in most of my blogs. CAD is a useful tool in explaining difficult technical subject matter, which is precisely why I use it so often. Next week we’ll explore in more depth the benefits of using this tool.
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Diesel Locomotive Brakes
June 6th, 2010|
In the past few weeks we’ve taken a look at both mechanical and dynamic brakes. Now it’s time to bring the two together for unparalleled stopping performance. Have you ever wandered along a railroad track, hopping from tie to tie, daring a train to come roaring along and wondering if you could jump to safety in time? Many have, and many have lost the bet. That’s because a train, once set into motion, is one of the hardest things on Earth to bring to a stop. In this discussion, let’s focus on the locomotive. A large, six-axle variety is shown in Figure 1.
Figure 1 – A Six-Axle Diesel-Electric Locomotive These massive iron horses are known in the industry as diesel-electric locomotives, and here’s why. As Figure 2 shows, diesel-electric locomotives are powered by huge diesel engines. Their engine spins an electrical generator which effectively converts mechanical energy into electrical energy. That electrical energy is then sent from the generator through wires to electric traction motors which are in turn connected to the locomotive’s wheels by a series of gears. In the case of a six-axle locomotive, there are six traction motors all working together to make the locomotive move. So how do you get this beast to stop? Figure 2 – The Propulsion System In A Six-Axle Diesel-Electric Locomotive You probably noticed in Figure 2 that there are resistor grids and cooling fans. As long as you’re powering a locomotive’s traction motors to move a train, these grids and fans won’t come into play. It’s when you want to stop the train that they become important. That’s when the locomotive’s controls will act to disconnect the traction motor wires running from the electrical generator and reconnect them to the resistor grids as shown in Figure 3 below.
Figure 3 – The Dynamic Braking System In A Six-Axle Diesel Electric Locomotive The traction motors now become generators in a dynamic braking system. These motors take on the properties of a generator, converting the moving train’s mechanical, or kinetic, energy into electrical. The electrical energy is then moved by wires to the resistor grids where it is converted to heat energy. This heat energy is removed by powerful cooling fans and released into the atmosphere. In the process the train is robbed of its kinetic energy, causing it to slow down. Now you may be thinking that dynamic brakes do all the work, and this is pretty much true, up to a point. Although dynamic brakes may be extremely effective in slowing a fast-moving train, they become increasingly ineffective as the train’s speed decreases. That’s because as speed decreases, the traction motors spin more slowly, and they convert less kinetic energy into electrical energy. In fact, below speeds of about 10 miles per hour dynamic brakes are essentially useless. It is at this point that the mechanical braking system comes into play to bring the train to a complete stop. Let’s see how this switch from dynamic to mechanical dominance takes place. A basic mechanical braking system for locomotive wheels is shown in Figure 4. This system, also known as a pneumatic braking system, is powered by compressed air that is produced by the locomotive’s air pump. A similar system is used in the train’s railcars, employing hoses to move the compressed air from the locomotive to each car.
Figure 4 – Locomotive Pneumatic Braking System In the locomotive pneumatic braking system, pressurized air enters an air cylinder. Once inside, the air bears against a spring-loaded piston, as shown in Figure 4(a). The piston moves, causing brake rods to pivot and clamp the brake shoes to the locomotive’s wheel with great force, slowing the locomotive. When you want to get the locomotive moving again, you vent the air out of the cylinder as shown in Figure 4(b). This takes the pressure off the piston, releasing the force from the brake shoes. The spring in the cylinder is now free to move the shoes away from the wheel so they can turn freely. We have now returned to the situation present in Figure 2, and the locomotive starts moving again. Next week we’ll talk about regenerative braking, a variation on the dynamic braking concept used in railway vehicles like electric locomotives and subway trains. _____________________________________________
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Dynamic Brakes
May 31st, 2010|
Last week we looked at how a mechanical brake stopped a rotating wheel by converting its mechanical energy, namely kinetic energy, into heat energy. This week, we’ll see how a dynamic brake works. Chances are you have directly benefited by a dynamic braking system the last time you rode in an elevator. But, to understand the basic principle behind an elevator’s dynamic brake system, let’s first take a look at the electric braking system in Figure 1 below.
Figure 1 – A Simple Electric Braking System Here the brake consists of an electric generator wired via an open switch to an electrical component called a resistor. The weight is attached to a cable that is wound around a pulley on the generator’s shaft. As the weight freefalls, the cable unwinds on the pulley, causing the pulley to turn the generator’s shaft. Unlike last week’s mechanical brake which required a good deal of effort to employ, a dynamic braking system requires very little. All that needs to be done is to close a switch as shown in Figure 2 below. When the switch is closed, an electrical circuit is created where the resistor gets connected to the generator. The resistor does as its name implies: it resists (but doesn’t stop) the electrical current flowing through it from the generator. As the electrical current fights its way through the resistor to get back to the generator, the resistor gets hot like an electric heater. This heat is dissipated to the cooler surrounding air. At the same time, the weight begins to slow down in its descent. But how is this happening? The electric braking system can be thought of as an energy conversion process. We start out with the kinetic, or motion energy, of the freefalling weight. This kinetic energy is transmitted to the electrical generator by the cable, which spins the generator’s shaft as the cable unwinds. Electrical generators are machines that convert kinetic energy into electrical energy. This energy travels from the electric generator through wires and a closed switch to the resistor. In the process the resistor converts the electrical energy into heat energy. So, kinetic energy is drawn from the falling weight through the conversion process and leaves the process in the form of heat. As the falling weight is drained of kinetic energy, it slows down.
Figure 2 – Applying the Electric Brake Okay, now let’s get back to dynamic brakes on elevators. An elevator is attached by a cable to a hoist that is powered by an electric motor. When it’s time to stop at the desired floor, the automatic control system disconnects the elevator’s electric motor from its power source and turns the motor into a generator. The generator is then automatically connected to a resistor like the one shown in the electric brake above. The kinetic energy of the moving elevator is converted by the generator into electrical energy. The resistor converts the electrical energy into heat energy which is then dissipated into the surrounding environment. The elevator slows down in the process because it’s being robbed of kinetic energy. When the dynamic brake slows the elevator down enough, a mechanical brake is introduced, taking over to bring the elevator to a complete stop. This two-fold process serves to reduce wear and tear on the mechanical brake’s parts, lengthening the operational lifespan of the system as a whole. Next time, we’ll tie everything together and show how mechanical and dynamic brakes work together in a diesel locomotive. _____________________________________________
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Brakes and Braking Systems
May 23rd, 2010|
Imagine driving in your car, you’re traveling at a speed of 65 mph and you’re coming up on a curve. You depress your brake pedal to negotiate the turn, and nothing happens… Scenarios just like this one have been in the news quite often lately, brakes which just aren’t operating correctly. We’ve heard the tales of terror, recounted by those unfortunate individuals who have been placed in this situation, but have we reflected on just why their brakes might have failed? Put most simply, a brake is a device whose purpose is to stop a body in motion. This important task is accomplished by converting the kinetic energy (energy of motion) into heat energy. This can be accomplished by either of two methods, mechanically or electrically. In today’s blog we’ll focus on the mechanical aspect. A simple mechanical brake is shown in Figure 1 below. In this arrangement kinetic energy is converted into heat energy when force is applied to a lever, causing a brake shoe to meet up with a rotating wheel. The brake shoe has a pad attached to its surface that makes direct contact with the wheel, and when the two come together great friction is produced. It’s this friction that will ultimately stop the object in motion. Friction turns the kinetic energy into heat energy.
Figure 1 – A Simple Mechanical Brake Friction at its simplest is a mechanical resistance to movement. Whenever two materials in motion come into contact with each other there is always some degree of friction. The extent to which friction is produced by their meeting is referred to as the “coefficient of friction.” The coefficient of friction varies according to the surface character of the materials coming in contact. For example, the coefficient of friction for the leather sole of your shoe on smooth ice is very low. This means you’ll do a lot of slipping when you’re trying to walk, and that’s because ice presents little friction to resist a smoothly soled shoe. But take this same shoe and apply it to the rough surface of concrete, and you’ll be walking quickly and efficiently. Coefficients of friction between different materials have been duly measured in laboratories and are tabulated for easy access in engineering reference books. Based on our simple example above, one would easily come to the conclusion that a high coefficient of friction is desirable when talking about brake shoes, specifically the one represented in Figure 1 above. The higher the coefficient of friction, the more the pad wants to grab the wheel, and the less force you will need to apply to the brake shoe to successfully come to a stop. That’s mechanical braking in a nutshell. Next time, we’ll focus on an electrical braking system known as a “dynamic brake.” _____________________________________________
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