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Last week we talked about an area of thermodynamics that concerns refrigeration cycles as presented through the example of an air conditioner. This week, we’ll learn about stoichiometry, which is concerned with the math behind chemical reactions, like those that take place during the burning of fuels. During the combustion process, heat energy is released from a fuel when the combustible elements in the fuel combine with oxygen. This is known as oxidation, or in common everyday language as burning. The most important thing to remember about oxidation is that it obeys the first law of thermodynamics. That is, mass cannot be created or destroyed. In a chemical reaction like combustion, particles of fuel and air are rearranged in space and then combine to form different substances. However, despite the rearranging, the mass that goes into the reaction must equal the mass that comes out. This conservation of mass is the basis of stoichiometry. For example, if pure carbon (represented by the chemical symbol “C”) is burned in pure oxygen (O2), you can represent the combustion process as: C + O2 → CO2 This is chemistry shorthand for representing how carbon and oxygen combine during burning to form carbon dioxide (CO2). The elements to the left of the arrow are known as “reactants” and the elements to the right are known as “products.” In stoichiometry, the mass of the reactants must equal the mass of the products. But, how do we quantify the mass of reactants and products? Now this is where it gets a little weird. To make use of our chemistry shorthand above, we have to consider something called moles. No, these aren’t the little furry creatures that tunnel under your lawn and eat your tulip bulbs. In stoichiometry a mole is considered to be 6.02×1023 molecules of a substance. That is 602,000,000,000,000,000,000,000 molecules! Okay, so we have one heck of a lot of molecules in a mole. So, what does that have to do with figuring out how much mass we are dealing with in the combustion process? Well, in order to make moles work for us, we have to take into consideration the differing molecular weights of substances. Molecular weight is the number of grams (g) of mass that are contained within one mole of a substance, like the element carbon in our example above. To help make stoichiometry more workable, scientists created a table that provides the molecular weight of all known chemical elements. This table is known as the Periodic Table of Elements, or the “Periodic Table” for short. Now going back to our example above, if we know from the Periodic Table that carbon has a molecular weight of 12 g per mole and oxygen has a molecular weight of 16 g per mole, then how many grams of carbon dioxide do we get by burning carbon in pure oxygen? The combustion process can be represented by this equation: C + O2 → CO2 (12 g/mole) × (1mole of carbon) + (16 g/mole) × (2 moles of oxygen) = 44 g of carbon dioxide This is a fairly straightforward example of how stoichiometry works. In reality, things can get far more complicated. In a power plant for example, fuels like coal contain substances in addition to carbon, such as hydrogen and sulfur, and they, too, must be factored into the stoichiometric accounting system. To further complicate things, fuels are usually burned in air, rather than pure oxygen. Air, too, contains substances other than oxygen, including nitrogen, argon, and molecules of water. These other substances’ presence in fuel and air make the combustion process more challenging to account for, because they all get mixed together, and they can combine into all sorts of other substances. Despite these complicating factors, the first law of thermodynamics must be obeyed, so the balancing act is still the same: mass of the reactants must equal the mass of the products. Once mechanical engineers use stoichiometry to figure out what’s going in and coming out of the combustion process, they can then use the data provided by chemical analysis of the fuel to calculate the heat energy that is released. They can also calculate the air required for proper combustion. This helps them to design things capable of delivering enough fuel and air to meet the heat input requirements for a diversity of power cycles, from the engine in your car to the coal fired power plant supplying electricity for your home. Next week, we’ll talk about psychrometric analysis. No, this has nothing to do with psychiatry. Psychrometrics involves the analysis of gas and vapor mixtures like air and water. _________________________________________________________________ |
Archive for December, 2009
Thermodynamics in Mechanical Engineering, Part IV, Stoichiometry
Sunday, December 27th, 2009
Thermodynamics In Mechanical Engineering, Part III, Refrigeration Cycles
Sunday, December 20th, 2009|
Last week we talked about an area of thermodynamics that concerns power cycles, an example of which can be found in a coal fired power plant. This week we’ll learn about another area of thermodynamics, that of refrigeration cycles. It’s been snowing in the Midwest, so the topic seems appropriate enough. A refrigeration cycle is obviously found in your refrigerator, but did you know that it’s also found in your air conditioner? Refrigeration cycles operate in Opposite Land as compared to power cycles. You know, the place where everything works in reverse. Instead of heat going into the cycle and electricity coming out, in a refrigeration cycle, electricity goes in and heat comes out—out of your refrigerator or air conditioned house, that is. Let’s consider an example of the simple air conditioner refrigeration cycle shown in Figure 1. The cycle has four important parts: an evaporator coil, a compressor, a condenser coil, and an expansion valve. All parts are connected by pipes, and the entire system is sealed up tight with refrigerant inside.
Figure 1 – A Simple Refrigeration Cycle Used In An Air Conditioner The compressor is the heart of the operation, so to speak. In our simple cycle, the compressor consists of an electric motor-driven piston that moves back and forth within a cylinder. The motor does work as the piston moves back and forth, and the compressor pumps refrigerant through the pipes, the condenser coil, the expansion valve, and the evaporator coil. Like your heart, the pump has check valves that keep refrigerant flowing through the system in one direction (counterclockwise in our example). Keeping the flow going in one direction is critical to the operation of the cycle, as we’ll see in a moment. The refrigerant is the life blood of the cycle. It is a chemical that is manufactured to have special thermodynamic properties. For example, it’s really good at quickly absorbing a lot of heat at low temperature, like the temperature of the air in your house. The evaporator coil in Figure 1 would be located on the inside of your home. As the refrigerant enters the evaporator coil, it is a mixture of liquid and vapor. Inside the evaporator coil, the liquid refrigerant boils off to a vapor as it absorbs heat from the room. Yes, that’s right, the refrigerant boils at room temperature! The heat absorption in the evaporator is helped along by using a fan to push room air across its coil. Warm air from the room gets sucked into the air conditioner and cool air blows out into the room. But that’s not the end of the story. That heat from the room has to somehow get outside of the house, where it can be disposed of. This is so the refrigerant can pick up another load of heat when it flows back through the evaporator coil. But disposing of the heat from the refrigerant isn’t easy, since heat naturally wants to flow from a hotter place to a cooler place. So how do you buck Mother Nature and get heat to flow from the inside of your house where it’s cool to the outside of your house where it’s hotter? It takes work, and that’s where the compressor comes into play. The compressor first pulls the refrigerant vapor out of the evaporator and raises its pressure and temperature. This heart, like our own, is a hard worker. As the vapor leaves the compressor and passes through the condenser coil located outside your home, it is in a state where it can easily give up its heat to the warm air outside. The release of heat is helped along by the use of another fan that serves to push the outdoor air across the outside surfaces of the condenser coil. Then, as heat is released to the outdoor air, the refrigerant condenses back into a liquid. After leaving the condenser coil, the liquid refrigerant passes through the expansion valve, where its pressure and temperature are reduced. The refrigerant is now ready to pick up a new load of heat in the evaporator coil, and the cycle repeats itself with the help of a working electric motor. Next week, we’ll continue our exploration of thermodynamics and narrow our focus onto an area known as stoichiometry, which is concerned with the math behind chemical reactions, like those that take place during the burning of fuels. Math is fun. Just keep repeating that to yourself. _________________________________________________________________ |
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.
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).
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).
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. _________________________________________________________________ |
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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