Steam Power

Steam Power

Miners have always had problems with flooding, and in 1698 Brit- ish engineer Thomas Savery (1650–1715) constructed what was probably the first steam engine, which was used to pump water.

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Another British engineer, Thomas Newcomen (1663–1729), developed a steam engine in the early 1700s that would become the forerunner of most of the later, more efficient machines. New- comen’s engine used a device called a piston.

In an ordinary steam engine, the piston is like a plunger that fits snugly in a cylinder, as shown in the figure on page 74. Steam from a boiler enters the cylinder in one direction and exerts pres- sure—a force acting over an area—on the piston, moving it for- ward. The moving piston pushes a jointed rod that has two parts. The part connected to the piston is called the connecting rod, and the other part is called the crankshaft and is attached to a heavy wheel, the flywheel. As the piston moves, the connecting rod and crankshaft transfer this motion into a rotation of the flywheel.

The piston can only move so far in the cylinder and soon reaches the limit—this is one stroke of the piston. If all the engine could accomplish is one little push, it would not be very useful, so when the forward motion is finished, the piston must back up and go through the same process again. But there is a problem: the steam that pushed the piston forward is still around, making it difficult for the piston to back up. Early steam engines cooled the cylinder, and the steam turned back into water, so there was no more pressure against the piston. But this meant the cylinder had to be reheated in order for steam to initiate the next stroke of the piston. James Watt (1736–1819), a Scottish engineer, realized it would be much better if the steam simply escaped. He developed valves to let steam enter the cylinder (through the inlet valve) and to escape (through the exhaust valve) when the piston was ready to come back. Although Watt did not invent the steam engine, his improvements were necessary to make the steam engine efficient enough to power the Industrial Revolution.

The steam comes from the boiler, a sturdy metal container that holds a quantity of water. The burning of some kind of fuel, such as coal or wood, raises the temperature of the water past the boil- ing point, producing steam—hot water vapor.

Why steam and not something else? Steam is effective because of its high energy; it contains the energy that was put into the sys-

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Expanding steam enters a steam engine’s cylinder and pushes the piston forward. A connecting rod and crankshaft link the piston to a wheel, the flywheel, whose rotation does work such as operate a pump or spin an axle.

tem to raise the temperature of the water, as well as the energy to raise the temperature of the steam. Water boils at 212°F (100°C)

Heat Engines 75

heat the temperature of steam can be raised, in which case it is sometimes referred to as superheated steam.

Chapter 1 described heat as a flow of energy related to the motion of atoms and molecules. Higher temperatures mean that the atomic and molecular components of a substance are fly- ing around or vibrating faster and with more energy. Some of this energy can be converted into work, which is what a heat engine does. But steam also has another source of energy—the latent heat of vaporization. As discussed in chapter 3, energy is required to break bonds holding atoms or molecules together. This is what happens when water is turned into steam, because the bonds holding water molecules together get broken and the molecules go flying off as a gas. The process does not destroy this energy (which is forbidden by the first law of thermodynamics), the energy simply exists as a different form, called the latent heat of vaporization. This is the energy contained by the state or phase of a substance. Water absorbs heat to turn it into steam, but the reverse process—when steam condenses back into water—returns this energy.

Steam can exert an enormous pressure. Water vapor fills a vol- ume of about 1,500 times greater than the same amount of liq- uid at normal pressure (the pressure of Earth’s atmosphere). This pressure can do work, which is defined in physics as exerting a force over a distance. In steam engines, the steam does work on the piston. But this work does not come free, because the first law of thermodynamics says that a system loses energy when heat flows out of it or when it does work. When steam pushes against the pis- ton, it does work, so it loses energy. As a consequence, its tempera- ture falls—the motion of its water molecules is “spent” doing this work. The boiler must replace this energy, which means the boiler must be kept going. Fuel is needed to keep the boiler hot. Without fuel the steam engine, or any heat engine, stops working.

Heat engines such as steam engines are similar to the heat pumps discussed earlier, except they run in opposite directions. Air conditioners and refrigerators are heat engines run backwards, as illustrated in the diagram on page 76. (A similar situation exists in electricity: an electric motor is an electric generator running in

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A heat engine and a refrigerator are two different machines. But box diagrams for both of these machines show their relationship—the engine does work from the “downhill” movement of heat from hot temperatures to cold (A), and refrigerators and air conditioners move heat “uphill” with the help of work done by electricity (B).

How much work a steam engine can do depends on several factors. Important factors include the average pressure, the area of the piston and the distance it moves, and how many strokes occur per minute. Further development of steam engines occurred as engineers squeezed all the power they could get out of these machines. Watt took the big step, but steam engines continued to

be refined. Better efficiency meant less fuel to do a given amount of work, saving the operator money. But the subject of thermodynamics seems full of limits—the first, second, and third laws of thermodynamics are often phrased in terms of something that cannot be accomplished—so there is reason to suspect the existence of a limit to the efficiency of a steam engine (or any heat engine). Much to the chagrin of engi- neers all over the world, such a limit exists.

But an understanding of the efficiency limit of heat engines produced a great deal of insight. (Some people say that it even gave rise to the science of thermodynamics.) British scientist James Joule

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(1818–89) showed that a given amount of work always produces

a certain amount of heat, and his experiments were important in formulating the first law of thermodynamics. But when people wondered if the opposite was true—whether a given amount of heat always produces a certain amount of work—the answer, to the dismay of efficiency-seeking engineers, was no.

Heat cannot be completely converted into work. This is the idea behind the second law of thermodynamics, as described earlier. This means that some heat—some energy, in other words—must

be wasted in a heat engine. But how much energy must be wasted? Efficiency requires as little waste as possible, so the question becomes one of finding the maximum efficiency of a heat engine. As described in the following sidebar, French engineer and scien- tist Sadi Carnot (1796–1832) found the answer, not by tinkering with steam engines, but rather by sitting down and thinking about the underlying physics. His ideas apply not just to steam engines but to all heat engines.

According to Carnot, given the temperature, T h , of the heat

source and the temperature, T l , of the exhaust environment, the

best efficiency, E, that any kind of heat engine can achieve is E=1–T l /T h (the temperatures must be in the absolute scale). An efficiency of 1 means that all heat is converted into work, but Car- not’s equation means that no heat engine can possibly ever reach this mark. In the process of converting heat into work, some of the heat must be exhausted into the environment. All heat engines waste some of their fuel by uselessly exhausting some of the energy into the environment. This cannot be avoided.

Carnot’s theory is true, but it puzzled scientists for many years. No one could figure out why physics requires some heat to be wasted in the exhaust of a heat engine. It seems almost mean- spirited, as if nature was thumbing its nose at engineers who are always striving to improve their machines. The reasons for Carnot’s theory involve a strange and interesting concept called entropy, to

be discussed in the next chapter of this volume. Although Carnot’s theory was not what engineers wanted to hear, it was an advance in physics and important to know for anyone designing a heat engine. The best that an engineer can do is make

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