chapter  4
17 Pages

Heat, steam and thermal efficiency

The foundation for the development of steam as an energy source was established in the seventeenth century. Otto von Guericke, the mayor of Magdeburg, Germany, created an apparatus consisting of two hemispheres that were very carefully fitted together. Using an air pump, von Guericke could create a vacuum inside the hemispheres. In a public exhibition, he showed that teams of eight horses hitched to each hemisphere could not pull the hemispheres apart. This demonstration showed that the air itself can exert a tremendous amount of force. Another of his experiments involved using an air pump to remove as much of the air as possible from a spherical copper container. The sphere suddenly crumpled, in what was the world’s first recorded ‘implosion.’ Reports of these experiments inspired other scientists to recognize that air pressure, pushing against a vacuum, had considerable force and could be harnessed somehow to do useful work, if only there were some convenient way to create a vacuum, as von Guericke had done inside the hemispheres. In Paris, Denis Papin had the idea that a possibly very useful way

of making use of steam would be with a piston and cylinder. Papin reasoned that water could be boiled inside the cylinder. The piston would be pushed upward, rising on the column of steam made from the boiling water. If the fire were removed, the steam would

condense, creating a partial vacuum inside the cylinder. The pressure of the atmosphere would force the piston down, just as it can crumple an evacuated copper sphere or prevent hemispheres from being pulled apart. If the piston is connected to something, then work can be done, e.g., raising a heavy load. The English inventor Thomas Savery then introduced the idea of

using a separate boiler to create steam, rather than boiling water in the cylinder itself. He recommended condensing the steam more thoroughly and more quickly by pouring cold water onto the cylinder. Steam forced into the cylinder pushes the piston up, against the weight of the atmosphere. When the steam condenses back to liquid water, the volume of the liquid will be about onethousandth the volume of the steam. Condensing the steam creates a near-vacuum in the cylinder. The weight of the atmosphere on the piston then pushes it down. By connecting this device to a water pump, energy in the steam could be used to do work, i.e., to operate the pump, and thereby pump water out of the mines. Because one of the serious industrial problems of Savery’s time was how to remove water from underground coal mines, Savery advertised his machine as the ‘Miner’s Friend.’ Thomas Newcomen, an English hardware dealer with little

formal education, and his friend John Calley, a plumber, tinkered for years with Savery’s Miner’s Friend. They developed an engine that involves using a heavy counterweight to raise the piston, and a better way of condensing the steam-a spray of cold water into the cylinder. Newcomen also realized that it is better to condense only part of the steam, keeping the cylinder fairly warm, and letting the engine work more quickly. Since power is determined by work done per unit time, decreasing the amount of time required to do a given amount of work makes the engine more powerful. Newcomen’s engine was the first new way of doing work since

the extensive adoption of waterwheels some seven hundred years earlier. Over the next half-century, Newcomen’s engine became widespread throughout Europe, for operating pumps and other kinds of machinery. Nevertheless, the engine had a number of flaws. It was cumbersome and very slow, sometimes taking several minutes for the piston to go up and down. Because of the crude manufacturing methods of the time, the engine leaked badly. The ability to do work was limited by the fact that the real effort was supplied by the

pressure of the atmosphere. The engine was extremely wasteful of energy because, at one point in its operating cycle, the cylinder has to be hot (full of steam) but at another point in the cycle the cylinder has to be cold (to condense the steam). The Newcomen engine was notorious for its high fuel consumption; no more than 1-2% of the chemical potential energy in the coal (used as fuel to make the steam) was transformed into useful work. A contemporary criticism was that it took an iron mine to build a Newcomen engine, and a coal mine to keep it going. Despite these flaws, Newcomen’s engine was an immediate commercial success. Forty years later more than a hundred were in use, mostly in pumping water from mines. A Newcomen engine used at a coal mine in Derbyshire (England) was in operation from 1791 to 1918. The last surviving Newcomen engine (in Yorkshire, England) was not scrapped until 1934, after operating for more than a century without needing extensive maintenance.It’s important to recognize that in the Newcomen engine the steam actually only functions as an intermediary agent. Steam is used to create the necessary vacuum. The pressure of the atmosphere pushing against the vacuum is the actual source of work. For that reason, the Newcomen engine is called an atmospheric steam engine, or an atmospheric engine. Despite its inefficiencies, Newcomen’s engine steadily gained

applications throughout Britain and Europe to such an extent that even universities began to acquire models of the engine to study. The model engine at the University of Glasgow had been broken, and was taken to the university’s scientific instrument maker, James Watt, for repair. He recognized the major problem of the Newcomen engine-that a single cylinder must alternate between being hot and being cold. This very inefficient operation wastes considerable amounts of fuel. He recognized that it would be much better to have two chambers-one, the cylinder with piston, that is always hot, and one that is always cold. This modification incorporated two major advances: By elim-

inating the temperature cycling (hot-cold-hot-cold, etc.) in the single cylinder, the efficiency of operation is greatly increased. In earlier designs pressure of the atmosphere moved the piston; the steam simply provided a convenient way of making a vacuum. The new device used the expansion of the steam to do the work of the engine so, strictly speaking, represents the first true steam

engine. A steam engine is an apparatus or device for converting heat (in steam) into kinetic energy. It is one representative of a more general class of devices called heat engines. Watt made a series of improvements, both to the earlier engine of

Newcomen and to his own original design for the steam engine. One was a separate chamber to condense the steam. A second was the invention of a system of gears that converted the reciprocating, up-and-down motion of the piston into rotary motion of a shaft or axle. The ability of Watt’s engine to drive rotating machines made it attractive to the cotton industry, which in those days was the main user of automatic machinery. It freed industry from a reliance on waterwheels for operating factories. Consequently, manufacturers could build larger factories, and could locate them nearer to sources of raw materials and population, rather than near sources of water. Watt’s further improvements included the invention of a gover-

nor that automatically controlled the engine’s output of steam, a gauge for determining pressure in the cylinder, and an ‘indicator’ for following changes in steam pressure during the operating cycle of the engine. Watt’s governor adjusted the amount of steam passing into the cylinders according to how rapidly the machinery was going. This is the first example of real feedback control of a machine. The Greek word for ‘governor’ is kybernetes. It has come into English as ‘cybernetics.’ Fundamentally, cybernetics means governorship. Watt’s governor and the safety valve invented by Papin were the two essential devices that made steam engine operation safe, at least preventing them from running out of control and possibly exploding. To improve efficiency, Watt installed a steam jacket around the

cylinder, to keep the cylinder as hot as the steam itself. These improved engines had fuel consumption one-third of that of a typical Newcomen engine for the same amount of work. This gain of efficiency made all the difference in accounting for Watt’s engine displacing Newcomen’s. Engineers of that era spoke of the ‘duty’ of an engine. The term derives from the practice of having humans or horses operate machinery by serving a turn of duty in a treadmill. For steam engine operation, the duty of the engine was expressed in terms of the amount of water pumped from a mine for a given quantity of coal that was burned to make the steam. James Watt measured the amount of work a horse could do by making it pull a

known weight, lifted over a pulley, up to a certain height. He conceived the ideas (discussed in Chapter 1) of work being the product of force and distance and of power being the rate of doing work. The name of the unit of power developed by Watt is, of course, the horsepower. The invention and continual improvement of the steam engine

meant that humankind was no longer dependent on draft animals, rivers, or the wind for accomplishing work. Nonetheless, Watt did not invent the steam engine out of nothing. Watt’s tremendous achievement was to improve upon the earlier, existing designs of men like Papin, Savery, and Newcomen to develop the first truly practical steam engine. Watt had a patent monopoly on the use of low-pressure steam in

engines. Richard Trevithick had the insight to realize that highpressure operation could offer significant advantages, not the least of which is that high-pressure engines would do the same work as a low-pressure engine but could be considerably smaller and lighter, and therefore cheaper. If a high-pressure engine could be smaller and lighter than a low-pressure engine doing the same amount of work, perhaps it could be made so small, and so light, that it would be portable. Watt engines were so heavy that probably they could not have done enough work to move themselves, let alone pull something behind. Trevithick realized that the moving piston of the steam engine could be made, with appropriate mechanical linkages, to turn wheels. He built a steam ‘road carriage’ in 1801, making the first trial run on Christmas Eve. Two years later he was driving through London at the princely rate of 16 kilometers per hour. In February of 1804 he mounted a steam carriage on rails, and hauled a load of ten tonnes, along with seventy men, on a 16kilometer trip in Wales. This was the first successful operation of a locomotive. By the 1850s, the steam engine had become the dominant source

of work in most possible areas of application. It dominated in pumping water from mines. It played a major role in operating machinery in factories. It was the source of locomotion on all the world’s railroads. It was beginning to displace sail as the means of propelling ships. Steam was the energy source of the Industrial Revolution. Watt’s steam engine operated the pumping machinery to get water out of mines, and operated the hoisting machinery to

lift coal and ores from the mines. This made it possible to get coal and ore cheaply from deeper and deeper seams. The steam locomotive made it possible to transport coal, ore, and other raw materials cheaply. The steam engine, applied to the blast furnace, allowed continuous production of cheap iron and steel. Iron and steel could be used to make a great variety of industrial machinery that could be operated cheaply and on a large scale by steam engines. The products of these manufactories could be transported cheaply on land by the steam locomotive, and internationally in steam-worked iron ships. A nineteenth-century steam engine capable of generating as

much power as a typical lawnmower engine would fill a two-story building. But, focusing on the inefficiency of the early steam engine overlooks its critical contribution to society. The steam engine freed humans from relying on energy sources supplied by nature (wind, water, or animal muscles). The steam engine was the first device that provided useful work from an energy source-steamthat could be created on demand by humans. Also, the steam engine was the first step in removing the constraints of geography. It was no longer necessary to build factories on the banks of fastflowing rivers, to dam a river for a water supply, or to find locations that offered reasonably steady wind. A steam engine could be erected any place that work was needed. There is still one constraint in using the steam engine: it provides

work by moving a piston up and down, or making a shaft or axle turn. This means that the engine itself must be very close to the device that it will operate. Whatever mechanism is being operated by the engine has to be directly, physically connected to it. One consequence is that every factory with steam-engine-driven machinery had to have its own engine, in the basement or yard, and its own supply of fuel, usually coal. A little over a century after Watt’s great invention, the second liberating step began to emerge from research in a completely different field of science-the study of electricity. This we will explore in future chapters.