PART TWO: POWER AND ENGINEERING 282 Figure 5.3: Corliss and drop valves. (a) Schematic arrangement of Corliss valves. The valve is a machined cylinder oscillating about an axis lying at right angles to the piston stroke. The upper valves are the steam and the lower valves the exhaust valves. The valves are loose on the valve spindle so they are free to find their seats under the action of the steam pressure, and free to lift off their seats to allow trapped water to escape. Reproduced with permission from R.A.Buchanan, and G.Watkins The Industrial Archaeology of the Stationary Steam Engine (Allen Lane, London, 1976). (b) Trip gear that controls the cut-off of the Corliss valve. The valve is opened by means of the oscillating link driven by the eccentric. When the connection at the points A and B is broken the valve closes under the action of the powerful spring. The moment of closure depends on the height h of the lever. As the engine speed increases, h increases and the valve closes earlier. Closure of the valve is assisted by atmospheric pressure acting on the exposed side of the dash- pot piston and the vacuum formed on the other side when the valve is opened. The exhaust valves are not provided with trip gear and the angular motion in one direction is an exact repetition of the motion in the other direction. STEAM AND INTERNAL COMBUSTION ENGINES 283 Reproduced with permission from D.A.Wrangham The Theory and Practice of Heat Engines (Cambridge University Press, Cambridge, 1951). (c) Schematic arrangement of equilibrium drop valves. The valves are mushroom shaped and seat as shown. Motion is in the vertical direction. Note that identical valves are mounted on the same stem so the pressures on the valve faces oppose one another and a balanced arrangement is obtained. Consequently, the valve operating force is only required to overcome friction and inertia. Because there are no sliding parts (cf. Corliss valve) the valve is well adapted to use with superheated steam. Reproduced with permission from R.A.Buchanan and G.Watkins The Industrial Archaeology of the Stationary Steam Engine (Allen Lane, London, 1976). Admission and exhaust of the steam was controlled from the beginning of the nineteenth century in most cases by the slide valve or in some cases, by the poppet valve. Both types of valve were subject to wire drawing because they did not close sufficiently quickly at the points of cut-off. In 1842, F.E.Sickels took out a patent on a quick-closing valve gear using poppet valves, and a trip gear to control the cut-off (see Figure 5.3), with gravity assisted closure (called ‘drop-valves’ by Oliver Evans). To save wear and tear on the valve seat and value face, Sickels used a water-filled dashpot to decelerate the valve smoothly as it approached the end of its travel. Sickels was not the only engineer who understood the advantages of rapid valve operation, and in about 1847, G.H.Corliss invented a quick-closing valve gear (patented in 1849) consisting of four flat slide valves, one inlet and one outlet at each end of the cylinder, but he did not persist very long with this valve gear. In order to simplify manufacturing and to reduce valve friction, engines built by his company after about 1850 used the oscillating rotary valve (see Figure 5.3) that is normally associated with his name. Corliss made an evenmore fundamentally important contribution to steam engine technology by replacing the inefficient method of throttle governing with the better method of adjusting to load variations by using the governor to control the cut- off, socalled cut-off governing. Engines using Corliss valve gear (commonly called Corliss engines) were built by Corliss, and his licencees from 1848, and by many others after the Corliss patent expired in 1870. This type of engine was extensively used for driving textile mill machinery where the close regulation of engine speed, that was ensured by the governor control of cut-off, was essential. The medium speed engine Automatic trip gear mechanisms do no operate satisfactorily at rotational speeds in excess of about 150rpm. Higher speeds became possible in 1858 when C.T. Porter combined his very sensitive governor with a positive action, PART TWO: POWER AND ENGINEERING 284 variable cutoff valve gear that had been invented in 1852 by J.F.Allen. In 1861, Porter and Allen formed a partnership to build engines using their two inventions. The outstanding feature of the Porter-Allen engine was its quiet, vibrationfree operation at all speeds, which resulted from Porter’s careful study of the dynamics of the reciprocating engine. Because of their high and closely controlled speed, engines of this type were extensively used from about 1880 onward for driving electric generators, e.g. Edison’s Pearl Street Station. The high speed engine Steam engines operating with piston speeds in excess of 3m/s (600ft/min) could be made sufficiently small, for a given power output, so that a significant reduction in first cost could be realized compared to slower-running engines, by the invention in the early 1870s of the shaft governor, which is usually mounted on the flywheel shaft. This operates by balancing the force of the governor spring against the centrifugal force. These had a tendency to hunt (an inability of the governor to locate a steady operating speed) because the rate at which they acted was independent of the rate of change of the load. This fault was overcome by the invention in 1895 by F.M.Rites of the inertia governor in which inertia forces augmented the centrifugal forces in the governor. With this modification the high speed engine could be applied where very close control of the engine speed was required, e.g. electric power generations. High speed engines were characterized by stroke: bore ratios less than unity, and power outputs that did not usually exceed 370kW (500hp). Typically, piston valves and cam-driven poppet valves were used for steam distribution. Lubrication was particularly important and in 1890 the Belliss & Morcom Co. in England introduced forced lubrication, which is now a standard feature of any high speed machinery. Many of the characteristic features of the high speed engine were carried over into the early internal combustion engines, so that contemporary engines of both types bear a considerable resemblance, both in superficial features and in certain details. Compound engines The zenith of steam engine design was in the multiple expansion engines that were developed in the second half of the nineteenth century. Such engines were used in the largest numbers for water supply system pumping and as marine engines, because efficiency was an important consideration in these applications. STEAM AND INTERNAL COMBUSTION ENGINES 285 The inverted engine, in which the cylinders are arranged above the crankshaft with the piston rod acting downwards, was the universal type in large multiple expansion engines, because of the need to economize on floor space. Stationary engines Stationary compound engines were employed for operating the pumps of public water supply systems and for turning electrical generators, as well as in textile mills, as rolling mill drives, mine hoisting engines and blast furnace blowing engines. In 1866 a pressurized water supply system using a pump, rather than an elevated reservoir or standpipe, started operating at Lockport, New York. This development, which was to have a significant effect on the history of steam pumping engines, was due to Birdsill Holly. The Lockport pumps were driven by a water-wheel, but a second installation in Dunkirk, New York, used steamdriven pumps. The compound engine was first applied to water pumping in 1848 by the Lambeth Water Works, London, but the employment of this type of engine was not extensive until E.D.Leavitt installed such a machine at Lynn, Massachusetts, in 1873. These engines were landmarks in both capacity and efficiency, and their performance was not surpassed until H.F.Gaskill introduced (1882) a pump driven by a Woolf compound steam engine at Saratoga Springs, New York. This was a compact, high capacity, efficient steam pump, which was very popular in the United States. The first triple expansion pumping engine (see Figure 5.2 (d) above) was built in 1886 for the City of Milwaukee. It was noteworthy in being designed by E.T. Reynolds of the E.P.Allis Co. (later Allis-Chalmers). He joined this company in 1877 from the Corliss Steam Engine Co., and was one of the chief proponents of engines using the Corliss valve gear after the Corliss patent expired in 1870. Two 7500kW (10,000hp) double tandem compound engines were designed in 1888 by S.Z.de Ferranti for use in the Deptford Power Station of the London Electricity Supply Company. This was a pioneering AC distribution system, but because substantial difficulties were encountered in placing it in service, Ferranti’s connection with the company was terminated, and the engines were never completed. Somewhat later, in 1898, the Manhattan Railway Co., which operated the overhead railway system in New York, purchased from the Allis-Chalmers Co. compound engines with horizontal high pressure and vertical low pressure cylinders. These unique engines, which were undoubtedly among the largest stationary engines ever built, and probably the most powerful (6000kW; 8000hp), are usually known as the Manhattan engines. PART TWO: POWER AND ENGINEERING 286 Marine engines It was the application of the compound engine that allowed the steamship to take over from the sailing vessel on the longest voyages. The consequent reduction in coal consumption made more space available on the ship for passengers and cargo, and fewer stops were needed to replenish the bunkers. Compound engines had been tried in small vessels between 1830 and 1840, but they were not fitted in ocean-going ships until 1854, when the Brandon was launched by Randolph, Elder & Co., of Govan on the River Clyde in Scotland. This had a Woolf compound engine with inclined cylinders and an overhead crankshaft, designed by John Elder. Saturated steam was supplied at 2.8bar (40psig), and in service the coal consumption was 2.13kg/kWhr (3.5lb/ihphr) which was about a 30 per cent improvement over the performance of vessels fitted with simple engines. The compound engine was quickly adopted by British shipping companies operating to ports in the East and in Africa. However, it was not until the 1870s that vessels operating on Atlantic routes were fitted with engines of this type. By about 1880 the compound engine was used almost universally in marine service, but between 1875 and 1900 triple expansion, and then quadruple expansion engines, were adopted for the largest vessels. The triple expansion engine was originally proposed in 1827 by Jacob Perkins, but no engine of this type was built until 1861 by D.Adamson; this was a stationary engine. The first application to a sea-going ship was by John Elder & Co., who in 1874 fitted an engine using steam at 10.3bar (150psig) with cylinders 580mm (23 inch)×1040mm (41 inch)×1550mm (61 inch) bore by 1070mm (42 inch) stroke in the Propontis. However, it was the Aberdeen, launched in 1880 by Robert Napier & Sons, that had the greatest influence on the history of the marine engine. The engine had cylinders 760mm (30 inch)×1140mm (45 inch)×1780mm (70 inch) bore by 1370mm (58 inch) stroke. It used steam at 8.6bar (125psig) and had an output of 1340kW (1800ihp). On its first voyage the coal consumption was 1.0kg/kWhr (1.7lbs/ihphr). This engine was the prototype for thousands of marine engines that were built until the middle of the twentieth century: Liberty ships were fitted with triple expansion engines. The first quadruple expansion marine engine was fitted in the County of York built at Barrow, Lancashire, in 1884, but this type of engine was not tried again until 1894 when the Inchmona was launched, this was a 707kW (948ihp) engine supplied with steam at 17.6bar (255psig) 33°C (60°F) superheat. The reciprocating steam engine undoubtedly reached its highest level in some of the quadruple expansion engines built for the very large ocean-going liners at the close of the nineteenth century. One of the most outstanding was the 29,830kW (40,000hp) engine produced for the twin-screw Kaiser Wilhelm II. STEAM AND INTERNAL COMBUSTION ENGINES 287 Uniflow engine The Uniflow engine, which represents the final stage of the development of the steam engine, was motivated by the problem of cylinder condensation and reevaporation, which was a serious cause of energy loss in the engine. Steam enters the engine cylinder and is immediately exposed to cylinder walls that have been cooled by the previous charge of steam, which had itself been cooled in consequence of its expansion during the working stroke of the piston. If the cylinder wall temperature is low enough, the incoming steam will condense on the cylinder walls, and energy is given up to the walls. As the expansion proceeds in the cylinder the steam temperature can fall below the cylinder wall temperature, resulting in re-evaporation of the condensed steam. However, because this occurs near the end of the stroke, very little of the energy thus returned to the steam is available to do work and it is carried away as the steam leaves the cylinder. The question of cylinder condensation and re-evaporation became the central concern of steam engine engineering from 1855 to 1885. Several developments alleviated this problem, either deliberately or incidentally: compounding; steam jacketing of the cylinders; superheating; and increasing inlet steam pressure. While these techniques were used on large marine and stationary engines, they were prohibitively expensive for the lower power single-cylinder engines that were widely used in industrial applications: an economical solution for engines of this type required a radically new design. The requisite development was the introduction of one-way steam flow (hence ‘Uniflow’ or ‘Unaflow’) in the cylinder, so that steam was admitted at each end of the cylinder and exhausted in the centre through circumferential ports in the cylinder wall uncovered by the piston (Figure 5.4). The Uniflow principle appears to have been proposed quite early in steam engine history (Montgolfier, 1825; Perkins, 1827), but serious consideration of the idea did not occur until T.J.Todd took out a British patent in 1885 on an engine of this type (he called it the ‘Terminal-exhaust’ cylinder). It is not clear if such an engine was ever built, so the practical realization of the Uniflow engine is usually credited to J.Stumpf. Uniflow engines were built in Europe, Britain and the United States. One of the most successful builders was the Skinner Engine Company of Erie, Pennsylvania. This company built Uniflow engines until the mid-1950s, and during the Second World War supplied them for naval craft (where they were popular because they generated less vibration than other types of reciprocating engines). The progress in the steam inlet pressure and the heat rate between the time of Newcomen (1712) and the end of the nineteenth century is indicated in Figure 5.5. The data for the earliest years do not have the precision and accuracy of the later period, nevertheless they are indicative of the general PART TWO: POWER AND ENGINEERING 288 trend. In the final decade of the nineteenth century, the introduction of regenerative feed-water heating, in which steam used in the cylinder jackets was returned to the boiler, had a marked effect in lowering the best values for the heat rate. In fact, the best of the final generation of reciprocating steam engines with cylinder steam-jacketing had heat rates that were comparable with those of contemporary non-regenerative steam turbines (typically 23.0×103 btu/kWhr; see Figure 5.10). STEAM TURBINES The steam turbine is a device that directly converts the internal energy of steam into rotary motion (see Appendix). It is also characterized by uniformity Figure 5.4: Schematic section of a Uniflow engine. The diagram shows the Uniflow steam path and the small difference in temperature between the steam and the cylinder wall throughout the piston stroke. Since the exhaust ports must not be uncovered until the piston reaches the end of the expansion stroke the piston length must equal its stroke. Hollow construction is used to reduce the piston weight. Because the return stroke of the piston is a compression stroke and because the compression ratio (expansion ratio for the expanding steam) is high, typically 46, there is a danger that the pressure of the residual steam in the cylinder could become high enough to dislodge the cylinder head cover. This is avoided by providing additional clearance space, in the form of a cavity in the cylinder head connected to the cylinder by a spring-loaded valve, with manual override for starting the engine, or by using various types of automatic and manual valves that divert the residual steam into the exhaust. Reproduced with permission from D.A.Wrangham, The Theory and Practice of Heat Engines (Cambridge University Press, Cambridge, 1951). STEAM AND INTERNAL COMBUSTION ENGINES 289 of turning moment and by the possibility of balancing it perfectly, which is important where high powers are involved, when a reciprocating engine would have correspondingly massive dimensions that could produce vibrations of an unacceptable magnitude. Because the steam flow through the turbine is continuous rather than cyclic, it is able to expand steam from a high pressure to a very low pressure, thereby maximizing the efficiency. This, because of the low density of the low pressure steam, would require the cylinder of a reciprocating engine to have impracticably large dimensions. The continuous flow characteristics of the steam turbine allow it to avoid the complex valve gear necessary in the reciprocating engine, which should Figure 5.5: The historical trend of reciprocating steam engine inlet pressure (boiler pressure) and power plant heat rate between 1800 and 1900. The data shown are based on marine practice, but are also representative of stationary engines. The upward trend beginning between 1850 and 1860 marks the introduction of the compound engine. To convert heat rate to efficiency multiply by 2.93×10-4 and invert. Adapted with permission from R.H.Thurston, A History of the Growth of the Steam Engine, Centennial edition (Cornell University Press, Ithaca, N.Y., 1939). PART TWO: POWER AND ENGINEERING 290 have made it attractive to the ancients, and indeed there is some historical evidence that a turbine was built in the first century AD by Hero in Alexandria. Later, in the seventeenth century, an Italian, de Branca, proposed another type of steam turbine. Unfortunately, these primitive machines suffered from the fatal defect that, for their times, their speed was too high to be either needed or useable. Consequently, the practical steam turbine had to await the development of the ability to design and construct high speed gears, or high speed electrical generators, or the formulation of such principles of fluid mechanics as would allow the energy of the steam to be utilized without high rotational speeds. None of this occurred until the end of the nineteenth century. 1884–1900: early history The simplest form of turbine (Figure 5.6 (a)), which was first demonstrated by the Swedish engineer Gustav de Laval in c.1883, is constructed by arranging the blades on a single revolving wheel in such a way that they turn the steam through an angle, thereby imparting a portion of the kinetic energy of the steam jets to the rotating blades with no change in steam pressure as it passes through them. This last is an important distinguishing characteristic of this type of turbine, which is sometimes known as an impulse turbine. In about 1888, de Laval made the crucial discovery that in order to extract the maximum amount of energy from the steam, the nozzle had to be of a converging-diverging form (see Appendix). This results in a very high steam velocity, leading to: (a) high rotational speeds (10,000–30,000rpm), which usually requires a speed reducing gear for its practical utilization; (b) wasted kinetic energy in the leaving steam; (c) large friction losses between the steam, the blades, and the turbine rotor. The losses, items (b) and (c), make the efficiency of the de Laval turbine inferior to that of a reciprocating engine operating with identical inlet and exhaust conditions. The principles of the impulse turbine, and its problems, were well known to nineteenth-century engineers, including James Watt. Consequently, until the close of the century it was never considered a serious competitor to the reciprocating engine. The steam turbine was first placed on a practical basis by C.A. Parsons. He took out a patent in 1884 on a turbine which avoided the limitations of the impulse turbine by using the steam pressure drop inlet to exhaust in small steps rather than one large step (see Figure 5.6 (b)), resulting in a much lower steam speed. However, there is an important difference between the Parsons and impulse turbines in the way in which the pressure drop is arranged in a stage (a pair of moving blades and stationary nozzles): in the former the pressure drop occurs in both the rotating and stationary elements, while in the impulse turbine the pressure drop only occurs in the STEAM AND INTERNAL COMBUSTION ENGINES 291 stationary element. The Parsons turbine is known as a reaction turbine, and since Parsons arranged for equal pressure drops to occur in both the stationary and rotating elements of each stage, it is called a 50 per cent reaction turbine. Parsons commenced experimental work on his turbine shortly after becoming a partner in 1883 in the firm of Clarke, Chapman & Co. In 1885 he constructed his first working turbine Figure 5.7 which drove an electrical generator at 18,000rpm having an output that appears to have been between about 4kW (5.4hp) and 7.5kW (10hp). The surprisingly high speed, considering the Parsons turbine was intended to avoid this feature of the de Laval turbine, is a consequence of the small dimensions of the first Parsons turbine. Thus, an increase in the mean blade radius from 4.4cm (1.75 inches) (estimated for Parsons’s first turbine) to 26.7cm (10.6 inches) would reduce the rotational speed to 3000rpm. The essential point is the lower steam speed in the Parsons turbine compared to the de Laval turbine. Thus, for comparable steam conditions, the steam speed in the de Laval turbine is about 760m/s, (2500ft/sec), whereas in the Parsons turbine it is about 116m/s (380ft/sec). From 1885 until 1889 development of the Parsons turbine was very rapid with machines of maximum output 75kW being ultimately produced. However, Parsons’s partners did not have his faith in the steam turbine, so in 1889 the partnership was dissolved. Parsons founded C.A.Parsons & Co. and proceeded to develop a radial reaction turbine that circumvented the patents controlled by his former partners (a truly remarkable achievement). In 1894, Parsons came to an agreement with Clarke, Chapman that allowed him to use his 1884 patents for the axial flow steam turbine, and, as a result, construction of the radial flow steam turbine stopped. Although the Parsons turbine was in many ways significantly superior to the de Laval turbine, it did have some disadvantages. In particular, it was difficult to build with outputs greater than 2000kW (2682hp). This was a consequence of the great length of the machine, which was necessitated by the numerous rows of moving and stationary blades that were required to keep the steam velocity low. As a result the rotor was particularly sensitive to slight mechanical and thermal imbalances that could lead to distortion and, hence, damage to the moving blades if they touched the turbine casing. The techniques that were eventually devised to overcome this problem are discussed below. In addition to the limitations in size that were encountered in the early Parsons turbine, there is a substantial loss in power output due to leakage through the clearance space between the rotating blades and the turbine casing, which is particularly serious in the high pressure sections of the turbine where the blade heights are small. In order to avoid this leakage problem and yet retain the advantages of the Parsons multi-stage concept, Auguste Rateau, a French engineer, designed sometime between 1896 and 1898 a multi-stage . points A and B is broken the valve closes under the action of the powerful spring. The moment of closure depends on the height h of the lever. As the engine speed increases, h increases and the valve. generated less vibration than other types of reciprocating engines). The progress in the steam inlet pressure and the heat rate between the time of Newcomen (1712) and the end of the nineteenth century. 5.5. The data for the earliest years do not have the precision and accuracy of the later period, nevertheless they are indicative of the general PART TWO: POWER AND ENGINEERING 288 trend. In the