The Evolution of Steam Engines

Abstract
The limitations of human "man-power", in terms of potential capability and endurance, have inspired the pursuit and development of alternative forms of energy and the adoption of man-made prime movers. Typical historical examples of such realistic imagination are the harnessing of natural resources including air pressure, in the form of wind, to propel water-borne craft, and to rotate windmills to power flour mills; the force of gravity, such as in the weight of falling water, to rotate "water-wheels", also used to power grain mills and irrigation systems. In the Netherlands, for instance, windmills have been used for hundreds of years for powering water pumps to drain low lying coastal plains as part of massive land reclamation projects. Subsequently, in the absence of blowing wind and/or falling water, the need for substitute man¬-made artificial forms of energy became evident. In the course of cooking food over a fire, vapor was observed rising from the open pot, carrying with it, heat energy and tasteful flavor. Eventually, this gave rise to the idea of covering the pot in order to trap the vapor, thereby intensifying the cooking process and retaining more of the flavor. This step in turn, resulted in intermittent bouncing of the pot lid, showing that the vapor had enough latent energy to briefly lift the weight of the pot lid against the force of gravity, allowing vapor to escape in short repetitive bursts. However, in this case, the potential capability of steam pressure as a form of alternative energy was not immediately recognized.

The earliest known application of steam pressure as a form of energy for purposes other than cooking can be traced back to about one hundred years B.C., when an Egyptian named Hero, experimented with an apparatus in which steam pressure was applied to make the device rotate. The basic configuration of his apparatus consisted of a small spherically shaped metal globe supported on a metal pipe passing horizontally through its center and connected at both ends to a steam kettle. Two small L-shaped pipes, pointing in opposite directions, were secured to the globe, diametrically opposite from each other and at right angles to the centerline of the supporting pipe. In operation, steam from the kettle was fed to the globe via holes in the axial pipe exposed to the inside of the globe. As steam pressure built up within the globe, it escaped to atmosphere via the L-shaped pipes, producing a "jet reaction" effect that caused the globe to rotate, as shown in Figure 1, comparable to that of contemporary rotating garden sprinklers.

Early Steam Engines
The successful application of steam as a form of energy is due to its inherent capability to expand in response to applied heat. The intensity of expansion is such that by the time the water ceases to boil, the resultant vapor has expanded to more than one thousand six hundred times greater than the original volume of water. One of the earliest known inventors who envisaged the feasibility of adapting this expansive capacity to the design of a device capable of producing "artificial energy", in the form that ultimately became known as a "Steam Engine", was John Savery, who applied for a patent for his invention in England in the year 1698. This was followed by the efforts of Thomas Newcomen, an English blacksmith in 1705, whose engines, although rather inefficient, were used primarily for pumping water out of coal mines. While working for the University of Glasgow, Scotland, in 1764, a mathematical instrument maker named James Watt, decided to raise the efficiency of contemporary steam engines with the use of an external condenser, for which he received his first patent in 1769. The purpose of the condenser was to "chill" the exhaust steam from the engine, and convert it back to water so that it could be re-used as "feed-water" for the boiler. His subsequent improvements included a throttle valve, a speed control governor and a heavy flywheel to maintain crankshaft momentum by dampening the jerky reciprocating motion at the apex of each piston stroke, to conform to more uniform rotary motion. But what is considered to be his most significant improvement was in replacing the single-acting piston with a double-acting piston, involving the alternating admission of steam to each end of the cylinder by means of a reciprocating slide valve.

Steam Engine Slide Valve Operation
Steam is supplied from the boiler, via the main stop valve to the High Pressure Steam Chest SC, at maximum operating pressure. As the Piston P, reaches the uppermost position in its stroke, the Slide Valve SV, moves down to expose the upper Steam Passage S, allowing steam within the Steam Chest to enter the cylinder above the Piston to push it down. Simultaneously, with the Slide Valve in its lowermost position, the lower Steam Passage S, is exposed to the cavity of the Slide Valve SV, allowing steam to vacate the lower part of the cylinder and flow to the Exhaust Passage E which leads to the Condenser. When the Piston moves down to the lowermost position in its stroke, the Slide Valve moves up to expose the lower steam passage, allowing steam to enter the cylinder from the Steam Chest and push the piston up. Simultaneously, with the Slide Valve in its uppermost position, the upper Steam Passage is exposed to the cavity of the Slide Valve, allowing it to vacate the upper part of the cylinder and flow to the Exhaust Passage which leads to the Condenser. The lower end of the Piston Rod is attached to a Crosshead that moves up and down within the Crosshead Guide, secured to the Engine Support Column. The Crosshead in turn, is joined to the upper end of a Connecting Rod, the lower end of which, is attached to a Crank Pin of the Crankshaft. The reciprocating vertical motion of the Piston Rod is transmitted via the Connecting rod to the Crankshaft, causing the latter to rotate. Likewise, the rod extending from the lower end of the Slide Valve, is attached to its own Connecting rod, the bottom end of which, is in turn attached to an eccentric disc on the Crankshaft, that provides the relatively short reciprocating stroke of the Slide Valve.

The Steam Age
This change in design resulted in an increase of horsepower in the order of 80 percent, for the same size cylinder. A double-acting steam engine piston and slide valve are shown in Figure 2. At this point it is worthy of mention that apart from the mechanical genius of so many inventors and engineers who contributed to the "Industrial Revolution" and subsequent developments, a significant factor that made the "Steam Age" feasible, was the abundance of coal. Fortunately for ship-owners, coal was mined in various countries around the world and became the primary fuel for steam boilers aboard ships, railroad locomotives and in stationary power plants, prior to the advent of fuel oil. Another significant historical event was the evolution of mechanically-propelled steam ships that eventually rendered sailing ships obsolete. Among the early pioneers who attempted to adapt steam powered propulsion to boats and ships were two American inventors, namely, John Fitch and James Rumsey. Unfortunately for Rumsey, his first boat broke down during its initial run in 1786. The following year Fitch demonstrated his version of a steam-powered vessel propelled by a series of mechanically-actuated paddles that worked, but was limited to a speed of only three miles per hour. Shortly thereafter, Rumsey showed off his new design based on a steam engine-powered water pump, from which the discharge exited through the transom to provide "water jet propulsion". The advent of steam engines also gave birth to railroad transportation in the form of several relatively comfortable passenger coaches pulled by a steam-powered locomotive, to replace the horse drawn carriage and stage coach for long distance overland travel.

A typical example of an early steam-powered railroad locomotive, built in Newcastle, England by George Stephenson in 1829, and capable of pulling a passenger train at 30 miles per hour, is the "ROCKET", shown in Figure 3. Although the American engineer Robert Fulton is believed by some to have been the inventor of the steam engine, he was not. His forte was in learning how to apply steam power profitably. During their sojourn in France, Robert Fulton and his partner Robert Livingstone, built a steamboat in 1803 that they operated on the River Seine in Paris. With the experience gained from this vessel Fulton returned to the United States and built a larger vessel named the CLAREMONT. Following her commissioning in 1807, Fulton's new vessel operated on the Hudson River between New York and Albany, a distance of 150 miles, providing a very dependable means of public transportation that earned $16,000.00 in her first year of service. During the second half of the nineteenth century, following the loss of several steam-powered vessels, ship-owners concluded that wooden hulls were unsuitable for heavy steel propulsion machinery, resulting in the birth of "Iron Ships", such as the S.S. GREAT EASTERN, launched in 1854, that was equipped with paddles and a screw propeller. But subsequently, the British Cunard Line launched a ship, the S.S. SERVIA, built entirely of steel, in 1881, since it was determined that steel is lighter, stronger and less brittle than iron. These factors combined to make the vessel considerably lighter, with more cargo capacity and capable of higher speed than her iron-built counter-parts of comparable size. With the availability of stronger hulls, ship-owners felt encouraged to progress from the conventional single-cylinder steam engine to the compound engine, employing two or more cylinders, to produce significantly more horsepower and higher speed.

Triple Expansion Steam Engines
In 1885, the Cunard Line proudly introduced the S.S. UMBRIA and the S.S. ETRURIA both equipped with more powerful steam reciprocating engines, and both capable of attaining a speed of 20 knots. In due course the compound engine evolved into the Triple Expansion, Double-Acting Steam Reciprocating Engine wherein steam is allowed to expand in three successive stages. Starting with the High Pressure cylinder, which is the smallest in diameter, then to the Intermediate Pressure cylinder, which is somewhat larger, and finally to the Low Pressure cylinder which is even larger. For example, a typical cylinder diametrical ratio would be 30 inches (H.P.); 50 inches (I.P.), and 80 inches (LP.). Given a hypothetical initial steam pressure in the order of 275 pounds per square inch, steam admitted to the HP cylinder expands enough to lower the pressure to about 150 p.s.i, at which it enters the IP cylinder wherein subsequent expansion lowers the pressure to about 50 p.s.i, from where it enters the LP cylinder and finally exhausts at approximately 13 p.s.i., into the condenser. Despite subsequent introduction of steam turbines and Diesel engines as maritime prime movers, around the turn of the century, triple-expansion reciprocating steam engines prevailed throughout the first half of the twentieth century, including World War II. A typical contemporary Steam engine is shown in Figure 4. This is a 2,600 brake horse power, Triple-Expansion Steam Reciprocating Engine, originally designed and built in England for marine propulsion. Engines of this type were used extensively for powering the hundreds of "Liberty" ships built in the United States during World War II. See Fig. 5. This engine design was successfully adapted for American production by the Hooven, Owens &amp; Rentschler Corp., during World War II and was subsequently built by eighteen different manufacturers in the U.S. The total weight of the engine is 270,000 pounds and it stands 19 feet high by 21 feet long, operating at a normal speed of 65 r.p.m., directly connected to a four-bladed propeller 18 feet in diameter, providing a vessel speed of 11 knots.

Steam Boilers
Much of the original development of boilers can be traced back to Edward Somerset who proposed what is currently recognized as being one of the earliest practical steam boiler designs in 1663, and which in turn encouraged various other would-be inventors to undertake the challenge. Subsequently, John Savery, noted for his steam engine of 1698, contributed several significant design improvements that were recognized by Thomas Newcomen and James Watt. Of the various fire-tube boilers the Scotch Boiler type is probably the most prevalent and has been used extensively for steam ships especially by shipbuilders on the River Clyde in Scotland, from where the term originated. The first marine application of this type of steam boiler is believed to have been the installation carried out on the S.S. MACGREGOR LAIRD in 1882. A Marine Scotch Boiler is a horizontal, cylindrical steel shell, internally fired steam generator and can be fitted with one, two or three furnaces.These are large bore corrugated steel cylinders extending horizontally within the lower half of the boiler shell, from the front end off the boiler to the lower half of the forward wall plate of the combustion chamber. The furnace is the space in which the fuel, either oil or coal is burned. Some boilers are built with a separate combustion chamber for each furnace, while others may have a single combustion chamber that is common to all furnaces. The rear wall plate of the combustion chamber is separated from and secured to the back end plate of the boiler by a series of horizontal stay bolts. The top of the combustion chamber is known as the combustion chamber crown and this is strengthened against the stress of steam pressure by several inverted "U" shaped girder stays located above the crown plate and secured thereto with through bolts. This arrangement allows the stress load to be dispersed from the horizontal crown plate to the vertical tube plate and rear wall plate of the combustion chambers. Fire tubes are installed above and parallel to the furnaces, extending horizontally from the front end of the boiler to the upper half of the forward face of the combustion chamber. Fire tubes conduct the hot gases of combustion from the upper half of the combustion chamber, to the forward end of the boiler, from where the gas flow is vectored upwards through a smoke box (uptakes) and eventually to the smoke stack. For additional structural support some of the fire tubes are made with extra heavy wall thickness to relieve some of the stress from the boiler shell tube plate and the combustion chamber tube plate, against the stress imposed by steam pressure. These are known as stay tubes. The interior of the boiler shell is partially filled with water that surrounds the furnaces, combustion chambers and fire tubes, to a level about ten or twelve inches higher than the combustion chamber furnace crown. The space above the fire tubes is reserved for containment of steam. For internal inspection, maintenance and repair purposes Scotch boilers are provided with elliptical manhole covers, secured in place by "U" shaped dog girders and bolts. The manhole opening periphery is roll-flanged inwards with a generous radius to increase rigidity and to provide an inward-projecting flat surface on the edge of the plate to form a solid seating area for the manhole covers. Fire Tube Boilers, see Fig. 6, have practically all been replaced for shipboard use and stationary power plants, by Water-Tube Boilers, see Fig. 7.

Steam Boiler Design
Contemporary steam boilers built in the United States are designed in accordance with specifications provided by the Rules of Construction of Fired and Unfired Pressure Vessels, contained in the Boiler Construction Code, published by the American Society of Mechanical Engineers (ASME). This design criterion is recognized by the majority of State Governments; the United States Coast Guard; and adopted by the National Board of Boiler and Pressure Vessel Inspectors; and by most casualty insurance underwriters involved in providing casualty/liability insurance for boilers and pressure vessels. Boiler and Pressure Vessel Design Code - The A.S.M.E. Boiler and Pressure Vessel Design Code, adopted by the National Board of Boiler and Pressure Vessel Inspectors, and by the United States Coast Guard, provides the following guidance:

P=TxSxE = 0.5 x 55,000 x 0.9 = 24,750 - 165 p.s.l. T = Thickness of shell, in inches or fractions thereof.

S = Allowable stress (of metal), in pounds per square inch.

E = Efficiency of welded or riveted joints.

F = Factor of Safety.

R = Radius of shell, in inches.

Transposition of Formulae, as per A.S.M.E.: As shown below, the formula can be transposed to determine the nominal value of any given factor, providing that all other factors are known.

P = TxSxE = 0.5 x 55,000 x 0.9 = 24,750 = 165 p.s.i. F x R 5 x 30 150

T = PxRxF = 165 x 30 x 5 = 24,750 = 0.5 S x E 55,000 x 0.9 49,500

S = PxRxE = 165x 30 x 5 = 24,750 = 55,000 T x E 165 x 5 825

R = SxTxE = 55,000 x 0.5 x 0.9 = 24,750 = 30 P x R 165 x 5 825

F = SxTxE = 55,00 x 0.5 x 0.9 = 24,750 =5 P x R 165 x 30 4,950

E = PxRxF = 165 x 30 x 5 = 24,750 = 0.9 S x T 55,000 x 0.5 27,500

NOTE: In cases where heavy pitting of the boiler shell is found, the depth of the deepest pit is subtracted from the original plate thickness. The resultant safe working pressure is then calculated to maintain the Safety Factor of 5, pending repair or replacement, and the boiler safety valve should be re-set accordingly.

Steam Turbine Engines
One of the earliest steam turbines was designed by a Swedish engineer named Carl Gustaf de Laval in 1887. This was an impulse turbine and built specifically for use in the dairy products industry in which de Laval was involved. Others soon followed, such as the reaction steam turbine designed in 1889 by Charles Parson (1854-1931) in England, and the multistage turbine built by an American, Charles 0. Curtis (1860-1953), and installed in a Turbo-Electric Power Plant in Chicago in 1903. This machine powered an electric generator that developed 5000 kW. (kilowatts), but only occupied about ten percent of the space previously required by the steam reciprocating engine that it replaced. About eight years after installing his first (land-based) Turbo-Electric Power Plants, that were of much higher thermal efficiency than their steam reciprocating counterparts, Charles Parsons, an English engineer, introduced the first marine steam turbine in 1897, installed aboard his yacht, the S.S. TURBINIA that attained a speed of 34 knots on her trial run during the Royal "Review of the Fleet', on the occasion of the Golden Jubilee of Britain's Queen Victoria in 1897. This was the first sea-going vessel to be powered by a steam turbine plant and it impressed several shipping companies including the Cunard Line, one of Britain's major passenger ship operators. As a result, some of them decided to investigate the feasibility of adapting such a prime mover to large oceangoing ships and in 1906 the Cunard Line commissioned two newly built large passenger liners powered by steam turbines, namely, the T.S.S. MAURITANIA and the T.S.S. LUSITANIA. Each of these vessels was about 790 feet in length with a registered tonnage of more than 35,000 tons, capable of 27 knots and both deployed on the North Atlantic run from Southampton to New York. Due to the relatively high ratio of horsepower-to-weight of the steam turbine, compared to that of steam reciprocating engines, the steam turbine became the main propulsion plant of choice for trans-oceanic passenger liners and for various types of naval vessels, including battleships and cruisers.

Steam Turbine Design Features
A steam turbine rotor consists of several stages of radial blades, (analogous to spokes on a wagon wheel), all mounted on a common shaft, and supported at both ends of the casing by white metal anti-friction bearings. See Fig. 8. Each successive stage is slightly larger in diameter than its predecessor, and all blades are positioned (equally), at a fixed angle relative to the shaft center-line. In between the rotor blade stages, there are casing blade stages, secured in place within circumferential grooves machined into the top and bottom casings. These casing blades are positioned (equally), at an angle of approximately ninety degrees to the angle of the rotor blades. In operation, when steam is supplied at maximum operating pressure to the high pressure turbine casing, it impacts the first stage of rotor blading, causing the rotor to spin. Leaving the first stage of rotor blading, the steam hits the first stage of casing blading and is deflected so as to impact the second stage of rotor blading. This sequence of impact and deflection occurs progressively at each stage throughout the turbine, with the steam expanding in volume and gradually losing pressure. In the case of marine propulsion turbines, the exhaust steam from the high pressure rotor is supplied to a low pressure rotor wherein a similar cycle of impact and deflection occurs, and finally, to the condenser wherein it is chilled and liquefied.

Due to the high speed of steam turbine rotor rotation, relative to that of conventional marine propellers, both turbine rotors, the high pressure and the low pressure, are connected to a "bull gear, a large diameter reduction gear, that reduces the output speed to a range compatible with that for which the propeller is designed. However, despite the previously mentioned advantages of steam turbine propulsion, the Diesel engine, either as a direct-drive prime mover, through reduction gearing or Diesel-Electric drive has become even more efficient and overall more economical for marine propulsion. Although Diesel engines have become the prime movers of choice all over the world, for merchant vessel propulsion and for many naval vessels, nuclear-powered submarines, see Fig. 9, are all propelled by steam turbine engines. In this case, the primary liquid cooling system for the nuclear reactor is passed through a heat exchanger wherein the secondary water cooling system absorbs sufficient heat (from the primary system), to produce high pressure steam (as in a boiler), that is then supplied to the main propulsion turbine rotors that rotate the propeller shafts via reduction gearing.