CHAPTER XXX

MOTIVE POWERS PRODUCED WITH NEW ECONOMY

Improvements in steam practice . . . Mechanical draft . . . Automatic stokers . . . Better boilers . . . Superheaters . . . Economical condensers . . . Steam turbines on land and sea.

In every industry a threshold question is how motive power may be had at the lowest cost. In this field within twenty years wholly new methods have been introduced, while old processes have been greatly amended. Thanks to economical water-wheels and generators, efficient transmission, and motors all but perfect, water-powers, as at Niagara Falls, now send electricity to thousands of distant workshops, to serve not only as an ideal means of actuation, but as a source of light, heat and chemical impulse. While electrical art has thus been marching forward, all the heat engines have been improved in every detail of construction. New valve-gears, economizers and superheaters, united with triple-expansion cylinders of the boldest dimensions, worked at pressures and speeds greater than ever before, combine to make the best steam engines to-day vastly more effective than those of a generation ago. And these engines are withal facing the aggressive rivalry of the steam turbines devised by De Laval, Parsons and Curtis, all much less heavy and bulky than engines, simpler to build and operate, while their motion is continuous instead of interrupted at every piston stroke.

Competing with steam motors are the new gas engines, twice as efficient in converting heat into motive power. For this reason and because much improvement seems to be feasible in their designs, and in systems for supplying them with cheap gas, their adoption on a large scale in the near future appears to be certain. Especially will this be the event should the turbine principle be as successfully applied to gas as to steam motors. Already gases from coke ovens and blast furnaces, formerly thrown away or used only in part, are being employed in gas engines with success.

To-day the production of motive power largely centres in stations so huge that they adopt with gain appliances too elaborate for use in small installations. At the power-house of the Interborough Rapid Transit Company, New York, for example, automatic machinery conveys coal from barges to vast bunkers under the roof, an even distribution being effected by self-reversing trippers. Twelve of the furnaces have automatic stokers. Ashes are removed by conveyors. Lubricating oil is pumped to high reservoirs whence it descends to flush all the bearings; it is then carried to filters from which it passes to another round of duty. It is plain that the huge scale of such a plant opens new doors to ingenuity, especially in the dovetailing of one service with another.

In some central stations, as at Findlay, Ada, and Springfield, Ohio, the exhaust steam is utilized for district heating, so that the generation of motive power is merged into the larger field of fuel economy treated as a whole. Where there is a profitable market for exhaust steam it pays to use a group of engines or turbines which are either non-condensing, or only some of which are condensing, for the aim is not simply to use the motor which asks least fuel, but to install such motors and heaters as together will earn most for the capital invested.

Steam Engines.

An experimental quadruple-expansion steam engine at Sibley College, Cornell University, has consumed but 9.27 pounds of steam of 500 pounds pressure per indicated horse-power, with a mechanical efficiency of 86.88 per cent. An Allis-Chalmers compound engine, tested December, 1905, at the Subway Power-house, New York, developed 7,300 horse-power from steam at 175 pounds pressure with a consumption of 11.96 pounds of steam per indicated horse-power. The cylinders were not steam jacketed and no reheaters were used. This engine has two horizontal high pressure cylinders, 42 inches in diameter; and two vertical low pressure cylinders, 86 inches in diameter; all of 60 inch stroke. The four cylinders work on the same crank pin, with the effect of two cranks at right angles to each other in superseded designs. A similar engine, less powerful, is shown opposite this page.

Mechanical Draft.

At this point let us put back the clock a little that we may understand why tallness in chimneys is much less in vogue for steam plants than formerly, and why this change is found to be well worth while. A device at least two centuries old is the smoke-jack, of which a specimen lingers here and there in the museums and curiosity shops of England. The rotary motion of its vanes, due to the upward draft from a kitchen fire, was employed to turn a joint of meat as it roasted in front of the coals. To-day the successors of this primitive heat-mill are the cardboard or mica toys which, fastened to a stove-pipe, or close to a lamp chimney, set at work a carpenter with his saw, a laundress with her sad-iron, and so on. These playthings show us the simplest way in which heat can yield motive power; because simplest it prevails almost universally, and yet it is wasteful in the extreme. Nobody for a moment would think of putting a wheel like that of a smoke-jack in a chimney so that the rising stream of hot gases might drive a sewing-machine or a churn, and yet for a task just as mechanical, namely, the pushing upward a chimney current itself, the heating that current to an extreme temperature is to-day the usual plan. Under good design the gases of combustion are obliged to do all the work that can be squeezed out of them; then and only then they are sent into the chimney. What if their temperature be so low, comparatively, that their rise in the stack, if left to themselves, is slow as compared with the rise in another stack of gases 300° hotter? One hundredth part, or even less, of the saved heat when applied through an engine to a fan will ensure as quick a breeze through the grate-bars as if the chimney gases were wastefully hot, and this while the chimney is but one eighth to one fourth as tall as an old-fashioned structure. This is the reason why mechanical draft is now adopted far and wide in factories, mills and power-houses. The advantages which follow are manifold: the plant is rendered independent of wind and weather, inferior fuels are thoroughly and quickly consumed, at times of uncommon demand a fire can be easily forced so as to increase the duty of the boilers. To-day in the best practice the feed water for the boilers is heated by the furnace gases just before they enter the stack; the piping for this purpose, formed into coils known as economizers, checks the chimney draft. This checking is readily overcome by mechanical draft, leaving the engineer a considerable net gain as fan and economizer are united. One incidental advantage in modern plants of sound design, and good management, is that they send forth but little smoke or none at all. With thorough combustion no smoke whatever leaves the stack.

Automatic Stoking.

The avoidance of smoke is promoted by the use of well designed mechanical stokers: two of the best are the Roney and the Jones models. The Jones apparatus forces its fuel into the fire from beneath, so that its gases, passing upward through blazing coal, are thoroughly consumed.

Boilers.

In large plants the boilers are usually of the water-tube variety, working at high pressures which may be increased at need. Mr. Walter B. Snow says:[38]--“Until the recent past the steam generator or boiler and the manner of its operation received far less attention than they deserved. Although under the best conditions over 80 per cent. of the full calorific value of the fuel may be utilized in the production of steam, this high standard is seldom reached in ordinary practice. Mr. J. C. Hoadley showed an efficiency of nearly 88 per cent. in his tests of a warm-blast steam-boiler furnace with air-heaters and mechanical draft, while Mr. W. H. Bryan has reported eighty-six tests conducted under common conditions with ordinary fuel, upon boilers of various types, which indicate an average efficiency of only 58 per cent., and have a range between a minimum of 34.6 per cent. obtained with a small vertical boiler, and a maximum of 81.32 per cent. with a water-tube boiler of improved setting. The possibilities of increased economy in ordinary boiler practice are thus clearly evident.”

[38] In his “Steam Boiler Practice.” New York, John Wiley & Sons, 1904. $3.00.

Superheaters.

A cardinal improvement in steam engineering of late years has been in perfecting superheaters; this advance owes much to the mineral oils now available for lubrication at temperatures which may be as high as 675° Fahr. As steam expands to perform work it falls in temperature and much of it condenses as water, with marked loss of efficiency, with harm to its containers by severe hammering. A superheater avoids this trouble by so raising the initial temperature of the steam that condensation either ceases altogether or is much lessened. The apparatus is usually a nest of tubes placed in the fire-box close to the boiler; or, the tubes may be heated by a fire of their own, away from the boiler. The Schmidt superheater has long, parallel bent tubes, connecting two parallel headers. It may be directly applied to locomotive boilers without essential modification, and without checking the draft. On the Canadian Pacific Railway about two hundred simple locomotives have been provided with superheaters, lowering the coal consumption to 87, 85, 83 and as little as 76 per cent. in comparison with compound engines having no superheaters. At St. Louis in 1904 the Pennsylvania Railroad conducted elaborate tests of diverse locomotives. The most economical compound engine each hour used 18.6 pounds of ordinary saturated steam per indicated horse-power. Aided by a superheater this consumption was reduced to 16.6 pounds, a saving of 10.75 per cent. See page 241. In Germany portable steam engines of 150 to 220 horse-power, superheating their steam 150° to 170° Centigrade above the temperature of saturation have, in compound types, reduced their demand for steam to 12.47 pounds per horse-power hour and, in a triple-expansion model, to 9.97 pounds. In all cases the steam pipe takes the shortest possible path between its superheater and its cylinder.

Improved Condensers.

By an improved design Professor R. L. Weighton of Armstrong College, Newcastle-on-Tyne, has doubled the efficiency of the surface condenser, and reduced its consumption of water 44 per cent. In his apparatus the condensing water enters at the base, and leaves at the top, after several circuits instead of but two as in the ordinary condenser. This new apparatus is drained off in sections, instead of allowing the condensed steam to accumulate at the bottom, as in common practice. This sectional drainage is effected by dividing the interior into diaphragms somewhat inclined to the horizontal, so that the water of condensation is removed as fast as formed and does not flow from the upper tubes over those beneath. The gain in this arrangement arises from the fact that the greater part of the condensation takes place in the upper part of a condenser, where the steam impinges first upon the tubes. The Weighton apparatus, in conjunction with dry air-pumps, shows a condensation of 36 pounds of steam per square foot of surface per hour, with a reduction of pressure to one twentieth of barometric pressure (1-1/2 inches as compared with 30), using as condensing water 28 times as much as the feed water, at an inlet temperature of 50° Fahr.

Steam Turbines.

For a long time, and well into the nineteenth century, water was lifted by pistons moving in cylindrical pumps. Meantime the turbine grew steadily in favor as a water-motor, arriving at last at high efficiency. This gave designers a hint to reverse the turbine and use it as a water lifter or pump: this machine, duly built, with a continuous instead of an intermittent motion, showed much better results than the old-fashioned pump. The turbine-pump is accordingly adopted for many large waterworks, deep mines and similar installations. This advance from to-and-fro to rotary action extended irresistibly to steam as a motive power. It was clear that if steam could be employed in a turbine somewhat as water is, much of the complexity and loss inherent in reciprocating engines would be brushed aside. A pioneer inventor in this field was Gustave Patrick De Laval, of Stockholm, who constructed his first steam turbine along the familiar lines of the Barker mill. Steam is so light that for its utmost utilization as a jet a velocity of about 2,000 feet a second is required, a rate which no material is strong enough to allow. De Laval by using the most tenacious metals for his turbines is able to give their swiftest parts a speed of as much as 1400 feet a second. His apparatus is cheap, simple and efficient; it is limited to about 300 horse-power. Its chief feature is its divergent nozzle, which permits the outflowing steam to expand fully with all the effect realized in a steam cylinder provided with expansion valve gear. Another device of De Laval which makes his turbine a safe and desirable prime mover is the flexible shaft which has a little, self-righting play under the extreme pace of its rotation.

The Parsons Steam Turbine.

Of direct action turbines the De Laval is the chief; of compound turbines, in which the steam is expanded in successive stages, the first and most widely adopted was invented by the Hon. Charles A. Parsons of Newcastle-on-Tyne. From an address of his to the Institute of Electrical Engineers, early in 1905, the following narrative has been taken:--

“In the early days of electric lighting the speed of dynamos was far above that of the engines which drove them, and therefore belts and other forms of gearing had to be resorted to. To make a high-speed engine, therefore, was of considerable importance, and this led to the possibilities of the steam turbine being considered. It was at once seen that the speed of any single turbine wheel driven by steam would be excessive without gearing, and in order to obtain direct driving it was necessary to adopt the compound form, in which there were a number of turbines in series, and thus, the steam being expanded by small increments, the velocity of rotation was reduced to moderate limits. Even then, for the small sizes of the dynamos at that time in use, the speed was high, and therefore a special dynamo had to be designed. Speaking generally, an increase of speed of a dynamo increases its output, and therefore it was obvious that such a high-speed dynamo would be very economical of material.

“These considerations led, in 1884, to the first compound steam turbine being constructed. It was of about 10 horse-power and ran at 300 revolutions per second, the diameter of the armature being about three inches. This machine, which worked satisfactorily for some years, is now in the South Kensington Museum. Turbines afterward constructed had two groups of 15 successive turbine wheels, or rows of blades, on one drum or shaft within a concentric case on the right and left of the steam inlet, the moving blades or vanes being in circumferential rows projecting outwardly from the shaft and nearly touching the case, and the fixed or guide blades being similarly formed and projecting inwardly from the case and nearly touching the shaft. A series of turbine wheels on one shaft were thus constituted, and each one complete in itself is like a parallel-flow water turbine, the steam, after performing its work in each turbine, passing on to the next, and preserving its longitudinal velocity without shock, gradually falling in pressure as it passes through each row of blades, and gradually expanding. Each successive row of blades was slightly larger in passage way than the preceding to allow for the increasing bulk of the elastic steam, and thus the velocity of flow was regulated so as to operate with the greatest degree of efficiency on each turbine of the series. . . . It constituted an ideal rotary engine, but it had limitations. The comparatively high speed of rotation necessary for so small an engine, made it difficult to avoid a whipping or springing of the shaft, so that considerable clearances were found obligatory, and leakage and loss of efficiency resulted. It was perceived that these defects would decrease as the engine was enlarged, with a corresponding reduction of velocity. In 1888 therefore several turbo-alternators were built for electric lighting stations, all of the parallel-flow type and non-condensing. In 1894 the machines were much improved, the blade was bettered in its form, and throughout greater mechanical strength was attained. . . . To-day (1905) under 140 pounds steam pressure, 100° Fahr. superheat, and a vacuum of 27 inches, the barometer being at 30 inches, the consumptions are in round numbers as follows:--A 100-kilowatt (134 horse-power) plant takes about 25 pounds of steam per kilowatt-hour at full load, a 200-kilowatt (268 horse-power) takes 22 pounds, a 500-kilowatt (670 horse-power) takes 19 pounds, a 1,500-kilowatt (2,010 horse-power) 18 pounds, and a 3,000-kilowatt (4,020 horse-power) 16 pounds (or 12 pounds per horse-power-hour). Without superheat the consumptions are about 10 per cent. more, and every 10° Fahr. of superheat up to about 150° lowers the consumption about 1 per cent.

“A good vacuum is of great importance in a turbine, as the expansion can be carried in the turbine right down to the vacuum of the condenser, a function which is practically impossible in the case of a reciprocating engine, on account of the excessive size of the low-pressure cylinder, ports, passages and valves which would be required. Every inch of vacuum between 23 and 28 inches lowers the consumption about 3 per cent. in a 100-kilowatt, 4 per cent. in a 500-kilowatt, and 5 per cent. in a 1,500-kilowatt turbine, the effect being more at high vacua and less at low.”

Marine Steam Turbines.

In 1894 Mr. Parsons launched his “Turbinia,” the first steamer to be driven by a turbine. Her record was so gratifying that a succession of vessels, similarly equipped, were year by year built for excursion lines, for transit across the British Channel, for the British Royal Navy, and for mercantile marine service. The thirty-fifth of these ships, the “Victorian” of the Allan Line, was the first to cross the Atlantic Ocean, arriving at Halifax, Nova Scotia, April 18, 1905. She was followed by the “Virginian” of the same line which arrived at Quebec, May 8, 1905. Not long afterward the Cunard Company sent from Liverpool to New York the “Carmania” equipped with steam turbines, and in every other respect like the “Caronia” of the same owners, which is driven by reciprocating engines of the best model. Thus far the comparison between these two ships is in favor of the “Carmania.” The new monster Cunarders, the “Lusitania” and the “Mauretania,” each of 70,000 horse-power, are to be propelled by steam turbines. The principal reasons for this preference are thus given by Professor Carl C. Thomas:--

Decreased cost of operation as regards fuel, labor, oil, and repairs.

Vibration due to machinery is avoided.

Less weight of machinery and coal to be carried, resulting in greater speed.

Greater simplicity of machinery in construction and operation, causing less liability to accident and breakdown.

Smaller and more deeply immersed propellers, decreasing the tendency of the machinery to race in rough weather.

Lower centre of gravity of the machinery as a whole, and increased headroom above the machinery.

According to recent reports, decreased first cost of machinery.[39]

[39] “Steam Turbines,” by Carl C. Thomas, professor of marine engineering, Cornell University, a comprehensive and authoritative work, fully illustrated. New York, John Wiley & Sons, 1906. $3.50.