Sidebilder
PDF
ePub
[blocks in formation]

ning of a race for speed supremacy that has
brought out a new 'record-breaker' almost every
year since.
As the vessels are increased in size
it takes less horse power per ton to drive them
at a given speed, and this has tended to augment
the tonnage of the great liners so that the dimen-
sions of the Great Eastern are now exceeded by

nearly a dozen vessels, built and building. The accompanying table gives the principal features of the largest ships belonging to the great steamship companies, transatlantic and transpacific.

MARINE ENGINES. Marine engines have during recent years tended to one general type. For special services on inland waters a great num

TABLE GIVING PARTICULARS OF THE LARGEST AND MOST IMPORTANT STEAMERS IN THE WORLD

[blocks in formation]
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]
[graphic][merged small][subsumed]

ONE OF THE TWO SETS OF MAIN ENGINES OF THE "KAISER WILHELM II."

Each set of engines develops 20,000 horse-power, and consists of two four-cylinder, three-crank, quadruple-expansion engines.

STEAM NAVIGATION.

ber of various specialties are found, but in seagoing steamers the type in almost universal use is the vertical (i.e., the piston moves vertically), inverted (i.e. the cylinder is above the crank), direct-acting (i.e. the connecting rod joins the crosshead directly to the crank), triple (or quadruple) expansion engine. Engines differ as regards fittings and attachments, length of stroke and revolutions, weight, speed, etc., but a description of the general type will give the essential features of all.

533

Naval engines are built lighter, have a shorter stroke, and run at higher speeds than those in the merchant service. In a triple-expansion engine the steam works expansively in three successive stages, in a quadruple in four. The reason for the introduction of the multiple-expansion engine is the greater economy obtained when steam is used expansively over a greater range. This cannot be efficiently accomplished in a single cylinder owing to various causes chiefly due to liquefaction, hence compound (two stage) engines were introduced, then the triple and quadruple. The economy gained by the compound over the simple is about 50 per cent., by triple over compound about 25 per cent., and by quadruple over triple about 10 per cent. In the quadruple the gain in economy is obtained by considerable increase in weight, so that for most services the real gain of the quadruple is questionable. The type of engines will depend largely on the steam pressure employed. For a pressure of 40 to 90 pounds a compound; up to 190 pounds the triple; above 190, the quadruple may be used if space and weight are not very important. At present the pressure used is ordinarily between 140 and 250 pounds per square inch above the atmosphere. A great advantage of multiple-expansion engines, and a cause for their adoption, is the more even turning effect and better balancing obtained. The triple-expansion engine has either three or four cylinders, more often three, arranged in successive order, H.P. (high pressure), I.P. (intermediate pressure), L.P. (low pressure), each cylinder being attached by means of its piston and connecting rod to its own crank on the crank shaft, which is usually made in interchangeable sections, one for each crank. Cranks are usually set at 120° from each other to obtain even turn ing effect. Four cylinders are used when the L. P. cylinder would be too large to be conveniently fitted or built, or to obtain a better balancing of the engine and reduce vibrations. The sequence of the cylinders is then high pressure, intermediate pressure, low pressure, low pressure; or, on what is known as the YarrowSchlick-Tweedy system, low pressure, high pressure, intermediate pressure, low pressure, with cranks set at right angles. Here the crank shaft is generally in two sections.

The course of steam in a triple-expansion engine would be as follows: Leaves main steam pipe, passes through separator, then throttle, and into high-pressure valve chest. The movement of the valve opens and closes the steam ports at pressure cylinder for 0.6 to 0.75 of the stroke and then cuts off. The steam in the cylinder then expands, continuing to move the piston. Just before the end of the stroke the valve opens to exhaust and at about the same time begins to allow steam to enter on the other side of the piston; this results in cushioning at the end of the stroke.

STEAM NAVIGATION.

The steam having been reduced about 50 to 60 per cent. in pressure and correspondingly in temperature, leaves the high pressure cylinder, and passes to the intermediate pressure receiver, which is really the intermediate pressure valve chest. From here it enters through the intermediate pressure valve and does its work in the intermediate pressure cylinder, being again reduced in temperature and pressure. On leaving the intermediate pressure cylinder the steam is generally at about atmospheric pressure. Then it is conducted to the low pressure receiver and goes through its third stage of working and expansion. On leaving the low pressure it goes through the exhaust pipe to the main condenser where it is condensed, and then as water and vapor it is pumped by the air pump to the hot well or feed tank, and thence to the boiler, where it is reëvaporated.

A plate illustrating a four-cylinder triple-expansion engine of a modern second-class cruiser for the United States Navy is shown with various The engine framing is supparts indicated. ported on a bed plate made in sections and bolted by holding-down bolts to structural parts of the vessel. The engine framing is made of steel columns braced by various cross rods. A more general practice is to have cast or wrought steel inverted Y frames on one side which support the guides, and in the merchant service the condenser is generally cast in one piece with lower The cylinders are portions of the Y frames. supported on top of framing and bolted to it by der is a separate casting; generally the valve various fastenings. In large engines each cylinchest is cast with the cylinder, making the whole a rather intricate casting. Cast iron is generally used for cylinders. Cylinders are fitted with liners, a, which form the bearing surface for the piston. The liners are bolted at the lower end to the bottom of the cylinder and the joint at the top is packed. Liners are of cast steel or cast iron; steel is stronger, but cast iron gives a better wearing surface. The space between the cylinder and liner is commonly used as a steam jacket. Lately the economy of steam jacketing, especially for fast-moving engines, has been questioned. All cylinders are fitted with covers, c, of cast iron or steel, and these are secured to the cylinders by bolts and nuts and the joints packed by gaskets. For large cylinders, smaller openings called bull's-eyes, d, are fitted for purposes of examination. The pistons are coneshaped, made of forged or cast steel or cast iron, and fitted with cast iron spring packing rings, e. The piston rod is secured to the piston by its taper and the piston rod nut, f, on top. The opening in the bottom of the cylinder for the piston rod is fitted with a stuffing box, g, supplied with some form of metallic packing. The lower end of the piston is secured to the crosshead h, made of forged steel, which has a slipper, i, sliding on the crosshead guide, j. The crosshead also has journals, k, for the upper end of the connecting rod. The lower end of the connecting rod is attached to the crank pin, l, by means of the crank pin brasses, m. The crank shaft is supported by the main bearings, n, which are supported by a bed-plate. All large bearings are lined with anti-friction metal.

The valves o for the H. P. and I. P. cylinders are piston valves, either single or double ported, hollow or solid. For the L. P., the

through such parts as the thrust bearing and crosshead guides, or sprayed on various other bearings where heating is likely.

The oil services on a modern engine are very elaborate, as all working bearings must be supplied with a lubricant. The best practice is the manifold system, where each bearing has a small pipe leading up to one of the several manifolds where it is fed by means of a wick. The manifold can be filled from a reservoir placed above the level of the engine. Besides oil, graphite and various preparations of tallow and grease are used for lubrication.

double-ported flat slide valve, p, fitted with a relief ring is used. Piston valves are employed with high pressure because in them the pressure is on all sides and there is no force holding the valve against its seat. They are fitted with spring rings to make them tight. Valves are made of cast iron, cast steel, or forged steel. The valve seats are generally liners of cast iron. The valve stem is secured to the valve by its taper or shoulder and the nut q and the upper end r of the rod is fitted as a guide. The lower end of the valve rod is connected by means of a bearing to the link block 8, which works in the link t. To the ends of the link are attached the eccentric rod u, and the lower ends of these rods are bolted to the eccentric straps v, which move around eccentrics w. The eccentrics are secured to the shaft and fitted so that the position can be slightly changed. There are two eccentrics, one to give go-ahead motion and the other backing. The link is moved by an arm attached to the reversing shaft a, which is operated by the reversing engine y, and this engine is controlled by the reversing lever. The link arm is attached to the adjustable cutoff block, by means of which the cut-off can be varied from .5 to .75 of stroke. This is the Stephenson gear, which is most generally used; others are Marshall's, Joy's, Morton's, etc.

In the cut shown, the air pump A is operated by a cross beam B attached to the L. P. crosshead. In many merchant vessels the circulating, bilge, and feed pumps are operated from such a beam. For large installments these pumps are as a rule independent and of the Blake, Worthington, Snow, or other patent type. The office of the air pump is to pump the condensed water and vapor from the condenser to the feed tank and produce a vacuum. Surface condensers are now always fitted, and the steam and condensed fresh water are kept separate from the circulating sea water. This keeps salt out of the system. The condenser consists of an approximately cylindrical vessel, having a water chamber and a tube sheet at each end. Brass tubes connect the two tube sheets and cold sea water is pumped through the tubes by means of the circulating pump, thus cooling and condensing the exhaust steam surrounding the tubes. Condensers are made of bronze or cast steel and the tubes of brass. Circulating pumps are centrifugal and operated by a vertical simple or compound engine.

The feed tank is generally fitted with a filter chamber for purifying the water. Feed heaters, using auxiliary exhaust steam to heat the water before reaching the boilers, are fitted for purposes of economy and make the service less hard on boilers. The feed pumps are vertical, single, or duplex plunger pumps. Cylinders are lagged (i.e. covered with non-heat-conducting material) to prevent loss of heat. All cylinders are fitted with relief valves set at appropriate pressures, drains for conducting off any water that may accumulate, and indicator pipes, cocks, and reducing motion for taking indicator cards. Pressure gauges are supplied to indicate the pressure in the steam pipe, the various receivers, and the vacuum in the condenser. Revolution counters are attached, which automatically record the number of revolutions of the engines. A water service is supplied, consisting of a system of piping by means of which sea water can be circulated

The steam pressures now used are 150-300 pounds. It is not likely that much higher pressures than 250 will be soon used, on account of the great strength of parts necessary to withstand the pressure, the difficulty of keeping tight joints, and the high temperature of steam, which heats the working surfaces and prevents proper lubrication.

The

Steam is expanded in triple-expansion engines 6 to 9 times; in quadruple, 8 to 12 times. ratio of the area of the H. P. to that of L. P. cylinder varies from 1 to 5 to 1 to 10, there being a greater ratio with increased pressures. The revolutions vary from 80 or 100 per minute in very long stroke engines to 400 to 500 in high-speed torpedo boats. The piston speed is limited to about 1000 feet per minute. The length of stroke for large merchant vessels is four to six feet; for naval engines not over four feet; with smaller engines the stroke is less. A relatively long stroke results in greater economy.

Of late the steam turbine is beginning to be introduced in place of the reciprocating engine for fast vessels. See STEAM TURBINE.

use

BOILERS. Modern marine practice is either to

the cylindrical fire-tube boiler carrying pressures of 150 to 200 pounds per square inch, or some form of water-tube boiler using pressures of 160 to 300 pounds. W.T. (water-tube) boilers are more largely used for naval purposes and fast passenger vessels and cargo vessels in fresh water, cylindrical boilers for general merchant service.

The substantial advantages of the cylindrical boiler are: reliability, simplicity; it is well made and generally understood; it can use salty or dirty water, and it will stand hard usage without serious loss or injury. The disadvantages are: great weight; steam cannot be gotten up or taken off quickly; it does not readily adjust itself to change of output; and heavy forced draught cannot be used.

The advantages of the water-tube boiler are: lightness; adaptability to high pressure; rapidity of raising steam or taking it off; it is readily adjusted to sudden change of output; forced draught can be used (in nearly all types); and repairs or removals are more easily made. Its disadvantages: it requires great care and attention; it cannot use salty or dirty water or experience hard usage: corrosion takes place very readily; it is complicated and many types require a large number of mechanical attachments; and being new, it is not well understood by men who handle it and best results are not obtained. The economy of fuel is about the same in the best of each type. For average running the cylindrical is probably the more economical.

The general form of a cylindrical boiler is the

« ForrigeFortsett »