Page:The New International Encyclopædia 1st ed. v. 18.djvu/625

STEAM NAVIGATION. of various specialties are found, but in sea-going 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.

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 turning 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 Yarrow-Schlick-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.

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 parts indicated. The engine framing is supported 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 portions of the Y frames. The cylinders are supported on top of framing and bolted to it by various fastenings. In large engines each cylinder is a separate casting; generally the valve chest 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 cone-shaped, made of forged or cast steel or cast iron, and fitted with cast iron spring packing rings, c. 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 ahso 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