the air-pump levers, A PL, which are keyed to the rocking shaft, RS. The inner end of the levers, A PL, are attached by short links to the crosshead, C, of the low-pressure cylinder piston rod, PR (see all three views). An enlarged section and plan of the air and the circulating pumps, with a detailed description, will be given in a future Lecture. As explained above under the heading "Course of the Steam," the condensed steam, on falling to the bottom of the condenser, is drawn therefrom through the surface condenser discharge pipe, S C, DP, and the air-pump suction valve, AP, S V, by the air pump, and transferred to the hot well, H, from which it is pumped by the feed pump, F P, into the boiler; but the air which comes from the surface condenser is allowed to escape through the side of the ship by a pipe leading from the top of the hot well, H. This pipe is not shown in the drawings. A jet condenser cold-water injection pipe, CIP (see front elevation and plan), with a rose pipe on its end, is fitted to the bottom of the exhaust pipe, EP', so that the steam may be condensed by the old method of jet-condensation, should the surface condenser or any of its auxiliary parts break down. Under ordinary circumstances, however, the double-acting circulating pump, CP, draws in cold water from the sea through the circulating pump suction pipe, CP, SP (see side elevation and plan), and forces it through the cold-water pipe, C W P, into the top of the surface condenser (see also front elevation) through the upper tier of horizontal tubes, and then back again through the lower set of tubes, and through the vertical discharge pipe, DP, out to sea again: thus keeping up a continuous circulation of cold water through all the condenser tubes, whereby the exhaust steam, as it spreads over these tubes on leaving the exhaust pipe, is condensed on their cold surface and gravitates to the bottom of the condenser. Turning Gear.-A worm wheel, W W, is attached to the afterend of the crank shaft, CS, and a worm, driven by a small engine, gears with this wheel. This arrangement is necessary for turning the engines while the vessel is lying in port, and the worm is fitted in such a way that it may be easily and quickly thrown out of gear before the main engines are worked by steam. Governor.-A marine-engine governor is fitted to the after bulkhead in a convenient position (not shown), and is driven by a rope from a V-grooved pulley keyed to the crank shaft just outside the worm wheel, W W. This governor actuates a valve near the throttle valve, T V, in the main steam pipe, and helps to prevent the engines racing when the ship is in a heavy sea. Gauges.-Three circular-faced gauges, G, are shown in the front elevation, fixed to the forward front main column, M C. One of these is the steam gauge for indicating the pressure of steam in the main steam pipe, or the initial pressure as it enters the highpressure cylinder valve casing; another is for indicating the pressure of steam in the intermediate receiver, B, between the high and low pressure cylinders or the back pressure on the highpressure piston; and the third is for indicating the vacuum in the condenser or the back pressure on the low-pressure piston. Each gauge is connected directly to the space wherein the pressure is to be ascertained by a small solid drawn copper pipe and cock, which is always kept open during the time that the engines are working. Starting Handles.-Immediately below the gauges, and at a handy height for the engineer on watch, are fixed four starting handles, SH. One handle is connected to a throttle valve in the main steam pipe near the governor throttle valve, T V, so that the engineer may suddenly check the speed of the engines, or stop them, or, by partially closing this valve, work the engines at any desired speed (with the stop valve full open and the link motion in full gear) as required. The second and third handles are connected respectively to the drain cocks of the valve casing and the jacket water trap below the high-pressure cylinder, so as to free these places from any water that may accumulate there, due to priming or condensation. The pipes leading from these drain cocks are taken directly to the hot well, H, so as to economize as far as possible all the condensed steam and have it pumped back to the boiler. The fourth handle is connected to an impulse piston valve, IV, connected to the low-pressure cylinder supplied with steam direct from the donkey boiler, so that live steam may be admitted to the top or bottom of this cylinder at pleasure, for the purpose of starting the engines over the dead centres, should they not move away directly when steam is applied in the ordinary way, and for the purpose also of warming up the whole of the parts connected with that cylinder. To Start the Engines. First, warm up slowly and carefully every part connected with the valve casings and cylinders, and then blow through the condenser so as to clear all the spaces of air and water, by partially opening the stop valve, S V, throttle valve, TV, the drain cocks, and the impulse valve, I V, attached to low-pressure cylinder, and moving the link motion first into forward, then into back, gear. (Sometimes a special set of cocks and pipes are employed for warming up and blowing through.) Second, open the stop valve and throttle valve as fully as may be required, and move the link motion into forward gear so as to make the engines go ahead. LECTURE XX.-QUESTIONS. 1. Make a complete free-hand sketch of the three views of the s.s. St. Rognvald's engines, as given in this Lecture; attach all the index letters, write out a complete index to parts, and describe in your own words the general arrangement of the whole engine, as well as how it is started and reversed, without referring to the text-book. 2. The diameter of the high-pressure cylinder is 36", and the low-pressure cylinder 70", in the St. Rognvald's engines. Find the ratio of the cross areas of the two pistons. Ans. 1: 3.76. 3. The diameters of the cylinders being 36" and 70", as stated in Question 2, and the stroke of each piston being 4': find the superficial area of the inside of the cylinder liners (neglecting clearance lengths and depth of pistons), and also the volume swept through by each piston. Ans. 4. Suppose that steam is cut off at half stroke in the high-pressure cylinder of the s.s. St. Rognvald's engines, what is the total ratio of expansion of the steam (neglecting clearance)? Ans. 7.52. 5. Steam is supplied at 90 lbs. pressure on the square inch by steam gauge to the high-pressure cylinder of the s.s. St. Rognvald's engines. Suppose that it is cut off at half stroke, and that the back pressure is 25 lbs. above atmospheric pressure; also, that steam is cut off at half stroke in the low-pressure cylinder, and that the back pressure is 4 lbs. absolute; how many revolutions per minute must the engines work at in order to develop 1500 I.H.P.? Ans. 38. 6. The cylinders of a compound engine are 25′′ and 45′′ in diameter. Find the ratio of their areas. Ans. I to 3:24. 7. The cylinders of a compound engine are 25′′ and 45′′ in diameter; with the same stroke, the mean effective pressure of steam in the smaller cylinder is 40 lbs., and in the larger cylinder 12 lbs. Which cylinder is doing most work, and by what percentage? Ans. The smaller cylinder; by 2·6%。• 8. What is the total ratio of expansion in a compound engine with cylinders 31" and 62′′ diameter, and the same stroke, when steam is cut off at half stroke in the high-pressure cylinder? Ans. 8 times. 9. Find the indicated horse-power of a compound engine, the cylinders being 27.5" and 48" diameter; stroke in each case 2'5'; revolutions per minute 75; the mean effective pressure in high-pressure cylinder being 37 lbs., and in low-pressure cylinder 7.35 lbs. Ans. 4008. 10. Steam is admitted into the high-pressure cylinder of a compound engine at 70 lbs. by gauge, and cut off at 4 of the stroke. The cubic capacity or volume swept through by the piston of the low-pressure cylinder being 3 times that of the high-pressure one: find the final pressure per square inch of the steam when it exhausts into the condenser, Ans. 113 lbs. absolute. II. Sketch in section the high pressure cylinder, with slide and expansion valve, as forming part of a compound cylinder marine engine. Describe briefly the arrangement of the engine, and how the condensation of steam is effected. (S. & A. Exam., 1893.) 12. Sketch and describe the escape valve as fitted to the cylinders of a marine engine. What is the use of such a valve? Show, by a sketch, where it is fixed. (S. & A. Exam. 1896.) LECTURE XXI. CONTENTS.-Details of Engines-Cylinders-Cylinder Covers-Stuffing Boxes, Glands, and Packing-Relief Valves-Steam Ports-Pistons -Piston Rods and Crossheads-Connecting Rods; with Specification for Details of s.s. St. Rognvald's Cylinders, &c. &c. In this and the two following Lectures we shall enter very fully into the details of a compound marine engine, illustrating our remarks by a series of figures reduced from the working drawings of the s.s. St. Rognvald's engines, kindly furnished to the author by the makers, Messrs. Hall, Russell, & Co., Aberdeen, as well as by extracts from the contract specification. Reference should be made when required to the three figures in last Lecture. Details of Engines-Cylinders.- Cast iron is the material universally employed for the construction of steam-engine cylinders. The inside of the cylinder barrel is frequently fitted with a thin liner, which is made of a hard close-grained material, capable of taking on a high polish and withstanding the rubbing action of the piston. If the liner becomes much worn, it may be taken out and replaced by a new one at a very small expense; or it may be re-bored, if only slightly worn. In small engines it is not usual to fit the cylinders with liners, but the metal of the cylinder barrel is made thicker than is necessary for strength at first, so that when the cylinder becomes much worn, it may be re-bored and fitted with a new piston. When the cylinder is to be steam-jacketed, a liner is now always employed, and steam is circulated round the annular space between the cylinder barrel and the liner. Cylinder liners are usually constructed of hard cast iron; but compressed steel liners, as manufactured by Sir Joseph Whitworth's patent process, have been largely employed, and have given satisfactory results. The method of fitting-in these liners will be readily understood by reference to the high-pressure cylinder of the St. Rognvald's engines, illustrated in the last Lecture. The interior of the cylinder barrel has a fitting strip at each end and at the middle, which projects from inch to inch above the interior surface of the barrel itself; and these strips are bored out so as to fit exactly similar strips on the external surface of the liner. The annular space between the liner and the cylinder barrel is therefore about 1 inch, and the hot steam from the boiler is passed round this space. The liner is usually fixed-in by an internal flange on its lower or inner end, which is sometimes recessed into a space in that end of the cylinder, and is attached to the cylinder end by screwed pins. To prevent waste of steam, it is necessary that the joints between the liner and the cylinder barrel should be steam-tight, and at the inner joint this may be effected by the use of red lead, when the liner is being fixed in its place. At the back or upper end, however, a small groove is usually bored out at the joint immediately above the fitting strip, and this groove is packed with soft rope, asbestos, or some of the other packings in general use. This packing is kept in position by a thin wrought-iron ring, which is fixed to the top of the liner. A very simple and efficient plan is to caulk a thin copper ring into a space bored out for it above the fitting strip. end Cylinder Covers.-One of the ends of a steam-engine cylinder, called the cover, is always bolted on, whilst the other is usually cast along with the cylinder barrel. It is the back or upper which is separate from the cylinder barrel. In large engines, and all jacketed engines, this cover is made hollow, and the flat sides are connected by ribs. In small engines, it consists simply of a circular plate of metal. This cover is held down by studs, which are screwed into a flange on the cylinder barrel, and are sufficiently strong to resist the full initial pressure of the steam acting on the area of the cover. The pitch of these studs must not be too great, since it then becomes difficult to keep the joint steam-tight. Stuffing Boxes and Gland Packings. It is of the greatest importance that piston rods and valve spindles should be kept thoroughly steam tight without any unnecessary loss in power, and wear and tear due to friction. The stuffing boxes for holding the packing are generally cast along with the cylinder or with the cover, but in the case of the lower ones in the piston rods of large engines, they are sometimes made separate and bolted on. The following figure shows very clearly how the stuffing boxes for the piston rods and the valve spindles of the s.s. St. Rognvald's engines are made and packed P with Bell's asbestos packing. INDEX TO PARTS. PR for Piston rod. SB NB P G B |