MARINE BOILERS.

LANCASHIRE BOILER—Fig. 18.

The disadvantage of the Lancashire boiler is the difficulty of finding adequate room for the two furnaces without unduly increasingthe diameter of the shell. Low furnaces are extremely unfavorable to complete combustion, the comparatively cold crown plates, when they are in contact with the water of the boiler, extinguishing the flames from the fuel, when they are just formed, while the narrow space between the fuel and the crown does not admit the proper quantity of air being supplied above the fuel to complete the combustion of the gases, as they arise.

On the other hand, though this boiler favors the formation of the smoke, it supplies the means of completing the combustion afterwards, as before explained, by means of the hot air from the second furnace.

galloway tubesFig. 18 (a)

Fig. 18 (a)

Another disadvantage is the danger of collapsing the internal flue already spoken of; this is remedied by the introduction of what are called the galloway tubes, illustrated in the cut shown onthis page, which is a cross section of the water tubes shown in Figs.18and20.

These tubes not only contribute to strengthen the flues but they add to the heating surface and greatly promote the circulation so important in the water space.

These descriptions and illustrations of the Lancashire boiler are of general value, owing to the fact that very many exhaustive tests and experiments upon steam economy have been made and permanently recorded in connection with this form of steam generator.

In theGallowayform of boiler the flue is sustained and stiffened by the introduction of numerous conical tubes, flanged at the two ends and riveted across the flue. These tubes, a sketch of which are given infig. 18 (a), are in free communication with the water of the boiler, and besides acting as stiffeners, they also serve to increase the heating surface and to promote circulation.

Figs. 19, 20.

The illustration (figs.18,19 and 20) give all the principal details of a Lancashire boiler fitted with Galloway tubes.Fig. 18represents a longitudinal section andfigs. 19 and 20shows on a large scale an end view of the front of the boiler with its fittings and also a transverse section. The arrangement of the furnaces, flues, and the Galloway tubes is sufficiently obvious from the drawings. The usual length of these boilers is 27 feet, though they are occasionally made as short as 21 feet.

The minimum diameter of the furnaces is 33 inches, and in order to contain these comfortably the diameter of the boiler should not be less than 7 feet. The ends of the boiler are flat, and are prevented from bulging outwards by being held in place by the furnaces and flues which stay the two ends together and also by the so-called gusset stayse,e. In addition to the latter the flat ends of the boiler have longitudinal rods to tie them together; one of these is shown atA,A,fig. 18.

The steam is collected in the pipeS, which is perforated all along the top so as to admit the steam and exclude the water spray which may rise to the surface during ebullition. The steam thence passes to the stop valveToutside the boiler and thence to the steam pipes to the engines.

There are two safety valves on top of the boiler onB(fig. 18), being of the dead weight type described hereafter, and the other,C, being a so-called low water safety valve. It is attached by means of a lever and rod to the floatF, which ordinarily rests on the surface of the water. When through any neglect, the water sinks below its proper level the float sinks also, causing the valve to open, thus allowing steam to escape and giving an alarm.Mis the manhole with its covering plate, which admits of access to the interior of the boiler andHis the mud hole by which the sediment which accumulates all along the bottom is raked out. Below the front end and underneath, the pipe and stay valve are shown, by which the boiler can be emptied or blown off.

On the front of the boiler (fig. 19) are shown, the pressure gauges, the water gauges and the furnace door;Kis the feed pipe;R,R, a pipe and cock for blowing off steam. In the front of the setting are shown two iron doors by which access may be gained to the two lower external flues for cleaning purposes.

In the Lancashire boiler it is considered advisable to take the products of combustion, after they leave the internal flues, along the bottom of the boiler, and then back to the chimney by the side. When this plan is adopted the bottom is kept hotter than would otherwise be the case, and circulation is promoted, which prevents the coldest water from accumulating at the bottom.

The Galloway (or Lancashire) boiler is considered the most economical boiler used in England, and is being introduced into the United States with success. The long traverse of heat provided (three turns of about 27 feet each) contributes greatly to its efficiency.

It may be useful to add the following data relating to this approved steam generator, being the principal dimensions and other data of the boiler shown infig. 18:

Steam pressure, 75 lbs. per sq. inch.Length, 27 feet.Diameter, 7 feet.Weight, total, 151⁄2tons.Shell plates,7⁄16inch.Furnace diameter, 33 inches.Furnace Plates,3⁄8inch.End plates,1⁄2inch.Grate area, 33 sq. feet.Heating surface:In furnace and flues450sq. feet.In Galloway pipes,30„In external flues,370„850sq. feet.

We have thus detailed step by step the improvement of the steam boiler to a point where it is necessary to describe at length the locomotive, the marine, the horizontal tubular and the water tube boilers, which four forms comprehend ninety-nine out of one hundred steam generators in use in the civilized world at the present time.

The boilers used on board steamships are of two principal types. The older sort used for steam of comparatively low temperature, viz.: up to 35 lbs. per square inch, is usually made of flat plates stayed together, after the manner of the exterior and interior fire boxes of a locomotive boiler.

Medium high pressure marine boilers, constructed for steam of 60 to 150 lbs. per square inch, are circular or oval in cross section, and are fitted with round interior furnaces and flues like land boilers. There are many variations of marine boilers, adapted to suit special circumstances.Fig. 22shows a front elevation and partial sections of a pair of such boilers andFig. 23shows one of them in longitudinal vertical section.

THE MARINE STEAM BOILER

Fig. 22.

Fig. 23.

It will be seen from these drawings that there are three internal cylindrical furnaces at each end of these boilers, making in all six furnaces per boiler. The firing takes place at both ends. The flame and hot gases from each furnace, after passing over the bridge wall enter a flat-sided rectangular combustion chamber and then travel through tubes to the front uptake (i.e.the smoke bonnet or breaching), and so on to the chimney.

The sides of the combustion chambers are stayed to each other and to the shell plate of the boiler; the tops are strengthened in the same manner as the crowns of locomotive boilers, and the flat plates of the boiler shell are stayed together by means of long bolts, which can be lengthened up by means of nuts at their ends. Access is gained to the uptakes for purposes of cleaning, repairs of tubes, etc., by means of their doors on their fronts just above the furnace doors. The steam is collected in the large cylindrical receivers shown above each boiler. The material of construction is mild steel.

The following are the principal dimensions and other particulars of one of these boilers:

Fig. 24is a sketch of a modern marine boiler, which is only fired from one end, and is in consequence much shorter in proportion to its diameter than the type illustrated in figs.22and23.

Marine boilers over nine feet in diameter have generally two furnaces, those over 13 to 14 feet, three, while the very largest boilers used on first-class mail steamers, and which often exceed fifteen feet in diameter, have four furnaces.

In the marine boiler the course taken by the products of combustion is as follows; the coal enters through the furnace doors on to the fire-bars, the heat and flames pass over the fire bridge into the flame or combustion chamber, thence through the tubes into the smoke-box, up the up-take and funnel into the air.

Fig. 24.

The fittings to a marine boiler are—funnel and air casings, up-takes and air casings, smoke boxes and doors, fire doors, bars, bridges, and bearers, main steam stop valve, donkey valve, safety valves and drain pipes, main and donkey feed check valves, blow-off and scum cocks, water gauge glasses on front and back of boiler, test water cock for trying density of water, steam cock for whistle, and another for winches on deck.

A fitting, called a blast pipe, is sometimes placed in the throat of the funnel. It consists of a wrought iron pipe, having a conical nozzle within the funnel pointing upwards, the other end being connected to a cock, which latter is bolted on to the steam space or dome of the boiler. It is used for increasing the intensity of the draft, the upward current of steam forcing the air out of the funnel at a great velocity; and the air having to be replaced by a fresh supply through the ash-pits and bars of the furnaces, a greater speed of combustion is obtained than would otherwise be due to simple draft alone.

Boilers are fitted with dry and wet uptakes, which differ from each other as follows:—The dry uptake is wholly outside the boiler, and consists of an external casing bolted on to the firing end of the boiler, covering the tubes and forming the smoke-box, and is fitted with suitable tube doors. A wet uptake is carried back from the firing ends of the boiler into its steam space, and is wholly surrounded by water and steam. The dry uptake seldom requires serious repair; but the wet uptake, owing to its exposure to pressure, steam, and water, requires constant attention and repair, and is very liable to corrosion, being constantly wetted and dried in the neighborhood of the water-line. The narrow water space between both front uptakes is also very liable to become burnt, owing to accumulation of salt. The flue boilers of many tugs and ferry boats are fitted with wet uptakes.

A superheater is a vessel usually placed in the uptake, or at the base of the funnel of a marine boiler, and so arranged that the waste heat from the furnaces shall pass around and through it prior to escaping up the chimney. It is used for drying or heating the steam from the main boiler before it enters the steam pipes to the engine. The simplest form of superheater consists of a wrought iron drum filled with tubes. The heat or flame passes through the tubes and around the shell of the drum, the steam being inside the drum. Superheaters are usually fitted with a stop valve in connection with the boiler, by means of which it can be shut off; and also one to the steam pipe of the engine; arrangements are also usually made for mixing the steam or working independently of the superheater.

A safety-valve is also fitted and a gauge glass; the latter is to show whether the superheater is clear of water, as priming will sometimes fill it up.

The special fittings of the marine boiler will be more particularly described hereafter as well as the stays, riveting, strength, etc., etc.

The use of the surface condenser in connection with the marine boiler was an immense step toward increasing its efficiency. In 1840 the average pressure used in marine boilers was only 7 or 8 lbs. to the square inch, the steam being made with the two-flue pattern of boiler, sea water being used for feed; as the steam pressure increased as now to 150 to 200 lbs. to the square inch, greater and greater difficulty was experienced from salt incrustation—in many cases the tubes did not last long and frequently gave much trouble, until the introduction of the surface condenser, which supplied fresh water to the boilers.

Fig. 25

The condenser is an oblong or circular box of cast iron fitted in one of two ways, either with the tubes horizontal or vertical; at each end are fixed the tube plates, generally made of brass, and the tubes pass through the plates as well as through a supporting plate in the middle of the condenser. Each end of the condenser is fitted with doors for the purpose of enabling the tube ends to be examined, drawn, or packed, as may be necessary. The tube ends are packed in various ways, and the tubes are made of brass, so as to resist the action of the water. The water is generally sucked through the tubes bythe circulating pump, and the steam is condensed by coming in contact with the external surface of the tubes. In some cases the water is applied to the external surface, and the steam exhausted through the tubes; but this practice is now generally given up in modern surface condensers. The packing round the tube ends keeps them quite tight, and in the event of a split tube, a wooden plug is put in each end until an opportunity offers for drawing it and replacing with a new one.

The condenser may be made of any convenient shape. It sometimes forms part of the casting supporting the cylinders of vertical engines; it is also frequently made cylindrical with flat ends, as infig. 25. The ends form the tube plates to which the tubes are secured. The tubes are, of course, open at the ends, and a space is left between the tube plate and the outer covers, shown at each end of the condenser, to allow of the circulation of water as shown by the arrows.

The cold water, which is forced through by a circulating pump, enters at the bottom, and is compelled to pass forward through the lower set of tubes by a horizontal dividing plate; it then returns through the upper rows of tubes and passes out at the overflow; the tubes are thus maintained at a low temperature.

The tubes are made to pass right through the condensing chamber, and so as to have no connection with its internal space. The steam is passed into the condenser and there comes in contact with the cold external surface of the tube, and is condensed, and removed as before, by the air pump, as may be readily seen in the illustration (p. 65.)

The advantages gained by the use of the surface condenser are: 1. The feed water is hotter and fresh; being hotter, it saves the fuel that would be used to bring it up to this heat; and being fresh it boils at a lower temperature. 2. Not forming so much scale inside the boiler, the heat passes through to the water more readily; and as the scum cock is not used so freely, all the heat that would have been blown off is saved. Its disadvantages are that being fresh water and forming no scale on the boiler, it causes the boiler to rust.

It is often said that one engineer will get more out of a ship than another. In general it will be found that the most successful engineer is the man who manages his stokers best. It is very difficult on paper to define what is meant. It is a thing to be felt or seen, not described. * * * * The engineer who really knows his business will give his fires a fair chance to get away. He will work his engines up by degrees and run a little slowly for the first few moments.

Water Tube Boiler.—Fig. 26.

A popular form of steam boiler in use in the United States and Europe is what is called the water tube boiler. This term is applied to a class of boiler in which the water is contained in a series of tubes, of comparatively small diameter, which communicate with each other and with a common steam-chamber. The flames and hot gases circulate between the tubes and are usually guided by partitions so as to act equally on all portions of the tubes. There are many varieties of this type of boiler of which the cut illustrates one: in this each tube is secured at either end into a square cast-iron head, and each of these heads has two openings, one communicating with the tube below and the other with the tube above; thecommunication is effected by means of hollow cast-iron caps shown at the end of the tubes; the caps have openings in them corresponding with the openings in the tube heads to which they are bolted.

In the best forms of the water tube boilers, it is suspended entirely independent of the brick work from wrought iron girders resting on iron columns. This avoids any straining of the boiler from unequal expansion between it and its enclosing walls and permits the brick work to be repaired or removed, if necessary, without in any way disturbing the boiler. This design is shown inFig. 26.

The distinguishing difference, which marks the water tube boiler from others, consists in the fact that in the former the small tubes are filled with water instead of the products of combustions; hence the comparison, frequently made, between water-tube andfire tubeboilers—the difference has been expressed in another way, “Water-tube vs. shell boilers,” but the principle of steam production in both systems remains the same; the heat from the combustible is transferred to the water through the medium of iron plates and in both, the furnaces, steam appliances, application of the draught, etc., is substantially the same. In another important point do the systems agree,i.e., in the average number of pounds of water evaporated per lb. of combustible; it is in the thoroughness of construction and skillfulness of adaptation to surroundings that produce the best results. Water tube or sectional boilers, have been made since the days of James Watt, in 1766, in many different forms and under various names. Owing, however, to the imperfection of manufacture the system, as compared to shell boilers, has been a failure until very recently; various patterns of water-tube boilers are now in most favorable and satisfactory use. The advantages claimed for this form of steam generator are as follows:

1. Safety from disastrous explosions, arising from the division of the contents into small portions, and especially from details of construction which make it tolerably certain that the rupture will be local instead of a general violent explosion which liberates at once large masses of steam and water.

2. The small diameter of the tubes of which they are composed render them much stronger than ordinary boilers.

3. They can be cheaply built and easily repaired, as duplicate pieces can be kept on hand. The various parts of a boiler can be transported without great expense, trouble or delay; the form and proportions of a boiler can be suited to any available space; and, again, the power can be increased by simply adding more rows of tubes and increasing the grate area.

4. Their evaporative efficiency can be made equal to that of other boilers, and, in fact, for equal proportions of heating and grate surfaces, it is often a trifle higher.

5. Thin heating surface in the furnace, avoiding the thick plates necessarily used in ordinary boilers which not only hinder the transmission of heat to the water, but admit of overheating.

6. Joints removed from the fire. The use of lap welded water tubes with their joints removed from the fire also avoid the unequal expansion of riveted joints consequent upon their double thickness.

7. Quick steaming.

8. Accessibility for cleaning.

9. Ease of handling and erecting.

10. Economy and speediness of repairs.

The known disadvantages of these boilers are

1. They generally occupy more space and require more masonry than ordinary boilers.

2. On account of the small quantity of water which they contain, sudden fluctuations of pressure are caused by any irregularities in supplying the feed-water or in handling the fires, and the rapid and at times violent generation of steam causes it to accumulate in the contracted water-chambers, and leads to priming, with a consequent loss of water, and to overheated tubes.

3. The horizontal or inclined water tubes which mainly compose these boilers, do not afford a ready outlet for the steam generated in them. The steam bubbles cannot follow their natural tendency and rise directly, but are generally obliged by friction to traverse the tube slowly, and at times the accumulation of steam at the heated surfaces causes the tubes to be split or burned.

4. The use of water which forms deposits of solid matter still further increases the liability to overheating of the tubes. It has been claimed by some inventors that the rapid circulation of the water through the tubes would prevent any deposit of scale or sediment in them, but experience has proved this to be a grave error. Others have said that the expansion of the tube would detach the scale as fast as it was deposited and prevent any dangerous accumulation, but this also has been proved an error. Again, the use of cast iron about these boilers has frequently been a constant source of trouble from cracks, etc.

The soot and ashes collect onthe exteriorof the tubes in this form of boilers, instead of inside the tubes, as in the tubular, and they must be as carefully removed in one case as in the other; this can be done by the use of blowing pipe and hose through openings left in the brick work; in using bituminous coal the soot should be brushed off when steam is down.

All the inside and outside surfaces should be kept clean to avoid waste of fuel; to aid in this service the best forms are provided with extra facilities for cleaning. For inspection, remove the hand holes at both ends of the tubes, and by holding a lamp at one end and looking in at the other the condition of the surface can be freely seen. Push the scraper through the tube to remove sediment, or if the scale is hard, use the chipping scraper made for that purpose.

Hand holes should be frequently removed and surfaces examined, particularly in case of a new boiler. In replacinghand hole caps, clean the surfaces without scratching or bruising, smear with oil and screw up tight.

The mud drum should be periodically examined and the sediment removed; blow-off cocks and check valves should be examined each time the boiler is cleaned; when surface blow-cocks are used they should be often opened for a few minutes at a time; be sure that all openings for air to boiler or fluesexcept through the fire, are carefully stopped.

If a boiler is not required for some time, empty and dry it thoroughly. If this is impracticable, fill it quite full of water and put in a quantity of washing soda; and external parts exposed to dampness should receive a coating of linseed oil. Avoid all dampness in seatings or coverings and see that no water comes in contact with the boiler from any cause.

Although this form of boiler is not liable to destructive explosion, the same care should be exercised to avoid possible damage to boilers and expensive delays.

Probably one of the first sectional boilers brought into practical use is one made of hollow cast iron spheres, each 8 inches in diameter, externally, and3⁄8of an inch thick, connected by curved necks 31⁄2inches in diameter. These spheres are held together by wrought iron bolts and caps, and in one direction are cast in sets of 2 or 4, which are afterwards drawn together so as to give more or less heating surface to the boiler according to the number used.

Owing to their multiplication of parts all sectional, including water tube boilers, should be made with unusual care in their details of construction, setting, fittings and proportions. It is to the attention paid to these “points” that the sectional boilers are now coming into more general favor.

The essential features of locomotive boilers are dictated by the duties which they have to perform under peculiar conditions. The size and the weight are limited by the fact that the boiler has to be transported rapidly from place to place, and also that it has to fit in between the frames of the locomotive; while at the same time, the pressure of the steam has to be very great in order that with comparatively small cylinder the engine may develop great power; moreover, the quantity of water which has to be evaporated in a given time is very considerable. To fulfil these latter conditions a large quantity of coal must be burned on a fire grate of limited area; hence intense combustion is necessary under a forced blast. To utilize advantageously the heat thus generated, a large heating surface must be provided and this can only be obtained by passing the products of combustion through a great number of tubes of small diameter.

The forced draught in a locomotive boiler is obtained by causing the steam from the cylinders, after it has done its work, to be discharged into the chimney by means of a pipe called the blast pipe; the lower portion of this consists of two branches, one in communication with the exhaust port of each cylinder. As each puff of steam from the blast pipe escapes up the chimney it forces the air out in front of it, causing a partial vacuum, which can only be supplied by the air rushing through the furnace and tubes.

The greater the body of steam escaping at each puff, and the more rapid the succession of puffs, the more violent is the action of the blast pipe in producing a draught, and consequently this contrivance regulates the consumption of fuel and the evaporation of water to a certain extent automatically, because when the engine is working its hardest and using the most steam, the blast is at the same time most efficacious.

LOCOMOTIVE BOILER.—Fig. 27.

The blast pipe is perhaps, the most distinctive feature of the locomotive boiler, and the one which has alone rendered it possible to obtain large quantities of steam from so small agenerator. The steam blast of a locomotive has been compared to the breathing apparatus of a man, and has rendered the mechanism described nearer a live thing than any other device man has ever produced.

On account of the oscillations, or violent motions to which the boiler of locomotive engines are subject, weighted safety-valves are not possible to be used and springs are used instead to hold the valves in place.

The locomotive form of steam boiler is sometimes used for stationary engines, but owing to extra cost and increased liability to corrode in the smaller passage they are not favorites.

DESCRIPTION OF PAGE ILLUSTRATION.

Infig. 27, F B represents the fire box or furnace; F D, fire door; D P, deflector plate; F T P, fire box tube plate; F B R S, fire box roof stays; S T P, smoke box tube plate; S B, smoke box; S B D, smoke box door; S D, steam dome; O S, outer shell; R S V, Ramsbottom safety-valve; F, funnel or chimney.

Fig. 28.

The crown plate of the fire-box being flat requires to be efficiently stayed, and for this purpose girder stays called fox roof stays are mostly used, as shown in the figure. The stays are now made of cast steel for locomotives. They rest at the two ends on the vertical plates of the fire-box, and sustain thepressure on the fire-box crown by a series of bolts passing through the plate and girder stay, secured by nuts and washers.Fig. 28is a plan and elevation of a wrought-iron roof stay.

Another method adopted in locomotive types of marine boilers for staying the flat crown of the fire-box to the circular upper plate is shown infig. 29—namely, by wrought-iron vertical bar stays secured by nuts and washers to the fire-box with a fork end and pin to angle-iron pieces riveted to the boiler shell.

Fig. 29.

The letters in this figure refer to the same parts of the boiler as do those infig. 27,i.e., F B to the fire-box, etc., etc.

It was formerly the custom to make the tubes much longer than shown in the fig., with the object of gaining heating surface; but modern experience has shown that the last three or four feet next the smoke box were of little or no use, because, by the time the products of combustion reached this part of the heating surface, their temperature was so reduced that but little additional heat could be abstracted from them. The tubes, in addition to acting as flues and heating surface, fulfil also the function of stays to the flat end of the barrel of the boiler, and the portion of the fire box opposite to it.

In addition to the staying power derived from the tubes, the smoke box, tube plate and the front shell plate are stayed together by several long rods.

The Horizontal Tubular Boiler.—Fig. 30.

TABLE OF SIZES, PROPORTIONS, ETC.:

DiameterofShell.LengthofShell.GaugeofShell.GaugeofHeads.NumberofTubes.DiameterofTubes.LengthofTubesSquarefeet ofHeatingSurface.NominalHorsePower.72in.19ft.4in.3⁄8in.1⁄2in.804in.18ft.0in.1,50010072„18„4„3⁄8„1⁄2„8631⁄2„17„0„1,50010072„17„4„3⁄8„1⁄2„1083„16„0„1,50010066„18„4„3⁄8„1⁄2„7431⁄2„17„0„1,3509066„17„4„3⁄8„1⁄2„923„16„0„1,3509060„18„3„3⁄8„1⁄2„783„17„0„1,2008060„17„3„3⁄8„1⁄2„763„16„0„1,1257560„16„3„3⁄8„1⁄2„773„15„0„1,0507060„16„3„3⁄8„1⁄2„703„15„0„9756560„16„3„3⁄8„1⁄2„643„15„0„9006054„17„3„5⁄16„7⁄16„603„16„0„9005054„17„3„5⁄16„7⁄16„563„16„0„8255554„16„3„5⁄16„7⁄16„523„15„0„7505054„16„3„5⁄16„7⁄16„463„15„0„6754554„16„3„5⁄16„7⁄16„403„15„0„6004048„17„2„5⁄16„7⁄16„503„16„0„7505048„16„2„5⁄16„7⁄16„483„15„0„6754548„16„2„5⁄16„7⁄16„423„15„0„6004042„16„2„1⁄4„3⁄8„363„15„0„5258542„15„2„1⁄4„3⁄8„323„14„0„4503042„14„2„1⁄4„3⁄8„283„13„0„3752536„14„2„1⁄4„3⁄8„3621⁄2„13„0„3752536„14„2„1⁄4„3⁄8„2821⁄2„13„0„3002036„13„2„1⁄4„3⁄8„2021⁄2„12„0„2251536„12„2„1⁄4„3⁄8„1421⁄2„11„0„15010

In estimating the horse power by means of the above table, 15 square feet has been allowed for each horse power, and the number of feet in each boiler is givenin round numbers. This table is one used in every-day practice by boiler makers.

THE FLUE BOILER.

Back to Index Next