FORMULE FOR ULTIMATE ELASTIC STRENGTH OF FLAT STAYED SURFACES.

PP'p

For Diameter of the Stay-bolts, Clark gives d' =

.0024

in which d' = diameter of screwed bolt at bottom of thread, P = = longitudinal and P' transverse pitch of stay-bolts between centres, p = internal pressure in lbs. per sq. in. that will strain the plate to its elastic limit, s = elastic strength of the stay-bolts in lbs. per sq. in. Taking s = 12, 14, and 8 tons, respectively for iron, steel, and copper, we have

For iron, d' = .00069
For steel,

PP'p, or if P = P', d' = .00069P √p;

PP'p,
PP'p,

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d' =
d' =

.00064P Vp;
00084P Vp.

་་ d'.00064 For copper, d' = .00084 In using these formulæ a large factor of safety should be taken to allow for reduction of size by corrosion. Thurston's Manual of Steam-boilers, p. 144, recommends that the factor be as large as 15 or 20. The Hartford Steam Boiler Insp. & Ins. Co. recommends not less than 10.

Strength of Stays.-A. F. Yarrow (Engr., March 20, 1891) gives the following results of experiments to ascertain the strength of water-space stays:

The above are taken as a fair average of numerous tests. Stay-bolts in Curved Surfaces, as in Water-legs of Vertical Boilers.-The rules of the U. S. 'Supervising Inspectors provide as follows: All vertical boiler-furnaces constructed of wrought iron or steel plates, and having a diameter of over 42 in, or a height of over 40 in, shall be stayed with bolts as provided by § 6 of Rule II, for flat surfaces; and the thickness of material required for the shells of such furnaces shall be de. termined by the distance between the centres of the stay-bolts in the furnace and not in the shell of the boiler; and the steam-pressure allowable shall be determined by the distance from centre of stay-bolts in the furnace and the diameter of such stay-bolts at the bottom of the thread.

The Hartford Steam-boiler Insp. & Ins. Co. approves the above rule (The Locomotive, March, 1892) as far as it states that curved surfaces are to be computed the same as flat ones, but prefers Clark's formulæ for flat stayed surfaces to the rules of the U. S. Supervising Inspectors.

Fusible-plugs.--Fusible-plugs should be put in that portion of the heating-surface which first becomes exposed from lack of water. The rules of the U. S. Supervising Inspectors specify Banca tin for the purpose. Its melting-point is about 445° F. The rule says: All steamers shall have inserted in their boilers plugs of Banca tin, at least 1⁄2 in. in diameter at the smallest end of the internal opening, in the following manner, to wit: Cylinder-boilers with flues shall have one plug inserted in one flue of each boiler; and also one plug inserted in the shell of each boiler from the inside, immediately before the fire line and not less than 4 ft. from the forward end of the boiler. All fire-box boilers shall have one plug inserted in the crown of the back connection, or in the highest fire-surface of the boiler.

All upright tubular boilers used for marine purposes shail have a fusible plug inserted in one of the tubes at a point at least 2 in, below the lower gauge-cock, and said plug may be placed in the upper head sheet when deemed advisable by the local inspectors.

Steam-domes.-Steam domes or drums were formerly almost universally used on horizontal boilers, but their use is now generally discontinued, as they are considered a useless appendage to a steam-boiler, and unless properly designed and constructed are an element of weakness.

Height of Furnace.-Recent practice in the United States makes the height of furnace much greater than it was formerly. With large sizes of anthracite there is no serious objection to having the furnace as low as 12 to 18 in., measured from the surface of the grate to the nearest portion of the heating surface of the boiler, but with coal containing much volatile matter and moisture a much greater distance is desirable. With very volatile coals the distance may be as great as 4 or 5 ft. Rankine (S. E., p. 457) says: The clear height of the "crown "or roof of the furnace above the gratebars is seldom less than about 18 in., and often considerably more. In the fire-boxes of locomotives it is on an average about 4 ft. The height of 18 in. is suitable where the crown of the furnace is a brick arch. Where the crown of the furnace, on the other hand, forms part of the heating-surface of the boiler, a greater height is desirable in every case in which it can be obtained; for the temperature of the boiler-plates, being much lower than that of the flame, tends to check the combustion of the inflammable gases which rise from the fuel. As a general principle a high furnace is favorable to complete combustion.

IMPROVED METHODS OF FEEDING COAL,

Mechanical Stokers. (William R. Roney, Trans. A. S. M. E., vol. xii.)-Mechanical stokers have been used in England to a limited extent since 1785. In that year one was patented by James Watt. It was a simple device to push the coal, after it was coked at the front end of the grate, back towards the bridge. It was worked intermittently by levers, and was designed primarily to prevent smoke from bituminous coal. (See D. K. Clark's Treatise on the Steam-engine.)

After the year 1840 many styles of mechanical stokers were patented in England, but nearly all were variations and modifications of the two forms of stokers patented by John Jukes in 1841, and by E. Henderson in 1843.

The Jukes stoker consisted of longitudinal fire-bars, connected by links, so as to form an endless chain, similar to the familiar treadmill horse-power. The small coal was delivered from a hopper on the front of the boiler, on to the grate, which slowly moving from front to rear, gradually advanced the fuel into the furnace and discharged the ash and clinker at the back.

The Henderson stoker consists primarily of two horizontal fans revolving on vertical spindles, which scatter the coal over the fire.

Numerous faults in mechanical construction and in operation have limited the use of these and other mechanical stokers. The first American stoker was the Murphy stoker, brought out in 1878. It consists of two coal magazines placed in the side walls of the boiler furnace, and extending back from the boiler front 6 or 7 feet. In the bottom of these magazines are rectangu lar iron boxes, which are moved from side to side by means of a rack and pinion, and serve to push the coal upon the grates, which incline at an angle of about 35° from the inner edge of the coal magazines, forming a V-shaped receptacle for the burning coal. The grates are composed of narrow parallel bars, so arranged that each alternate bar lifts about an inch at the lower end, while at the bottom of the V, and filling the space between the ends of the grate-bars, is placed a cast-iron toothed bar, arranged to be turned by a crank. The purpose of this bar is to grind the clinker coming in contact with it. Over this V-shaped receptacle is sprung a fire-brick arch.

In the Roney mechanical stoker the fuel to be burned is dumped into a hopper on the boiler front. Set in the lower part of the hopper is a "pusher" to which is attached the "feed-plate" forming the bottom of the hopper. The "pusher," by a vibratory motion, carrying with it the feed-plate," gradually forces the fuel over the "dead-plate" and on the grate. The grate-bars, in their normal condition form a series of steps, to the top step of which coal is fed from the "dead-plate." Each bar rests in a concave seat in the bearer, and is capable of a rocking motion through an adjustable angle. All the grate-bars are coupled together by a "rocker-bar." A variable back-and-forth motion being given to the "rocker-bar," through a con

necting-rod, the grate-bars rock in unison, now forming a series of steps, and now approximating to an inclined plane, with the grates partly overlapping, like shingles on a roof. When the grate-bars rock forward the fire will tend to work down in a body. But before the coal can move too far the bars rock back to the stepped position, checking the downward motion, breaking up the cake over the whole surface, and admitting a free volume of air through the fire. The rocking motion is slow, being from 7 to 10 strokes per minute, according to the kind of coal. This alternate starting and checking motion is continuous, and finally lands the cinder and ash on the dumping-grate below.

Mr. Roney gives the following record of six tests to determine the comparative economy of the Roney mechanical stoker and hand-firing on return tubular boilers, 60 inches x 20 feet, burning Cumberland coal with natural draught. Rating of boiler at 12.5 square feet, 105 H. P.

Evaporation per pound, dry coal from and at 212° lbs

Three tests, hand-firing. Three tests, Stoker. 11.89 12.25 12.54 54.6 66.7 84.3

10.36 10.44 11.00

H.P. developed above rating, % 5.8 13.5

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Results of comparative tests like the above should be used with caution in drawing generalizations. It by no means follows from these results that a stoker will always show such comparative excellence, for in this case the results of hand-firing are much below what may be obtained un ler favorable circumstances from hand-firing with good Cumberland coal.

The Hawley Down-draught Furnace.-A foot or more above the ordinary grate there is carried a second grate composed of a series of water tubes, opening at both ends into steel drums or headers, through which water is circulated. The coal is fed on this upper grate, and as it is partially consumed falls through it upon the lower grate, where the combustion is completed in the ordinary manner. The draught through the coal on the upper grate is downward through the coal and the grate. The volatile gases are therefore carried down through the bed of coal, where they are thoroughly heated, and are burned in the space beneath, where they meet the excess of hot air drawn through the fire on the lower grate. În tests in Chicago, from 30 to 45 lbs. of coal were burned per square foot of grate upon this system, with good economical results. (See catalogue of the Hawley Down Draught Furnace Co., Chicago.)

Under-feed Stokers.-Results similar to those that may be obtained with downward draught are obtained by feeding the coal at the bottom of the bed, pushing upward the coal already on the bed which has had its volatile matter distilled from it. The volatile matter of the freshly fired coal then has to pass through a body of ignited coke, where it meets a supply of hot air. (See circular of The American Stoker Co., New York, 1898.)

SMOKE PREVENTION.

A committee of experts was appointed in St. Louis in 1891 to report on the smoke problem. A summary of its report is given in the Iron Age of April 7, 1892. It describes the different means that have been tried to prevent smoke, such as gas-fuel, steam-jets, fire-brick arches and checker-work, hollow walls for preheating air, coking arches or chambers, double combustion furnaces, and automatic stokers. All of these means have been more or less effective in diminishing smoke, their effectiveness depending largely upon the skill with which they are operated; but none is entirely satisfactory. Fuel-gas is objectionable chiefly on account of its expense. The average quality of fuel-gas made from a trial run of several car-loads of Illinois coal, in a well-designed fuel-gas plant, showed a calorific value of 243,391 heat-units per 1000 cubic feet. This is equivalent to 5052.8 heat-units per lb. of coal, whereas by direct calorimeter test an average sample of the coal gave 11,172 heat-units. One lb. of the coal showed a theoretical evaporation of 11.56 lbs. water, while the gas from 1 lb. showed a theoretical evaporation of 5.23 lbs. 48.17 lbs. of coal were required to furnish 1000 cubic feet of the gas. In 39 tests the smoke-preventing furnaces showed only 74% of the capacity of the common furnaces, reduced the work of the boilers 28%, and required about 2% more fuel to do the same work. In one case with steam-jets the fuel consumption was increased 12% for the same work.

Prof. O. H. Landreth, in a report to the State Board of Health of Tennessee (Engineering News, June 8, 1893), writes as follows on the subject of smoke prevention:

As pertains to steam-boilers, the object must be attained by one or more of the following agencies:

1. Proper design and setting of the boiler-plant. This implies proper grate area, sufficient draught, the necessary air-space between grate-bars and through furnace, and ample combustion-room under boilers.

2. That system of firing that is best adapted to each particular furnace to secure the perfect combustion of bituminous coal. This may be either: (a) "coke-firing," or charging all coal into the front of the furnace until partially coked, then pushing back and spreading; or (b) "alternate side-fir ing"; or (c) "spreading," by which the coal is spread over the whole grate area in thin, uniform layers at each charging.

3. The admission of air through the furnace-door, bridge-wall, or side walls. 4. Steam-jets and other artificial means for thoroughly mixing the air and combustible gases.

5. Preventing the cooling of the furnace and boilers by the inrush of cold air when the furnace-doors are opened for charging coal and handling the fire.

6. Establishing a gradation of the several steps of combustion so that the coal may be charged, dried, and warmed at the coolest part of the furnace, and then moved by successive steps to the hottest place, where the final combustion of the coked coal is completed, and compelling the distilled combustible gases to pass through this hottest part of the fire.

7. Preventing the cooling by radiation of the unburned combustible gases until perfect mixing and combustion have been accomplished.

8. Varying the supply of air to suit the periodic variation in demand. 9. The substitution of a continuous uniform feeding of coal instead of intermittent charging.

10. Down-draught burning or causing the air to enter above the grate and pass down through the coal, carrying the distilled products down to the high temperature plane at the bottom of the fire.

The number of smoke-prevention devices which have been invented is legion. A brief classification is:

(a) Mechanical stokers. They effect a material saving in the labor of firing, and are efficient smoke-preventers when not pushed above their capacity, and when the coal does not cake badly. They are rarely susceptible to the sudden changes in the rate of firing frequently demanded in service.

(b) Air-flues in side walls, bridge-wall, and grate-bars, through which air when passing is heated. The results are always beneficial, but the flues are difficult to keep clean and in order.

(c) Coking arches, or spaces in front of the furnace arched over, in which the fresh coal is coked, both to prevent cooling of the distilled gases, and to force them to pass through the hottest part of the furnace just beyond the arch. The results are good for normal conditions, but ineffective when the fires are forced. The arches also are easily burned out and injured by working the fire.

(d) Dead-plates, or a portion of the grate next the furnace-doors, reserved for warming and coking the coal before it is spread over the grate. These give good results when the furnace is not forced above its normal capacity. This embodies the method of “coke-firing" mentioned before.

(e) Down-draught furnaces, or furnaces in which the air is supplied to the coal above the grate, and the products of combustion are taken away from beneath the grate, thus causing a downward draught through the coal, carrying the distilled gases down to the highly heated incandescent coal at the bottom of the layer of coal on the grate. This is the most perfect manner of producing combustion, and is absolutely smokeless.

(f) Steam-jets to draw air in or inject air into the furnace above the grate, and also to mix the air and the combustible gases together. A very efficient smoke-preventer, but one liable to be wasteful of fuel by inducing too rapid a draught.

(g) Baffle-plates placed in the furnace above the fire to aid in mixing the combustible gases with the air.

(h) Double furnaces, of which there are two different styles; the first of which places the second grate below the first grate; the coal is coked on the first grate, during which process the distilled gases are made to pass over the second grate, where they are ignited and burned; the coke from the first grate is dropped onto the second grate: a very efficient and economical smoke-preventer, but rather complicated to construct and maintain. In the second form the products of combustion from the first furnace pass through

the grate and fire of the second, each furnace being charged with fresh fuel when needed, the latter generally with a smokeless coal or coke: an irra. tional and unpromising method.

Mr. C. F White, Consulting Engineer to the Chicago Society for the Prevention of Smoke, writes under date of May 4, 1893:

The experience had in Chicago has shown plainly that it is perfectly easy to equip steam-boilers with furnaces which shall burn ordinary soft coal in such a manner that the making of smoke dense enough to obstruct the vision shall be confined to one or two intervals of perhaps a couple of minutes' duration in the ordinary day of 10 hours.

Gas-fired Steam-boilers.-Converting coal into gas in a separate producer, before burning it under the steam-boiler, is an ideal method of smoke-prevention, but its expense has hitherto prevented its general introduction. A series of articles on the subject, iliustrating a great number of devices, by F. J. Rowan, is published in the Colliery Engineer, 1889-90. See also Clark on the Steam-engine.

FORCED COMBUSTION IN STEAM-BOILERS.

For the purpose of increasing the amount of steam that can be generated by a boiler of a given size, forced draught is of great importance. It is universally used in the locomotive, the draught being obtained by a steam. jet in the smoke-stack. It is now largely used in ocean steamers, especially in ships of war, and to a small extent in stationary boilers. Economy of fuel is generally not attained by its use, its advantages being confined to the securing of increased capacity from a boiler of a given bulk, weight, or cost. The subject of forced draught is well treated in a paper by James Howden, entitled, "Forced Combustion in Steam-boilers" (Section G, Engineering Congress at Chicago, in 1893), from which we abstract the following: Edwin A. Stevens at Bordentown, N. J., in 1827, in the steamer North America," fitted the boilers with closed ash-pits, into which the air of com bustion was forced by a fan. In 1828 Ericsson fitted in a similar manner the steamer "Victory," commanded by Sir John Ross.

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Messrs. E. A. and R. L. Stevens continued the use of forced draught for a considerable period, during which they tried three different modes of using the fan for promoting combustion: 1, blowing direct into a closed ash-pit; 2, exhausting the base of the funnel by the suction of the fan; 8, forcing air into an air-tight boiler-room or stoke-hold. Each of these three methods was attended with serious difficulties.

In the use of the closed ash-pit the blast-pressure would frequently force the gases of combustion, in the shape of a serrated flame, from the joint around the furnace doors in so great a quantity as to affect both the efficiency and health of the firemen.

The chief defect of the second plan was the great size of the fan required to produce the necessary exhaustion. The size of fan required grows in a rapidly increasing ratio as the combustion increases, both on account of the greater air-supply and the higher exit temperature enlarging the volume of the waste gases.

The third method, that of forcing cold air by the fan into an air-tight boiler-room-the present closed stoke-hold system-though it overcame the difficulties in working belonging to the two forms first tried, has serious defects of its own, as it cannot be worked, even with modern high-class boiler-construction, much, if at all, above the power of a good chimney draught, in most boilers, without damaging them.

In 1875 John I. Thornycroft & Co., of London, began the construction of torpedo-boats with boilers of the locomotive type, in which a high rate of combustion was attained by means of the air-tight boiler-room, into which air was forced by means of a fan.

In 1882 H.B.M. ships "Satellite" and "Conqueror" were fitted with this system, the former being a small ship of 1500 I.H.P., and the latter an ironclad of 4500 I.H.P. On the trials with forced draught, which lasted from two to three hours each, the highest rates of combustion gave 16.9 I.H.P. per square foot of fire-grate in the "Satellite," and 13.41 I.H.P. in the "Conqueror."

None of the short trials at these rates of combustion were made without injury to the seams and tubes of the boilers, but the system was adopted, and it has been continued in the British Navy to this day (1893).

In Mr. Howden's opinion no advantage arising from increased combustion over natural-draught rates is derived from using forced draught in a closed ash-pit sufficient to compensate the disadvantages arising from difficulties