manner in which it is set and fired, the quality of the fuel used, quantity of grate-surface, amount of heating-surface, draught, etc., while the amount of water used will depend entirely on the engine, provided the steam is dry. The evaporation in tubular boilers, stationary, locomotive, and marine,-under good conditions, is about 8 to 9 lbs. of water to 1 lb. of coal; in flueboilers, 6 to 7; but the average result is about 25 per cent. below this. The nominal loss of fuel in boilers is rarely less than 30 per cent., and is frequently as high as 50. Taking the lowest estimate at 30 per cent., it may be illustrated as follows: The amount necessary to produce a draught, including the flame which escapes into the chimney, 20 per cent.; particles of coal falling through the grates, 5 per cent.; loss arising from the formation of carbonic oxide, 3 per cent.; loss induced by radiation, 2 per cent.

The common estimate of the quantity of water necessary to produce one horse-power is one cubic foot; the amount of heatingsurface necessary to evaporate one cubic foot of water in an hour has been found, by experiment, to be about 14 square feet to square foot of grate-surface, under the most favorable conditions. It has grown into a custom, in estimating the horse-power of steamboilers, to allow 14 square feet of heating-surface to square foot of grate-surface; but the evaporative performance of steam-boilers varies very much, as in one boiler a cubic foot of water may be evaporated in an hour by 9 square feet of heating-surface to square foot of grate-surface, while another will take double the amount. In locomotives, the proportion of heating-surface to grate-surface is about 50 to 1; in marine-boilers, about 28 to 1; ordinary cylinder-boilers, about 15 to 1; flue-boilers, 18 to 1; tubular-boilers, from 20 to 24 to 1; and in sectional- or patentboilers, about 30 to 1.

The tendency of water to foam in marine-boilers is frequently attributed to the presence of dirt, or other saline matter, in the water; but it is often induced by want of proper relations between the heating-surface, steam-room, and water-space of the boiler, as, when the discharge of steam is out of proportion to the steam-room

in the boiler, the high temperature required to generate steam with sufficient rapidity to supply the demand causes violent boiling, and the agitation is greater when the relation between the temperature and pressure is most disturbed. This is often the case with tug-boats just starting to tow a heavy vessel. Boilers with a large amount of heating-surface and small steam-room generally foam.

Marine-boilers are generally surmounted by a dome, and, though domes do not add much to the cubical capacity of the steam-room, they have the effect of superheating the steam, or imparting to it an extra heat, which greatly increases its expansive force, and renders it less liable to condense in the passages between the boiler and the cylinder.

Fittings of marine-boilers.-The fittings of marine-boilers are the funnels, air-casings, uptakes, smoke-box and fire-doors, gratebars, bearers and bridges, main steam-pipe and stop-valve, donkeyvalve, safety-valves and drain-pipes, main- and donkey-feed checkvalves, blow-off- and scum-cocks, water-gauges, test water-cock, steam-valves for whistle, and winches.

Bursting Pressure of Cylindrical Steam-Boilers.

The force which will rupture a cylindrical boiler depends upon the diameter and the pressure of the steam; hence, the total pressure to be sustained is equal to the diameter, multiplied by the pressure per square inch of surface, multiplied by the length of the boiler. The shorter the tube, and the smaller the diameter, the greater its power of resistance, and vice versa. No matter what the diameter of a boiler may be, the transverse, or cross pressure tending to tear it asunder, will always be double the longitudinal pressure.

Rule for finding the bursting pressure of cylindrical boilers with riveted seams.

Multiply the tensile strength of the iron (which should be taken at 50,000 lbs. per square inch of section) by 56, if single riveted, 38*

THE ENGINEER'S HANDY-BOOK.

449

and by 70, if double riveted, and divide by the diameter of the boiler, multiplied by the number of pieces of metal, that would make one square inch of cross section; the product will be the bursting strain.

For instance, what pressure will it require to rupture a cylindrical boiler with riveted seams, diameter 12 inches, thickness of iron inch? 50,000 x 56

12 × 4

583.33 x 2 =

1166 66 lbs., about one-fifth of

which would be the safe working-pressure.

Rule for finding the strain exerted in a longitudinal direction by the pressure of steam in a boiler.

Multiply the area of the head by the pressure in pounds per square inch, and divide the product by the circumference of the boiler, and by the number of thicknesses of iron that would make one square inch of cross section; the quotient will be the strain.

Example. Diameter, 12 inches. Area, 113.09 square inches. Pressure, 11663 lbs.

113.09 x 1166.66

37.69 ÷ 4

=

14014 lbs. per square inch of sectional area

in a longitudinal direction.

Rule for finding the strain exerted in a transverse direction by the pressure of steam in a boiler.

Multiply the pressure per square inch by the diameter, and also by the number of thicknesses of metal it will take to make one square inch of cross section, and divide the product by 2, because the boiler has 2 sides.

of sectional area in a transverse direction.

The power of any steam-boiler to resist strain depends upon the thickness and quality of material, character of the workmanship, and the shape of the parts subjected to strain.

The above cut represents the arrangements most generally employed for bracing marine steam-boilers, and includes the vertical and horizontal, angle, toggle, dome, and crown braces; as well as the buckles, crow-feet, angle-irons, girths, stay-bolts, and leg braces. The tubes answer for braces for the tube-sheets; the crow-feet for the crown and dome; the proper strength for the braces of marineboilers may be found by multiplying the number of square inches exposed to the pressure of the steam by six times the steam-pressure to be carried.

1

Rules.

Rule for finding the safe working-pressure of iron boilers. - Multiply the thickness of iron by 56,* if single riveted, and 70, if double riveted; multiply this product by 10,000 (safe load): then divide this last product by the external radius (less thickness of iron); the quotient will be the safe working-pressure in pounds per square inch, which, if multiplied by 5, would give the bursting pressure.

In the foregoing rule, the tensile strength of the iron is taken at 50,000, as it has been repeatedly proved by experiment that boiler-plate possesses less tenacity than the same iron would have if rolled into bars.

Rule for finding the internal strain to which boilers are subjected when under pressure. - Multiply the surface of the plate required for one square inch of sectional area by the pressure of steam in lbs. per square inch; multiply this result by the diameter of the boiler in inches, and divide by 2, which gives the strain per square inch of sectional area to which the boiler is subjected.

The surface of boiler-plate required for one square inch of sectional area will depend upon the thickness of plate; thus, iron inch thick will require 4 superficial inches to make one square inch of sectional area; iron inch thick will require 2, and so on.

Rule for finding the pressure per square inch of sectional area on the crown-sheets of steam-boilers. Multiply the width of the crownsheet in inches by its length in inches; multiply this product by the pressure of the steam in lbs. per square inch by the gauge; divide by 2, if inch iron, and so on according to the thickness. Rule for finding the aggregate strain caused by the pressure of steam on the shells of boilers. Multiply the circumference in inches by the length in inches; multiply that product by the pressure in pounds per square inch. The result will be the aggregate pressure on the shell of the boiler.

*Multiplied by 56, because the iron loses 44 per cent. of its strength in the process of punching. Double-riveted seams equal '70 of the original strength.