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The power of the steam-engine is measured by that of the horse. A horse-power, as fixed by Watt, is equal to 33,000 lb. avoirdupois, raised one foot high per minute ; and one day's work of a horse, is this power, acting through eight hours. The pressure of our atmosphere is reckoned as equal to that of thirty perpendicular inches of mercury; or 14-701b. per square inch, or 11.55 lb. per circular inch. To find the Horse's power of an Engine, according to
the Rule given by Mr. Watt. From the Diameter of the cylinder in inches, subtract 1, square the remainder, multiply the square by the velocity of the piston in feet per minute, and divide the product by 5640. The quotient will be the number required.
Proportion of the Cylinder.—The best proportion is when the length is twice the diameter; because the cooling surface is then least, in proporton to the content of steam.
Proportion of the Air Pump und Condenser.-In double condensing engines, these are made, by Boul ton and Watt's rule, each to measure one-eighth the content of the cylinder.
Velocity of the Piston to produce the best effect.In engines working the steam expansively, 100 times the square root of the length of the stroke in feet, is the best velocity in feet per minute.
In engines not working expansively, 103 times the square root of the length of the stroke in feet, is the best velocity in feet per minute.
To find the quantity of Water required for Steam and Injection.-Multiply the area of the cylinder in feet, by half the velocity in feet for single, and by the whole velocity in feet for double engines. Add 1-10th for cooling and waste; and this, divided by 1497 (at the common pressure on the valve of 21b.
circular inch), wil give the quantity of water required for steam per minute.
The quantity of water for injection should be 24 times that required for steam.
The diameter of the injection-pipe should be 1-36th part of that of the cylinder.
The valves should be as large as practicable.
The boiler should be capable of evaporating about 12 gallons per hour for each horse power.
NON-CONDENSING, OR HIGH PRESSURE ENGINES.
The length of the cylinder should be at least twice its diameter.
The velocity of the piston, in feet per minute, should be 103 times the square root of the length of the stroke
in feet; or 100 times, if the steam is worked expan. sively.
The area of the cylinder should be, to the area of the steam-passages, as 4800 is to the velocity of the piston, found as above.
Form and Direction of Steam-pipes.-Enlargements in steam-pipes succeeded by contractions, always retard the velocity of the steam—more or less according to the nature of the contraction—and the like effect is produced by bends and angles in the pipes. These should therefore be made as straight, and their internal surface as uniform and free from inequalities as may be practicable. The following proportions of velocity, from Mr. Tredgold, will exemplify this The velocity of motion that would result
from the direct unretarded action of the column of fluid which produces it, being unity
1000 or 8 The velocity through an aperture in a thin plate by the same pressure is
.625 or 5 Through a tube from two to three diame.
ters in length, projecting outwards .813 or 6.5 Through a tube of the same length, projecting inwards
.681 or 5.45 Through a conical tube, or mouth-piece,
of the form of the contracted vein .983 or 7.9
The construction and arrangement of the Marine Steam Engine necessarily differ from that of the ordi. nary condensing Engine, on account of the peculiar form of the floating structure in which it is placed, and of the absence of that solid support which can be obtained for Engines on land. The importance of ef fecting economy of room and weight on board a steamvessel, has led to the adoption of various methods of communicating motion to the paddle wheels; and vertical, oscillating, and other varieties of Engine haye been introduced, with more or less success; but the more general form is that of the beam or lever Engine, the position of the beam being reversed on being placed on each side of the bottom of the cylinder. The arrangement of the condenser, air-pump, &c., is also necessarily accommodated to the space in which the machinery is required to be fixed.
The following Dimensions are given by Mr. Russell, for the Cylinders of Marine Engines of various power: For 10 horse power, 20 inches diameter, 2 ft. Oin. stroke. .. 20
2 ft. 6 in. 32
3 ft. 2 in. .. 40 35
3 ft. 6 in. .. 50 40
4 ft. O in.
For 60 horse power, 43 inches diameter, 4 ft. 3 in.stroke. .. 70 46
4 ft. 6 in. .. 80 49
4 ft. 9 in. .. 90 52
5 ft. O in. ..100 . 55
5 ft. 6 in. .,125 59
6 ft.O in. :.150 62
6 ft. 3 in. ..175 66
6 ft.6 in. ..200
7 ft. O in. ..250 76
7 ft.6 in. ..300 82
8 ft. O in. ..350 87
8 ft. 6 in. ..400 92
9 ft. 2 in. ..500 100
10 ft.O in.
Economy of Steam-jackets. The following Table presents the results of three experiments made in France to ascertain the economy of steam-jackets to the cylinders of Engines, in the consumption of fuel. In the 1st, the steam first entered the jacket round the cylinder, and passed from thence into the cylinder. In the 2nd, the steam entered the cylinder directly, without passing into the jacket. In the 3rd, the steam entered both the cylinder and jacket directly, by means of separate communications between them and thc boiler. The result shows an increase in the consumption of fuel of nearly fivesevenths, in the second experiment, over that in the first