air pump A, of a single engine, arranged as is most convenient for exhibiting the parts. The steam enters from the boiler to the cylinder by the pipe S, through the valve o; and presses down the piston P, which is supposed to be taken at the time of its descent; the steam below it goes into the condenser, and is condensed by the jet which plays into it. The air pump bucket p is descending in the air and vapour which the pump had received from the condenser during the previous ascent. When the piston is at the bottom of the cylinder, a motion is given to the rod O which shuts the valves a and c, and opens the valve b; there is then a communication open by the pipe E, between the top and bottom of the cylinder, and the pressure of the counter weight must be sufficient to overcome the friction of the piston, and expel the steam from the upper to the lower side of the piston; the action of the counter weight has also to expel the air and water of condensation through the valve Q by means of the air pump. The mode I have shewn of placing the valves and moving them by a single motion is not Messrs. Boulton and Watt's, but is one intended to render the motion of the steam from the upper to the under side of the cylinder more quick, by the pipe E being exhausted: the motion of the valves is simple and easily balanced. The valves of Messrs. Boulton and Watt are similar to Fig. 5, but they move them independently of one another, and this ought to be the case for an engine to work expansively, unless a separate valve acted on by a regulator be used to cut off the steam, (see Sect VIII.) An elevation of Boulton and Watt's single engine is represented in Plate XII. as applied to raising water. 407.—The proportions of the parts. The length of the cylinder should be twice its diameter, (art. 329.) The velocity of the piston in feet per minute should be ninety-eight times the square root of the length of the stroke, (art. 342.) The area of the steam passages should be equal to the area of the cylinder, multiplied by the velocity of the piston in feet per minute, and divided by 4800, (art. 154.) The air pump should be one-eighth of the capacity of the cylinder, or half the diameter and half the length of the stroke of the cylinder, (art. 351,) and the condenser should be of the same capacity. The quantity of steam will be found by multiplying the area of the cylinder in feet by half the velocity in feet; with an addition of one-tenth for cooling (art. 160,) and waste; and this divided by the column of the steam corresponding to its force in the boiler, (art. 121,) gives the quantity of water required for steam per minute, from whence the proportions of the boiler may determined, (see Sect. III. art. 224, and 227.) At the common pressure of two pounds per circular inch on the valve, the divisor will be 1497. The quantity of injection water should be twenty-four times that required for steam, (art. injection pipe one thirty-sixth of the diameter of the air pump bucket should be as large as they can be foot valves not less than the same area. For the proportions of the beams and D D be 284;) and the diameter of the cylinder. The valves in the made, and the discharge and other parts for strength, (see Sect. VII;) and the modes of regulation and management (see Sect. VIII.) 408.-The power of the single engine may be ascertained as follows: The effective pressure on the piston is less than the difference between 1.000 First, by the force producing the motion of the steam into the Second, by the cooling in the cylinder, (art. 160,) and pipes, Third, by the friction of the piston and loss by escape (art. 474.) passages Fifth, by the force required to open and close the valves, raise ⚫007 ⚫038 •05 '007 • 100 •100 •100 ⚫402 •598 The force of the steam in the boiler is commonly thirty-five inches of mercury, that of the uncondensed steam, (temp. 120°,) is 3.7 inches, hence, 35 x 598 20·93′ inches, and 20·93 — 3·7 = 17·25, or 6'66 pounds is the mean effective pressure on the piston; and when the steam in the boiler is of any other force, the mean effective pressure may be determined in the same manner. 409.-RULE. Multiply the mean effective pressure on the piston by the square of its diameter in inches, and by half the velocity in feet per minute, and the product is the effective power in pounds raised one foot high per minute. Divide by 33000 and the result is the number of horses' power. Example. Let the force of the steam in the boiler be thirty-five inches of mercury, the diameter of the cylinder forty-eight inches, and half the velocity 135 feet, per minute. Then the mean pressure is 666 pounds, and 6-66 × 48a × 135 = 2,071,526 pounds raised one foot, or 1.27 cubic feet per minute, or 76-2 cubic feet per hour; and by (art. 190,) 76-2 × 8-22 = 626.4 pounds of caking coal,* or 626.4 9.94 pounds of coals per hour for each horse power. 410. The application of the single engine is limited by the nature of its action to raising water or other works admitting of an inefficient returning stroke, but for these purposes it has great advantages. I would suggest as an improvement, that the condensation should be effected as described for the atmospheric engine, (art. 400,) and that it should always act more or less by expansion; the full effect of expansion cannot however be obtained unless the action be equalized by a proper arrangement of the pressures and counter weight. 411.-Single engine acting expansively. When the single engine acts expansively, it is necessary to determine the point of the stroke at which the steam should be cut off. Now the pressure on the piston should never be less than the mean moving force, otherwise it would be overpowered, and the column of water would descend again. Consequently we may adopt this analogy. As the whole force of the steam in the boiler is to one, so is half the greatest effective force on the piston added to the resistance from friction, &c. to the portion of the stroke at which the steam should be cut off. Thus, if the force in the boiler be thirty-five inches of mercury, and the resistance of the uncondensed steam 3.7 inches, then 3·7 + ( 35 × ·402 ) = 17·77 inches, the loss of power from friction, &c. (art. 408,) and consequently pressure on the piston at the end of the stroke; therefore 35: 26-38 :: 1': '75 of the coal. This is equivalent to raising 17,600,000 pounds one foot by a bushel of coals, or 198,000 pounds by one pound of stroke. The steam will obviously act expansively in its ascent in the same proportion, whence a less counter weight is necessary. 412. To find the mean pressure on the piston in an expansive engine. Let the portion of the stroke made when the steam is cut off be 1 n. Then the nth part of the whole force in pounds per circular inch, of the steam in the boiler, multiplied by the 2-3 times the common logarithm of n, added to 3, is the mean moving force or pressure; which is to be used in the rule, (art. 409,) for finding the power, and also for adjusting the load. Example. Suppose the steam to be cut off at three-fourths of the stroke, then 1 n three-fourths, or n= 1.33, and its logarithm is 0-125156; the whole force being thirtyfive inches of mercury, or 13.5 pounds per circular inch, we have 3 × 13·5 × (2·3 × 125156+ ·3 ) 4 5.95 pounds per circular inch for the mean pressure. = 13.5 x 441 = 413.-The velocity should be found by the rule (art. 343,) and the quantity of steam will be as much less than that required for an engine working at full pressure as the portion of the stroke at which the steam is cut off is less than the whole stroke; and in other respects the quantity of water, fuel, water for condensation &c. should be determined by the rules in (art. 407.*) The counter weight will be less in the same ratio, as the pressure on the piston is less than it is in a common engine. Owing to a larger sized engine being required, the expansive method is not valued as it ought to be, except when the force of the steam in the boiler is increased, and this I would recommend to the extent of two atmospheres, but not higher. 414.-The double engine of Boulton and Watt. It has been already shewn in what the double engine differs from a single one, (art. 389.) The parts are shewn in Fig. 1. Plate V. where C is the cylinder; the steam enters at S, and passes into the upper part of the cylinder at F, or into the lower part at D, as in Fig. 3; Fig. 1. shewing the piston in * Taking the example of (art. 409,) we find 22,000,000 pounds may be raised one foot by a bushel, or nearly 250,000 pounds by one pound of coal; and I do not think more has been actually done with low pressure steam by a single engine. the state of descending and Fig. 3, as ascending. From the lower part of the cylinder in Fig. 1, the steam escapes through D, into the condenser B, where it is condensed by a jet of cold water, which plays into it constantly; and the uncondensed gases and water pass through the valve G, during the ascending stroke, and during the descending one they pass from the lower to the upper side of the pump bucket, through its valves, and are drawn up by the ascending stroke, and expelled at the valve Q into the hot well. When the steam piston P ascends, the steam from the upper part of the cylinder, passes through F down the pipe E to the condenser. The steam passages D, and F, are opened and closed by a D-slide, so called from its plan resembling the letter D; it is moved by the rod O, by tappets or other methods, (see Sect. VII.) where the different methods are described. In small engines the steam passages are frequently opened and closed by cocks, in larger ones by valves, or slides, the species of which and the pistons and other parts are described in Sect. VII. 415.-The proportions of the parts for a double engine acting with full pressure. When the case to which the engine is applied will admit of it, the length of the cylinder should be twice its diameter, (art. 329.) The velocity of the piston in feet per minute, should be found by multiplying the square root of the length of the stroke by 103 for machinery, or by 98 for raising water, (art. 337 and 342.) The area of the steam passages should be equal to the area of the cylinder multiplied by the velocity of the piston in feet per minute, and divided by 4800, (art. 154.) The air pump should be one-eighth of the capacity of the cylinder, or half the diameter, and half the length of the stroke of the cylinders; (art. 351,) and the condenser should be of the same capacity. The quantity of steam will be found by multiplying the area of the cylinder in feet by the velocity in feet, with an addition of one-tenth for cooling and waste; and this divided by the volume of the steam corresponding to its force in the boiler, (art. 121,) gives the quantity of water required for steam per minute, from whence the proportions of the boiler may be determined, (see Sect. III. art. 224, and 227;) at the common pressure of two pounds per circular inch on the valve, the divisor will be 1497. The quantity of injection water should be twenty-four times that required for steam, (art. 284,) and the diameter of the injection pipe one thirty-sixth of the diameter of the cylinder. The valves in the air pump bucket should be as large as they can be made, and the discharge and foot valves not less than the same area. For the proportions of the beams and other parts for strength, (see Sect. VII;) and the modes of regulation and management (see Sect. VIII.) |