The line under the diagram and parallel to the atmospheric line is 18ths distant, and represents the perfect vacuum-line, the space between showing the amount of force with which the uncondensed steam or vapour opposes the ascent or descent of the piston at every part of the stroke.

67. How is the uniform force of the steam estimated from the diagram taken?

Divide the diagram in the direction of its length into any convenient number of equal parts, through which draw lines at right angles with the atmospheric line, add together the lengths of all the spaces taken in measurements corresponding with the scale on the indicator, divide the sum by the number of spaces, and the quotient is the mean effective pressure on the piston in pounds per square inch.

Thus, the diagram taken shows an effective pressure of steam above the atmospheric line, of 6.28 pounds per square inch, and an effective pressure of 12.2 pounds underneath, or a uniform effective force of 18.48 pounds per square inch on the piston; hence, suppose the engine to have a cylinder of 70 inches in diameter, and the piston

moving at a velocity of 231 feet per minute, the effective power of the engine may be estimated as follows::

18.48—2.5 15.98 pounds per square inch,

=

after deducting 2.5 pounds for friction, &c., then 702 × 7854 × 231 × 15·98

33,000

400-146 horse power.

14204929-914

33,000

68. The relative volumes of steam and water being known, also a given quantity or volume of steam expended in a given time, what will be the amount of water evaporated for the volume of steam expended?

RULE. To the temperature of the steam in degrees of Fah., add 459, multiply the sum by 76, divide the product by the elastic force in inches of mercury, and the quotient is the volume of steam compared with the volume of water.

Ex. Suppose 500 cubic feet of steam at a temperature of 240.7° be required for an engine per minute, the quantity of water in theoretical estimation necessary for the production of that amount of evaporation is 240.7° 51 inches of mercury elastic force.

=

240-7459 × 76

53177

= 1042.7 volume

51

51

of steam to one of water.

And at the common atmospheric pressure one cubic inch of water produces about one cubic foot of steam, and at all pressures bears a relative 1728 × 500

proportion; hence it follows that

864000 1042.7

1042.7

-828.6 cubic inches of water required.

Or, from the table of steam generated under different pressures, steam at a temperature of 240.7°, the cubic inches of water in a cubic foot of steam equal 1.658; hence 1.658 x 500 = 829 cubic inches of water.

69. For a steam-engine, is the preceding rule applicable by which to determine the capacity of feed-pump for a boiler?

No; it is merely the quantity of water evaporated in theory; practice shows leakage, priming, and other contingencies to provide against; hence the feed-pump ought not to be less than three and a half times the estimated capacity for theoretical evaporation. The capacity for a feed-pump is obtained by the following rule:

Multiply the capacity of the cylinder in cubic inches by the total pressure of the steam in pounds per square inch, divide the product by 4800, the quotient equal the capacity of pump in cubic inches.

Ex.-Suppose a condensing engine with a cylinder 33 inches diameter, and a stroke of five feet, or 60 inches, the total pressure of steam 21 pounds, or six pounds above the atmospheric pressure, what must be the pump's capacity?

70. That being the proper capacity, how are the dimensions of pump tested or determined?

If the length of the stroke is given, divide the capacity by the stroke in inches, and the square root of the quotient multiplied by 1.12837 equal the diameter in inches.

Ex. Suppose the capacity of a pump to be 224.5 cubic inches, and an unchangeable length of stroke of 11 inches given, to find the necessary diameter.

224.5

11.

=

20.4 × 1.12837-5.1 inches nearly.

71. At what temperature ought the condenser to be kept, or otherwise, at what temperature ought the condensed water to be delivered into the hot well of a condensing engine, that the engine may produce a maximum of effect by condensation?

At 100°, when the water for injection is not less than 50°. But if a reduction of temperature can be obtained by the use of colder water, then a better effect will be produced when the condensed water is delivered at about 80°. For a land condensing engine, the capacity of the cold water pump commonly is one forty-eighth the capacity of the cylinder.

=

But suppose the temperature of the injection or condensing water to be 32° and 80° respectively, the relative proportions of each for condensation to maintain the condenser at 100° will be, 100° - 80° 20, and 100° - 32° = 68; consequently the available temperatures of the waters to condensation 20° and 68°, or the quantities for injections are as 20 to 68, and in like proportion at any other given temperature.

=