330. _Q._--Is not a good vacuum in an engine conducive to increased power?
331. _Q._--And is not the vacuum good in the proportion in which the temperature is low, supposing there to be no air leaks?
332. _Q._--Then how could Mr. Watt find a temperature of 100° in the water drawn from the condenser, to be more beneficial than a temperature of 70° or 80°, supposing there to be an abundant supply of cold water?
333. _A._--Because the superior vacuum due to a temperature of 70° or 80° involves the admission of so much cold water into the condenser, which has afterward to be pumped out in opposition to the pressure of the atmosphere, that the gain in the vacuum does not equal the loss of power occasioned by the additional load upon the pump, and there is therefore a clear loss by the reduction of the temperature below 100°, if such reduction be caused by the admission of an additional quantity of water. If the reduction of temperature, however, be caused by the use of colder water, there is a gain produced by it, though the gain will within certain limits be greater if advantage be taken of the lowness of the temperature to diminish the quantity of injection.
334. _Q._--How do you determine the proper area of the injection orifice?
_A._--The area of the injection orifice proper for any engine can easily be told when the quantity of water requisite to condense the steam is known, and the pressure is specified under which the water enters the condenser. The vacuum in the condenser may be taken at 26 inches of mercury, which is equivalent to a column of water 29.4 ft. high, and the square root of 29.4 multiplied by 8.021 is 43.15, which is the velocity in feet per second that a heavy body would acquire in falling 29.4 ft., or with which the water would enter the condenser. Now, if a cubic foot of water evaporated per hour be equivalent to an actual horse power, and 28.9 cubic inches of water be requisite for the condensation of a cubic inch of water in the form of steam, 28.9 cubic feet of condensing water per horse power per hour, or 13.905 cubic inches per second, will be necessary for the engine, and the size of the injection orifice must be such that this quantity of water flowing with the velocity of 43.15 ft. per second, or 517.8 inches per second, will gain admission to the condenser. Dividing, therefore, 13.905, the number of cubic inches to be injected, by 517.8, the velocity of influx in inches per second, we get 0.02685 for the area of the orifice in square inches; but inasmuch as it has been found by experiment that the actual discharge of water through a hole in a thin plate is only six tenths of the theoretical discharge on account of the contracted vein, the area of the orifice must be increased in the proportion of such diminution of effect, or be made 0.04475, or 1/22d of a square inch per horse power. This, it will be remarked, is the theoretical area required per actual horse power; but as the friction and contractions in the pipe further reduce the discharge, the area is made 1/15th of a square inch per actual horse power, or rather per cubic foot of water evaporated from the boiler.