SECTION X. STEAM-THEORY OF THE STEAM ENGINE-MARINE BOILERS BOILER MOUNTINGS-MARINE ENGINES - TYPES OF ENGINES-SLIDE VALVE AND GEAR-INDICATOR DIAGRAMS-EXPANSIVE WORKING AND COMPOUND ENGINES -SURFACE AND JET CONDENSERS-PROPELLORS-INSTRUCTIONS FOR STEAM LAUNCHES AND TORPEDO BOATS. THEORY OF THE STEAM ENGINE. Mechanical work consists in the overcoming of resistance by the action of force. It is measured by the product of the resistance, and of the distance through which the resistance is overcome. In British units, the resistance is usually expressed in pounds and the distance in feet; the product is then expressed in foot-pounds. The unit of work, or foot-pound, is the work done in overcoming one pound resistance through one foot distance. Energy is a term meaning capacity for doing work. The energy, or working capacity represented by the action of a given force through a given distance, is also measured in foot-pounds, by taking the product of the force in pounds and of the distance in feet through which it acts. Mechanical Energy is the energy a moving body possesses in virtue of its motion. The object of the steam engine is to develop mechanical energy, and apply it to the performance of work. The term Power is used to express the amount of work done in a given time. The customary unit is one horse power, which represents a performance of 33,000 ft.-lbs. per minute. In the operation of the steam engine the energy exerted on the piston during any number of complete revolutions is equal to the work done in the same time, whenever the engine is running at a steady speed of revolution. The power may therefore be estimated by finding the product of the mean driving force acting on the piston, and of the distance through which this force acts in a given time. Only the effective driving force is taken into account in this calculation; i.e. the force available after back pressure has been allowed for. Let p = mean effective pressure on piston (lbs. per sq. inch), ..px A = mean effective driving force on piston in lbs., n = .. lx n = ..px Axl x n = number of strokes per minute, number of feet traversed by piston per minute, and Indicated horse-power - PXA XIX n 33,000 This is the actual power developed in the cylinder, it is called indicated horse-power because the mean effective pressure is found by means of the indicator. Heat. The principal physical effects, produced in substances by the application of heat to them, are change of temperature, change of volume, and change of condition or state, by melting, or by vaporization. While a change of state is being produced by heat, the temperature of the substance remains unaltered. Heat thus employed in producing changes in substances without alteration of their temperature is called latent heat, while heat so employed as to produce change of temperature is called sensible heat. Measurement of Heat.—Heat is usually measured by the change of temperature it is capable of producing in water. The British Thermal Unit is the quantity of beat necessary for raising the temperature of 1 lb. of pure water through 1o Fahrenheit. This is practically (not strictly) independent of the temperature of the water. Thus 100 B. T. U. will raise 100 lbs. of water through 10; 1 lb. through 100°; 10 lbs. through 10°, and so on. The Mechanical Theory of Heat is the theory by which heat is regarded as a form of energy. This theory is founded on the facts that heat may be produced by the expenditure of mechanical energy in various ways, and vice versâ; and, as proved experimentally by Dr. Joule, that in overcoming friction, the quantity of heat produced is in all cases strictly in proportion to the quantity of mechanical energy expended, viz. 1 B. T. U. per 772 ft.-lbs. The quantity 772 ft.-lbs. is therefore regarded as the equivalent of the unit of heat, and is called the mechanical equivalent of heat or Joule's equivalent. Steam. In the ordinary operation of the steam engine, steam is produced in the boiler under a certain pressure, which is kept constant by proper management of the fires. During a portion of the piston's stroke in either direction steam is freely admitted to the cylinder, an equal quantity of steam being meanwhile produced in the boiler; a certain amount of work is thus done by the displacement of the piston through the volume of the steam admitted. The steam in the cylinder is then cut off from communication with the boiler, and is allowed to drive the piston, by expansion, through the remainder of the stroke. Additional work is thus done by the further displacement of the piston during expansion. Finally the steam escapes from the cylinder to the condenser or into the air, and the piston is driven back by the action of steam, which is admitted and allowed to expand in a similar manner on the other side of the piston. The following are some of the more important facts in connection with these processes. 1. The temperature of steam in the normal condition is dependent on its pressure, and is the same as the boiling point of pure water under that pressure. The greater the pressure, the higher is the temperature. 2. The density of steam in the normal condition is approximately proportional to its pressure. At the atmospheric pressure the volume per pound is 26.36 cubic feet. 3. The heat expended in producing steam of temperature To from water of temperature to is, per pound of steam, Sensible heat (for raising water to boiling point) = T - t B. T. U.'s. = Total heat required (sensible + latent) = T − t + 966 -7 (T — 212). 4. The total heat required to convert water of any given temperature into high pressure steam is only a little more than that required to convert the same weight of water into low pressure steam; the difference is about 3 B. T. U. per degree difference of temperature. 5. The latent heat of evaporation is spent in doing work. The great increase of volume accompanying evaporation is resisted externally by the pressure on the water, and internally by the cohesion of its particles. The latent heat does the work of overcoming these resistances. The external work of evaporation, however (i.e. the work of overcoming external resistance) represents only a small fraction of the latent heat, the greater part being absorbed in doing internal work. approximately. The exact relation 6. When steam is allowed to expand, its pressure falls according to the 1 law, pressure c between the volume, pressure and volume of expanding steam, however, depends on the condition of the steam during expansion. The total work done in driving the piston through v cubic feet is equal to p xv x 144 ft.-lbs. where p is the average steam pressure (lbs. per sq. inch) exerted through the volume v. 7. This work is obtained by the disappearance of an equivalent quantity of heat. The work done before cut-off is in reality the external work of evaporation, and is due to that portion of the latent heat which disappears in doing external work; while the work done during the expansion is due to heat expended by the steam itself. This expenditure of heat by steam during expansion, though directly beneficial (inasmuch as it produces an equivalent amount of work which is entirely additional to the work obtainable from the same steam without expansion) is the cause of a partial condensation of the |