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through which the force acts. This product is called "work" and is numerically computed by multiplying the force in pounds by the distance in feet through which the force acts. The resulting computations are then expressed in foot-pounds (ft.-lb.) Thus, if the mean effective pressure, P, in a cylinder is measured in pounds per sq. in. and the piston has an area of A sq. in., it follows that the total force or pressure acting in the direction of the motion of the piston is PA. When this force has pushed the piston the length of its stroke, L ft., the work accomplished is
PLA ft. lb., since this is the product of the force and the distance through which the force acts. If there are N working strokes per minute, the ft. lb. of work accomplished every minute are now seen to be PLAN.
The mention of the words "per minute" in the last statement now indicates to us that the time taken to perform a given. quantity of work in engineering practice is of vast importance. Consequently this fact necessitates still another unit of measurement, namely that of power. Power is defined as the time rate of doing work. The horsepower is the basic unit. When 550 ft. lb. of work are performed per sec., or 33,000 ft. lb. per minute,
a horsepower is said to be developed. Hence, since in the above engine cylinder PLAN ft. lb. per min. are being developed, the horsepower is computed as follows:
Thus, in Alameda, California, a certain Diesel oil engine has a piston area of 113.15 sq. in., a stroke of 1.5 ft., a mean effective pressure of 77.3 lb. per sq. in., and each cylinder makes 125 working strokes per minute. Hence, each cylinder develops 77.3 X 1.5 X 113.15 X 125 33,000
In a later discussion the particular power units employed in steam engineering practice will be considered in minute detail, such, for instance, as the horsepower, the boiler horsepower, and the myriawatt.
Various Types of Energy Employed for Useful Work.-Another important consideration is that of the physical characteristic of a body which enables it to perform work. This physical quality possessed by a body which enables it to perform a definite quantity of work is spoken of as its energy. Energy then is the capacity for work. In general we meet with two great classes of energy. One is that of kinetic energy, or energy of motion. According to Law 2, if the motion of a body be changed, a force is required. Hence a body actually in motion possesses kinetic The other type of energy is known as potential, or energy
of position. Thus steam moving with a high velocity, by the nature of its kinetic energy, is enabled to drive the wheels of an impulse turbine. On the other hand, crude petroleum when heated so that it will unite with the oxygen of the air gives out
energy in the form of heat, which may be caused to do useful work. The energy inherently latent in the crude pertroleum is known then as potential energy. Engineering practice is largely concerned with the harnessing of various forms of energy. Looking about us in nature and in modern engineering accomplishment, we may see numerous instances of energy. The steam engine and steam turbine indicate a form of mechanical energy; the incandescent light, or the dynamo, that of electrical energy; the evolving of heat in the burning of crude oil, that of chemical energy; the human conducting of affairs, that of human energy; the rays of light from the sun, dissipating eternally 10,000 h.p. over each acre of the earth's surface, that of solar energy, and so on indefinitely. Modern investigation has conclusively established the fact that all types of energy are interchangeable, and though some types of energy are more readily convertible into other types, yet the basic law is true that no energy in sum total is ever destroyed, and on this basis, or law, known as conservation of energy, practically all of our engineering formulas and computations are evolved.
The conversion of the chemical energy of crude oil into heat energy of the furnace and thence into steam largely concerns our attention in this discussion. Thus each pound of California crude oil will be found in later articles to contain approximately 18,500 British thermal units of heat energy. This energy of one pound of oil when wholly converted into mechanical energy is sufficient to lift a person weighing 150 pounds through a vertical skyward journey of some 18 miles. Hence the study of the application of such enormous reservoirs of energy, latent in crude petroleum, will prove intensely interesting and instructive.
Bearing in mind these fundamental laws, we should now be able to see mentally the exact changes of energy that are going on in the modern power plant; first as chemical energy in oil, next as latent heat energy in furnace gases, then as latent heat energy in steam, next as energy of motion in the moving parts of the power generating apparatus, where the final transformation into electrical energy is brought about.
THEORY OF PRESSURES
N the preceding discussions we have seen that a force is said to be acting whenever the physical conditions are such that the velocity of a body tends to be changed in magnitude or direction. If two opposing forces are equally balanced, there is simply
a tendency to change motion and such a force is known as a pressure. This opposing force in the case of a gas or vapor under pressure is supplied by the walls of the containing vessel. Pressures then constitute an important phase of steam engineering practice.
The Steam Gage.-In steam engineering practice heavy pressures, that is pressures above the atmosphere, are usually measured by means of an instrument known as a steam gage. This gage consists of a piece of hollow metal bent into a circular shape which, under pressure, tends to straighten out, see Fig. 16. This straightening effect is proportional to the pressure under which the boiler is working. A rack and pinion movement, placed on the end of this curved piece of metal in the steam gage, causes the needle of the gage to indicate pressure readings. By comparing this gage with a definite standard its accuracy is ascertained.
The Difference Between Absolute Pressure and Gage Pressure. There is a point at which a gas is said to exert no pressure. This expanded condition of a gas has never been wholly realized in practice, yet this very beginning point or zero value is most convenient in expressing pressure valuations and such denotations are known as absolute pressure values. The steam gage attached to the boiler does not read absolute pressure values, but
such pressure readings are known as pounds pressure per sq. in. (gage) which means that one must add the absolute pressure of the atmosphere, Pa, to the gage reading, P., in order to ascertain the true absolute pressure P under which the boiler in generating steam. Thus
FIG. 16.-Interior and exterior view of steam gage, showing principle of operation.
Thus, if a pressure gage of the steam boiler reads 186.4 per lb. sq. in. and the pressure of the atmosphere is found to be 14.6 lb. per sq. in., the absolute pressure under which the boiler is operating is
The Column of Mercury. The most accurate method of measuring small pressures such as the pressure of the atmosphere and condenser vacuum pressure is by means of a vertical column of mercury. In its simplest form this consists of a long glass tube closed at one end and filled with mercury. The tube is then inverted and the open end placed in a vessel of mercury exposed to the atmosphere or condenser as the case may be, as shown in Fig. 18.
In the case of atmospheric pressure determi- adjuster for the nation the mercury will at once lower itself in the long tube until the height of enclosed mercury above that in the vessel is sufficient to balance the pressure from the