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.

CHAPTER III

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

P

=

186.4 + 14.6

=

201.0 lb. per sq. in. 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.

steam gage.

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

atmosphere without. If the barometer be at sea-level and the temperature of the mercury column 32°F., the height of mercury

FIG. 18. The principle of the atmospheric barometer, the condenser vacuum and the measurement of pressures above the atmosphere.

will now measure exactly 29.921 inches for such standard conditions.

Vacuum Pressures. It has already been pointed out that measurement of pressure by means of the steam gage indicates a pressure over and above that exerted by the atmosphere and consequently to ascertain the true absolute pressure of the fluid under measurement we must add to the gage reading the atmospheric pressure of the day. And so in the measuring of the pressure of a condenser, unavoidably there has grown up a similar but opposite custom in which the pressure is measured down from the atmosphere. Such a reading is known as a vacuum pressure. In order then to ascertain the absolute pressure P under which a condenser is operating it is necessary to subtract the vacuum pressure reading P. from the atmospheric pressure reading P.. Thus

Thus if a condenser is operating under 28.5 in. of vacuum and the atmospheric pressure is 29.92 in., we mean that the actual air and steam still undisposed of in the condenser exert an abso

lute pressure equivalent to the difference between 29.92 and 28.50 which is 1.42 in. of mercury.

Confusion in Pressure Units.-We now see that readings in inches of mercury for low pressure and pounds pressure per sq. in. for high pressure are expressions that are not at all comparable to each other and hence their interrelation becomes an endless source of confusion.

Relationship of Pressure Units.-By careful measurement of the atmosphere at sea-level, scientists have established that the height of a mercury column with the mercury at 32°F. in temperature is 29.921 in. Such a column of mercury one square inch in cross-section weighs 14.696 lb. This gives us at once a method by which we may transfer inches of mercury Im into pounds of pressure per square inch P. Thus

Inches of Water and Pounds Pressure per Square Inch.Very slight pressures are often measured in inches of water above or below atmospheric pressures. Thus, in determining the draft of a chimney, a "U" tube is inserted into the chimney, and the height of the unbalanced portion of the water column indicates the draft in the chimney in inches of water. Since a column of water 1728 in. high and one square inch in cross-section at 100°F. weighs exactly 62 lb., the inches of water I may be converted into lb. pressure per sq. in. P by the formula

The Thirty Inch Vacuum.-In engineering practice a thirty inch mercury vacuum is considered to be the point of absolute zero in pressure. This is not strictly true, however, for we have just seen that such an absolute zero point is reached under a vacuum pressure of 29.921 in. of mercury. The reading of the column of mercury in this case is taken when the mercury is at a temperature of 32°F., which is the standard temperature for scientific measurement. If, however, we change our standard to that of 58.4°F. the same weight or pressure of mercury now measures just 30.0 in. This temperature is more nearly that of the condenser room where atmospheric pressures are read and since it makes a column of even thirty inches in height, we shall adopt such a reading at 58.4°F. as standard for absolute vacuum meas