« SebelumnyaLanjutkan »
From the tables it is seen that in this instance p v = 19.05, v1 = .016.
74.6 883.5 B.t.u.
.. External Work 21.16 144 (19.05 - .016) .016) .. Internal Work Entropy of Water.-In certain advanced problems in steam engineering, engineers and physicists have found it convenient. to invent fictitious qualitites of steam. While many have endeavored to give a physical interpretation of entropy, perhaps it is clearer for the student to consider it as merely a mathematical fiction which, however, often becomes extremely useful for the representation of steam engineering problems and indeed assists wonderfully in their solution.
On this assumption, entropy may be defined as such a quantity that when plotted against absolute temperatures the area under the curve connecting all such points will numerically represent the amount of heat supplied to one pound of matter in order to accomplish the indicated change in temperature. Thus in the instance at hand if one should plot a curve with ordinates representing absolute temperatures and with abscissas representing the entropy for each corresponding temperature, the area under this curve would be exactly 199.1 units. For it takes 199.1 units of heat energy to raise one pound of water from 32°F. to 231°F .or on the absolute scale from 491.6°F. to 690.6°F.
By analysis in higher mathematics it is found that entropy of water may be quite closely computed by the formula
Wherein is the entropy of water, T2 the absolute temperature at the end of the heat application and T1, the absolute temperature at the beginning which is usually taken at the melting point of ice or 491.6°F. on the absolute scale. Thus in this instance
T2 log T
231 + 459.6)
32 + 459.6
2.306 log10 (491.6)
The values in the steam tables were arrived at by a slightly more accurate process than this by taking into account the fact that the specific heat of water is not constant as heat is added.
The Entropy of Evaporation. Since the temperature remains constant during the evaporation of water into dry saturated
steam, it is evident that the entropy curve in this case would simply be a rectangle as shown in the illustration wherein one dimension is of length T and the area swept off is of L units. Hence, the entropy for heat of evaporation is evidently
FIG. 40. The temperature entropy diagram.
By the invention of a fictitious quality of water and steam, known as entropy, the plotting of a diagram is made possible, so that an area represents heat added. Thus, in the diagram above, the abscissas are entropy and the ordinates absolute temperatures. The area abcf is exactly 180 units, which is the heat required to raise water from 32°F. to 212°F. Similarly, the area fcde is 970.4 units, which is the heat required to evaporate one pound of water at 212°F. into steam at 212°F.
Total Entropy. The sum of the entropy value for water and for heat of evaporation is called the total entropy of dry saturated steam. This is evidently arrived at numerically by adding together the two preceding columns. Thus, total entropy=entropy of water + entropy of evaporation
...Total entropy Tables for Superheated Steam.-In later pages of the steam tables are to be found data relative to superheated steam. subsequent chapter will deal largely with superheated steam computations, we shall delay the consideration of superheated steam tables until the reader has been more thoroughly grounded in other fundamental computations of dry saturated steam.
HOW TO COMPUTE BOILER HORSEPOWER
HAT energy is never created or destroyed is a fundamental postulate of modern engineering practice. All of our machines and driving mechanisms are, then, simply devices by means of which we may convert one form of energy into another form to suit our convenience or meet the demands of industrial activity. Thus an electric generator does not create energy but is merely a device whereby energy existing in the waterfall or in the steam turbine may be converted into electrical energy. Neither does the energy exist inherently in the waterfall, but due to the emission of heat from the sun, this water has first been drawn from the ocean into the clouds to be later deposited on the lofty mountain peaks. Due to this superior position it is enabled to develop water power energy and thus transfer the energy of the sun's rays into more useful form to ease man's burdens. And so with the steam boiler, we have fundamentally a mechanism by which energy latent in fuel oil or other combustible is first given out as heat energy of combustion to be immediately converted into latent heat energy of steam.
FIG. 41. How James Watt would have standardized a mechanical horsepower at the Pan
The Meaning of the Word "Rating."-The rapidity with which this conversion of one form of energy into another form may be accomplished is known as the rating of the mechanism involved. Thus a small boy may by means of a block and tackle hoist a huge weight to the top of a modern sky-scraper and at a later observation one may see a team of horses straining to their
utmost to accomplish the same task. By close inspection, however, it will be found that the small boy has by means of intervening pulleys been able to take from thirty to forty times longer to accomplish what the horses did in a comparatively short time. Hence power, the basis of comparative effort, is the time rate of doing work.
The Development of the Word "Horsepower."-After his invention of the steam engine, James Watt soon found that
FIG. 42. A close up view of the filling pipes for the oil storage reservoirs of the Southern California Edison Company's Long Beach Plant. These valves are under control of the oil company from whom the oil is purchased.
he must devise some unit or measuring stick, as it were, with which to measure the power of his mechanism. As he was a pioneer in the art, he had to cast about for some convenient unit to adopt. What more natural unit should he consider than that of the draft horse? After watching a horse drawing up large cakes of ice into an ice house by the use of a snatch block, it occurred to him that when the horse pulled up a fairly good load he must be doing a certain amount of work. After making several experiments he found that by adding more sheaves to
the blocks the horse could raise a greater load but it took more time to do it. He found that the average dray horse was able to raise a load of 550 lbs. at the rate of 60 ft. per minute, or to do 33,000 ft. lbs. of work per minute. This unit Watt called a horsepower and applied it to the measurement of the power of his steam engines.
The Boiler Horsepower.-In the early days of the steam engine the principle of the conservation of energy had not been firmly established. Indeed that heat was a form of energy at all was a debated question for many years after the steam engine became of vast practical importance.
FIG. 43.-Steam flow meter, recording pressure gage and indicating pressure gage, Station C, Pacific Gas and Electric Company, Oakland, California.
Hence, since the energy latent in steam was not then known to be the underlying reason for the power driving action of the steam engine, the first rating of the boiler was made on the basis of power development in the engine which received its supply of steam from the boiler in question. Thus a boiler that could supply steam to operate a steam engine developing 50 indicated h.p. was said to be a 50 h.p. boiler. Later it became evident, due to the rapidly increasing efficiencies of the steam engine that such a rating was wholly variable. It was found, however, that under ordinary working conditions a boiler which could evaporate