CHAPTER VIII

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 Panama-Pacific Exposition.

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.

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

THE MECHANICAL HORSEPOWER

FIG. 44. The unit of power in modern steam engineering.

THE KILOWATT

FIG. 45. The unit of power in electrical engineering, which is 1.34 times the mechanical horsepower.

THE BOILER HORSEPOWER

FIG. 46. The unit of power in boiler practice, which is 13.14 times the mechanical horsepower.

THE MYRIAWATT

FIG. 47. The unit of boiler rating proposed by certain national engineering societies, which is 13.4 times the mechanical horsepower.

30 lb. of steam per hr. at 70 lb. pressure and taking feed water at 100°F. could usually operate a 1 h.p. engine, consequently this mode of boiler rating became popular.

In 1884, the American Society of Mechanical Engineers adopted the following definition for the boiler h.p.: That a boiler evaporating 34.5 lb. of water at 212°F. into steam at 212°F. per hr. should be known as a 1 h.p. boiler.

The Conversion of Boiler Horsepower to Mechanical Horsepower Units. In later years the principle of the conservation of energy finally became well established and when engineers began to compute the actual energy represented in a mechanical horsepower as originally adopted by James Watt and then compare this to the energy represented in the steam generated by what was known as a one horsepower boiler, it was found that the boiler horsepower represented the conversion in unit time of over thirteen times the energy represented in the mechanical horsepower unit acting over the same unit of time.

It is instructive to follow this computation as it will familiarize the reader with these two distinct units. Let us then proceed to an analysis. The mechanical horsepower unit is defined as a performance of work or conversion of energy at the rate of 33,000 ft. lb. per minute. Since 1 B.t.u. of energy has been found to have its equivalent in 777.5 ft. lbs .of mechanical work, it is seen. that 33,000 ft. lb. of work per minute, or 1,980,000 ft. lb. of work per hr. may be represented by 2547 B.t.u. per hr. From the definition of the boiler horsepower above mentioned, as that adopted by the American Society of Mechanical Engineers, it is seen that since it requires 970.4 B.t.u. to evaporate 1 lb. of water at 212°F. into steam at 212°F., one boiler horsepower represents 34.5 970.4 B.t.u. per hr. or 33,479 B.t.u. of heat energy per hr. Hence, when we compare the boiler horsepower with the ordinary horsepower it is seen that the boiler horsepower represents a unit which is 13.14 times larger than the ordinary horse

power.

In recent

The Myriawatt as a Basis of Boiler Performance. years, due to the tremendous growth in the electrical industry, engineers have recognized the inconsistencies of the boiler horsepower unit and an effort has been made by the national engineering societies to make a more rational standard of rating. As a consequence, the American Institute of Electrical Engineers has proposed that the Myriawatt be adopted as a standard of boiler