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 horsepower.

The Myriawatt as a Basis of Boiler Performance. In recent 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

rating instead of the Bl. h.p. A Myriawatt is the power equivalent of 10,000 watts or 10 kw. which converted into heat units become 34,150 B.t.u. per hr. Although it is still to be remembered that the Myriawatt does not yet make output and input of electrical units expressible in like quantities, since output is usually expressed in kilowatts, still the factor of 10 furnishes a basis. readily convertible and makes possible a change in units without materially upsetting the old boiler h.p. range of capacity.

If, then, a boiler evaporates M pounds of steam per hour and the total heat of each pound of steam so evaporated be H and the heat of liquid represented in the feed water be h, then the rating of a boiler in Myriawatts is evidently

Myriawatts

=

M(H-ht)
34,150

(1)

Relationship of Boiler Horsepower and Myriawatts.-Similarly, since one boiler horsepower is equivalent to heat absorption of 33,479 B.t.u. per hour and a myriawatt to 34,150 B.t.u. per hour, then we may convert a rating in Myriawatts to a rating in boiler horsepower or vice versa by the relationship:

The Builder's Rating. In the commercial evolution of the steam boiler there has grown up a method of rating boilers by "rule of thumb" process. It is evident that the area of the steam generating surface of the boiler actually exposed to the heated gases of the furnace has something to do with the capacity of the boiler. For different designs of boilers, however, the particular factor to be applied varies widely. It has become of common acceptance, however, that 10 sq. ft. of boiler surface exposed to the furnace heat shall be considered on this rule of thumb comparison as equivalent to one boiler horsepower. Hence to compute the builder's rating of a boiler we must compute the area in square feet of the surface exposed to the furnace. By dividing this area A by ten we arrive at the Builder's Rating:

.. Bl. h.p. (Builder's rating) =

A
10

(3)

As a detailed illustration, let us take the case of a Parker boiler installed at the Fruitvale Power Station of the Southern Pacific Company in Oakland, California.

This boiler is made up of three banks of tubes with two drums above, half exposed. In detail we compute as follows:

Hence, we have that the builder's rating of this boiler should be

To Compute Actual Boiler Rating. Since it is seen from the fundamental definition of the boiler horsepower that the standard reference boiler generates its steam from water at 212°F. into steam at 212°F., we must next develop a factor by which we can reduce ordinary boiler performances of high temperatures and pressures to this fictitious standard before we can proceed further. The next chapter will be devoted to this consideration.

CHAPTER IX

EQUIVALENT EVAPORATION AND FACTOR OF EVAPORATION IN FUEL OIL PRACTICE

tures are taken.

N the previous chapter it was seen that as the fundamental definition of the boiler horsepower is based upon a fictitious boiler that receives its feed water at 212°F. and then evaporates it into dry saturated steam at 212°F. and atmospheric pressure, we must now develop some factor by which we can reduce boiler performances as actually met with in practice to this fictitious standard.

In order also to compare the steaming qualities of two different boilers or indeed to compare the same boiler under different conditions of water supply and steam

FIG. 48.-Piping in boiler set- generation, it is necessary that some ting where superheat tempera- standard of comparison be adopted. Thus a boiler under its normal condition of operation may be found to evaporate 13.61 lb. of water per lb. of oil fired per hour when taking its feed water at 169.1°F. and converting it into superheated steam at a temperature of 527°F. and a pressure of 185.3 gage. On the other hand, the identical boiler, when steaming under overload conditions of a feedwater temperature of 174.1°F., a superheat temperature of 536.9°F. and gage pressure of 194.1 lb. per sq. in. may be found to evaporate only 13.17 lb. of water per lb. of oil fired, even though the same quality of oil be used in each instance. It is evident then from sight that to compare these two evaporative quantities without taking account of the actual heat transferred from the fuel to the steam in the boiler would be a possible source of error.