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-hs)
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:

Rating in boiler horsepower
Rating in Myriawatts

=

34,150
33,479

(2)

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

FIG. 48.-Piping in boiler setting where superheat tempera

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 generation, it is necessary that some 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.

The Standard that Has Been Adopted. To avoid inconsistencies and to develop some rational method of comparison, engineers have found it convenient and accurate to reduce all evaporative quantities of a boiler to a definite standard. In order to follow out this standardized comparison, all steam generating performances of boilers read as if the boiler took its feed water at 212°F. and atmospheric pressure, and converted it into dry saturated steam at 212°F. and atmospheric pressure, as set forth in the standard definition of the boiler horepower in the last chapter. It is clearly evident that no such theoretical

FIG. 49.-Platform scales and tanks for water measurement.

The boiler immediately to the right of the platform scales is under test. The tank below the platform scales into which the water is emptied after being weighed, is utilized to furnish all water for the boiler during the test. At the beginning of the test a hooked gage registers the height of the water in this tank, and at each hourly period thereafter sufficient water is weighed and emptied into it from the tanks above to maintain this exact level. By means of these data, properly taken, the factor of evaporation and the boiler horse-power are easily computed.

boiler has ever existed, yet this standard of comparison is found very convenient. Thus in any case of boiler performance, if M. represents such an equivalent or comparative standardized evaporation in lbs. of water per lb. of fuel, and M, the lb. of water actually evaporated in the boiler under conditions of test, we may now invent a factor to be known as the factor of evaporation, Fe, whereby such performances may be readily reduced:

w

(1)

In the same way, the equivalent evaporation of water per hour may be computed from the formula

wherein Men and Much represent hourly conditions of evaporation.

Let us next analyze the factor of evaporation and see how we may actually compute its value for any given case. We have previously found that in the operation of the boiler, steam appears in three different conditions or qualities, namely in what is known as dry saturated, wet steam, or superheated steam. Let us then consider the valuation of the factor of evaporation for these three distinct instances.

Dry Saturated Steam.-In the case of dry saturated steam, the water enters the boiler already possessing a heat of liquid h corresponding to its entrance temperature which may be readily found in the steam tables. This water is next converted into dry saturated steam which has a total heat (H.) corresponding to the pressure at which the evaporation takes place. Consequently the actual heat which has been transferred from the boiler shell to the water is (H. h) heat units. But to evaporate one pound of water at 212°F. into dry steam at 212°F. requires 970.4 heat units. Hence if Mw pounds of water are evaporated under test conditions, the number of pounds M. under standardized condi(He - hs) 970.4

tions would evidently be Mw

Therefore for dry sat

urated steam

F. (dry saturated steam)

=

(H. - hs)
970.4

(3)

Thus in the case of a boiler which takes its feed water at 101.8° F. and converts it into dry saturated steam at 180 lb. pressure per square inch, from the steam tables we find that He is 1196.4 and h, is 69.8, hence the factor of evaporation is

970.4

Wet Steam. In the case of wet steam all of the water entering the boiler is not converted into steam. As a consequence a certain portion of heat (h. — hƒ) is required to raise the temperature of the water from entrance temperature t, to the temperature of evaporation to and if only X. parts of a lb. are then evaporated into steam, only X.L. B.t.u. are required to accomplish this result. Hence, the total heat required per lb. of water so evaporated is (he + XeLehs).

e

As a consequence the factor of evaporation in this case may from similar reasoning be expressed by the formula

(he + XeLe - hƒ)

(4)