the various readings by footing up the total and dividing by the number of readings noted. The columns for the water fed to the boiler and the oil fed to the furnace are footed and as in the hourly sheets previously described, the totals from these sheets which are noted on this general log sheet should now check-that is, the total gross should equal the sum of the total tare and total net columns. The reader is to bear in mind that the actual notations to be made in any particular test are not all set down in this general log sheet suggestion, for the information desired and the purpose of the test must in each given case determine these factors. The sheet will, however, serve as a general guide for such matters.

The Plotting of Test Data.-As the test proceeds hour by hour, it is very instructive and helpful to keep a diagrammatic log sheet also. By this means a glance will often reveal certain irregularities that may be righted at their incipiency. Such a log sheet is shown in Fig. 196 and by reference to it the reader will observe how the history of a test may be simply and clearly set forth.

CHAPTER XXXVIII

THE HEAT BALANCE AND BOILER EFFICIENCY

The steaming qualities of a boiler are best set forth by measuring its so-called efficiency. The efficiency of a boiler is the relationship between the heat absorbed per pound of fuel fired and the calorific value of 1 lb. of fuel. Thus although each pound of fuel consumed in steam production is found to have a calorific value of 19,450 B.t.u. in the numerical illustration for this chapter, that portion alone of this heat which is actually represented in the steam itself is of economic value.

In the illustrative test which is made use of throughout this chapter, it will be found that of this 19,450 B.t.u. represented in each pound of oil only 14,076.56 go toward power generation. It is then useful and instructive to analyze the losses in a boiler and see through what channels this heat has been dissipated. The major portion of these losses may be easily computed by means of data taken in the test. Those which cannot be mathematically computed are thrown under the column entitled "Stray Losses," and are made to represent such an amount that the total losses together with the useful heat generated in the boiler represent the heat from 1 lb. of fuel.

Let us then examine the various channels of heat transfer going on in the boiler and see how the details of the heat balance are set forth. In this discussion H, will represent the calorific value of 1 lb. of fuel oil under test.

(a) a. The Total Heat Absorbed by the Boiler.-As has been previously shown, the equivalent evaporation of a boiler per pound of oil represents the number of pounds of water which would be evaporated into steam per pound of oil if the water was at 212°F. and under atmospheric pressure, and this water then converted into dry saturated steam at the same temperature and pressure. It is self-evident then that the total heat absorbed by the boiler for each pound of oil burned in the furnace is equal to the equivalent evaporation multiplied by the heat necessary to convert 1 lb. of water into steam under conditions just mentioned.

This quantity of heat has been found by Marks and Davis to be 970.4 B.t.u., as set forth in previous discussions. Representing this in a formula the total heat H, absorbed by the boiler per pound of fuel is

in which H, is the total heat absorbed by the boiler per pound of dry fuel, M. the equivalent evaporation per pound of oil, and L. the latent heat of evaporation at 212°F., which is 970.4 B.t.u. Hence, if the equivalent evaporation of a boiler is found by test to be 28,225 lb. of water per hour, and if the measurement of oil shows that 1872 lb. of oil have been consumed

b. Heat Absorbed by Boiler for Atomization. In ordinary practice of fuel oil combustion, there are three methods of atomization employed. In the larger power plants the use of steam for atomization purposes, or in other words, the diverting of steam from the boiler into the furnace in order to atomize the oils, seems to have by far the preference. It is proposed to alter the rules of the American Society of Mechanical Engineers so that the heat represented by the steam used in atomization must be subtracted from the total heat absorbed by the boiler in order to compute the net evaporative power of the boiler. Hence to make this computation we must know the number of pounds of steam used in atomization per pound of oil burned. Methods of arriving at this result have been described in Chapter XXXV.

8

Calling M, the pounds of steam used in atomization per pound of fuel burned, H, the total heat per pound of steam so used, and h1 the heat in the entering feed water, and H. the heat absorbed by the boiler per pound of fuel in atomizing the oil, it is evident that

a

(2)

Thus it has been found in the test under description that 0.530 lb. of steam were utilized in atomization per pound of oil. Saturated steam at a temperature of 381.9° was used. From the steam tables such steam is found to have a total heat of 1198.08

B.t.u. The entering feed water was at a temperature of 169.1°F. and has a heat of liquid amounting to 136.87 B.t.u. We find by substitution that the heat absorbed in atomizing the oil is computed as follows:

Ha = 0.530 (1198.98 - 136.87)

= 562.44 B.t.u.

c. Net Heat Absorbed by Boiler for Power Generation.— Since then the heat utilized in atomization must be subtracted from the total heat absorbed by the boiler, to ascertain the net heat H absorbed by the boiler for power generation, we have the following formula:

n

(b) Loss Due to Water in the Fuel.-All fuels contain a certain amount of moisture. It is evident that since it requires considerable heat to convert this moisture into steam and then to send it forth from the chimney in a superheated condition, a definite loss is thereby sustained in boiler operation. This moisture must first be raised to 212°F., then converted into steam, and then heated to the temperature of the outgoing chimney gases. If we let M be the proportion by weight of moisture in the 1 lb. of fuel, to the temperature of the oil entering the burner, to the temperature of the escaping gases, and Hm the loss due to moisture in the fuel per pound of fuel burned, we may write at once an equation representing this loss.

Thus

w

Hm Mw [212 to 970.4 +0.47 (t, 212)]

=

(4)

The reasons for this formula are seen by inspection. To raise each pound of moisture from to to 212° F. would require as many B.t.u. as the raise in temperature, in other words (212 – t。) B.t.u. Again, to evaporate each pound would require 970.4 B.t.u., and as 0.47 of a B.t.u. are required to superheat 1 lb. of steam 1° in temperature at atmospheric pressure, each pound of steam superheated to the temperature of the outgoing chimney gases would require 0.47 (t - 212) B.t.u. Therefore, the total heat required for M pounds would be as indicated in the formula above by summing up these separate components.

го

Thus in the test under consideration, let us assume that the fuel contains 1 per cent. of moisture; that its entering temperature

is 96°F., and that the temperature of the escaping gases is 400°F. Hence

0.01 [212 -96 + 970.4 + 0.47 (400 - 212)]

B.t.u.

= 11.67

(c) Loss Due to Water Formed by Burning Hydrogen.In the chapter on chimney gas analysis, it was seen that the Orsat Apparatus is so constructed that the vapor or superheated steam formed by the burning of the hydrogen content in the fuel is condensed into water upon entering the burette; hence the Orsat analysis indicates only dry flue gases and takes no account of the percentage of steam actually present in these gases. It is seen then that the moisture formed by the burning of hydrogen must also create a loss as it journeys upward through the boiler. Assuming H to be the heat lost due to the moisture formed by the burning of hydrogen by following identically similar processes of reasoning just employed in the considerations of the loss due to the moisture in the fuel, we find that each pound of moisture formed by the burning of hydrogen requires

[212 to 970.4 + 0.47 (t, - 212)] B.t.u.

From the principles of chemistry each pound of hydrogen combines with 8 lb. of oxygen, thereby forming 9 lb. of water or steam. This relationship gives us a ready means of computing the weight of water vapor formed by the burning of hydrogen, although the Orsat analysis failed to do so. Assuming M to be the proportion by weight of hydrogen per pound of fuel oil burned, we have (5)

=

9M [212 to + 970.4 + 0.47 (t, — 212)]

By referring to the test data, we find that the fuel analysis shows 0.11 lb. of hydrogen per pound of fuel, that the temperature of entering air is 84° and the temperature of the escaping gases 400°, therefore

(d) Loss Due to Heat Carried away by Dry Gases. From the Orsat analysis, as was seen in Chapter XXXIII on the Computation of Combustion Data, the pounds of dry gas passing up the chimney per pound of fuel burned may be easily computed by means of several different formulas. It is found by experiment