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Date of test..

Duration of test, 12 hours.

Boiler, return tubular, 72 in. diameter, 18 ft. long, 62-41⁄2 in. tubes. Kind of coal, Pocahontas; average steam pressure.. 85

Weight of coal consumed

lbs.

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Weight of water apparently evaporated..

II, 100 107,187

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Weight of dry ash returned

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Moisture in the coal..

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9.65 10.61

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Water evaporated into dry steam, from and at 212°.. 117,788
Water evaporated per lb. of coal, actual conditions..
Water evaporated per lb. of coal, from and at 212°.
Water evaporated per lb. of combustible, from and

at 212

Water evaporated per lb. of dry coal, from and at 212°
Water evaporated per hr. per sq. ft. of heating surface
Coal burned per sq. ft. of grate surface per hour
Horse power developed by boiler during test..
Temperature of feed water, average...
Temperature of chimney gases, average.
Square feet of grate surface..

Square feet of heating surface.

Ratio of grate surface to heating surface.

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11.81 10.82 " 6.22 25 284.5

141°

400°

36

1576

43.7

The results obtained will be taken up in their regular order beginning with, first, water evaporated into dry steam from and at 212°. As it may be of benefit to some, a short definition of the meaning of the above expression is here given.

The term "equivalent evaporation," or the evaporation from and at 212°, assumes that the feed water enters the boiler at a temperature of 212° and is evaporated into steam at 212° temperature and at atmospheric pressure. As for instance, if the top man hole plate were left out or some other large opening in the steam space allowed the steam to escape into the atmosphere as fast as it was generated. Owing to the variation in the temperatures of the feed water used in different tests, and also the variation in the steam pressure, it is absolutely necessary that the

results of all tests be brought by computation to the common basis of 212° in order to obtain a just comparison.

The process by which this is done is as follows: Referring to the record of the test it is seen that the steam pressure average was 85 lbs. gauge pressure, or 100 lbs. absolute, and that the temperature of the feed water was 141°. Referring again to Table No. 5, physical properties of steam, it will be seen that in a pound of steam at 100 lbs. absolute pressure there are 1, 181.8 heat units, and in a pound of water at 141° temperature there are 109.9 heat units. It therefore took 1181.8109.9 = 1071.9 heat units to convert one pound of feed water at 141° into steam at 85 lbs. pressure. To convert a pound of water at 212° into steam at atmospheric pressure, and 212° temperature requires 965.7 heat units, and the 1,071.9 heat units would evaporate 1,071.9 ÷ 965.7 = 1.11 lbs. water from and at 212°. The 1.11 is the factor of evaporation for 85 lbs. gauge pressure and 141° temperature of feed water, and by multiplying "water corrected for moisture in the steam" (see record of test), 106, 116 lbs., by 1.11, the weight of water which could have been evaporated into steam from and at 212°, is obtained, which is 117,788 lbs. The factor of evaporation is based upon the steam pressure and the temperature of the feed water. in any test and the formula for ascertaining it is as H - h follows: Factor = in which H 965.7

+

=

total heat in

It

the steam, and h = total heat in the feed water. is used in shortening the process of finding the evaporation from and at 212°, and Table No. 6 gives the factor of evaporation for various pressures and temperatures.

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1.043

1.047

1.027 1.030 1.032 1.035 1.037 1.039 | 1.041
1.039 1.042 1.045 1.047 1.050 1.052 1.054 1.056 | 1.059

212

200

173

164

1.077 1.080 1.083

191 1.049 1.052 1.054 1.057 1.059 1.061 182° 1.058 1.061 1.064 1.066 1.069 1.071 1.067 1.070 1.073 1.076 1.078 1.080 1.085 1.087 1.090

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Second, water evaporated per pound of coal actual conditions = water apparently evaporated divided by coal consumed = 9.65 lbs. No accurate estimate regarding the quality of the coal or the efficiency of the boiler can be made from this figure (9.65 lbs.). It can be used, however, in estimating the cost of fuel for generating the steam; as, for instance, if the boiler is supplying steam to an engine that uses 30 lbs. of steam per horse power per hour, it will require 30+ 9.65 = 3.1 lbs. of coal per horse power per hour; the "actual conditions" under which the boiler is being operated being the pressure of steam required by the engine and the temperature of the feed water.

=

=

Third, water evaporated per pound of coal from and at 212° water evaporated into dry steam from and at 212° divided by coal consumed 10.61 lbs. This figure is the proper one to use in comparing the relative economic values of different varieties of coal tested with the same boiler or boilers.

Fourth, water evaporated per pound of combustible from and at 212° = water evaporated into dry steam from and at 212° divided by weight of combustible = 11.81 lbs. This result is the one to be used for ascertaining the efficiency of the boiler, and the percentage of efficiency is found by dividing the heat absorbed by the boiler per pound of combustible by the heat value of one pound of combustible. average heat value of bituminous and semi-bituminous coals is not far from 15,000 heat units per pound of combustible. In the evaporation of 11.81 pounds of water from and at 212° the heat absorbed was 11.81 x 965.7 = 11,404.9 heat units. The efficiency of e boiler therefore was = 76 per cent.

11,404.9 × 100

15,000

=

The

In like manner to ascertain the efficiency of the boiler and furnace as a whole, the water evaporated from and at 212° per pound of coal is taken. Thus, 10.61 × 965.7 = 10,246 heat units absorbed from each pound of coal. Now assuming that there were 13,500 heat units in each pound of the coal used in the test, the per cent of efficiency of boiler and furnace was 10.246 X 100 = 75.9.

13,500

Fifth, water evaporated per pound of dry coal from and at 212° water evaporated into dry steam from and at 212° divided by coal corrected for moisture. Thus, 117,788 10,878 10. S2 lbs. This result is useful for calculating the results of tests of the same grade

+

=

of coal, but differing in the degree of moisture in each.

Sixth. Boiler horse power. The latest decision of the American Society of Mechanical Engineers (than whom there is no better authority) regarding the horse power of a boiler is as follows: "The unit of commercial horse power developed by a boiler shall be taken as 34%1⁄2 units of evaporation per hour. That is, 34% lbs. of water evaporated per hour from a feed temperature of 212° into steam of the same temperature. This standard is equivalent to 33,317 B. T. U. per hour. It is also practically equivalent to an evaporation of 30 lbs. of water from a feed water temperature of 100° F. into steam of 70 lbs. gauge pressure."

According to this rule the horse power developed by the boiler during the test under consideration = water evaporated into dry steam from and at 212°, 117,788 lbs 12 hrs. + 34.5 = 284.5 horse power.

QUESTIONS

1. What is the primary object of an evaporation test?

2. Name four other important points which can be determined by evaporation tests.

3. In making a test of the efficiency of the boiler and furnace, what precautions should be observed? 4. How is the heating surface of water tube boilers estimated?

5. What length of time should a test be conducted? 6. In case the boiler is fed by an injector, what precautions are necessary?

7. If the steam contains any moisture, what should be done?

8. How is the weight of combustible determined?

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