Engineering Bulletin No 1: Boiler and Furnace Testing | Page 6

that only a little more than half of the heat in the coal is usefully employed in making steam.
The chart shown in figure 3 is given to save the work of figuring the efficiency. If the equivalent evaporation per pound of coal is calculated and the heating value of the coal is known, the boiler efficiency may be found directly from the chart. At the left-hand side locate the point corresponding to the equivalent evaporation and at the bottom locate the point corresponding to the heating value of the coal. Follow the horizontal and vertical lines from these two points until they cross, and note the diagonal line that is nearest to the crossing point. The figures marked on the diagonal line indicate the boiler efficiency.
Take the case just worked out, for example. The equivalent evaporation is 7.5 pounds and the heating value of the fuel is 13,500 B. t. u. At the left of the chart locate the point 7.5 midway between 7 and 8 and at the bottom locate the point 13,500 midway between 13,000 and 14,000. Then follow the horizontal and vertical lines from these two points until they cross, as indicated by the dotted lines. The crossing point lies on the diagonal corresponding to 54, and so the efficiency is 54 per cent.
BOILER HORSEPOWER OR CAPACITY.
The capacity of a boiler is usually stated in boiler horsepower. A boiler horsepower means the evaporation of 34.5 pounds of water per hour from and at 212�� F. Therefore, to find the boiler horsepower developed during a test, calculate the evaporation from and at 212�� F. per hour and divide it by 34.5.
Take the test previously mentioned, for example. The evaporation from and at 212�� F. or the equivalent evaporation, was 7.5 pounds of water per pound of coal. The weight of coal burned per hour was 5,000 �� 10 = 500 pounds. Then the equivalent evaporation was 7.5 �� 500 = 3,750 pounds per hour. According to the foregoing definition of a boiler horsepower, then--
3,750 Boiler horsepower = ----- = 109. 34.5
The "rated horsepower" of a boiler, or the "builders' rating," is the number of square feet of heating surface in the boiler divided by a number. In the case of stationary boilers this number is 10 or 12, but 10 is very commonly taken as the amount of heating surface per horsepower. Assuming this value and assuming further that the boiler tested had 1,500 square feet of heating surface, its rated horsepower would be 1,500 �� 10 = 150 boiler horsepower.
It is often desirable to know what per cent of the rated capacity is developed in a test. This is found by dividing the horsepower developed during the test by the builders' rating. In the case of the boiler tested, 109 horsepower was developed. The percentage of rated capacity developed, therefore, was 109 �� 150 = 0.73, or 73 per cent.
HEATING SURFACE.
The heating surface of a boiler is the surface of metal exposed to the fire or hot gases on one side and to water on the other side. Thus, the internal surface of the tubes of a fire-tube boiler is the heating surface of the tubes, but the outside surface of the tubes of a water-tube boiler is the heating surface of those tubes. In addition to the tubes, all other surfaces which have hot gases on one side and water on the other must be taken into account. For instance, in a fire-tube boiler from one-half to two-thirds of the shell (depending on how the boiler is set) acts as heating surface. In addition to this, the surface presented by both heads, below the water level, has to be computed. The heating surface of each head is equal to two-thirds its area minus the total area of the holes cut away to receive the tubes.
COST OF EVAPORATION.
The cost of evaporation is usually stated as the cost of fuel required to evaporate 1,000 pounds of water from and at 212�� F. To find it, multiply the price of coal per ton by 1,000 and divide the result by the product of the equivalent evaporation per pound of coal and the number of pounds in a ton.
Suppose that the cost of the coal used in the foregoing test was $3.60 per ton of 2,000 pounds. The equivalent evaporation per pound of coal was 7.5 pounds. Therefore the cost of evaporating 1,000 pounds of water from 180�� F. to steam at 100-pound gage, is--
$3.60 �� 1,000 ------------- = $0.24, or 24 cents. 7.5 �� 2,000
TABLE OF TEST RESULTS.
After the test has been made and properly worked up, as heretofore described, collect all the results of the test on one sheet, so that they can be kept in convenient form for reference and for comparison with later tests. A brief