TempTeol Deg. Fahr 50 бо 70 80 STEAM PRESSURE BY GAUCE. - 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260, 270 280 290 300 Example: Water evaporated from and at 212° per pound of combustible, 7.48 pounds. Per cent of refuse in coal, 6.71. Per cent of moisture in coal (lignite), 39.76. Cost of coal per ton, delivered, $1.70. Find cost of evaporating 1,000 pounds of water. Solution: To find weight of combustible required to evaporate 1,000 pounds of water, divide 1,000 by 7.48. 1,000 7.48 133.69 pounds combustible to evaporate 1,000 pounds of water from and at 212°. = 53.53. Total per cent moisture and refuse in coal, 46.47. Per cent combustible in coal, 100 - 46.47 Coal required to furnish 133.69 pounds combustible, 133.69.5353249.7 pounds. Cost of 249.7 pounds coal at $1.70 a ton, 249.7 $0.212. $1.70 2,000 Hence it cost 21.2 cents to evaporate 1,000 pounds of water from and at 212° F. Heating Value of Coal and Efficiency of Boilers. The total amount of heat in a pound of coal is measured by a coal calorimeter. Table VII gives the heating value of a number of different grades of coal. The number of heat units absorbed in the water in the boiler for a pound of coal can be found from the evaporative test. This number divided by the number of heat units in a pound of the coal used gives the per cent of combined efficiency of the boilers, furnaces and grates. Examples: 1. A test shows that 1 pound of coal as fired evaporates 4 pounds of water, and it requires 1,024 B. t. u. of heat to raise 1 pound of water from the temperature of the feed-water to steam at boiler pressure. The coal contains 6,029 B. t. u. per pound. What is the combined efficiency of boiler, furnace and grates? Solution: 1,024 X 4 4,096 B. t. u. absorbed in water in boiler per pound of coal burned. 4,096 6,029 0.68. Hence efficiency is 68 per cent; that is, the water absorbs 68 per cent of the heat that is in the coal. 2. If the coal used in a test has 11,290 B. t. u. per pound, and 1 pound of coal evaporates 7.3 pounds of water from and at 212°, and the temperature of the feed-water is 200°, what is the efficiency of boilers, furnaces and grates? 12 Hint: Heat of steam at 212° from 200° is 965.7 + 977.7 B. t. u. This result is higher than can be obtained in practice. Answer, 86.5 per cent. = Comparative Value of Fuels. The value of fuel depends on the number of heat units it contains. For example: North Dakota lignite coal was purchased at $1.70 a ton, delivered. In another plant coal was purchased at $2.15 a ton. The first coal contained only 6,029 B. t. u. per pound. The second contained 11,290 B. t. u. per pound. Which coal was the cheaper? It is clear that the second coal contained nearly twice as many heat units per pound as the first, and the cost was less in proportion to amount of heat. The first coal supplied 12,058,000 heat units per ton -12,058,000 heat units for $1.70. The second coal supplied 22,580,000 heat units per ton, or this amount of 11,290 heat for $2.15. If we take X $1.70 we get $3.18; 6,029 that is, if we buy the first coal at $1.70 we pay $3.18 for the same amount of heat that we get in a ton of the second coal. We get as much heat for $2.15 when we buy the second coal as we get for $3.18 when we buy the first coal. Example: Which is cheaper, Pocahontas coal, 15,094 B. t. u. per pound, at $2.90 per ton, or Pennsylvania semibituminous coal, 13,155 B. t. u. per pound, at $2.65 per ton? Answer, Pocahontas is cheaper. Cost of Power. We give here some practical examples in finding cost of power, which do not require the evaporative test. Q. A plant generates electricity at 220 volts, 700 amperes. How many watts and how many kilowatts of power does it give out?-Answer, 700 x 220 154,000 watts 154 K. W. Q. This plant in a 30-day run consumes 277.2 tons of coal (2,000 pounds to the ton). How many pounds of coal does it consume in 30 days? — Answer, 554,400 pounds. Q. How many pounds in one day?- Answer, 18,480 pounds. Q. How many pounds in one hour? Answer, 770 pounds per hour. Q. How many pounds per kilowatt hour?- Answer, since the plant generates 154 K. W., it will give out 154 kilowatt hours in one hour. Dividing 770 by 154, we get 5 pounds of coal per K. W. hour. Q. What is the cost of the coal burned in 30 days at $3 per ton? Answer, $831.60. Q. 9,000 cubic feet of water are added to the system in the 30 days. How many gallons of water does this equal? (1 cubic foot equals 7.5 gallons.) - Answer, 67,500 gallons. Q. What was the cost of the water at 12 cents per 1,000 gallons? Answer, $8.10. - Q. Cost of oil, waste and grease was $42; labor account, $540; interest, depreciation and taxes, $210; |