CHAPTER LXXIII STEAM CONSUMPTION Q. What is the relation between steam consumption per hour per horse-power and train speed? A. It increases with the speed. Prof. Goss's tests show it as a minimum at 35 miles an hour (188 revolutions or 752 feet piston speed a minute on the engine tested by him). Q. What is the relation between steam consumption per hour per horse-power and point of cut-off? A. The former varies with the point of cut-off; and according to Goss is not minimum at shortest cut-off, except at 55 miles an hour. (296 revolutions or 1,184 feet piston speed a minute.) Q. How high does the back pressure sometimes reach in locomotives? A. 15 lbs. above atmosphere. Q. What effect has this on the value of or gain by expansion? A. It occasions a loss. Q. How can such a loss be overcome? A. By increase in initial admission pressure. Q. What prevents utilization of this method? A. The difference between the increased boiler weight and cost and the permissible decrease in weight and cost of cylinders and some moving parts. Q. With 240 lbs. boiler pressure and open throttle, what would be the minimum consumption of saturated steam? A. Goss found it to be 24 lbs. per H. P. hour. Q. What would be the maximum? A. 26.3 lbs. Q. With 120 lbs. gage pressure what should be the minimum and maximum steam consumption per H. P. hour? A. 27.5 and 33.8 lbs. respectively. Q. Where did Goss find the most effective cut-off for the 120-lb. pressure? A. At the eighth notch, or 35 per cent of the stroke. Q. What did he find the best notch with 200 lbs.? Q. For pressures above 160 lbs., what speed was found the most economical of steam? A. Forty miles an hour. Q. Is the increase in saving proportionate to the increase in pressure? A. No, the successive percentages of saving decrease. Q. From the mechanical point of view, what are the disadvantages of high initial pressures and expansion rates? A. They cause such varying pressures on the piston as to produce wheel slip; also racking of the moving parts. Q. In how many ways can the efficiency of a steam engine be expressed? A. In steam per hour, steam per horse-power hour, thermal units per minute, thermal efficiency percentage, mechanical efficiency percentage, efficiency ratio or potential efficiency percentage, cylinder efficiency, commercial efficiency, and duty. Q. What is the difference between actual and indicated steam consumption? A. One is the actual steam represented by the weight of water evaporated, the other that calculated from the indicator card. Q. How is the heat consumption expressed? A. By the above the maximum temperature at which the condensation could be returned to the boiler (inapplicable to locomotives). Q. What is the thermal efficiency? A. The proportion of heat turned into useful work to that supplied (measured above the heat in the exhaust). Q. What is the mechanical efficiency? A. The ratio of the actual horse-power developed to the indicated. Q. What is the efficiency ratio? A. The ratio of the thermal efficiency to that of an ideally perfect engine. Q. What is the effect of increased piston speed on the water rate? A. To decrease it. On one engine the steam used ran down from 18 lbs. per i. h. p. hr. at 200 ft. per minute to 141⁄2 lbs. at 850 ft.; in another, from 21 lbs. at 200 ft. to 17 lbs. at 850 ft. Q. What are the fuel consumptions in the various classes of service? A. 0.169 lb. of coal per ton mile in freight, 0.194 in express passenger, and 0.335 in express local passenger service. (N. Y., N. H. & H. R. R.) Q. How many pounds of coal per indicated H. P. for passenger service? A. Average 4.37 for express and 4.61 for local. (This includes all idle engine time in the 24 hours.) CHAPTER LXXIV FUELS* Q. State the relationship between heating value and fixed carbon percentage of coal. A. According to Mahler, it is as follows: Q. Give an idea of the composition of the various fuels used in locomotive furnaces. A. Kent's classification is according to the percentages of dry combustibles, as follows: Q. What are the advantages of fuel oil? A. Reduction of hostler service; elimination of ash-pit * See also "Combustion" and "Firing." service; pressure more readily controlled; fuel waste through grate and stack diminished; smoke lessened; cinders done away with; no danger of setting fire along right of way. Q. What is lignite? A. An early formation of coal, containing more than 50 per cent of volatile matter in the combustible portions; coming in age of formation between peat and bituminous coal, which latter has 25 to 50 per cent of volatile matter in its combustible portions (that is, less ash and moisture). Q. Give the average weights per cubic foot of various solid fuels. A. Large bituminous coal, 52 lbs.; run of mine bituminous, 54; large anthracite, 54; buckwheat anthracite, 52. Q. Give the cubic contents of a short ton of coal. A. Bituminous, 38.4 cubic feet; anthracite, 37. Q. Of a long ton. A. Bituminous, 43 cubic feet; anthracite, 41.5. Q. Give the weight and volume of fuel oil (crude petroleum). |