Triple-expansion Engines-Condensing-Steam. 12 10 5% 8% 13% 12 370 14 277 246 6% 10% 16% T212 19 16 9 14% 22% 18 Jacketed. Horse-power Horse-power Horse-power Horse-power when Cut when Cut when Cut 35 42 48 44 53 59 57 72 84 45 53 62 56 67 76 73 92 107 67 79 92 83 100 112 108 137 159 87 103 120 109 131 147 141 180 208 222 125 148 172 156 187 211 203 257 299 185 154 183 212 192 231 260 250 317 368 158 206 245 284 258 310 348 335 426 494 138 277 329 381 346 415 467 450 571 663 32 120 357 424 491 446 535 602 580 736 854 34 112 458 543 629 572 686 772 744 944 1095 42 93 670 796 922 838 1006 1131 1089 1383 1605 80 877 1041 1206 1096 1316 1480 1424 1808 2099 10 16 25 20 112 18 281 24 13 22 3312 28 15 24% 38 43 17 27 20 33 52 23 38 60 48 Mean effec. press.,lbs. 16 19 22 20 24 27 26 33 38.3 when Cutting off at 34 Stroke in First Cylin der. 120 140 160 lbs. lbs. lbs. 81 97 110 104 123 140 154 183 208 201 239 272 289 343 390 356 423 481 477 568 645 640 761 865 825 981 1115 1058 1258 1430 1551 1844 2096 2028 2411 2740 37 44 50 No. of expansions.... Type of Engine to be used where Exhaust-steam is needed for Heating.-In many factories more or less of the steam exhausted from the engines is utilized for boiling, drying, heating, etc. Where all the exhaust-steam is so used the question of economical use of steam in the engine itself is eliminated, and the high-pressure simple engine is entirely suitable. Where only part of the exhaust-steam is used, and the quantity so used varies at different times, the question of adopting a simple, a condensing, or a compound engine becomes more complex. This problem is treated by C. T. Main in Trans. A. S. M. E., vol. x. p. 48. He shows that the ratios of the volumes of the cylinders in compound engines should vary according to the amount of exhaust-steam that can be used for heating. A case is given in which three different pressures of steam are required or could be used, as in a worsted dye-house: the high or boiler pressure for the engine, an intermediate pressure for crabbing, and low-pressure for boiling, drying, etc. If it did not make too much complication of parts in the engine, the boiler-pressure might be used in the high-pressure cylinder, exhausting into a receiver from which steam could be taken for running small engines and crabbing, the steam remaining in the receiver passing into the intermediate cylinder and expanded there to from 5 to 10 lbs. above the atmosphere and exhausted into a second receiver. From this receiver is drawn the low-pressure steam needed for drying, boiling, warming mills, etc., the steam remaining in receiver passing into the condensing cylinder. Comparison of the Economy of Compound and Singlecylinder Corliss Condensing Engines, each expanding about Sixteen Times. (D. S. Jacobus, Trans. A. S. M. E., xii. 943.) The engines used in obtaining comparative results are located at Stations I. and II. of the Pawtucket Water Co. The tests show that the compound engine is about 30% more economical than the single-cylinder engine. The dimensions of the two engines are as follows: Single 20" x 48"; compound 15" and 30" x 30". The steam used per horse-power per hour was: single 20.35 lbs., compound 13.73 lbs. Both of the engines are steam-jacketed, practically on the barrels only with steam at full boiler-pressure, viz. single 106.3 lbs., compound 127.5 lbs. The steam-pressure in the case of the compound engine is 127 lbs., or 21 lbs. higher than for the single engine. If the steam-pressure be raised this amount in the case of the single engine, and the indicator-cards be increased accordingly, the consumption for the single-cylinder engine would be 19.97 lbs. per hour per horse-power. Two-cylinder vs. Three-cylinder Compound Engine.— A Wheelock triple-expansion engine, built for the Merrick Thread Co., Holyoke, Mass., is constructed so that the intermediate cylinder may be cut out of the circuit and the high-pressure and low-pressure cylinders run as a two-cylinder compound, using the same conditions of initial steam-pressure and load. The diameters of the cylinders are 12, 16, and 24 inches, the stroke of the first two being 36 in. and that of the low-pressure cylinder 48 in. The results of a test reported by S. M. Green and G. I. Rockwood, Trans. A. S. M. E., vol. xiii. 647, are as follows: In lbs. of dry steam used per I.H.P. per hour, 12 and 24 in. cylinders only used, two tests 13.06 and 12.76 lbs., average 12.91. All three cylinders used, two tests 12.67 and 12.90 lbs., average 12.79. The difference is only 1%, and would indicate that more than two cylinders are unnecessary in a compound engine, but it is pointed out by Prof. Jacobus, that the conditions of the test were especially favorable for the two-cylinder engine, and not relatively so favorable for the three cylinders. The steam-pressure was 142 lbs. and the number of expansions about 25. (See also discussion on the Rockwood type of engine, Trans. A. S. M. E., vol. xvi.) Effect of Water contained in Steam on the Efficiency of the Steam-engine. (From a lecture by Walter C. Kerr, before the Franklin Institute, 1891.)-Standard writers make little mention of the effect of entrained moisture on the expansive properties of steam, but by common cousent rather than any demonstration they seem to agree that moisture produces an ill effect simply to the percentage amount of its presence. That is, 5% moisture will increase the water rate of an engine 5%. Experiments reported in 1893 by R. C. Carpenter and L. S. Marks, Trans. A. S. M. E., xv., in which water in varying quantity was introduced into the steam-pipe, causing the quality of the steam to range from 99% to 58% dry, showed that throughout the range of qualities used the consumption of dry steam per indicated horse-power per hour remains practically constant, and indicated that the water was an inert quantity, doing neither good nor harm. Relative Commercial Economy of Best Modern Types of Compound and Triple-expansion Engines. (J. E. Denton, American Machinist, Dec. 17, 1891)-The following table and deductions show the relative commercial economy of the compound and triple type for the best stationary practice in steam plants of 500 indicated horse-power. The table is based on the tests of Prof. Schröter, of Munich, of engines built at Augsburg, and those of Geo. H. Barrus on the best plants of America, and of detailed estimates of cost obtained from several first-class builders. The figures in the first column represent the best recorded performance (1891), and those in the second column the probable reliable performance. The table on the next page shows the total annual cost of operation, with coal at $4.00 per ton, the plant running 300 days in the year, for 10 hours and for 24 hours per day. Increased cost of triple-expansion plant per horse-power, including boilers, chimney, heaters, foundations, piping and erection. 7.50 Taking the total cost of the plants at $33.50, $36.50 and $41 per horsepower respectively, the figures in the table imply that the total annual saving is as follows for coal at $4 per ton: I. A compound 500 horse-power plant costs $18,250, and saves about $1630 for 10 hours' service, and $4885 for 24 hours' service, per year over a single plant costing $16,750. That is, the compound saves its extra cost in 10-hou service in about one year, or in 24-hour service in four months. 2. A triple 500 horse-power plant costs $20,500, and saves about $114 per year in 10-hour service, or $826 in 24-hour service, over a compound plant, thereby saving its extra cost in 10-hour service in about 1934 years, or in 24-hour service in about 234 years. Highest Economy of Pumping Engines, 1900. (Eng. News Sept. 27, 1900.) *These figures do not include the heat saved by the economizer; including this they are 163,912,800; 187.8; 22.58. The Nordberg engine had a series of feed-water heaters taking steam respectively from the exhaust, from the low-pressure cylinder, and from the third, second, and first receivers. The feed-water was thereby treated successively to 105°, 136°, 193°, 260°, and 311° F. The coal consumption of the Chestnut Hill engine was 1.062 lbs. per I.H.P. per hour, including the coal used by the fan, stoker, and economizer engines. This is the lowest figure yet recorded. Steam Consumption of Sulzer Compound and Tripleexpansion Engines with Superheated Steam. The figures on the next page were furnished to the author (Aug., 1902) by Sulzer Bros., Winterthur, Switzerland. They are the results of official tests by Prof. Schröter of Munich, Prof. Weber of Zurich, and other eminent engineers. NOTES.-A, B, C, D, tandem engines at electrical stations A, Frankfort a/M.; B, Zurich; C. Mannheim; D, Mayence. E. F. tandem engine with intermediate superheater: E, Metallwarenfabrik, Geislingen, Würtemberg; F, Neue Baumwoll-Spinnerei, Hof, Bavaria. G, H, engines at electrical stations, Berlin G, Moabit station, horizontal 4-cyl.; H, Louisenstrasse, 4-cyl. vertical. COMPOUND ENGINES. Steam Consump- Efficiency tion in Pounds. Dimensions A 1500 30.5 and 85 130 356 26.4 49.2X59.1 1800 850 13.3 14.90 21.30 0.895 0.851 132 428 26.4 842 12.05 13.52 19.48 0.891 0.842 122 482 26.6 1719 12.42 13.24 18.72 0.939 0.903 B 1050 26.8 and 100 108 455 26.8 1167 13.10 13.77 19.72 0.951 0.004 to 43.3X51.2 83 136 357 28 481 13.00 14.68 21.30 0.886 0.830 750 13.10 14.14 20.35 0.926 0.877 1078 14.10 14.95 21.30 0.932 0.892 515 11.32 12.70 18.69 0.894 0.824 132 533 27.8 788 11.52 12.38 17.90 0.931 0.875 134 545 27.2 1100 11.88 12.50 17.92 0.951 0.902 86130 358 28.2 1076 14.10 14.82 21.25 0.951 0.902 129 358 28 1316 14.50 15.10 21.55 0.9600.915 132 496 28.31071 11.73 12.33 17.70 0.951 0.903 1021 15.37 16.30 23.40 0.943 0.893 do., non-cond'g 136 527 Relative Economy of Compound Non-condensing Engines under Variable Loads.-F. M. Rites, in a paper on the Steam Distribution in a Form of Single-acting Engine (Trans. A. S. M. E., xiii. 537). discusses an engine designed to meet the following problem: Given an extreme range of conditions as to load or steam-pressure, either or both, to fluctuate together or apart, violently or with easy gradations, to construct an engine whose economical performance should be as good as though the engine were specially designed for a momentary condition-the adjustment to be complete and automatic. In the ordinary non-condensing compound engine with light loads the high-pressure cylinder is frequently forced to supply all the power and in addition drag along with it the low-pressure piston, whose cylinder indicates negative work. Mr. Rites shows the peculiar value of a receiver of predetermined volume which acts as a clearance chamber for compression in the high-pressure cylinder. The Westinghouse compound single-acting engine is designed upon this principle. The following results of tests of one of these engines rated at 175 H.P. for most economical load are given : WATER RATES UNDER VARYING LOADS, LBS. PER H.P. PER HOUR. Horse-power.. ......... .... 210 170 140 115 100 80 50 22.6 21.9 22.2 22.2 22.4 24.6 28.8 18.4 18.1 18.2 18.2 18.3 18.3 20.4 Efficiency of Non-condensing Compound Engines. (W. Lee Church, Am. Mach., Nov. 19, 1891.)-The compound engine, non-condensing, at its best performance will exhaust from the low-pressure cylin der at a pressure 2 to 6 pounds above atmosphere. Such an engine will be limited in its economy to a very short range of power, for the reason that its valve-motion will not permit of any great increase beyond its rated power, and any material decrease below its rated power at once brings the expansion curve in the low-pressure cylinder below atmosphere. In other words, decrease of load tells upon the compound engine somewhat sooner, and much more severely, than upon the non-compound engine. The loss commences the moment the expansion line crosses a line parallel to the atmospheric line, and at a distance above it representing the mean effective pressure necessary to carry the frictional load of the engine. When expansion falls to this point the low-pressure cylinder becomes an air-pump over more or less of its stroke, the power to drive which must come from the high-pressure cylinder alone. Under the light loads common in many industries the low-pressure cylinder is thus a positive resistance for the greater portion of its stroke. A careful study of this problem revealed the functions of a fixed intermediate clearance, always in communication with the high-pressure cylinder, and having a volume bearing the same ratio to that of the high-pressure cylinder that the high-pressure cylinder bears to the low-pressure. Engines laid down on these lines have fully confirmed the judgment of the designers. The effect of this constant clearance is to supply sufficient steam to the low-pressure cylinder under light loads to hold its expansion curve up to atmosphere, and at the same time leave a sufficient clearance volume in the high-pressure cylinder to permit of governing the engine on its compression under light loads. Economy of Engines under Varying Loads. (From Prof. W. C. Unwin's lecture before the Society of Arts, London, 1892.)-The general result of numerous trials with large engines was that with a constant load an indicated horse-power should be obtained with a consumption of 11⁄2 pounds of coal per indicated horse-power for a condensing engine, and 134 pounds for a non-condensing engine, figures which correspond to about 134 pounds to 2 pounds of coal per effective horse-power. It was much more difficult to ascertain the consumption of coal in ordinary every-day work, but such facts as were known showed it was more than on trial. In electric-lighting stations the engines work under a very fluctuating load, and the results are far more unfavorable. An excellent Willans noncondensing engine, which on full-load trials worked with under 2 pounds per effective horse-power hour, in the ordinary daily working of the station used 71⁄2 pounds per effective H.P. hour in 1886, which was reduced to 4.3 pounds in 1890 and 3.8 pounds in 1891. Probably in very few cases were the engines at electric-light stations working under a consumption of 41⁄2 pounds per effective H.P. hour. In the case of small isolated motors working with a fluctuating load, still more extravagant results were obtained. |