September 17, 1915. THE COLLIERY GUARDIAN. 571 meters by which consumers pay for supply from cor- porations or companies. This instrument may be used as an alternative to the chart recorder, but its record is less instructive; it is very useful, however, for record- ing the total volume of air delivered by a compressor during any given period. These meters may also be furnished' with automatic compensation for pressure and temperature. A combination may be made in one meter of the graphic recording and the integrating arrangements, and the further addition of a graphically recording pressure gauge may be made. Measurement, the Key to Efficiency.—It is suggested that the minimum requirements in respect of meters for a colliery are represented by the following :—A recording meter should be provided for the measurement of the X H) 'B I Fig. 1. Curve, Showing the. Volume of Free Air Gompresslo Plh Minute Per Brake Horse Power at Various Gauge Pressures so " 4d' 50 60 TO 80 90 100 Lbs Per o" Volume of free air per minute compressed to various pressures. Curve A is based on guaranteed performances of compressors working at full load, and under test condi- tions. Curve B is based on the assumption that the average performances of compressors as in general service, in their existing condition, and under varying loads, is 17 per cent, less than is shown by curve A. total volume of air delivered by each compressor; the pipe line serving each underground district should have a chart-recording meter, or provision for readily attach- ing such an instrument; and at individual air motors and at gate-end valves for coal cutters, provision should be made for connecting at any time a rate-of-fiow indicator. With this modest equipment, which is neither unduly expensive nor difficult to manage, the compara- tive efficiency from day to day or week to week can be kept under notice and control. Measurement is the key— the only key—to efficiency in the use of compressed air power; without this key, it is futile to expect that any approximation to the attainable efficiency will be realised in colliery practice. Reliability of Air Meters.—Air meters are not more immune from suspicion than meters of other kinds, but, when suitably used, they are accurate, under working conditions, within quite small limits of error. A meter with an error of 3 per cent, is quite good enough to detect a loss by leakage of 20 to 30 per cent., or to discover the use or misuse by a machine of 100 per cent, more air than enough. Units. — Measurement applied to compressed-air engineering will develop a demand for some new defini- tions and terms, and would be encouraged by facilities for concise statements of quantities and values. The lack of nomenclature is in itself evidence of indifference to measurement in relation to the compressed-air system. The problem is properly an international one, and, after more imperious questions are adjusted, the unhappily postponed International Engineering Congress may afford an opportunity for appointing a committee to consider the subject. Cost of Compressed Air. Cost of Power. — The costs of generating power at collieries are as diverse as the conditions under which collieries are worked. Many collieries, large and small, new and old, find it convenient and economical to pur- chase power from supply companies, at costs of about 0*50d. to 0*55d. per electrical unit, and this provides a rough standard of value of power at collieries. With power at 0-525d. per unit used in motors of an average efficiency of 88 per cent., at, say, three-quarter full load, the cost to the user is 0*45d. per brake horse-power Curve Showing Cost of looo Cubic Ft of Free Air Compressed tq Different Gai~e. Pressures . Cost Per B H P Hour Tak> n at 20 30 40 60 70 80 90 loo Lea per $o. Inch Fig. 2.—Cost of Compressed Air. hour. For present purposes, the convenient figure of |d. per brake horse-power hour may, therefore, be adopted as generally applicable. Volume of Air Compressed per Brake Horse-power Hour.—It may be helpful to colliery officials to have, in familiar and easily remembered terms, an expression for the value of compressed air in units of power and money. The curve (fig. 1) represents the volume of free air per minute compressed to various pressures per brake horse- power. This curve is merely empirical, and is based upon such full load figures as are guaranteed by responsible makers of compressors, but for our purposes it is more serviceable than a calculated theoretical curve. It may be taken, for example, that with a modern com- pressor, in' good condition and working at full load, 1 brake horse-power will compress 6 cu. ft. of free air per minute to 601b. gauge pressure. It is quite safe to say that the average colliery compressor, in its present condition and under variable load, compresses 17 per cent, less air than would a new and up-to-date compressor in first-rate condition, working at full load. Cost and Volume.—We thus arrive, for example, at the figure of 5 cu. ft. of free air per minute compressed to 60 lb. per brake horse-power, and this is equivalent to 300 cu. ft. per brake horse-power hour. With power costing |d. per brake horse-power hour, the cost of compressed air at 60 lb. pressure is thus |d. per 300 cu. ft. The curve (fig. 2) gives the approximate.cost per 1,000 cu. ft. of free air compressed to various pressures where power costs -J-d. per brake horse-power hour. Tests of Air Consumption. Lack of Data.—So far as the writer is aware, there are no published data concerning quantitative tests of air consumption of longwall coal cutters in operation, and on the relation between expenditure of air power and area undercut. Method of Testing.—The whole of the meter tests were made with a Venturi rate of flow indicator, of which the makers guaranteed accuracy within 1 per cent. In a number of the more recent tests, another instrument of the same principle, but of more robust and convenient form, was also inserted in the pipe line in series with the Venturi tube, and at sufficient distance apart from it to avoid mutual interference. Very satis- factory correspondence of the indications of the two meters was observed. Coal Cutter Tests.—Air consumption tests of about forty coal cutting machines were made. Fifteen to twenty series of co-related observations were made, and recorded with reference to most of the machines tested. The machines were of various types, and by various makers, and of ages ranging between new and ten or 1 2 3 4 5 6 7 8 9 10 II 12 13 14 IS Coal Cuties l£FER£HCI NATURI OF holinc materia : dia OF I Frailih 1 HOSE lengthc trailing HOSE nYAROS NUHMI OF OBSER- VATION! AIR Reseat Fohpres ORSULK Phessur at Cate ENO HACHIK! Standing PRESSUEl at Cat £ ENO HACHIM Cutting PttS&At DROP IN HOSE PRESSUKI Coal IuTTE Cutting SPEED IN INCHES PERMIN DEPTH OF IN INCHES AREA CUT Per min IN SO YDS :us ft ■REE All PER Minute Cub ft Free Air PER SQ YaroCit A HARO Clod 2'V 40 12 64 60 54 NO FlTTINGON MACHINE FOR ■RESSUREGAUS 14 06 64 ■6944 681 980 B HARO Clod 2V 42 12 64 59-4 52 12 7 65 59 •6434 695 1084 C very HARO CLOD 2’V 42 12 64 52 393 89 60 4086 599 1461 D VERY HARO CLOO 2^’ 36 12 64 556 3976 4 25 60 •1967 714 3642 E 5 Ria COAL rr 12,z2 20) 2o) 12 64 52 8 45 14 9 71 17 •8144 580 715 F COAL 2 75 20 74 51 407 15 7 25 69 51 83 •2756 652 2367 G HARD Clod 2* 47 18 74 453 426 270 15 6 3 9 63 6 •1917 714 3728 H COAL 2* 70 17 74 61-25 5625 21-25 35 7-4 49 75 287 4 50 1568 1 HARO CLOO 2’ 82 20 70 6571 6427 29-72 36 55 8 II 65 89 •412 897 2176 J HARO CLOO 2k2 40 12 70 672 65 * 46 6 9 46 64 66 47 1146 2435 K HARO Clod 2' 80 20 70 70 6BI 284 39 6 6 86 62 8 •332 797 2402 L HARD ClOO 2* 20 15 70 65 62 ★ 55 II 36 6338 564 1029 1825 M Coal 2 40 II 65 65 53 14 8 382 13 66 49 512 735 1435 N Coal 2’ 40 10 65 65 64 17 6 46 1 24-85 50 6 •964 831 862 0 Coal 2’ 40 15 54 42 35 13 22 9 8 66 •499 543 1089 P FiRECLA 2V 35 15 54 43 29 S NoFitting on MACHINE FOR tressure gauge 96 72 534 478 896 Q Fireclay 2’t 35 IS 54 36 255 6 0 72 •333 413 1241 R COAL Fireclay 2’ 20 9 SO 46 31 16 15 16 64 32 7 •42 656 1562 S Brassy COAL 2’ 62 7 50 48 47-4 21-4 26 14 0 40 6 •433 681 1554 T Coal 2’ 41 5 50 46 45 5 13 5 32 33'5 42 1085 753 694 u Coal Fireclay 2* 38 3 50 45 39 17 5 21 5 22-6 30 -523 658 1258 . v COAL 'IRECLAY 2* 30 20 60 55 50 20 30 16 5 70 •90 530 620 * ENGINE THROTTLE NOT FULL OPEN Table I.—Test Records of Coal-cutters: General Table of Costs. twelve years old, and were operated under diverse con- ditions. All the tests were made without any tuning up of the plant, or any disturbance of the usual method of operating it. The air consumption was measured at the gate end valve, and therefore included the air, if any, lost through leakage from the hose or couplings. Tables I., II., and IV. summarise the records of most of the tests. The figures on these tables may exhibit apparent inconsistencies; they certainly exhibit anomalies, but they are a faithful record of the observed conditions. Abnormal cases were individually investi- gated, and the causes of irregularity were located; but space permits reference only to the more prominent features. Interpretation of Test Records.—Notwithstanding the considerable number of the test records, the diversity of the conditions of power supply, of the state of the machines, and of the nature of the holing materials in the individual cases, impose the necessity for caution in basing upon them deductions for general application. It is, however, permissible to say that the tests indicate, if they do not determine, certain conclusions. In the first place, it may be said that with longwall coal cutters of the three well-known types, when in good order and operating under conditions for which they are suitable, the air consumption per minute, or per square yard cut, is not greatly affected by the type. In the selection of a machine, the matter of type may therefore be con- sidered with sole regard to the mining conditions, with- out reference to relative air consumption. The comparatively low air consumption per minute of some of the machines was due to— (a) The machines being designed for moderate power; (b) The air pressure being suitable for the machine; (e) The valves and pistons being in good order; and (d) The trailing hose being airtight. Where the air consumption per minute was high, the causes were usually due to— (a) Defective condition of the valves and pistons; (b) The air pressure being too high or too low; and to (c) Leaking air hose. The high consumptions per square yards cut were due to the same causes as high air consumption per minute, plus the consequences of very hard holing material or badly formed or blunt cutters, of unintelligent driving, of trailing hose being too small in bore or too great in length, of the air pressure being too low as an alterna- 1 Coal Cutter RFFERFWI 2 Mat' ire Holinc Coal URECLKY 3 3iametct OF Frau inc; Hose T Length of Trailing rtOSt Yds 26 5 Numbei Of OBSER VATIOH5 33 6 Aip FfmaeC ■ATuFlg Fkh“ 62* 7 Press ENO IN L&S 362 8 pRE*=.S DROP >N II 5 9 Preha at enu CuT-rtR IN LB6 24 7 IO Cuf’fR SPEED IN INCHED PER MN. 12 ii Depth FW Nenes 34 i? CUN Ft free fclR PER MIN 893 13 Cup ft rREi All PF <5 SO Y1- Cut 2808 2 H r 55 30 61* 392 19-9 19 3 I0& 33 63S 2326 3 £ Ji T 40 18 61° 36 14 22 IBS 34 475 980 4 Fireclay 2’ 55 18 60* 38 18 20 2® 33 SI5 IOIO 5 Fireclay 2’ 39 18 59’ 309 174 135 262 29 618 1054 6 Fireclay 2’ 37 20 62* 35-6 13 226 19-2 33-4 478 967 THE FIGURES OVEN in COLUMNS 6 Toft ARE AVERAGES OF THE Number of observations stated