THE COLLIERY GUARDIAN ________________________________________________________________________________ July 21, 1916. 125 CURRENT SCIENCE AND TECHNOLOGY. Compressibility of Natural Gas. Experiments on the compressibility of natural gas at pressures up to 35*5 atmospheres have been carried out by Messrs. G. A. Burrell and I. W. Robertson for the U.S.A. Bureau of Mines (Technical Paper No. 131), in order to determine the bearing of the facts on larger problems involved in the transportation and use of the gas. The gas examined was the natural gas supplied to the city of Pittsburg, its composition being :—Methane (CH4), 84’7 per cent.; ethane (C2H6), 9-4; propane (C H8), 3’0; butane (C4H10), 1*3; and nitrogen (N2), 1’6 per cent. The results are stated on the basis of water-free gas. The actual water-vapour content, by volume, of the samples tested, by ash plant is finding much favour on account of the absence of moving parts in its design, but the capital cost is very much greater (about three times) than that of bucket conveyors. Suction plants have not yet been long enough in use to warrant any definite pronounce- ment being made as to the repair bill. The cost for operating suction ash plant works out at about 5d. per ton handled, with energy at Id. per Board of Trade unit. Suction plant is now made to do other work. A modern boilerhouse calls for comparatively large overhead ash storage, from which ashes can be dropped into wagons or other vehicles for their removal.. The removal and dis- posal of ashes is becoming a serious matter. A few is impossible to overheat them, no matter how hard the boiler is worked. The slow part in the path of heat travel is from the hot gases to the boiler tube. It is tins part of the path that is responsible for the slow rate of heat transmission in boilers as now designed and operated. Anything that will increase the rate of heat importation by the hot gases to the boiler tubes will almost directly increase the rate of working the boiler. ________________ MINING EMPLOYMENT STATISTICS. passing them over phosphorus pent- oxide, was 0-7 per cent. The apparatus used consisted of a reservoir containing mercury, which could be forced into a series of three tubes by screwing up a plunger. One of the tubes served as a high-pressure, and another as a low-pressure air § manometer. The third tube contained the natural gas, the compressibility g of which was to be measured. Each tube was calibrated by. inserting a o small thread of weighed mercury, and g forcing this along the entire graduated portion. The volume of this mercury § having been calculated from the g weight, the bore of the tube through g its entire length was easily deter- g mined. Two air manometers were calibrated to serve as pressure gauges, the low - pressure manometer being calibrated against a long, open-tube manometer, and the high-pressure manometer against the low-pressure manometer. The results plotted in the accom- panying curve show that the gas is more compressible than an ideal gas (methane), the deviation amounting to as 30,000 25,000 20,000 15,000 10,000 5; 000 10 CORRECTIONS, PER CENT 85 15 95 39.48 35.5 32.90 26.32 19.74 13.16 6.58 0 as much as 15 pei* cent, at 35-5 atmospheres. A correction has been applied for the compressibility of air. As to the application of these results, it should be noted that sometimes natural gas is measured at pres- sures as high as 40 atmospheres (about 6001b. per years ago ashes had a market value, and 582.3 much as 4d. or 5d. a ton could be got for them. Then a time came when engineers were glad to get rid of them free of charge. To-day they have difficulty in getting the ashes from our stations carted away at 2s. a ton, paid to the carting contractor who undertakes their removal.— The Board of Trade Labour Gazette states that employ- ment in coal mining continued very good during June. There was an increase of 1,892, or 0-4 per cent, in the number of workpeople employed compared with a month ago, but a decrease of 217 on a year ago. Of the 530,470 workpeople included in the returns, 132,983, or 25-1 per cent, of the total, were employed at pits working 12 days during the fortnight to which the returns relate: while a further 338,287, or 63-8 per cent., were employed at pits working 11 but less than 12 days. The highest average number of days worked was in North Wales (5-86 days), and the lowest average was in Northumberland (5-43 days). Average No. of days worked per week by the collieries in fort- night ended Inc. M or dec. ( —) in June 1916, on a Work- people Districts. em- ployed in June 1916* a,. 1916. 1916. 1915. a^°- a£°- England Wales. Days. Days. Days. Days. Days. Northumberland ... 45,436...5’43...5’48...5'24 -0*05 +0’19 Durham ........... 92,281...5'59...5’57...5’49... +0’02 OTO Cumberland ...... 5,531. ..5’50... 5’76... 5'78...-0*26 -0’28 South Yorkshire ... 61,008...5*81...5 87...5*78... -0’06... +0’03 West Yorkshire.... 25,198...5*75...5’79...5’67 ..-0’04 0*08 Lancs. & Cheshire... 47,724...5*55...5*77...5’74...-0*22 ..-0T9 Derbyshire ........ 31,637...5’49...5’87 ..5*73...-0’38.. -0*24 Notts and Leicester 33,878...5*56...5’65..5’42...-0*09.. +0T4 Staffordshire ...... 27,398...5’75...5’82...5*81...-0’07...-0’06 W ar wick, Worcester and Salop........ 8,708...5’82...5’80...5 84... +0 02...-0*02 Gloster & Somerset 5,598...5’67. .5’97.. 5’91...-0’30...-0’24 North Wales ...... 7,617.. 5’86...6’00., 5’97...-0’14 -0’11 South Wales & Mon. 94,231...5’51...5’93 5’93...-0’42.. -0’42 Total ...... 486,245...5 60 . 5’76...5’67...-0T6...-0’07 Scotland. West Scotland .... 20,131 5’47...5’51...5 22...-0’04... + 0’25 The Lothians ...... 2,163 5’57...5’47...5’35...+0T0.. +0’22 Fife ............... 21,459 5’56. ..5’58. ..5’47...-0’02.. +0’09 square inch), and that many millions of cubic feet are measured at pressures of 20 to 30 atmospheres (300 to 4501b. per square inch). In computing the volume of gas, the assumption is always made that Boyle’s law applies; that is, that the product of pressure and volume is a constant at all pressures. Hence in measurements under high pressure, the error introduced is of great magnitude. For instance, suppose 100,000,000 cu. ft. of gas a day is measured at 375 lb. pressure (25*6 atmo- spheres). According to the curve, the gas is 11 per cent, more compressible at 3751b. (25-6 atmospheres) than at atmospheric pressure. This means that each day 11,000,000 more cubic feet of gas is measured than is supposed. If no correction is applied in measuring the gas, a distributing company that buys natural gas at high pressure and sells it at low pressure may sell much more gas than it pays for. Coal Handling in Large Power Stations. An electric power station of 50,000 kw. capacity and burning 200,000 tons of coal per annum requires coal- handling facilities for a supply of 1,000 tons of coal per day, together with facilities for the handling of ashes to the extent of 100 tons per day. This means, as pointed out by Mr. W. W. Lackie, engineer and manager of the Glasgow Corporation Electricity Department, a railway siding capable of holding from 100 to 120 full trucks, and probably another siding of equal dimensions to hold the empty trucks. The provision of an adequate railway siding is therefore a costly adjunct in the lay-out of a large power house, owing to the serious addition to the site which it entails. The length of railway sidings fora 50,000 kw. station would not be less than half a mile., or an addition to the site equivalent to 4,400 sq. yds. The wagons have to be tipped and coal-conveying plant has to be provided capable of handling 100 tons per hour. Coal-breaking machinery should be installed. Large coal storage accommodation is advisable, and will prove economical. Not less than from two to four months ’ fuel supply should be retained. This marginal stock enables coal to be delivered in fairly regular quantities through- out the year, and may result in lowering coal prices. In one Glasgow station we have a coal transporter fitted with a 1-ton grab. This transporter cost ^£2,600, and it handles coal at the rate of 40 tons per hour, and the'cost of energy, labour, and repairs on it brings the cost of handling coal in this way to just under Id. per ton. In another station truckloads of coal are elevated 30 ft. at one end of the coal store to an overhead platform, whence the coal is tipped into an overhead hopper or to the coal store, and the empty trucks lowered by a second elevator at the other end of the coal store. These two elevators, complete wtih electric'ally-operated capstans, cost <£2,000. The inclusive cost of handling coal in this way ’is 3|d. per ton, the higher cost being due to the amount of labour necessary for handling the trucks. Where coal has to be carried a short distance from coal store to boiler-house, bucket or tray conveyors appear to be the right thing. On the other hand, for long distances, the telepherage system works out slightly cheaper, although the human element, with its attendant problems, enters more largely into the telepherage systehm. For the removal of ashes, bucket conveyors have proved costly in upkeep on account of the wear of the axles and pulleys, caused by the fine, hard grit of ash dust; but even with this drawback, bucket or tray conveyors seem to be the cheapest method of removing ashes. Suction Engineering. Heat Transmission Through Boiler Tubes. • In a technical paper—No. 114—U.S.A. Bureau of Mines, Messrs. H. Kreisinger and J. F. Barkley describe some experiments on heat transmission through boiler tubes. The measurements were made with thermo-couples, embedded in the metal of a bottom tube at a point where the hot pro- ducts of combustion entered among the tubes. A couple was also placed in a tube in the top row at a point where the gases left the boiler. The hot junctions of the couples used in measuring the temperatures of the inside and outside surface of the tubes were formed by embedding both wires of each couple separately in the metal of (the tube. A hole about 0-001 in. larger in dia- meter than the wire was drilled in the tube to a depth of about Yft-in., and the end of the wire was secured with a special hollow tool, the wire being passed through the tool and bent aside through a groove at the top, so that the hammer did not strike the wire. The two wires of each couple made a junction through the metal of the tube, and the couple measured the temperature of an infinitesimal layer of metal at the surface of the tube irrespective of the depth of the wire in the wall. The thermo-couple wires were placed in a l|-inch tube of thick glass, which passed out of the boiler through a brass stuffing-box in the hand-hole cover. The leakage of steam and water was slight, but, owing to the glass tubing being attacked by the boiler water and gradually becoming soft and opaque, disintegrating in a few days, it was replaced by small porcelain tubes, which the boiler water and steam did not appreciably affect. The differ- ence in temperature between the inner surface of the boiler tubes and the boiler water was measured by a differential couple, consisting of a short piece of con- stantan wire, one end of which was embedded in the boiler tube, the other end being joined inside the tube to a copper wire, extending outside the boiler and forming the other kind of the couple. The potential set up in this couple was due to the difference of temperature between the two junctions, and therefore was a measure of the temperature difference between the inside surface and the boiler water. The temperature of the boiler water was also measured, independently, by a couple made by placing a constantan wire in a J-inch copper tube and fusing them together at one end. Beyond the junction, the constantan wire was insulated from the copper tube by a glass tubing slipped over the wire and into the copper tube. This couple was inserted in the boiler tube through a stuffing-box fitted into a hand-hole cap. The construction prevented all disturbance from galvanic action and electric leakage through the boiler water. The prominent feature in the results of the temperature measurements was that the temperature of the boiler tube is within 10 to 20degs. of that of the boiler water, and that the temperature of the tube is affected very little by the temperature of the hot gases, but follows the tem- perature of the boiler water. As the temperature drop along the path of heat travel is nearly proportional to the resistance to heat travel, the resistance appears to be very high from the hot gases to the gas side surface of the tube and very low from this surface to the boiler water. This indicates that a boiler tube can transmit Total ...... 43,753. .5’52 . 5’54...5’35...—0’02 . +0’17 Ireland .......... 472.5’56 5’28...4’94...+0’28...+0’62 Total, U.K_____ 530,470.5’59...5’74...5’64...-0T5...-0’05 * At the collieries included in the table. Compared with the previous month, employment showed an appreciable decline in Cumberland, Lanca- shire, and-Cheshire, Derbyshire, Gloucester and Somer- set, and South Wales and Monmouthshire. This may be accounted for to some extent by the fact that a certain amount of time was lest ew.ing to holidays. In the Lothians and in Ireland there was an improvement. In the other districts there v-as little change. Compared with June 1915, employment showed an improvement in Northumberland, Durham, Yorkshire, Nottingham, and Leicester, Scotland and Ireland, but a decline in every other district; the decline was most marked in South Wales and Monmouthshire. Inc. ( + ) or dec. ( —) in June 1916 on a Average No. of days worked per Work- week by the pits Description »:,e0’>le in fortnight “ p^ed _______ in June June May June 1916 * 24, 27, 26, 1916. 1916. 1915. Days. Days. Days. Month Year ago. ago. Days. Days. ........ 6,610...5’62...5’90...5’85...-0’28...-0’23 Coking ............. 28,164...5’78...5 78...5 74... — ...+0’04 Gas.................. 36,301...5’50 . 5’56...5’41. -0’06 -rO’09 House............... 60,174 . 5’61...5’73.. 5’59... —0’12 +0’02 Steam................ 181,528...5’57...5 79...5’70...-0’22 -0T3 Mixed................ 217,693...5’63...5’72...5’64...— C’09...—0’01 All descriptions ... 530,470...5’59...5’74...5’64... —0’15 . -0’05 * At the collieries included in the table. Compared with a month ago, employment showed a decline at pits producing all classes of coal, except at coking coal pits, which showed no change. Compared with a year, ago, there was some improvement at coking, gas, and house coal pits, but a decline at all other pits. The exports of coal, coke, and manufactured fuel during June amounted to 3,540,000 tons—a decrease of 321,000 tons on a month ago and of 221,000 tons on a year ago. Iron Mining.—In iron mines employment continued good. Returns received for each of the three periods named below, relating to the same mines and open works in each case, show that the number of workpeople employed in June 1916 was 13,490—an increase of 314 (or 2-4 per cent.) compared with May 1916, but a decrease of 271 (or 2-0 per cent.) compared with June 1915. The returns are summarised by districts in the following table :— Districts. Average No. of days worked per week by mines in fortnight ended Work- people em- ployed ________ in June June May June 1916. 24, 27, 26, 1916. 1916. 19)5. Days. Days. Days. 5,837 ...5’61...5’85..5’64.. Inc. (+) or dec. ( —) in June 1916 on a ___________ Month Year ago. ago. Days. Days. -0’24... —0’03 very easily all the heat that can ever be imparted to it by the hot gases, and so long as the tubes are kept free from scale, oil and other deposit and filled with water, it Cleveland......... Cumberland and Lancashire ...... 4,614 ...5’92 ..5 94...5’86...—0 02...+0’06 Scotland ......... 760 ...5’99.. 5’59.. 5’56... +0’40... +0’43 Other districts ... 2,279 ...5’96 . 5’89...5’99...+0’07...-0’03 All districts..... 13,490 ...5’79...5’88.. 5’77...-0’09... + 0’02