September 24, 1915. THE COLLIERY GUARDIAN. 623 gation was made, facilities were afforded for testing the pressure fall in pipes conveying metered volumes of air. Piping was not available for tests of a complete range of the sizes in general use, but the opportunity was taken of testing the sizes that were available. The paucity of experimental data on fall of pressure in pipes endows the results with some interest. The pipes were laid out and jointed above ground, and a valve was fitted at the discharge end to regulate the flow of air, which issued to the atmosphere. Tests were made of 300 yds. of 6 in. bore piping, and the results are plotted on the curves shown in fig. 7. An interesting feature of these curves is the difference of fall of pressure with the same volumes of free air at different pressures, and therefore at different densities. Tests were similarly made with 200 yds. of 3 in. bore piping, and the results are given on fig. 8. Fall of Pressure in Valves.—In connection with the tests referred to in the preceding paragraph, a number of tests were also made in order to ascertain the fall of pressure due to introduction in the line piping of right qI 5 v> Uj ce 3500 40CO 450C 5OOO ,'500 2000 2300 ' 3Cco Cuaic Feet of Free Air per min .Curves showing fall of pressures, IN 0" DIA PIPING 300 YPa LONG. Fig. 7.—Fall of Pressure in 6 in. Pipes. angle bends, and of valves and cocks of various types. These tests fully confirmed the view that some of the valves and cocks commonly used are the cause of serious loss of pressure—that they are in effect reducing valves. The curves (fig. 9) give the results of one series of tests. The types of valve and plug cock referred to in the tests are illustrated by figs. 10 and 11. It will be seen from fig. 9 that the penalty of forcing 750 cu. ft. of free air per minute at a pressure of between 30 and 35 lb. through the tortuous pasages of a 3 in. valve of the common type illustrated by fig. 10 is the loss of about 4 lb. pressure. The fall of pressure in this valve casing is due less to constriction of area than to the air at high velocity being forced twice to change abruptly and to a right angle its direction of flow, and. to the baffling sur- faces of the wings of the valve itself. The loss in the plug cock, which had a rectangular port of area 15 per cent, less than that of a 3 in. circle, was less serious, but amounted to about H lb., and was no doubt partly due to the flow being retarded by the change of section. (The areas of the rectangular ports of 2 and 2|in. plug cocks in use were respectively 27 and 25 per cent, less than the areas of the pipes for which the cocks were intended.) The moral of these tests is that only valves of the full- bore parallel flow, or sluice type, should be used in the piping. At the coal cutter a plug cock has the advan- tage that it may be instantly closed in case of emer- gency. A not infrequent accidental cause of fall of pressure is access of dirt to pipes in process of erection. Stones and other dirt enter the open ends of the pipes while lying on the ground, and if the pipes are not blown through before connecting the valves, the dirt is trapped at such valves as fig. 10, and causes serious fall of pressure. 800 IOOO I1OO HOO 1600 1800 2000. Cubic Feet of Free Air Per Minute. ■Curve. Showing Fall of Pressure • in £oo Yards of 5 Oia Piping, Pressure at Discharge 33 Lbs. Fig. 8.—Fall of Pressure in 3 in. Pipes. The Most Suitable Pressure for Coal Cutters.—There is a present tendency to use higher surface pressures in connection with coal cutters. The wisdom of this course is questionable. For rock drills and other tools which are man-handled, lightness is one of the chief desiderata. For such tools, high air pressure means small cylinders and more easily manipulated machines, and pressures of 80 to 1001b. are usual, and 110 to 1201b. are occasional. In relation to in-bye service, the surface pressure is not usually increased with the view to raising efficiency of the compressed air system, nor to delivering air to the coal cutters at a pressure higher than was formerly known to be necessary for the machines. The increase is made with the object of providing so large a margin of pressure that, notwithstanding excessive fall of pres- sure in transmission, the machines shall in all circum- stances receive air at adequate pressure. Increase of the surface pressure is subject to several objections, namely :—(a) The higher the pressure is, the greater is the rate of loss of volume of air by leakage; (5) air supplied to the machine at pressure higher than normal is not used economically; (c) in practice, the use of high pressure at the surface, to compensate for loss in transmission, is inevitably attended by wide fluctua- tions of pressure at the machines. The curves on fig. 5 show by their increasing steepness at the upper end of the pressure scale how the rate of leakage loss increases. The provision of surface pressure greatly in excess of that required by the machine also tends to increase pressure losses by condoning, by almost encouraging, extravagant fall of pressure in pipes. Most of the longwall coal cutters in general use will do their best with a pressure of about 35 lb. at the machine (not at the gate end valve), and it is desirable that the pressure at the machine should not be more than 40 nor less than 30 lb. A variation of 25 per cent, is a fair allowance. The problem is to supply air to the machines at the requisite pressure, and to maintain approximate uniformity of pressure (within the limits of 12| per cent, above and 12| below normal) under the varying service conditions. Speaking generally, it is beyond doubt that a solution of the problem is reasonably practicable. In the section on coal cutters it is shown that the best economy of air at the machine is obtained at rela- tively low pressures, and in ordinary circumstances nothing is to be gained by exceeding a pressure of 35 lb. at the stop cock. Referring to fig. 4, the comparatively small amount of energy rejected by the exhaust is due to the low average pressure of 20 lb. at the machines. Higher pressure would in this case have improved the performance of the machine. A pressure of 35 lb. being required at the machine, there appears to be no valid reason why a surface pressure of 60 lb. should be exceeded; where the pipe lines are not very long, a less pressure should be sufficient. This view is con- firmed by analysis of the pressure losses, and in this connection Tables I., II., and IV. are instructive. 2 o IC 20 16 18 400 _3o■ > Flett 3 I ^ipinc 3o) 3o Pressure 600 Curve Showing ai 0 ’Soo 3nly. I AMO 3_01' _0>A of Pre ssure. _Lf:S Ordinary Coch . 1600 1*200 1400 Cubic Feet of Free Air Per Minute Fig. 9.—Fall of Pressures in Valves and Cocks. Fig. 11.—3 in. Plug- cock. Fig. 10.—3 in. Valve. The tables certainly contain examples that are excep- tional in respect of high and of low pressure, and cases of unusually large fall and unusually small fall of pres- sure; these tend to mutually compensate in the over-all average. Table VII. presents the average pressures at the compressors and at the coal face, and is based on columns 6 to 10 of the tables. It may be pointed out here that a small gain of pressure is derived from aero- static head due to the column of compressed air in the shaft. The amount of this gain is the difference between the weight of the compressed air column at the working pressure and the weight of a column of equal height of atmospheric air. In a shaft 700 yds. deep, the gain at a mean gauge pressure in the pipe of 601b. per sq. in. is 4-5 lb. at 62 degs. Fahr., a quite appreciable advan- tage. It is believed that the figures in Table VII. roughly represent the average practice in regard to the propor- tionate losses in the mains and sub-mains, in the branches, and in the trailing hose, although divergencies from the mean are frequent and wide. We arrive at an approximate average loss of 12| per cent, between the compressors and the junction with the branch, and 12| per cent, in the branch to the gate end, equal to a fall of 25 per cent, of 52*25 lb., equal to 12*75 lb. By the use of sub-mains of pipes of suitable sizes, and of 4 in. instead of 3 in. pipes for branches to individual machines, the average loss would certainly be reduced to 10 lb. The trailing hose for conveying the air from the gate end valve to the coal cutter is an item of such import- ance that it is treated in a separate section. It may be said here, however, that if the pressure fall in the hose were limited to, say, 6 or 71b., the pressure at the coal cutters would be more than is necessary, and the sur- face pressure might be reduced. Trailing Hose. Fall of Pressure in Air Hose.—One of the most notable features of the test records is the large loss of pressure in the trailing hose. The examples (1) in Table I. and (10) in Table IV. are interesting, as illustrating on one hand how very great the fall of pressure in the hose may be, and, on the other hand, how little it may be. Fig. 12 gives curves plotted from the results of tests of fall of pressure in hose pipes with varying volumes of free air, at a discharge pressure of 251b. The advan- tage of hose of large diameter requires no more forceful illustration. Leakage from Air Hose.—The loss of air from leaking hose and defective couplings was in many cases serious. It is to be remembered that although a coal cutter works intermittently, and in the aggregate only during about half the full shift, the leakage from the hose is continuous, and is in fact higher while the machine is not cutting, because the pressure is then higher. Loss of 100 to 200 cu. ft. of air per minute from the hose is not uncommon. Among the examples metered were cases of losses of 400, 520, and of 840 cu. ft. respectively. Diameter and Length of Hose.—There is no part of a compressed-air coal-cutting equipment upon which an Average Pressure at the Coal. Pace based on all the Examples given in Tables I H & C7 CCWRISPONOINC COLUMN in Tables 6 7 8 9 10 Pressure at Compressor* Pressure at Gate Enos Machines Stano6 Pressure at Gate - Enos MACHINES CuTT TIN, Pressure Pali IN TRZ.ILING HOSE Pressure at Coal CurrcM Number of Examples on which Average; ARE &ASEO 38 32 38 28 3! Average Pressure 52 25 lbs 47 LBS 39 5 lbs 15 lbs. 24 lbs Average Fall of Pressure 5 25 lbs (2 75lbs IS LBS PROPORTION or Surface Pressure 10 /o 25 % x — 29% 4g7o — J- 100% Table VII.—Averages of Table I., II., and IV. attempt at reform could be more readily and inexpen- sively made than on the trailing hose. The size of hose in most general use is one of 2 in. bore. With such hose, the writer has shown that the average fall of pressure is 15 lb. The loss of pressure can only be reduced by increasing the diameter, or by shortening the length, or by doing both. To what extent can the diameter be increased, or the length reduced? It is quite certain that | in. increase of diameter might be conceded to efficiency; it is probable that (except in the thinnest seams) | in. of diameter might be conceded. Fig. 12 shows how great would be the benefit to efficiency if the inconvenience of handling 2| in. bore hose were incurred. Hose of 3 in. diameter has been tried in a few cases, but the advantage of increased economy as compared with 2| in. hose did not compensate for the difficulty of handling so unwieldy a hose as 3 in., with its bulky couplings. The standard length of hose is 40 yds., comprising two 20-yd. lengths coupled together. Wherever the dis- tance between the gates permits, the length might with advantage be reduced to two 15-yd. lengths, making 30 yds. in all. The shorter length would reduce the fall of pressure by 25 per cent., and it would, especially if the size were 2|in., be more easily handled, and the shorter a hose is, the less liable it is to damage. Hose of 30 instead of 40 yds. would involve more piping in gates, but the expense of this would be recouped by increased efficiency and improved durability of the hose. Before leaving the subject of trailing hose, it may be pointed out that the hose must withstand the full surface pressure, and that the difficulty of the hose problem is not mitigated by higher pressures than 60 lb., especially if hose of larger diameter were adopted. Hose Couplings.—The hose couplings in general use are defective in design, in that the thickness of the spigot z IO o Teo ul a ~wo Joo 4-oo Ron ~ soo Too 800 800 Cubic Feet of Free. Air je.r Min Curves Shcwins Fall in Pressure, in 2o Yards of Trailing Hose Without Couplings with 25 L6S Pressure; on Discharge Fig. 12.—Fall of Pressure in Trailing Hose. inserted in the bore of the hose materially constricts the fair-way for the air. The coupling should be of a form which does not intrude upon the bore. In this con- nection the insertion of iron piping and the clamping of the hose to it in repair of damage should be discouraged. (To be continued.) Hull Coal Exports.—The official return of the exports of coal from Hull to foreign countries for the week ending Tuesday, September 14, 1915, is as follows :—Amsterdam, 1,634 tons; Dunkirk, 393; Deauville, 913; Dieppe, 1,350; Gefle, 1,969; Gothenburg, 3,458; Harlingen, 1,150; Havre, 683; Oxelosund, 1,485; Oporto, 1,462; Rotterdam, 972; Rouen, 24,647; Stockholm, 2,086; Svancke, 178; Treport, 858—total, 43,238 tons. The above figures do not include bunker coal, shipments for the British Admiralty, nor the Allies’ Governments. Corresponding period September 1914 —total, 52,904 tons.