1198 THE COLLIERY GUARDIAN. June 14, 1918. FLOW OF AIR THROUGH SMALL COAL AND OTHER BROKEN MATERIAL/ By J. T. Storrow. In the discussion on Dr. Haldane’s paper on “ The Spontaneous Firing of Coal,” f a member raised the question concerning the laws governing the rate of flow of gases through broken materials. As this ques- tion is of special interest in mining work, it becomes important to know more definitely under what con- ditions the square root law holds, and when this is replaced by the direct law. At the suggestion of Dr. Haldane, experiments have been carried out to ascertain, if possible, the influence of the size of broken materials upon the rate of flow under different conditions of pressure. Experimental stoppings of different lengths of broken materials were made in a steel tube A—B (see figure) 1-81 in. in diameter (or 2*6 sq. in. in cross- section) and 9 ft. in length, held in a vertical posi- tion. The stoppings were supported in the tube by means of a wire gauze platform C fastened 4 in. from the lower end of the tube. The pressure of the air was taken from a branch entering the main tube below the retaining platform at b and measured on either of two gauges, one for pressures higher than 1 in. of water, the other for pressures from 1 in. downwards. The gauge for high pressures was of the ordinary U-tube type, but for the low pressures more sensitive readings were required, and these were obtained by using an inclined spirit gauge G, capable of being read to 0’002 in. of water guage. The pres- sure side of the gauges in each case was connected to a branch a, which entered the main tube above the stopping. The air, which was drawn through the stopping by a small motor-driven rotary exhauster, was measured by passing it through a dry gas meter M. In order to obtain steady pressures on the tube, a T-piece fitted with two taps Tx and T2 was placed in the circuit between the exhauster and the meter, as shown in the diagram. By regulating these two taps, and keeping the motor under constant load, any given pressure could be obtained showing fluctuations of less than 0’010 in, of water-gauge on the lower pressures, and of 0’1 in. on the higher pressures. The rate through varying lengths of different sizes of the material was measured with various ventilating pressures by noting (by means of a stop watch) the time taken for a definite quantity to pass through the meter. The quantity passing in cubic feet per minute was calculated from these figures. These Table I.—Air passing through Blank Tubes, with Retaining Gauze, etc., as in Subsequent Experiments, in Cubic Feet per Minute. Water- gauge. Through 16 ft. of 0’5 in. diameter tube. Through 9 ft. of 0’5 in. diameter tube. Through 9 ft. of 1’81 in. diameter tube. In. Direct. Exp. a/' Direct. I Exp. a/ Direct. 1 I Exp. a/ 0’02 0’13 0T8 — ! 2'10 — 0’04 0’26 0’23 0'18 0’36 0’31 0’25 4’20 2 98 2'97 0’06 — — — — — 4’47 3’65 3’65 0’08 0’46 0’40 0’32 0’62 0’57 0’44 — — — OTO 0’50 0’47 0’45 0’71 0’67 0’64 — — — 0’20 0’94 0’70 0’66 1’34 0’92 0’95 — — — 0’40 1’40 0’98 0’99 1’84 1’32 1’30 — — — 0’60 1’47 1’21 1’20 1’98 1’62 1’62 — — — 0’80 1’61 1’43 1’40 2'16 1’87 1’87 — — — 1’00 1’79 1’58 1’60 2’34 2'10 i i 2’09 — — — Table II.—Air Passing through Coal Slack of f to j in. Size, in Cubic Feet per Minute. Diameter of tube, 1'81 in. W ater- gauge. Through 8 ft. • Through 4 ft. Through 2 ft. Through 1 ft. In. Direct. Exp. a/ Direct. Exp. a/ Direct. Exp. Direct. Exp. 0’1 0’18 0’40 0’59 0’91 0’2 0’36 0’29 0’26 0'80 0’61 0’56 1'18 0’89 0'83 1’82 1'38 1’28 0’3 0’44 0'37 0’36 0’92 0’79 0’75 1’34 1’13 1’09 206 1’70 — 0’4 0’50 0’45 0’43 1'05 0’94 0’91 1’50 1’35 1’30 2’27 1’98 1’96 0’6 — — — 1’41 1’20 1’15 2’02 1’71 1’65 2'97 2’44 2’40 0’8 0’90 0’68 0’65 1’60 1’41 1’39 2’28 1’97 1’97 3’25 2’82 2’82 1’0 0’85 0’79 0’76 1’76 1’59 1’58 2’46 2’22 2’20 3'52 3 16 3’15 2’0 1’58 1'18 1T2 3'18 2’27 2’26 4’44 3’08 3'14 — — — 3’0 1’78 1’50 1’45 — — — — — — — — — 40 2’00 1’76 1’73 — — — — — — — — — 6’0 2’65 2'16 2'16 — — — — — — — — — 8’0 2’88 2'48 2’49 — — — — — — — — — A M Experimental Apparatus. results are recorded in Tables II. to V. inclusive under the columns headed “ Exp.,” on either side of which are given the figures calculated from the pressure according to the direct and the square root law re- spectively from the immediately preceding experi- mental result. From preliminary experiments it was found that the square root law did not hold exactly for tubes of | in. diameter when the pressure was less than | in. of water. In the case of a 2 in. pipe, however, the square root law held good at pressures down to 0’05 in. of water, as shown in Table I. These experiments show that under the experimental conditions the finer the material, the slower the air current, or the greater the length of stopping, the more nearly is the direct law obeyed, and it is evident that the material need not be extremely fine for this to happen. Now, in settled goaf the material is very tightly packed, and also contains a considerable proportion of fine particles, due to crushing. It follows, there- fore, that in any leakage through this material in a goaf the rate of flow of the air will be directly dependent on the pressure, since the latter will be very low and the distance travelled great. Any varia- tion in the ventilating current, therefore, will pro- duce a more marked effect in the goaf than in the airway, since the air current in the latter is governed by the square root law. If the goaf contains finely crushed coal, it is very desirable to keep the venti- lating pressure as low as possible, and thus prevent the larger ventilation increases from taking place in it. The most common cause of sudden changes in pres- sure is the occurrence of falls at the working face. These falls naturally cause pressure in the goaf, and a pressure which may be maintained for some time, owing to the difficulty of clearing the airway; conse- quently the higher the ventilating pressure is in such a case, the greater in direct proportion is the amount of additional flow through the ^carbonaceous material in the goaf, and in mines subject to spontaneous fires, the greater is the danger of heating. * Report to the Doncaster Coalowners’ Committee : Presented to the meeting of the Institution of Mining Engineers. t Trans. Inst. M.E., 1916-17, vol. liii., p. 194. Table III.—Air passing through Coal Slack of I to I in. size. Diameter of tube, 1'81 in. Water- gauge. In. Through 8 ft. Through 4 ft. Through 2 ft. Through 1 ft. Direct. Exp. Direct. Exp. a/ Direct. Exp. Direct. Exp a/~ 0’1 0'16 0’19 0’36 0'55 0'2 0’32 0’25 0 23 0’38 0’31 0’27 0'72 0’57 0’51 1’10 0’86 0'78 0’3 0’38 0’33 0 31 0’46 0’40 0’37 0 85 0’73 1’55 1’29 112 1'05 0’4 0’44 0’39 0'38 0’54 0’49 0’46 0’98 0’88 0’85 1’49 1'32 1’29 0'5 0’49 0’46 0’44 — — — — — — — — — 0'6 0'55 0'50 0’50 0'73 0’68 0’60 1’32 1'11 1’08 1’97 1'69 1'61 0'8 0’67 0’60 0’58 0'91 0'79 0’78 1'48 1'30 1’28 2’25 1'98 1’96 1’0 0’75 0’68 0'67 0'99 0’97 0’89 1’63 1'50 1’46 2’47 2'23 2’21 2’0 1’36 1’06 0’96 1'94 1’39 1'38 3’00 2’13 2'11 4'46 3'13 3'16 3’0 1'59 1'36 1'30 — — — — — — — — — 40 1’82 1'58 1’57 — — — — — — — — — 6’0 2'37 1’99 1'93 — — — — — — — — — 0'0 2’65 2'30 2'29 — — — — — — — — — Table IV.—Air passing through Coal Slack of £ to in. Size. Diameter of tube, 1’81 in. Water- gauge. Through 8 ft. Through 4 ft. Through 2 ft. Through 1 ft. In. Direct. Exp. a/" Direct. Exp. a/ Direct. Exp. a/~ Direct. j Exp. a/“ 0’1 0'009 0'03 0’07 ! 0'18 0'2 0'018 0’016 0'013 0’06 0'06 0'04 0'14 0’13 0'10 0'36 0'31 0'25 0’4 0'036 0'031 0'023 0'12 0'11 0'08 0'26 0'23 0'18 0'62 0'52 0'44 0’8 0'062 0’058 0 044 0'22 0'21 0'15 0'46 0'39 0'32 1'04 0'86 0'73 1'0 0’072 0 071 0'065 0'26 — 0'23 0'49 0'46 0'35 1’07 1'00 0'95 2'0 0'142 0’155 0’100 0'46 0'41 0'32 0’92 0'75 0'65 2’00 1'54 1'41 4'0 0’310 0 272 0’219 — — — — — — — — — 8'0 0'544 0’461 0’385 — — — — — — — — — Table V.—-Air passing through Coal Slack of to in. size. W ater- gauge. Through 8 ft. Through 4 ft. Through 2 ft. Through 1 ft. In. Direct. Exp. V- Direct. Exp. a/ Direct. Exp. a/ Direct. Exp. a/ 0'1 0'0015 0’003 0'006 0'0154 0’2 0'003 0'003 0’002 0'006 0’005 0'004 0’012 0’012 0'009 0'031 0'031 0'022 0'4 0'006 0'006 0’004 0'011 0'011 0'008 0'024 0'022 0'017 0’062 0’063 0'044 0'8 0'012 0'011 0’008 0'022 0’023 0 016 0’045 0’049 0'032 0'126 0’122 0'089 1'0 0’014 0’016 0’013 0’029 * 0'028 0'025 0’061 0'060 0'055 0'153 0'150 0 137 2’0 ' 0'032 0'031 0’023 0’057 0'054 0'040 0'120 0'120 0’085 0'300 0’284 0’212 4'0 0'061 0’061 0'044 0'109 0'110 0’077 0'240 0’233 0T70 0'524 0'517 0’402 8'0 0T21 0'118 0’086 0'221 0’215 0'156 0'466 0'406 0'329 1’034 0'882 0'731 10'0 0'14’7 0'146 0’138 0'269 0 270 0’241 0’507 0’500 0'454 1’212 1'152 1’035 20’0 0'293 0’273 0'207 0’540 0'500 0'382 — — — (11 in ches) — Table VI.—Cubic Feet per Minute of Air Passing through 1 Ft. of Material. Diameter of tube, 1’81 in. Water-gauge. In. Coarse coal dust. Common sand. Fine sand. Stone dust. Soil (dry). 0'5 0’016 0'011 0'005 0’0044 1'0 0'034 0’021 0'011 0'0009 0'0091 2'0 0'073 0'037 0'022 0’0019 0'0175 4'0 0'150 0'073 0’046 0’0041 0'0340 8'0 0'294 0’147 0'092 0'0086 0’0650 Degree of fineness. Per cent. Per cent. Per cent. Per cent. Per cent. Left on 10-mesh sieve 19'50 18'7 50'23 Through 10 ,, 30 „ — 8’50 1'15 26'1 16'95 „ 30 „ 60 „ 100 50'96 57'35 12’4 12’83 „ 60 „ 90 „ 19'56 36'85 5'5 13'13 „ 90 „ 200 „ 1'14 3'90 6’5 5'97 „ 200 — 0'24 0'75 30'8 10'89 A great reduction in the number of heatings, and for the last two years the prevention of serious fires, has been practically effected at the Brodsworth Main Colliery by reducing the ventilating pressure and paying greater attention to the stopping-up of old roads, holes caused by falls, etc. The reduction of