January 14, .1916, THE COLLIERY GUARDIAN. 69 plants now being pat down. The theory of the centri- fugal fan was very similar to that of the centrifugal pump; the understanding of the one helped the under- standing of the other. Mr. Thomas’s paper was made more useful by the fact that centrifugal pumps were now being used in large numbers in mines, and it was important that the men who had to work with them should know something about the principle on which they were constructed. With regard to the question of the varying observations obtained on the Pitot tube, he would suggest that the discrepancy might be due to eddies. He made a number of tests on a fan a few years ago, and found it rather hard to get figures which agreed with one another, because of the difficulty of fixing upon a place where there was a uniform current of air. It was most important that the measurements should be taken at a point where the flow of air was uniform, because if there was pulsation, or eddying, erratic results would be obtained. It was better, therefore, to take the measurements some distance from the fan, providing there was no leakage taking place in between, and a point 100 yds. away would perhaps be satisfactory, because in the interval the air would have settled down into a uniform flow. Referring to Table I., Mr. Thomas had pointed out that some of the figures in his ratios did not agree very well, but his (Mr. Bolton Shaw’s) view was that the ageement was very close indeed, and indicated very careful and accurate reading. If the figures were from actual readings, and not from the straightening out of curves, they were very close indeed, because they did not vary more than 3 per cent, at the most. Mr. Thomas explained that they were absolute observations. Mr. Bolton Shaw continued, that with regard to ;he rising and falling of the water gauge, according to the ordinary theory of centrifugal pumps and fans, the pressure depended on the centrifugal force set up in the air as it came from the periphery of the fan, so that, theoretically different factors would be obtained with fans with backwardly turned blades, fans with radial blades, and fans with forward-angle blades. With the backward-angle blade a fall of pressure ought to result directly air began to pass through; with the radial blade a constant pressure ought to be obtained; and with a forward-bladed fan a rising pressure would result, provided there was no friction in the fan. Mr. Thomas Bryson, of the Royal Technical College, Glasgow,.submitted a written contribution, in the course of which he said the author had not over-estimated the .importance of the subject with which he dealt. Mining engineers seemed not to have devoted that sufficiency ol study which was necessary for the acquirement ol familiarity with the subject, though doubtless many would have been glad to have become more intimate with the scientific side of it had the opportunity been provided in the days of their apprenticeship. With possibly one exception, the modern textbooks on coal mining betrayed either a very casual acquaintance with scientific fan testing, or an entire absence of the analytical faculty required for the proper interpretation of results. The author had shown that what was sometimes called the initial or total depression was not constant at a con- stant speed, and that it could not be measured when the “ fan-eye ” was closed to prevent the passage of air, through the fan. With regard to the apparent dis- crepancy between the accepted theory and the figures obtained with the experimental fan, he (the speaker) had recently been carrying out a series of experiments with a view to determining, if possible, the factors which controlled that discrepancy; and the results obtained so far indicated that the factors of speed and resistance had an important bearing on the question. Beyond ;hat he was not prepared to go for the present. In calculating the “horse-power of ventilation ” the author made use of the dynamic and maximum static depressions, but in the absence of line diagrams, and in view of the fact that depression was used in calculating the horse-power of ventilation and the overall efficiency of the plant (which latter was recorded as being 70 and 81'5 per cent.), he (Mr. Bryson) wnuld be glad if the author would explain how those observa.ions of depression were made, and why he believed it necessary to take cognisance of the pressure on the fan exit. The author claimed that the figures submitted verified the rules that “ the velocity of quantity of air produced is pro- portional to the square root of the water gauge, and to the cube root of the horse power of the ventilation. ” He agreed that so far as testing in the laboratory was con- cerned, the above statements were true, but Mr. G. L. Kerr, in his “ Practical Coal Mining,” on page 393, stated that the first law enunciated above was only approximately correct except at very high velocities. It was not quite clear whether or not the law was true for very high velocities. Mr. Mowat, in his excellent paper (Transactions I.M.E., Vol. xliv., p. 92) on “ Facts and Theories Relating to Fans,” demonstrated that, so far as actual mine ventilation was concerned, both laws were true. Mr. Thomas might with advantage have tested, those points on a mine installation. With regard to the determination of the value of the coefficient of contraction of a. fluid jet, the speaker’s own results agreed very closely with those of the author, but he had arrived at the conclusion that the value of the coefficient was largely dependent on (the ratio of the area of the testing orifice to its perimeter, and could not, ■ herefore, be stated to have a particular value. Although the author had dealt only with a few of the questions which were capable of investigation in the mining laboratory, he had probably done sufficient to show that the experimental fan was far from being an ornament, and that in the hands of the scientific investigator much useful information might be obtained by its use. Mr. Thomas deserved the thanks of the institution for his valuable addition to its Transactions. The discussion was adjourned. The Value of the Experimental Fan in the Mining Laboratory.* ; By DAVID E. THOMAS, B.Sc. The theories of mine ventilation and fan construction have always been of interest to mining men. Since the earlier days of mining, considerable progress has been made in this important branch of mining engineering. Fans have undergone considerable change, with the result that the modern fan shows a much improved efficiency; but theoretical laws laid down by the earlier authorities have become so generally accepted, that the need of experimental verification occurs to very few. It has been customary in the past, and even so to-day, for mining engineers to trust, to a great extent, to fan makers to supply them with ventilating machinery of the most efficient type. Fan makers have done their work well, but it must be admitted that comparatively little has been accomplished by mining men in investi- gating for themselves the theories of mine ventilation and design of fans, and fan makers have allowed very little information to leak through. It may safely be said that few branches of mining engineering have been so barren of increased knowledge in recent years as mine ventilation, yet ventilation is one of the most important problems that the mining engineer and his officials have to solve. It is a scientific maxim that no theoretical conclusion should be accepted a,s a definite truth unless accom- panied by experimental verification, and this maxim is impressed on the young mining student of to-day. Wherever proof is practically possible, the student is encouraged to demonstrate for himself all theoretical deductions. In our modern mining schools laboratories are equipped for this purpose, and among other apparatus the experimental mine fan forms a very important factor. The aim and object of this paper consist in laying before the society the nature of the work which can be successfully carried out in our mining schools, and the possible economic value of the work in mining engineer- ing. With this end in view, the writer submits a brief record of the results achieved at the St. Helens Mining School, Gamble Institute, St. Helens. In setting up a mine fan for purely experimental work in a laboratory, it is obvious.that a reproduction of the conditions of the actual mine is practically impossible; a model mine would be necessary, the ground space needed would be considerable, and the expense would be prohibitive. The well-known theory developed by Murgue, however, affords a method by which the diffi- culty may be overcome. Murgue assimilates the total resistances to the passage of air through a mine to ihe resistance offered to the passage of the same amount of air, wffien subjected to the same water gauge, through an orifice in a thin plate. Therefore, the construe;ion of lengthy ducts may be dispensed with if a thin plate is used to represent the resistances of the mine. It is with this principle in view that a fan has been erected at the Gamble Institute. The fan itself is of the Keith single-inlet type, with -a runner 12|in. in diameter, driven by a continuous-current 220-volt motor on the same shaft. A shunt regulator enables one to obtain a speed variation ranging from 870 to 1,600 revolutions per minute. A “fool-proof ” switch auto- matic overload release, and two fuses are attached to the panel, with the necessary voltmeter and ammeter. To the inlet side of the fan is attached a wooden rectangular duct measuring 18 in-, by 18 in. in internal cross section, and 16 ft. in length. At a distance of 4 ft. from its outer end, a slot has been provided to accommodate a sheet iron plate, in. thick. A number of these sheet iron plates are provided, with an aperture cut in each of varying shape and dimensions. Some of the plates have circular apertures, while others have rectangular apertures. One plate is blank, so that when it is inserted in the slot it closes entirely the fan inlet. The resistance offered to the passage of the air to the fan, or, in other words, the “equivalent orifice ” of the mine, can be altered at will by merely changing the resistance plates. The arrangements of the duct on the exhaust or outlet side are intended merely to obviate the discomfort, to the students, of air being discharged horizontally across the room. A connection between the inlet and the outlet ducts, controlled by doors, affords a simple means of reversing the air current. Anemometers are used for measuring the velocity of the current, and an ordinary water gauge indicates the depression produced. It is found, however, that the anemometer readings are subjected to an error due to the surface presented by the hand and the anemometer in the duct; but. the simpler laws of ventilation - can be readily verified by the use of the anemometer, as the errors • are fairly constant. It has been found more satisfactory to use the Pitot tube, with an inclined water gauge (1 in 10), for the more advanced investigations. The results-presented Jn Tables I., II., and III. were obtained by its use. The Pitot tube used is of a highly improved pattern. Both the dynamic and ihe static water gauges can be obtained by means of this instrument. In order to obtain the dynamic water gauge, the Pitot tube is connected at the T-piece, by two lengths of india- rubber tubing, to the two limbs of the inclined water gauge. Care is taken that the T is held parallel to the direction of the current, while the inner tube faces the current. The reading of the inclined water gauge is noted. The vertical plane of the duct or airway is divided into a number of imaginary sections, as would be done when measuring with an anemometer, and readings are taken in each section. The sum of all the readings obtained is taken, and the velocity deduced. * Paper read before the Manchester Geological and Mining Society. The following calculation shows that a dynamical water gauge of 1 in. produces a current of 4,000 ft. per minute :— 1 in. water-gauge = 1 in. of water, — —1 —in. of air-column at 32° F. or 0° C. 0-001293 = 773 in. of air-column. = 773’||^ in. of air-column at 16° C. = 818-38 in. V2 = 2yH, = 2 x 322 x ; or V = V64-4 x 68-19 = 66*27 ft. per second, = 3,976’2 ft. per minute. Supposing that the mean dynamic water gauge is 0*705, and the area of cross section of airway is 0’7 sq. ft.* then— The quantity of air passing, in cubic feet per minute, the water gauge being proportional to the square of the velocity, • =0’7 x 4,000 x v 0’705 cu. ft., = 2,332 cu. fr. per minute. If the temperature of the air is 16 degs. Cent., the error involved by taking the value 4,000 will only amount to 0’3 per cent, in the actual velocity. The maximum static water gauge may be obtained by first observing the water gauge on the inlet duct, and then observing the water gauge on the exhaust. For example— Mean static water gauge on inlet ■= —1’26 ,, ,, ,, ,, ,, exhaust = 4-0*16 Maximum static water gauge —1’10 As the dynamic water gauge represents work done by the fan, it is added to the maximum static water gauge. For example— Mean dynamic water gauge ... ... =0’70 Maximum static water gauge set up by fan =1’10 Total ......................... 1’80 With these remarks and the figures shown in Table I., ihe method of calculation will be clear.* It may not be amiss to issue a note of warning against the use of a crude form of Pitot tube, as the results are often misleading. It sometimes happens that the ordinary water gauge is discarded when, as a matter of fact, the results obtained with it are truer than those obtained with such a type of apparatus. The Pitot tube in its present form will not by any means displace the anemometer in the mine, as it does not afford a more ready means of measuring the velocity of the air current in the mine than is now available by the use of that instrument. Undoubtedly, where extreme accuracy is desired, more reliable results can be obtained with the Pitot tube. Results Obtained with the Experimental Fan. It will be noticed that the figures submitted verify the following rules embodied in Atkinson’s formula— ksv'1 p =-----. a The velocity or quantity of air produced is proportional to the square root of the water gauge, and to the cube root of the horse-power of ventilation. It is also apparent that the quantity produced is directly pro- portional to the speed of the fan. The writer desires to draw attention to the close agreement existing between the theoretical opening of the mine, as calculated from Rateau’s formula, and the actual opening when a resistance plate with a circular aperture is used : wherein 0 u represents the opening of the mine, in square feet; Q = quantity, in cubic feet per second; g = acceleration due to gravity, in feet per second per second; and H = head of column of air, in feet. Mr. Murgue’s formula is : a = —