In

of Koppers ovens, and again in this regard he must say that the cost of repairs depended greatly upon the working conditions, which varied considerably at different plants. The great bulk of Koppers ovens which had been erected during the last four years, that was, since the oven was introduced, had not cost 1d. in repairs. The heating of the ovens, as would have been seen, was so uniform that the oven-walls were subjected to a much smaller variation in temperature than in any other system, and the wear-and-tear was therefore correspondingly less. cases where the coal was charged into the ovens in a very wet state, or where the coal (or the water in the coal) contained alkaline salts, the wall-bricks suffered severely; and any oven working under such conditions would require frequent repairs, especially those systems which were not uniformly heated. He could safely state that the cost of repairs of a battery of Koppers ovens, working under normal conditions, would be lower than that of other systems, and further, that no serious repairs would be necessary during the first four or five years. After this period of time, it would probably be necessary to renew the lower courses of bricks in the oven-walls.

The CHAIRMAN (Mr. J. C. Cadman) moved a vote of thanks to Mr. A. V. Kochs for his interesting paper. He might mention that in some districts, particularly in North Staffordshire, slackcoal, which was formerly almost given away, was now washed and made into a valuable coke, through the introduction of bye-product coke-ovens.

Mr. ISAAC HODGES seconded the vote of thanks, which was heartily carried.

The Rev. G. M. CAPELL read the following paper on "The Application of Duplicate Fans to Mines":

THE APPLICATION OF DUPLICATE FANS TO MINES.

BY THE REV. G. M. CAPELL.

The general idea has hitherto prevailed that little or any advantage could be gained by using two fans, on one upcastshaft, as compared with running one of them singly at the same. speed. There are records in the Transactions* which appear to confirm this idea, that whether one or two fans are run at an equal speed, the result will practically be the same; but in all those past experiences the fans were not placed on the mines with a view to their working together. On thinking over the question, the writer came to the conclusion that the disposing of two fans on a mine, either a single or a double inlet, in such relative positions to each other that the passing of the air would correspond to the action of a double-inlet fan, was possible, and would produce the results of a double-inlet fan with other advantages.

The positions of the fans, A, and A, could be arranged so as to resemble the action of a double-inlet fan, by using the parting of the air-drifts between the inlets (figs. 1 and 2, plate xvii.). Another condition must be considered, namely, that in doubleinlet fans, A, and A, (fig 3), with a central dividing-plate, each fan is practically a separate machine, so far as each inlet is concerned. There is, however, a great difference between a doubleinlet fan and two separated fans, namely, that the double-inlet fan, working in a single casing, has a constant condition of watergauge in that casing; whereas two fans, working in duplicate, might have a variation of the water-gauges in the casing, arising from small difference of speed, or in those variations which affect air-currents in coming to the inlets of a fan. This is remedied by a regulating passage, with a valve connecting the two fancases in the new arrangement. With this arrangement for balancing the case-pressures, the two fans can be relied on to act as if

*A Duplex Arrangement of Centrifugal Ventilating Machines," by Mr. W. Cochrane, Trans. Inst. M. E., 1891, vol. ii., page 483.

they were running in one casing. The equalizing or regulating passage, F, is shown with its valve, g, placed between the two fans (figs. 1, 2 and 3, plate xvii.).*

In working a fan by tri-phase current, now so much in use, a difficulty presents itself, namely, the great loss of power at weekends and other off-days, by being unable to regulate in a simple manner the speed of the fan and the consumption of current. In the application of two single-inlet fans, say, 15 feet in diameter, to a mine, at 220 revolutions per minute, the two fans would each produce 250,000 cubic feet of air per minute at 6 inches of water-gauge in the inlet-passage; and this would unite in a common water-gauge, beyond the division between the inlets, that is, in the main drift, D. A model of the arrangement shows very clearly, when one motor-driven fan is stopped and the automatic drift-door is closed, by the minus or suction water-gauge of the working-fan, still running at the same speed, and using the same power as when creating 6 inches of water-gauge, that the single fan will pass 66 per cent. of the volume of air produced by the two fans, working together at the same speed, at 28 inches of water-gauge. Consequently, the speed of a tri-phase motor need never be changed in these duplicate fans, and the cost of working the fans can, in a minute, be reduced to half, or even less than half, at pleasure, while 66 per cent. of the full daily circulation of air is secured in the mine. The difficulty of reducing the speeds of tri-phase motors first turned the attention of the writer to this new application of duplicate fans.

It may be of interest to state that the actual cost of the two duplicate fans is very little more than that of a single fan to do the same maximum work. It will doubtless be seen that great advantages will result from applying this system especially to new mines, where one fan will probably be sufficient to ventilate the mine for, say, 10 years; and the other fan can be added, as the water-gauge of the mine, the length of the air-ways and the rubbing-surface increase. The two fans can then be worked together in unison, and produce the higher water-gauge required, with a larger volume than that produced by the single fan.

The reason why one fan running alone at the same speed as the two fans produces up to 75 per cent. of the volume produced

* British patent, April 28th, 1906, No. 9,952; and United States patent, November 6th, 1906.

by the two fans, on this duplicate system, is very simple. If the volume, say, 400,000 cubic feet per minute at 6 inches of watergauge, is produced by two fans at equal speeds: then, if 300,000 cubic feet per minute be passed, the water-gauge, due to 300,000 cubic feet under the same mine-conditions, will be reduced from 6 inches to 3.375 inches, and the horsepower in the air will be 163.6. When one fan is producing half of 400,000 cubic feet at 6 inches of water-gauge, the horsepower in the air is 189, leaving a wide margin for loss of useful effect due to the large volume per revolution passing through the fan under the lower water-gauge, because the water-gauge produced by the fan at its normal speed is transformed so largely into volume: a fact well known to engineers who have tested fans on a mine, under varying water-gauge conditions.

Duplicate fans, for the largest volumes and high water-gauges, are now in construction; and, when they are running, the formulas relating to their work will be compiled so as to enable them to be easily adapted to the varying conditions of mines.

The CHAIRMAN (Mr. C. C. Leach) asked what results would be obtained by the two fans, where one fan delivered the air through the other.

The Rev. G. M. CAPELL said that, in such a case, the same effect was obtained as in a series pump. He had calculated the anticipated results of the two fans long before they were made: he had then worked the details into a practical form, and it was possible to get almost any desired result at varying watergauges. The duplicate fans as shown in figs. 1 and 2 (plate xvii.) had not yet been tested on a mine; but an electricallydriven plant was in course of erection.

Mr. ROBERT CLIVE (Doncaster) asked what would be the effect if one fan was run at a higher speed than the other.

The Rev. G. M. CAPELL said that the fans would be electrically driven, so that the two fans would run closely to the same speed. The speed would have to be reduced at once, should there be any great differences of speed. The fans would work efficiently, if the difference of water-gauge between the two fans did not exceed inch; but a difference of water-gauge exceeding inch

would cause air to be drawn from one fan to the other, and then the balancing air-passage and its valve, between the fan-cases, would come into action.

*

Mr. J. C. B. HENDY (Etherley) wrote that some years ago he made experiments with duplicate Waddle fans at Pleasley colliery. He (Mr. Hendy) did not believe in running two fans on the same fan-drift, in regard either to efficiency or to economy. The Pleasley fans were not run together, the idea being simply to have a spare fan always in readiness.

Mr. D. MURGUE (St. Etienne, France) wrote that the ideas expressed by Mr. Capell seemed very judicious. Undoubtedly, in theory, the volume of air furnished by two fans revolving at precisely the same speed could not be greater than the volume furnished by a single fan. But, in practice, it was otherwise, as the friction of the air in the various parts of the fan must be taken into account, for it caused a diminution in the volume which the fan was capable of producing. Now, when two fans were at work, only half the volume was produced by each, diminishing therefore by a half the total loss of useful effect due to friction, and increasing consequently in a noticeable proportion the volume of air passed through them. This increase was especially noticeable when the two fans were of small dimensions and were working on a mine with a large orifice, such as were most British pits. But in a mine with a small orifice, such as were very frequent in France and Belgium, two fans of large size would hardly yield a volume of air notably greater than that yielded by a single fan. It was, therefore, quite possible that in a mine with a large orifice, two fans being set in motion at a constant speed by tri-phase current, 66 per cent. of the normal volume of air would be obtained with a single fan and at much less cost.

Mr. SYDNEY F. WALKER (Bath) said that he would like to get down to the bed-rock of the whole matter. As he understood the results, each of the fans exhausted 200,000 cubic feet of air at 6 inches of water-gauge, that was, together they produced 400,000 cubic feet of air at 6 inches of water-gauge; and singly, each fan produced 300,000 cubic feet of air at 33 inches of watergauge; but, when running together, the two fans produced a water-gauge of 6 inches, that was, 3 inches each.

* Trans. Inst. M. E., 1891, vol. ii., page 535.