174 THE COLLIERY GUARDIAN. July 26, 1918. to be no reason to doubt that the supply of a number of collieries from a central station is economically sound. The coal consumption of 2| lb. per unit is by no means the best that can be obtained. With the recent development of the steam turbine in large units, which has taken place chiefly in America during the last two years, the coal consumption has been reduced to 1| lb. per unit. The company have recently placed an order for a 5,000 kw. turbine, 350 lb. pressure, 750 degs. super- heat, with the guaranteed consumption of 10-2 lb. of steam per kw. hour on full load, as compared with a consumption of 13-4 lb., which is the guaranteed figure of their best machine at present. It is esti- mated that with the boiler house improvements the consumption at the new station will not be more than If lb. per unit. If this figure be taken and applied at Britannia, then the consumption would be as follows : — Table C.—Output taken for Week ending November 24, 1917—12,529 tons of Coal (gross). Percentage Plant. Units consumed. Average load.* of output consumed. Pumping 134,490 .... 800 .. 0-82 Compressed air ... 150,030 .... 893 .. 0'93 Winding 94,440 .... 562 . 0k58 Ventilating 16,080 .... 95 .. 0'10 Miscellaneous 11,260 .... 70 .. 0'07 Total 406,300 .... 2,420 .. 2'50 * Based on 168 hours per week. This is a result which cannot be equalled by any steam driven installation. It has previously been mentioned that the power stations supplying this colliery were too small to allow of three-phase winding motors being used, and it was necessary to resort to a balancing set. It will be observed that the units used for winding have in- creased but little from the sinking period to the coal- winding period. This proves that the power is largely consumed in running the converters and Ilgner wheels. hours at the colliery, so that both the turbo-generator and the turbo-compressor have to be run constantly; whereas if electric reciprocating compressors are used, only a turbo-generator and small electric compressor are run, which is cheaper and more efficient than the other method. The characteristic curve of a turbo-compressor is a very bad one; it will take no overload, and below a 60 per cent, load the air has to be blown to waste to avoid surging. The turbo-compressor is only an efficient machine between full load and three-quarter load, and it falls away very rapidly below that. Now if turbo-compressors are to be used, it is essen- tial that the arrangements are such that the load is a constant one, which is usually not the case at a colliery. But by coupling up a number of collieries by air mains, a constant load can be obtained, so that the future for turbo-compressors appears to be in a central compressing station, which must have a number of turbo-compressors, so that the plant running can be adjusted to the demand. Generally, the hourly demand at a colliery drops at night and week-ends (depending on circumstances) to about a quarter of the coal-winding shift load. This question of compressed air central stations leads to the study of the question of transmission of com- pressed air. There appears to be a general idea among mine managers that transmission of compressed air is inefficient, but this is not the case where pipes are of adequate size. The drop of pressure in large pipes is much less than in small, because the area of skin friction is less in proportion to the area in large pipes than it is in small. At two adjoining collieries under the writer’s charge, one colliery had two compressors—one of a capacity of 5,000 cu. ft. of air per minute, and the other 8,000 cu. ft. of air per minutewhile the other colliery has one compressor of 5,000 cu. ft. of air per minute with no standby. As the whole output of this pit depended on the air supply, either a new compressor had to be put in or a pipe line laid on the surface between arrangement has been working for three years, and has been very satisfactory. Sinking. The shafts are 21 ft. in diameter. Very great diffi- culties were encountered, and at one time the quan- tity being pumped continually amounted to over 2,000 gallons per minute, mostly from between 450 and 640 yards. The power consumed continuously in pumping at its maximum was 1,000 kw. per hour. The water was dealt with by vertical centrifugal pumps slung in the shaft, pumping to horizontal centrifugal pumps in the headings fixed in the lodge rooms. Cages and Guides. The arrangement of cages and guides is the same as at Penallta Colliery. The cages in both shafts are double decked, with two trams in each deck, taking a total of six tons of coal. The guides on the south pit are rar, 95 lb. per yard, and in the north pit (the upcast) rope guides’of 1J in. diameter. The main arches at the shaft bottom at the adjoining collieries had given a great deal of trouble; they had been built of brick, and in this case it was decided to use concrete, and for this purpose blocks 3 ft. by 9 in. by 9 in. were made in moulds and tapered to the amount necessary for an arch of the requisite diameter, namely, 22 ft. Behind these blocks ordinary concrete, reinforced with .steel, were put in to fill up solid to the strata, this concrete being 3 ft. in thickness, making, with 3 ft. block, a total thickness of 6 ft. The arch is shown in fig. 2, and it will be seen that the floor is not an invert but a solid block of concrete, 8 ft. thick, and reinforced with old rails. These arches have now been in 3J years, and have shown no sign of failure. Conveyors. When the workings were opened out it was decided to use conveyors of the jigging type, to be driven by compressed air, because a larger yield of coal per yard of face per day could be obtained than with the stall system. The yield per yard of face per day for Fig. 2.—South Pit Bottom. A much better consumption figure would be obtained by A.C. motors supplied direct from the mains, provided the peak load was not too high a propor- tion of the total power station load. If this system were largely adopted in the coal field, it is essen- tial that the supply should be obtained from large central stations supplying a number cf winding engines, so that they together will have a combined load which will approach the constant. Air Compressing. The question of economical air compressing is a very important one in the South Wales coal field. At Britannia there are three constant speed compressors, as follows: One 2,500 cu. ft., one 3,500 cu. ft., and one 6,000 cu. ft. of free air per minute, and one variable speed unit of 5,000 cu. ft. of free air per minute, so that it is possible to run the constant speed machine on full load for almost any power demand, the variable speed compressor taking up the fluctuations in the demand, which ensures the maximum efficiency under all conditions. Up to the present the electrical compressor has been at a disadvantage owing to having to operate at a constant speed, and this type of compressor is not efficient except from half to full load. Lately the variable speed motor has been developed, which is of great assistance in this problem of electrically driven compressors, which will now compare very favourably with steam driven compressors, at least down to 65 per cent, load, for the following reason. A unit of electricity can be generated at 10-2 lb. of steam, which is equal to 7-65 lb. per horse-power hour. Allowing liberally for conversion losses, a brake horse-power on the compressor shaft should be obtained for the expenditure of 8-5 lb. of steam per horse-power. The best consumption figure per b.h.p. in a recipro- cating engine and that in a Uniflow is 10-9 lb. Turbo-Compressors. The efficiency of the turbo-compressor does not appear to be any better than the turbo-generator and electric compressor, and it has this disadvantage: compressed air and electric power are required at all the two collieries. The latter plan was adopted, a 10 in. main being laid a distance of 3,600 ft. This main will deliver 5,000 cu. ft. with a drop in pres- sure of only 2| lb.—that is, from 62| lb. to 60 lb.— and has kept the colliery going for more than a week when its compressor had broken down. Similarly, Britannia Colliery is connected with Bargoed Colliery —a distance of 6,000 ft.—by a 9 in. main, which is used in case of failure of compressing plant at either colliery, and also for week-ends and night loads. The joints in both mains are welded so as to avoid leakages. Now if these arguments are sound, then in the future steam boilers will disappear from the collieries and the power will be supplied by a large central station, with great economy in coal consumption. But until the capital cost of machinery is reduced to approximately the point at which it stood before the war, it will not be possible to go on with these schemes, as the cost of converting the existing machinery would be prohibtive. When these electric supply stations are under consideration, the question of the supply of compressed air should also be considered, as it would be of great benefit to the coal field if every colliery could obtain a supply without having to make its own, which would entail running another machine, and which, owing to the nature of the load, must operate part of its time at low efficiency. Air Lock. As both shafts are used for winding, and both have double decks with simultaneous decking, the cover- ing of the upcast to prevent air leakages was a matter of some difficulty. Fig. 1 shows the arrangement in use. There is a cemented brick casing with double doors at each end, the distance between the doors being sufficient to allow four trams to pass through at once, so that they are operated once for each wind. The trams having passed through the first door, which is open, and switch is put in, it closes No. 1 door and opens No. 2. The doors cannot be open at the same time. After the trams have passed through, the switch is again put in, and this closes No. 2 door and opens No. 1 ready for the next set of trams. This the colliery averages 16 to 17 cwt., or from 12,000 to 13,000 tons are got per week from 2,400 yards of working face. The yield per yard of face would be greater if more men were available. Another point in favour of conveyors was that the pit was being opened out just when the war com- menced, and, horses being unobtainable, mechanical haulage had to be resorted to; and the conveyor lends itself to mechanical haulage from the face because the output is concentrated at one point. Mechanical haulage from the face in stall work has failed because of the difficulty of running a rope into each road for one tram. The output per engine is too low with this method, but with the conveyor an output up to 250 tons per day can be brought to one loading place, and this quantity of coal justifies the use of a hauling engine. To show what the average yield per loading place is, an output of 12,500 tons for the week was obtained from fourteen loading places; two of these were developing faces, and so lowered the general average. The maximum output from one loading station in a week is rather over 1,609 tons, the length of the face being 200 yards. One would expect to find a great saving in ripping costs with a conveyor, but that does not usually mature, and unless some price is fixed at a much lower figure than that in stalls, there will be a loss, for there is the cost of shifting the conveyor and loading the coal into trams at the conveyor end to be met. There is also the capital cost of the conveyors, pipes, the maintaining of same, and the power for driving them. The collier obtains a very great benefit; all the coal he cuts has only to be placed on the conveyor, imme- diately behind him, instead of in stall work, where he has to carry his coal some yards and then lift it up into the tram; that is to say, the conveyor does for him what the collier’s boy usually does in a thin seam. A shaking conveyor uses a considerable amount of power, and on tests made recently on a conveyor 152 yards in length, air pressure 60 lb., conveying 2 in. in the yard down-hill, the result was as follows: —