122 THE COLLIERY GUARDIAN. January 21, .1916. CURRENT SCIENCE Coke as a Boiler Fuel. Owing to the shortage of labour and the closing of various foreign markets, measures have been taken in Germany to prevent the possibility of a serious coal famine, and. to utilise the large quantities of coke which would' .normally be exported. Successful trials are reported to have been made with coke for steam raising, and a number of German electricity supply stations are said to be using coke for that purpose. Where a mixture of coal and coke is used, no particular trouble is experi- enced beyond that of obtaining and stoking a reasonably uniform mixture of the'two fuels, but the use of coke, alone is a less simple matter, particularly if mechanical stokers be used. Coke cannot be broken and screened so that it can be used satisfactorily in sprinkler stokers, whilst on travelling grates the difficulty is to ignite the gas-free fuel as it enters the combustion chamber. Normally, an incandescent arch of refractory material over the point of entry will gasify the fuel, and the burning gases keep the arch hot, thus rendering the whole action of ignition continuous. With coke this action fails, and the fuel ignites late and imperfectly, if at all, as the grate advances. f Section of Coke-burning Furnace. This difficulty is.claimed to have been overcome by the Belani. grate shown in the accompanying illustration (Zeitschrift des Vereincs dcutscher Ingenicurc). The lower part of the coke feed hopper’is lined with refractory bricks and fitted with an inclined grate at the front, and a flue at the back leading into the combustion chamber of the boiler. This hopper furnace preheats the fuel above, and raises it to incandescence before passing it on to the travelling grate. The bulk of the unutilised heat passes beneath the boiler, and is there utilised. This grate is said to have been tested with satisfactory results on a water tube boiler in the Dusseldorf Elektrizi- tatswerke. , . Other tests on . a boiler with ordinary horizontal grate showed, that coke may be a better fuel than coa] of greater calorific value. The heating surface was 1,680 sq. ft., and a flaming coal of 13,000 British rhermal units calorific value raised 3’4 lb. of steam per hour per square foot of heating surface, or 61b. per lb. of coal; on the .other hand, coke (dust to 2J in.) of 12,600 British thermal units per lb., raised 3*8 lb. per square foot of .heating surface, or 6'6 lb. per lb. of coke. The highest measured combustion chamber temperature and, the .flue gas exit temperature were respectivly 1,220 degs. and 376 degs. Cent, with coal; and 1,145 degs. and 473 degs. Cent, with coke. The superiority of the coke is due to its requiring a smaller surplus of air for com- bustion, and suffering no loss from unconsumed gases or tar.' ' Coal Washing. The keynote, of efficiency is intelligent supervision. And such supervision must be systematic and thoroughly carried out over the whole plant. In normal time, and with cheap coal, there is, undoubtedly, a very large amount of waste allowed to go on. At the washery itself, conditions are not.always what they might be. In many, cases. certain definite guarantees are given with regard to the efficiency of the washery. These especially refer to (1) the amount of coal passing away with, the dirt; and (2) the amount of dirt left in the coal. Unfor- tunately, in many cases the guarantees are merely superficial, and no method.of arriving at a definite result is previously agreed upon, so that actually the guarantees are not worth the paper they are written on. With one type of washer the undertaking given by the makers is that the coal passing away with the dirt shall not exceed per cent, of the latter, whilst the washed coal shall not have an ash content of more than 6-5 per cent. . It is only with the Latter figure that the majority of coke oven managers, particularly concern themselves. There, is, .of course, an obvious reason for such concern. As soon as the ash content of the washed coal rises unduly, this is reflected in the coke; and dirty coke in turn is soon noticed at the blast furnaces. With many types of washers, good clean coal can be produced by so regulating the washer that a larger proportion of coal passes away with the dirt. In order to overcome this loss/re-washing of the first dirt has been introduced, but even when this 'is done, the results are not always so -satisfactory as they mieht be. With certain classes of coal, if a good quality of washed coal is to be made, an unduly high proportion is lost in the dirt. With the ordinary type. of washer it is .not possible to eliminate this coal, owing, to its being too closely intermingled with shale, .so that the specific gravity' of the mixture is too near to that of dirt to allow of its separation in the AND TECHNOLOGY. ordinary way. Other parts of this raw coal have only sufficient dirt embedded to raise thd specific gravity a little above that of coal itself, the dirt therefore passing away with the washed, coal, and raising the ash content of the latter. Experiments have been made with a class of coal difficult to' wash, and in the ordinary state—no: re-washed—containing a high proportion of carbonaceous matter. On crushing the latter, and then re-washing, a good amount of coal was recovered from it, and the dirt from this second washing was of a normal charac.er. In the experiments, a solution of calcium chloride, having a specific gravity of 1’39, was used. The specific gravity of a piece of good coal was 1’26, and its ash content 4*28, whilst -the specific gravity of shale was 2-7, and, on combustion, the ash content was 91*04 (samples dried at 105 degs. Cent.). Percent. (1) Raw coal unwashed................... ash 14*95 After first washing—Coal, 89*8 per cent., „ 8*9 Dirt, 10*2 „ ,, 69 8 (2) Coal crushed and rewashed— Coal, 97*0 ,, „ 6*4 Dirt, 3*0 ,, „ .89 5 - (3) Dirt from (1), crushed and re washed— Coal, 14*4 „ ,, 10*5 Dirt, 85*6 ,, ,, 80’2 Had the first washing only been carried out, the yield of washed coal would have been 89*8 per cent, of the weight of raw coal, with an ash content of 8*9 per cent. By re-washing, the yield is about 88*6 per cent, of the weight of raw coai, but the ash content is reduced to a little over 6*4. The advantage of re-washing would be more marked in the case of a much dirtier coai—say, one containing over 20 per cent, of ash. From the point of view of economical working, therefore, it is necessary to watch both ends of the washery, and particularly to •reduce the amount of coal lost in the dirt. Frequent testing is necessary, and it is advantageous to provide the man in charge of the washery with simple testing apparatus. For quick tests, chloroform is a good medium to use. This has a specific gravity of about 1*4, and has the added advantage that if accurate tests are required, and the coal is to be used for ash tests afterwards, it can easily be dried. A tall cylinder of about 200 c.c. capacity, fitted with a large draincock, is used. This is about two-thirds filled with chloroform, and the coal or dirt to be tested is dropped into it a little at a time. When the whole of the sample has been dropped in, it should be gently agitated, when it will be found that the coal will float on the chloroform, whilst the dirt will have sunk. (Gas World.) Electrical Troubles and their Remedies. In a paper read before the West of Scotland branch of the Association of Mining Electrical Engineers, Mr. T. Anderson gave an account of troubles he had met with on electrical apparatus in mine service. In one case, a 200-yd. length of three-core paper insulated, lead covered, 3,000-volt cable, which ran overhead from a power house to a transmission pole, was continued by bare conductors to a substation at a mine about a mile away. While the plant was on full load, the circuit breaker in the power house opened repeatedly, so the cable was tested with a 1,000-volb megger, and showed a short circuit between two phases. After many attempts to locate the fault, it was decided to try and burn it out with high-tension current. The cable was connected to the high-tension side of a transformer, with a heavy copper fuse in circuit, and the switch closed. This blew through both lead and iron armour of the cable, about 10 ft. from one end. It was assumed that moisture had got in at the end and crept along the paper insulation, thus causing the trouble. In another case a small squirrel cage induction motor, driving a water pump, was found to be running con- siderably below,normal speed. Examination of the motor windings, etc., showed everything apparently normal. The motor was then disconnected, and reached its normal speed quite readily, but it was found that the shaft could easily be held by hand. An examination of the rotor showed a light film of oil and dirt, between the bars and end rings, which had set up a high resist- ance in the rotor circuit, and caused the reduction of speed on load. The next case dealt with referred to two similar 150 kw. two-phase alternators, which, when run in parallel, carried a large circulating current, which did- not reach the external circuit. After making various tests, it was determined to try and find whether there was any difference in the voltage waves of the two machines, and as no oscillograph was available, a lamp was connected across one phase of each machine, which was then rotated slowly.. In one case the lamp light fluctuated in synchronism with the revolutions, instead of maintaining a. steady value, thus indicating a lack of uniformity of magnetic flux around the machine; and examination showed that the air gap at the top of the rotor was double that at the bottom. On raising the bearings to equalise the gap, the trouble disappeared. In two other instances troubles were traced to centri- fugal force. In the first, a 500 kw. revolving field generator failed to excite when running, and it was found that a metal sleeve on a joint between the end of a field coil and a lead contained no solder whatever, and centri- fugal force separated the wires, thus opening the circuit. In the second case, some rotor strap connections were separated by centrifugal force, owing to rivets working loose, through the solder melting under the heat pro- duced at the imperfect joint. A simple method of locating faults in a mining loco- motive armature, with an earthed return, was described, only an ordinary ammeter being required. The arma- ture is taken apart from its field circuit (so that it cannot revolve when current is passed) . and fixed on a cradle arrangement with the brushes in position. The con- troller, fields, and armature are then wired up exactly as for working with an ammeter in circuit, and a nearly full load current is sent round the circuit. The arma- ture is then turned slowly round by hand. If there are any open circuited coils, there will be a flash at the point of open circuit, and short circuited coils will very soon become hot. One side of the system being per^ manently earthed, any earthing on any of the controller or armature winding will again cause flashing. A “ Fireless ” Dynamite Thawer. Mr. J. H. Carse (Mining and Engineering World) gives the following method for thawing frozen dynamite. Bore one or two 4-in. auger holes in the bottom of a barrel, in which place 2 to 3in. of straw, hay, sawdust, or anything that will allow water to filter through easily. A round oil can, capable of holding at least 5 galls, of water, is filled with boiling water, and placed in the centre of the barrel on top of the 2 or 3 in. of filling. The purpose of the auger holes in the bottom of the barrel is to allow any water to drain out that might accidentally be spilled. In filling the can, it is a good plan to leave an air space of about 1^