116 THE COLLIERY GUARDIAN. July 21, 1916. or less extent, depending upon the flux density, distance, and power factor. Transmission may be divided into two classes, the “ direct method,” in which the prime mover which develops the electrical energy is coupled direct to the machine or machines it operates, as is usual with direct- current systems, and the “ indirect method,” in which the pressure or nature or both are transformed. For certain purposes, direct current has many advan- tages over alternating current, as in cases such as hauling, or any duty where a high starting torque or variable speed is required, but it has the disadvantage that a commutator with the attendant risk of sparking is necessary. Of late years three-phase alternating current has been preferred, as it is possible in the case of squirrel-cage motors to have an open type of motor, which, under normal working, is free from all sparking trouble. Alter- nating current, moreover, lends itself readily to trans- formation as regards voltage while still retaining the same periodicity by means of static transformers. Experience has proved that where a number of induc- tion motors and static transformers are used the power factor is lowered and a lagging current is set up in the mains, particularly when motors have been generously proportioned and are lightly loaded. This lagging current decreases as the load on the motor increases, but it is a serious loss with most power installations. To neutralise the resulting loss, the most usual method is to couple a synchronous motor across the mains, and to •over-excite it until the power factor reaches the desired amount; and this has the additional advantage that it balances the flux on the phases. In large central power stations feeding a large area, the transmission losses are naturally greater than if the same area were fed by a number of smaller stations, and in view of the w’aste heat or coke oven gas available at collieries, it is evident that much economy might be effected by linking up a number of power stations and collieries. Taking, for instance, the district round Shef- field, Barnsley, and Wakefield, if all the collieries in this area were linked up to the central power station, fuller use might be made of each generating plant, and, by reason of the larger amount of power pooled, high load peaks would have less effect on the system. There would not be the same necessity for spare plant, as, in the event of a stoppage, power could be drawn from the mains, and a surplus of power finds its way to the power company’s mains, and finds a market. The cost 24 23 27 "D#>'Sr£A v 500 /OOO /5OO ' ZOGO £500 3000 3500 4000 Cafac/ttCoa Rw/vg 122 $ r r 3 I* § * Fig. 4.—Relation between Full Load Water Rates and Turbine Rating. per unit for inflowing and outflowing current could be adjusted so as to preserve equilibrium at any desired point. Charts of the load factor obtaining with various, industrial undertakings show that their combined load on the generating plant is less than the sum of their maximum loads. With a possible 100 per cent, demand, the maximum load on the generating plant is not more than 45 per cent. If this latter is true for a maximum demand of 100,000 horse-power, how much more would it apply when a greater power were involved? Transformers. Considerable saving in copper can be effected for long distance transmission by using a high voltage, and trans- formers are sometimes installed to step the voltage up to the required pressure. They may be divided into four classes :— Static transformers, for converting alternating current into alternating current of the same periodicity but a different voltage; Rotary converters, for converting alternating current into direct current; Static transformers with rotary converters, for con- verting high-tension alternating current into direct current; Motor generators, for converting alternating current into alternating current of a different frequency. The first of these is usually used for stepping up the station voltage to such pressure as may be suitable for transmission, a similar or other suitable unit being used at the other end of the line to supply such machinery as it may be desired to operate. Rotary converters are generally used for transforming the line voltage to such as may be required for the operation of tramways, tex- tile machinery, etc. Static transformers with rotary converters are generally used in a case where the line voltage is dangerously high for inexperienced handling. The motor generators of the fourth class are but rarely used, it having become almost universal to use three- phase alternating current of 50 periods. Gas Engines. As a means of generating power the gas engine is about equal in point of cost with the most modern type of steam generator, including modern turbines. The comparison between reciprocating steam engines such as are found at collieries, and the low efficiency of which has already been referred to, shows an immense waste of fuel and heat value. At the Carville works of the Newcastle and District Electric Supply Company current is generated at a cost of 0T32d. per unit produced, and the coal cost to produce a unit is only 0-lld. That is, the coal cost per horse-power hour is considerably under 0-ld. Where collieries have by-product coke ovens the surplus gas evolved in the coking process furnishes a source of power which if not utilised is wasted. With a good class of bituminous coal and regenerator ovens, 50 per cent, or more of the gas in the coal is surplus, so far as the heat required for the coking process is concerned. At Whamcliff e Silkstone Collieries, with 35 non- regenerator and 15 regenerator ovens (Simon-Carves system) 1,300 horse-power are being obtained daily and utilised in gas engines. If the whole 50 ovens had been regenerator ovens, 2,000 horse-power would be easily available from the same quantity of coal. The calorific value of gas is about 430 British thermal units, and although difficulties owing to the richness of the gas were at first experienced with the gas engines, they have now been overcome, and no trouble has been experienced from this cause for some time past. The power is used both for generating electricity and compressing air, as well as for driving a ventilating fan. Horse-power. Electricity .......... 800—1,000 Compressed air........ 280— 300 Ventilating fan....... 220— 220 1,300—1,520 If this power had to be generated from steam on the load factor and conditions prevailing, the lowest con- sumption which could be expected is some 2| lb. of coal per horse-power hour, which, with coal at 6s. per ton, would cost <£3,400 per annum. The actual saving was probably more than three times that amount, as a great quantity of obsolete plant was thereby stopped, the steam consumption of which was much higher than that corresponding with 2^ lb. of coal per horse-power hour. Much has been written to prove that gas engines are thoroughly reliable machines, but such is somewhat cold comfort for one who has a troublesome case to handle, and it occurs to the writer that a few troubles from his own experience, and the remedies applied, may be useful. Cylinders and piston rods are always lubricated by fresh oil, and not more than is sufficient for the purpose should be used. Excess oil will form carbon, with the attending risk of ignition. A two-cycle double-acting engine of 300 brake horse- power which operated satisfactorily for some three years on a load varying from nil to 20 per cent, overload, developed trouble due to pre-ignition when given a steady load of two-thirds full load due to the accumulation of carbon deposit on the cylinder heads and piston, and practically choking the exhaust ports. The result was insufficient air scavenge, causing the engine to be “ over gassed,” and the dirt due to incomplete combustion was deposited in the cylinder. The whole trouble has been permanently cured by cutting down the cylinder lubrica- rion by one-half, thereby saving some £6 yearly in oil, and, strange as it may seem, reducing the" wear on the cylinder walls and piston rings. Bearings are sometimes fed by drop-feed lubricators, but, except for the smaller sizes of engine, this system is not to be recommended, owing to the high oil consump- tion. The method to be preferred with large engines is forced feed, to supply sufficient oil to flood the bearings, the drainage falling into the bedplate, whence it is drawn through filters, pumped through the oil cooler, and returned to circulation. The oil consumption with this system is as low as 3 gals, per 24 hours for 1,000 horse- power, and renders local heating almost impossible. Pre-ignition may occur from several causes, viz., incorrect mixture, overload and misfiring, incandescent matter in the combustion chamber, or short-circuiting of the distributor and faulty setting of the magneto. Incorrect Mixture.—This may be so proportioned as to burn in a manner more or less active (depending on the proportion of gas to air) for a considerable time. In the case of a four-cycle engine, if the mixture is such that combustion continues during the expansion and exhaust strokes, the fresh charge diawn into the cylinder (which on the same load is identical in composition) meets the incandescent contents of the cylinder, and, in its turn, burns throughout the inlet stroke. This burning or combustion becomes far more intense on the compression stroke, and forms a “false pre-ignition.” It is but rarely that this trouble is encountered except with gas of a high hydrogen content, such as coke-oven or town gas. It has been shown that with a mixture of 8-63 per cent, of the total volume occupied by town gas the maximum pressure of 2081b. per sq. in. was not attained until 1*7 seconds after ignition occurred. With an engine running at as low a speed as 150 revolu- tions per minute, if this burning continued for half a second i.t would be sufficient to cause the ignition of the following charge. Back-firing may also occur from the same cause, but in such cases the mixture must be just rich enough to allow the burning to race the flow of mixture through the inlet valve and fire the charge on the opposite side. Dilution of the charge with a sufficient quantity of inert gas is sufficient to overcome this trouble, which has been known to crack three or four massive breach castings and cause the engine to be laid idle for months; but since the introduction of the inert (exhaust) gas no excessive strain appeared, and no further trouble was experienced. Overload and Misfiring.—Running an engine above its normal load will naturally give rise to excessive heating, which may cause pre-ignition. In such a case the load must be reduced to normal. With a multi- cylinder engine overload may occur, due to the refusal of some of the cylinders to take up their proper load, thus throwing an undue share of the work on to such cylinders as are in good working order. This refusal to carry the load may be due to a throttling of the charge or misfiring. The importance of reliable ignition cannot be over-estimated. It is, in fact, so important that most makers fit ignition plugs in duplicate, and in some cases in triplicate. Incandescent matter in the combustion chamber generally takes the form of carbonised oil, tar, and such dust as is drawn from the atmosphere with the inlet air. The presence of sulphur in the gas will cause accumu- lated dirt to bind together and adhere to the cylinder u I Q fO £O 30 40 50 50 70 30 90 /OO LOAD FACTOR % Fig. 5. walls. Asbestos joints, if allowed to project beyond the jointing faces into the cylinder, may become incandescent and cause pre-ignition. Overheated ignition plugs are a common source of trouble, and the writer has experi- enced much difficulty since the war, owing to the impos- sibility of procuring steatite .insulators. Porcelain insu- lators proved a failure, because, though sufficiently good non-conductors of electricity, they are not, like steatite, capable of dissipating the heat from the central terminal, and therefore, as soon as the central terminal became sufficiently hot pre-ignition occurred and the cylinders cracked. This trouble has, it is hoped, been permanently cured by the adoption of Lodge ignition plugs, which are mica insulated, and are so designed that expansion or contraction of the central pin does not alter the spark cap, which, with an engine operating on a varying load, is a very important matter. Short-Circuiting.—It is well known to motorists that the regularity and reliability of the ignition is dependent on the magneto. It is just the same with any multi- cylinder power gas engine. In the case of a high- tension magneto, with its distributor, a very small amount of carbon, dust or dirt is sufficient to short- circuit adjacent terminals, which would, of course, cause ignition to occur at an incorrect time. Where the wiring from the distributor to adjacent cylinders is run in iron or steel tubing, it is a common thing for current to be induced into one wire by current flowing through another wire enclosed in the same tube. It is imperative that magnetos should be reliable, and inflexibly connected to the timing shaft, as the slightest alteration of the setting of the magneto renders breakdown imminent. Safety Rules for By-Product Coke Ovens.—The Industrial Commission of Ohio recommends that the following rules should be adopted on by-product coking plants :—(1) Hoppers should be guarded in such a manner as to prevent man from falling. (2) Exposed moving parts of coal and coke handling machinery to be guarded as prescribed by general code. (It is recommended that grease cups be extended to the outside of the guards to facilitate oiling.) (3) Some method of shutting off power to be installed near coal hoppers, to enable workmen to shut down hopper shakers in case of accident. (4) All approaches leading to rooms or buildings where coal dust or gas may accumulate to bear warning signs forbidding smoking or open lights. (5) All elevated walks, stairways, and platforms to be guarded by standard railing. (Note exception under rule 6.) Would recommend that “ standard railing ” be specified as follows : To be not less than 3| ft. in height, with intermediate rail midway between top rail and floor, and shall have toe-board or toe-plate at base at least 3 in. in height. (6) Exception to rule 5 to be made for platforms at both front and back of coke ovens. (7) Quench- ing cars and lorry cars to be equipped with automatic warn- ing signals. (8) All power transmission machinery to be guarded by standard guards. (9) Reversing machines and counterweights on oven platforms should be enclosed by solid or mesh guards.