July 21, 1916. THE COLLIERY GUARDIAN. 115 for non-conducting covering appears to be “ slag wool ” (or silicate cotton), both on account of its high non- conductive and non-;nflammable properties. Steam Engines. The requirements of collieries do not lend themselves in case of the older colliery to the installing of engines of very economical type. In the case of a colliery steam- driven throughout, the surface engines may be classed as : (1) Winding engines, (2) hauling engines, (3) pump- ing engines, (4) air compressing engines, (5) electric generating engines, (6) fan engines, (7) miscellaneous small units for various purposes. Fig 1.—Lid on Flue Cover Adapted to Lancashire Boilers. All these, as shown in detail in Prof. Schultze’s paper, are extremely uneconomical, and, generally, by the nature of their work, must be so, the only exceptions being constantly running engines, like the fan engine and electric generating engines, the latter being frequently turbines, and having the advantage of con- densation. Winding Engines. Amongst the steam engines used at collieries, the most important are the winding engines, which are of relatively large size, and work intermittently. They have to overcome a great amount of inertia to' raise and accelerate the load to be raised, which has scarcely reached its maximum speed when retardation takes place. The load factor in such an engine is very low (about 7 to 15 per cent.), depending on the load, speed of wind, and depth of shaft. It is impossible to make a winding engine economical in the steam consumption per horse-power hour, though the working is improved by balancing the rope, variable cut-off gear, and com- pounding, where the steam pressure warrants; but when all this has been done, the winding engine remains an extravagant steam eater. Condensation in the valve g C - I 400- =§ - ^300- K *0 Q 20c- y //oa- I - - 90C — I - $ | eoc- 3 ' Fig. 2.—Relative Loss with Different Non- conducting Coverings. (1) Bare pipe; (2) asbestos composition; (3) kisol and hair felt; (4) magnesia; (5) slagwool; (6) silk; (7) cork (slabs). chests, cylinder, and the steam pipes, are practically as much as with an engine using steam continuously. Some of the older winding engines were under power to such an extent that nothing less than an approximately rectangular diagram was sufficient to obtain the neces- sary starting torque. Hauling Engines. Hauling engines are usually working up to their full capacity, if not overloaded. If the engine is drawing coal up an incline, it is loaded up to nearly the last tub it can haul. With an endless rope system of haulage, the conditions are somewhat more favourable, and in some cases expansion gear is fitted, but even in such cases the result is but moderately economical. Small air-driven haulages underground are always most extra- vagant users of compressed, air, which is, in turn, as low in efficiency as 25 per cent. The over-all efficiency of such plant must be exceedingly low, in view of the long ranges of pipes and the poor class of joints gener- ally used. Common asbestos rings and common rubber rings permit a gradual escape of air; and there appears to be, after all, something on the welded joint theory. Air Compressing Engines. The average torque of a variable speed compressor is constant for constant pressure, and for this reason steam cylinders may be proportioned, so as to give the most satisfactory form of diagram. The speed of such com- pressors must be varied as the demand for compressed air fluctuates, and this point is naturally detrimental to the efficiency of the plant on the steam side. A corresponding increase in the efficiency on the air com- pressor side might reasonably be expected by reason of the compression curve approximating nearer to the isothermal curve than it would when running at high speed, but from some tests recently taken by the writer, this is discounted by the fact that on slow speeds the air has time to get heated to an appreciable extent, previous to its being compressed at all. Electric Generating Engines. Prime movers for the generation of power take various forms, from the horizontal slow-speed engines, belted to generators, to the modern high-speed direct coupled type. Belt or rope-driven sets are not recommended, owing to their low efficiency, inability to take high-load peaks, cost of upkeep and space occupied. Radiation losses are Fig. 3.—Steam Consumption of 6,000 kw., 1,500 Rev., Three-phase 10,000 Volt Turbo- alternator. 24 25 2/ /2 /& /4 /7 /5 /6 TT/r&o Generator onnt 'A " Cond/t/ons. STEAM PRESSURE=/5OL&5 PERO'GUAGE ■STEAM CONSUMPTION PER KW. NOUR UNDER"A" COND/T/ONS AND VACUUM *= 26 INCLUDING INCLUDING AUXILIARIES AUXILIARIES VA CUUM = 23 " BAROMETER =30' ---|_—4-,---1 h—|------------ Power eactor -7 \ AS '"A* EXCEPT STEAM SATURATED 2000 3000 4000 3000 6000 7000 Machine, Load /CPK ^TfAM CONSUMPTION PER RXU. F^HCUR UNDER "^CONDITIONS considerably higher for unit power as compared with the high speed sets, on account of the larger heated surface which must be exposed to the atmosphere on account of the lower speed. Vertical high-speed reciprocating sets have in the last few years reached a high state of efficiency and reliability, and form the most efficient steam-driven prime mover up to about 300 kw. The makers have designed these engines that reversal of the stresses is so. gradual that forced lubrication is sufficient to keep the bearing surfaces from coming into contact. For powers above 500 kw. turbines are the most suitable. Fan Engines. Fan engines operate on constant loads, and develop a large number of horse-power hours for their size, or, in other words, their load factor is high. Such engines naturally give a better return for money spent on them, and may be generally classed as the most favourable case at collieries. Miscellaneous Engines. At almost every colliery there are a large number of small steam engines, for a great variety of purposes— screen engines, shop engines, feed pumps, and many others. All are probably uneconomical in view of their small size, but more serious still is the condensation in the steam pipes leading from the boilers to the small engines. There is sometimes a big drop in the steam pressure with these small units, and oftener a high back pressure. Valve setting is usually left until the engine refuses to carry its load. Taking all these circumstances into consideration, is it not inevitable that the cost of producing power in collieries by the use of steam alone must be exceedingly wasteful, and would be much more economically dealt with by electric motors? To analyse how the fuel consumed is used—and the writer has taken out the figures in the case of Wharn- cliffe Silkstone Collieries—the coal consumed over a period of 18 days was 839 tons, which it was calculated produced (839 by 2,240 by 11,840) = 22,251 millions British thermal units under the conditions existing at that time. Of these heat units produced it is calculated that 1-7 per cent., or 378 millions, were utilised in the per- formance of useful work, the balance (about 22 millions) being lost in boiler losses, radiation, latent heat, inefficient engines and the general conditions of opera- tion. The demand for power was very unfavourable to by the various engines :— Approximate Hours horse-power, per or ~ ' 4 Jintermittent. Total units per cent. Intermittent.. 14’2 Constant Avge. Max. in use. . 100 900 ... 16 ... an economical result. The winding and hauling engines worked intermittently, especially the former. The following table gives the percentage of total heat units absorbed Power used for Winding ........ Hauling......... 265 ... 550 ... 16 ... „ 17 6 Air compressing... 200 ... 350 ... 24 ... Variablespeed 17’6 Fan............. 235 ... — ... 21 ... Constant . 26'2 Other engines... 100 ... 400 ... 12 ... Various . 25’66* * The high consumption of the various engines is due to their small size and condensation resulting from long pipe connections. Condensation, Compounding, and Turbines. Wherever a plentiful supply of good water is avail- able, considerable economy may be effected from con- densation, which is, indeed, essential where steam tur- bines are used: and where the water supply is limited, recourse must be had to water coolers. Condensers are of two classes—“ surface ” condensers and “ jet ” condensers—the former being preferred where the condensate is required for boiler feeding if ■ the cooling water is of a readily scaling or corrosive nature. Scale on the inside of the tubes seriously impairs the efficiency, unless ample cooling surface is provided, and necessitates the expense and delay of scaling. Water of a corrosive nature may be used in connection with brass, Muntz metal, or electrically galvanised tubes; and it is certainly better that corro- sion should occur in the condenser than in the boiler. At a large number of collieries water is pumped from several different seams, some of which water is suitable for boiler feeding and some otherwise. In such cases jet condensing holds the field. The majority of jet con- densers are so arranged that the scale is more easily removed than with surface condensers, as there is but a fraction of the area to be scaled. Until recently some doubt was expressed as to the safety of jet condensers, on account of the possibility of their flooding back on to the prime mover. Various arrangements have been devised with the object of rendering this impossible, such as a vacuum breaker, operated by a float and mounted in a small vessel adjacent to the condenser. The accumulation of scale in this device often rendered the vacuum breaker inoperative, and the present-day practice is to use a condenser of the rotary type, with which, on the accumulation of an undue quantity of water in the impeller, greater centrifugal force results, due to the higher specific gravity of the mixture in the impeller. With this arrangement, provided the speed of the impeller remains practically constant, no flooding back is to be feared. Turbines. For powers above 500 kw., the most suitable and economical steam-driven prime mover is the turbine coupled direct to its electric generator. Present-day method is practically the result of a compromise between such factors as steam economy, strength, cost of construction, size, weight, factor of safety, leakage losses, and steam temperature. Up to the present, impulse turbines have met with great success on account of their simple form, which is not liable to distortion with the pressure and tempera- ture necessary for their efficiency; but it would appear that the reaction turbine should give better results, pro- vided it were possible to use steam of the same temper- ature as the impulse turbines. Theoretically, the reaction turbine is the best, but the impulse turbine has the advantage that -there are no guide blades to cause friction, and it occupies less space. The Ljungstrom turbine, for instance, with a steam pressure of 180 lb. superheated to 650 degs. Fahr., and 28| in. of vacuum at full load with barometer at 30 in., has an average steam consumption per kw.-hour of :— Normal capacity Per cent, of full load. of plant. Kw. 100 75 50 25 1,000 12’7 .. . 13 5 .. . 14’52 .. .. 11’28 1,500 11’92 .. . 12’51 .. . 13’68 .. .. 16’83 3,000 11’36 .. . 11’8 .. . 12’76 .. . 15’25 5,000 10’89 .. . 11’31 .. . 12’23 .. .. 14’6 It will be seen that the consumption per kw.-hour can be reduced by using large units at a high load factor. For example, with a 1,500 kw. set, saving 1 lb. of steam per kw.-hour over another set :— Units generated per year on 100 per cent, load- factor ...................................... 13,140,000 Pounds of steam saved per year ................ 13,140,000 Total heat per lb. of steam ............B.T.U. 1,308 Heat absorbed per lb. of steam ......... ,, 1,280 Heat required from ccal with 75 per cent, boiler efficiency .............................B.T.U. 1,707 Weight of coal 12,500 British thermal units per lb. of steam ............................. lb. 0T365 Total weight of coal saved per year ........tons 802 Value at 10s. per ton per annum .................... .£401 Fig. 3 gives the steam consumption of a 6,000 kw. turbo-generator under different conditions of load, and the effect of superheating and high vacuum. Fig. 4 gives the relationship between full load water rates and turbine rating under varying conditions as to vacuum and superheat. A glance at these charts shows hpw imper- ative it is to superheat the steam, and make the fullest possible use of high vacuum. Engineers are sometimes inclined to treat a small saving lightly, but the diagram shown in fig. 5, giving the capital value of a plant saving only 1 lb. of steam per kw.-hour, rises high enough to be well worth consideration. Electricity for Power Distribution. The advantages of generating power on a large scale have long been appreciated, and in this direction it has been demonstrated that central stations are generally able to provide power at a very low figure. This advantage must, however, be to a certain extent discounted by reason of the transmission losses which occur to a greater