E steamers with success and similar general types of apparatus have been running in the Roumanian service. Chapter XVIII THE MIXED SYSTEM OF COAL AND LIQUID FUEL COMBUSTION TH fuel. HERE is more in the mixed system than mere convenience. The use of solid and liquid fuel in the same furnace together modifies the conditions for each For coal the efficiency of combustion is better, and for oil the heat is better utilized. Combustion on the grate may be imperfect, but the oil atomizer so mixes up the gases from the grate with the air admitted through and above it, that combustion is much improved and the excess of air is used by the oil. Where the oil is only a fifth of the coal; the coal equivalent of the oil appears enormous. According to M. Bertin, where 5 kilos. of coal would ordinarily develop each 7,800 calories, they will produce 9,200 calories, a gain of 7,000 calories. The excess of air supplied with the 5 kilos. of coal would be 20 cubic metres, and this would suffice for the added kilogram of oil, and this would produce 11,000 calories with no further air supply. A total of 18,000 calories, compared with the original output of 7,800 calories per kilo. of coal, makes the ratio of oil to coal appear 2·31., Obvi ously a part of this is due to coal, but it may fairly be credited to the system. The limit of perfect use of air is found when the oil is one third of the coal, and the ordinary four metres cube of excess air still furnishes the theoretical 11 metres cube for the oil, the apparent equivalence of coal and oil becomes— These ratios are not perhaps secured in practice, but they serve to point to the possible advantages of the mixed system and what should be aimed at. With half and half coal and oil the ratio becomes 1-77, a figure that has been approached in certain experiments at Indret. Ratios of 3 and over which have been claimed cannot, as M. Bertin says, be justified on any hypothesis. Nor is the total consumption of the oxygen supplied at all closely approached in general practice. The proportion of free oxygen to carbonic acid is an indication of the excess of air admitted. The ratio of the air admitted to that used is These figures neglect the hydrogen. With coal burned at the rate of 100 kilos. per metre2 of grate, if the oxygen measures 8 per cent., and with 200 kilos., say 5 per cent., the fire is too thin or the draught too great. With 1 or 2 per cent. of carbonic oxide the fire is too thick and the draught poor. Both oxygen and CO present together indicates bad furnace arrange ments. A test at Indret of the trial boiler of the Jeanne d'Arc with coal alone gave the following results The same boiler on the mixed system gave the results below a better result, after all, than the mixed system produced. Though not yet beyond the experimental stage in the British Navy, the mixed system has been tried and abandoned in the Italian Navy. It has been installed in the German Navy also. In calculating the apparent effect of mixed fuel, M. Bertin assumes the case of a boiler working 1 hour and a weight of water = a per kilo. of coal ordinarily, b=the water evaporated per kilo. of mixed fuel, x=the evaporation attributed to one kilo. of oil, C=weight of coal burned per metre of grate, D= oil The vapour produced by C + D of mixed fuel, assuming a to be as in the ordinary coal fired boiler, will be Ca + Dx. Tests in the Furieux made to determine R gave the following results The figure 1.56 was greater than the figure found for oil when used alone, but this was not confirmed by tests at Cherbourg made on a Godard boiler with too forced a draught and badly arranged oil sprays, for the effect b of the mixed fuel was even inferior to that of coal alone, which shows how much the efficiency depends on good arrangement. The value of R was sought at Indret by M. Brillie in a series of tests extending from the end of 1896 to early in 1900, in view of applying mixed firing to boilers of Du Temple Guyot type in the new cruisers. The atomizers had air induction passages as in the Orde atomizer, fig. 10, but no air heating. The flames kept short and the heat kept well in the furnace, and high values of R were reached as 1.6 for a rate of combustion of 100 kilos. of coal and 50 kilos of oil per metre of grate.1 The tests, however, were too short for exactitude. Other tests made only upon engine power are, however, available. Let c be the coal per horse power ordinarily. Then d takes the place of ce in the production of one horse power, so that c-e The following table, a resumé of Navy tests on the locomotive type of boiler or torpedo boat No. 109 at Cherbourg, is of interest 1 Kilos per metre2 ÷ 5 = pounds per square foot nearly. h. 15mm 13mm 12mm 25mm | 0,388 1,358 1,302 0,94 1,09 0,494 1,58 26mm 29mm 50mm 1,354k 1,354k 1,506k 0,721 0,652 1,219 0,405 0,474 0,655 0,434 1,118 1,195 1,307 1,653 1,33 0,66 1,07 The interest lies in the falling off at high pressures, the furnace being too short satisfactorily to burn the oil at such rapid movement of the draught. Where 60 kilos. of oil were used to 80 kilos. of oil with draught but little forced, R was found to be.1.5, and the mixed system took the place of forced draught, with a result equal to the combustion of 170 kilos. of coal only, a result thought very encouraging. Very discordant results were obtained on the Milan, the Surcouf, the Pakin, and the Forbin. On the Milan especially oil proved very unsuitable to the furnaces of the Belleville boiler, as might be anticipated. On the Surcouf, on the contrary, the result of mixed fuel was to reduce total fuel consumption nearly to half that of coal alone. M. Bertin does not express any final opinion on mixed systems, but claims that where employed it is essential to success that all the details should be simple so as to avoid the danger of error on the part of a little trained personnel, such as the opening or closing of certain valves, always in their power to do. |