The following are a few particulars of temperatures of flames: (Franklin Inst.,' 1878, vol. lxxvi. p. 205.) Bunsen burner using 1 vol. gas, 24 air gave 1 vol. gas, 24 air, and I nitrogen gave 2,480° F. 2,150, 1,890 Bunsen burner, 1 vol. gas, 24 air and CO2, gave 2,010 H. Le Chatelier, Comp. Rend.,' 1892, vol. cxiv. p. 470. Bessemer process of steel-making . 2,984° F. 2,876 Electric light 2,390 3,272-3,812 The Temperature of Dissociation. When this temperature is reached chemical compounds are split up. For sympathetic ink and cupreous hydrate it is low, for gypsum it is higher, also for carbonate of lime, sulphate of iron, &c. For steam and for carbonic acid it is very high and has not yet been determined. From this it would follow that the higher the initial temperatures of the fuels and air the less heat is evolved, until a point is reached where no combustion takes place. This is far higher than any temperature to be met with even in a steel melting furnace, but its existence must not be lost sight of where heated air or gas is used, for the expected benefit will not be fully realised, nor will the flame temperatures be as high as expected. Furnace Temperatures. Hardly any experiments have yet been made to determine furnace temperatures, but since the introduction of H. Le Chatelier's electric pyrometer (M. E.,' 1891, p. 547, plate 115) and MM. Mesuré and Noël's optical one (I. and S. I.,' 1889, p. 251) at least one of the difficulties has been removed. The first of these instruments is a thermopile, the heated junction of the circuit consisting of wires made of two different alloys of platinum. The optical pyrometer consists of a tube in which a quartz plate is fixed near the centre, and a Nichol's prism at either end. One of these can be turned round its own axis. On looking at a heated object, and slowly turning the prism, it will be noticed that the colour changes from green to red and vice versa. The angle at which this change takes place is noted, and is the measure of the temperature. Care has to be taken not to let any daylight be reflected from the hot object, otherwise the readings will be those of the temperature of the sun. Both these instruments have to be graduated according to some reliable standard. FIG. 104 Air Thermometer.-If properly manipulated there is probably no more accurate measure of temperature than an air thermometer. All the permanent gases expand at the rate of 3665 between the freezing and the boiling point of water. This is at the rate of 0020611 per °F. It has been assumed that the absolute zero is reached when the volume of air is reduced to nothing. At the above rate this would be found to be 491° below the freezing point of water, or -459° F. Therefore if t is the Fahrenheit temperature of an object, its absolute temperature would be T=t+459°. The instrument shown in fig. 104 is based on the above facts. It consists of a small pear-shaped platinum bulb, having a small pin-hole at its apex. It is carefully filled with nitrogen gas, enclosed in fire clay, and placed in the furnace whose temperature is to be measured. When sufficiently heated the whole instrument is plunged into cold or boiling water, and is kept there point downwards till it has acquired the temperature of its surroundings. The protecting scale of fire clay has of course fallen off. The bulk is now carefully weighed, first in its present condition, viz. partly filled with water, then when quite full, and then when quite empty. The ratio of the total internal volume of the bulb, which, while in the furnace, was full of hot air, to the volume of air remaining after cooling in the water bath, is exactly the ratio of the absolute furnace temperature to that of the bath. Any other pyrometer can be graduated by placing it alongside the above. Pyrometers. Descriptions of various pyrometers will be found in the following publications. (See also p. 115.) J. Wilson, M. E.,' 1852, p. 53, mentions one which consists of a piece of platinum, to be thrown into a calorimeter while hot. M. Launy, Comp. Rend.,' 1869, vol. lxix. p. 347. The pressure exerted by the liberated carbonic acid gas from heated lime is measured. T. Carnelly and T. Burton, 'I. and S. I.,' 1884, p. 195. Various pyrometers. Prof. J. Wiborg, 'I. and S. I.,' 1888, ii. p. 110. An exposed bulb is filled with compressed air. M. Santignon, Génie C.,' 1890, vol. xvi. p. 328. This pyrometer consists of a bent metal tube which is inserted in the heated space, and a steady current of water is made to flow through it. Messrs. Heish and Folkard, 'Iron Age,' vol. xxxvii. p. 25. Platinum bulb air pyrometer. Le Roberts Austin, ' C. G.,' 1892, vol. cx. p. 152, describes Callendar's electric resistance pyrometer sensitive to ° C. up to a red heat. Le Chatelier's thermo-couple is sensitive to 2° F. up to 1,800° F. Chatelier's optical pyrometer, Mesuré and Noël's optical pyrometer. Landholt and Börnstein (1894) give a long list of melting temperatures of various alloys and metals. Temperatures in the Fuel.-It has already been explained that the temperature of a flame is proportional to the percentage of carbonic acid it contains till a point is reached where carbonic oxide is formed. H Fig. 105 illustrates, in a rough way, the conditions which may be expected in the interior of a thick fire.1 Of course this picture is mere guesswork, but it shows that by measuring the temperatures at various depths, or by analysing the gases at these points, some valuable information might still be obtained as to the process of combustion (compare p. 302). Prof. A. Ledebeur (Stahl und Eisen,' 1882, vol. ii. p. 356) made the following interesting experiments, in which the temperatures of the burning charcoal (contained in a tube) were carefully regulated. To a certain extent these experiments modify the above calculations: With thin fires it is quite possible that sufficient air for complete combustion passes through the fuel. In this case the conditions above the line A do not exist, and it is not necessary to admit air above the bars; but with thick fires, particularly with a weak draught, the upper layers of fuel effect the reduction of carbonic acid to carbonic oxide, and air must be admitted above. It will also readily suggest itself that if green coal is thrown on a fire which is still evolving much carbonic oxide, the latter will be cooled to such an extent that it cannot ignite when coming in contact with the upper air. Fires should, therefore, be allowed to burn as low as possible before recoaling. In blast furnaces no free oxygen and hardly any carbonic acid are found at a greater distance than two feet from each tuyer (p. 302). It might seem to be an advantage, even in cases of forced draught, to keep the fires thin, and thereby prevent the formation of combustible gases. This, however, is not the case, for, unless the grate surface were very much increased, there would be a serious loss, due to the blowing away of unburnt coal particles. A direct test for CO is described in Comp. Rend.,' 1897, vol. cxxiv. p. 621, and 'C. E.,' vol. exxix. p. 474. Effects of Draught.-In cases of complete combustion the weight of the products is sometimes thirty times the weight of fuel burnt. Now, suppose that the consumption is raised from 20 lbs. per square foot per hour to 40 lbs.; then the velocity of the escaping gases, which amounts to 600 lbs. per hour = 10 cubic ft. per second, would be twice as great, and the power to carry away solid particles would have been increased about fourfold. If, however, the fires are kept thick and all the gases leave the fuel as carbonic oxide, their weight is reduced to 270 lbs., and the temperature being slightly lower, and the bulk proportionately less, the power to lift up and carry away small particles of coal is reduced to about one-fifth. Of course even the thickest fires in a marine boiler will allow free oxygen and much carbonic acid to pass, so that the above favourable condition is hardly approached; but at any rate it is clear that with forced draught fires should be kept thick, restricting the admission of air under the bars, keeping the temperatures high, and giving much fuel contact to the carbonic acid gases, so as to convert them into combustible ones. A very large proportion of air should then be admitted at the doors or bridges. Naturally the horizontal draught will be excessive unless the furnaces are very large. The air which is admitted for the combustion of the gases should be as hot as possible, and when admitted at the bridges the utmost has probably been done in this direction by having passed it under the fire bars. When admitted at the doors artificial heating is the only means of warming it, but, as all draught over the fire bed has the effect of blowing away particles of coal and cinder, this trouble would only be aggravated if the air temperature and bulk were increased. An additional power to do harm is given to the draught if much air is also driven through the bed of coal, whereby all the smaller particles are carried to the top. This can only be prevented by a thick bed of coal, and this again is only possible with large furnaces. Forced and Natural Draught.-A few formulæ are necessary for explaining this subject, but they are of the simplest. v2 = 2.g.h. Here v is the velocity of air, h column of air measured in feet, corresponding to the draught pressure, and g = 32-2 feet is the acceleration due to gravity. Instead of an air column or mercury column, it is usual to express draught pressure in inches of water. The respective weights of mercury, water, and air being 13-60, 1, and 001293, the formula is changed into where H = v2 = 4,140 H = 56,300 M, pressure measured in inches of water, and M = pressure measured in inches of mercury. In practice the friction and bends. will seriously reduce the velocity, and the co-efficients have to be reduced by about 25%. It is also evident that with the same draught an incandescent, and therefore lighter, gas is more easily moved than a cold one. If T denotes its absolute temperature, while 491° is the absolute temperature at 32° F., then v2 8.5. H. T theoretically, or say 6. H. T practically. = It is as well to mention that a more general formula, but without the above correction, would be Here m is the molecular weight of a gas. This is 14.37 for air, about 16 for oxygen, 14 for nitrogen, 22 for carbonic acid, and 9 for steam. In order to find V, the volume of air discharged per second, the sectional area A, in square feet, of the orifice or channel has to be multiplied by the velocity v. The weight Q is found by multiplying this volume into the weight of a cubic foot of air, and introducing the correction for temperature. Q=A. 97.3 H H or roughly 100. AA from which it follows that the pressure H required for delivering Q lbs. of hot air per second through the section A is Resistances in Furnaces.-This formula has been used in estimating some of the values in the following table, in which an attempt is made to illustrate what takes place in two furnaces, whose diameters are respectively 30 ins. and 45 ins., with 6-ft. grates. The sectional areas of the ashpits and over the fuel would be 16 and 37 sq. ft. 20 lbs. per square foot is to be the coal consumption under natural and 40 lbs. under forced draught. The products of combustion will weigh about 24 times as much as the fuel, and it is assumed that two-thirds of the air passes through the fuel, and the other third through the door. If the third is admitted at a temperature of 32° F., it will be reasonable to assume that the temperatures in the fuel and in the flame are about 2,500° F., and that as they pass into the tubes this has been reduced to 1,000 F., while on entering the funnel it has fallen to 600° F. Doubtful though some of the following results are, they will at any rate serve to make comparisons, and then there can be no question that the large furnace has the advantage. |