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site for their conversion into carbonic acid gas and water vapour. From these considerations it will be evident that in the combustion of fuel, under ordinary conditions, there is always a great waste of heat actually generated and available. The total waste is considerably greater in the combustion of hydrogen than it is in the combustion of carbon, amounting in the one case to 32’6 per cent., and in the other to 24 per cent, of the total heat of combustion, but still the evaporative efficacy of hydrogen is nearly four times as great as that of carbon. In the combustion of hydrocarbons under these conditions, whether they be solid, liquid, or gaseous, the total amount of heat generated will be determined by the relative proportions of the carbon and hydrogen they contain. The amount of hydrogen in such substances generally ranges from one-seventh to one-fourth by weight, and for such limits the corresponding amounts of heat generated by their combustion, and their theoretical evaporative power would be as follows ;—■ Hydro carbon burnt. Carbon. Hydro gen. Total heat of combustion. Equivalent evaporation of water. at 212° at 60° lb. lb. lbs. Heat units. lbs. •86 x 14,500 = 12,470 ■14 x 62,032 = 8,684 21,154 21-9 18'8 ■75 x 14,500 = 10,775 1 ( •25 x 62,032 = 15,508 26,283 27'1 23-3 It must be remembered that these results are above the truth, for this calculation does not take into account the quantity of heat expended in effecting the decomposition of the hydrocarbon, i.e., the separation of the carbon from the hydrogen, nor does it make allowance for the circumstance that the quantities of heat calculated as being generated by the hydrogen, are calculated according to the heat-producing power of gaseous hydrogen. The results given above, as ex pressing the theoretical evaporative powers of these hydrocar bons, are therefore too high by an amount corresponding to the heat requisite to decompose the hydrocarbons and to convert the hydrogen from the liquid state it has in the hydrocarbons, to the gaseous state it has in the vapour resulting from their combustion. The difference between the theoretical evaporative power of hydrocarbons comprised within these limits of composition, and their evaporative efficacy, will be determined by the rela tive proportions of carbon and hydrogen they contain, just in the same manner as shown already, so far as relates merely to the mode in which the heat generated is disposed of amongst the combustion products constituting the furnace gas resulting from their combustion. And itis here necessary to notice another circumstance of considerable importance as regards the ad vantageous application of fuel, and especially hydrocarbon fuel. The following tabular statement will show the manner in which the heat that is consumed in producing a chimney draught, is distributed amongst the combustion products con stituting the furnace gas :— Combustion of Carbon. Furnace gas from 1 lb. carbon. Quantities of heat in furnace gas. Equivalent evapora tion of water at 212° Fahr. Carbonic acid gas lbs. 3’67 Heat units. 600° x *8 = 480 lbs. Nitrogen gas . . 8-94 600° x 2'2 = 1,320 1-4 Surplus aii- ... 11'61 600° x 2'8 = 1,680 1'7 24*22 3,480 3-6 It will be seen from this table that while the total waste of heat in the furnace gas from the combustion of 1 pound of carbon, is equivalent to 3’6 pounds of steam, more than one- half of that heat is consumed in raising the temperature of the surplus air supplied for diluting the combustion product in the furnace. Consequently, any arrangement by which this surplus supply of air could be dispensed with, and combustion maintained at the same rate, would have the effect of reducing the waste of heat to the extent of 50 per cent., and economis ing the heat generated by the carbon of the fuel to the extent of nearly 12 per cent. Herein consists the advantage gained by blowing the air into a furnace, instead of drawing it in by means of a chimney ; for in that ease the supply of air may be reduced to just enough to support combustion, and at the same time the temperature of the furnace gas may be so far reduced, either within the flues or tubes of the boiler, or in a feed water heater, as to render the greater part of the heat contained in it effective for production of steam. The possibility of economising, in this way, the heat gene rated by combustion of carbon is by no means unimportant ; but it is of far greater importance as regards the heat generated by combustion of hydrogen ; for in that case the total waste of heat arising from the discharge of the furnace gas at 600 deg. Fah. above the temperature of the air supply is equiva lent to above 12 pounds of steam per pound of hydrogen burnt, and nearly one-half of this waste heat is consumed in heating the surplus air supply. Therefore by dispensing with this surplus air, and cooling the furnace gas in a feed-water heater, a saving of something like one-fourth of the total available heat might be effected. A further advantage would also result from the increased temperature of combustion, viz., 4,692 deg. Fah. for carbon, and 4,922 deg. Fah. for hydrogen, and the consequent more ready transmission of heat from the combustion product to the water in the boiler. Combustion of Hydrogen. Furnace gas from 1 lb. hydro gen. Quantities of heat in furnace gas. Equivalent evapora tion of water at 212° Fahr. lbs. Heat units. lbs. Water vapour . . 9'00 600° x 4-3 = 2,580 2-7 Nitrogen gas . . . 26’78 600° X 6-6 = 3,960 4-1 Surplus air . . . 34-78 600° x 8’3 = 4,980 51 70’56 Latent heat 1 11,520 11-9 of water > 8,695 9-0 vapour..) 20,215 20-9 The combustion of the carbon and hydrogen of fuel pre sents another point of difference, which is important as regards the extent to which the available heat is, under ordinary con ditions, capable of being rendered effective in producing steam. This difference is due to the presence of water vapour in the furnace gas, resulting from the combustion of hydrogen. As a consequence of this circumstance a large amount of heat is absorbed and rendered ineffective for producing steam. From the foregoing table, representing the disposition of heat amongst the furnace gas, it will be seen that every pound of water-vapour in the furnace gas corresponds to a waste of heat sufficient to produce rather more than 1| pound of steam ; and hence it will be evident how great is the disadvantage result ing from the presence of water in the furnace gas, whether originating from hydrogen burnt or from damp fuel or other wise. The volumes of the air supply and combustion products for the extreme cases of carbon and hydrogen are as follow:— Air supply at C0« F. Uombustiou prpdueU Pound. - Pounds. Cubic feet. Pounds. Cubic feet. Carbon . . . 1 24 320 25 630 Hydrogen . . 1 69 960 70 2,044