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114 THE CIVIL ENGINEER AND ARCHITECT’S JOURNAL. [April 1, 1868. these combustion products, etc., one degree, and it amounts to— 25,22°F. = 14,500 5.75 In the combustion of hydrogen, under the same conditions) the products constituting the furnace gas amount to about 70 pounds per pound of hydrogen burnt, and they require the following quantities of heat to raise their temperature one degree of Fahrenheit’s scale :— Specific Heat. Pounds. Heat units. Heat units. Water vapour 9 x - 475 — 4'27500 Nitrogen gas 26'78 x '245 = 6'56110 Surplus air 34'78 x '238 = 8'27764 70'76 19'11374 Consequently, the increase of temperature resulting from the combustion of hydrogen is - 9 791° F = 62,032 ~ 8,695 ’ ' 19'114 So far, therefore, as relates to increase of temperature the effect produced by the combustion of hydrogen under these conditions is not much greater than that produced by the com bustion of an equal weight of carbon, notwithstanding the great difference in the actual quantities of heat generated, as shown below:— Total heat generated. Available heat. Increase of temperature. Carbon . . . Hydrogen . Pound. 1 1 Heat units. 14,500 62,032 Heat units. 14,500 53,337 2522°F. 2791°F. We have now to consider what portion of the available heat is, under ordinary conditions, effective in producing steam. The heated furnace gas, resulting from the combustion of the carbon or the hydrogen of fuel is the medium by which the heat generated is transferred to the water in the boiler; and if it could be managed that, between the moment of combustion, and the time when the furnace gas resulting from it is dis charged into the chimney, the whole of the available heat could be communicated to the water in the boiler, the evaporative effect realised might then be equal, or nearly equal, to the theo retical evaporative power of the fuel burnt. But this is never the case in ordinary practice. The extent to which the available heat could, in any case, become effective in producing steam by direct transmission to the boiler, must, of course, be limited by the temperature cor responding to the pressure at which steam is to be raised. If that were 50 lbs. per square inch, the furnace gas could not be cooled down below 360 deg. F. before being discharged from the heating surface of the boiler into the chimney. The quan tities of heat which would in such a case pass away in the furnace gas, without being directly effective in producing steam in the boiler, would amount to 12 per cent, in the combustion of carbon, and to 10'8 per cent, in the combustion of hydrogen, as follows:— Quan tity burnt. Fur nace gas. Quantity of heat requisite to produce increase of tempera ture = 300°. Equivalent evaporation of water at 212° F. Pound. Pounds. Heat units. Pounds. Carbon . . 1 25 300° x 5'750 = 1,725 *1'8 Hydrogen . 1 70 300° x 19'114 = 5,734 5'9 These quantities of heat would therefore be wasted as regards production of steam, except in so far as they might be applied in heating the feed-water supplied to the boiler. But, when, as in ordinary practice, the supply of air for sup porting combustion is maintained by the draught of a chimney, the temperature of the furnace gas eannot in any way be reduced below about 660 deg. F. without interfering with the draught of the chimney, and thus a waste of heat is occasioned considerably larger than that just mentioned as being the minimum waste. In very many instances the furnace gas is discharged into the chimney at a temperature very much more than 600 deg. F. above the temperature of the external air, and then the waste of heat is of course even still greater in proportion as the tem perature of the gas is higher. In the case of furnace gas, discharged at 600 deg. F. above the temperature of the air supplied to the furnace, this waste amounts to 24 per cent, of the available heat resulting from the combustion of carbon, and to 22 per cent, of that resulting from the combustion of hydrogen ; these amounts being equivalent to the evaporation of 3'6 lbs. of water at 212 deg. F. per pound of carbon burnt, and to 11'9 lbs. of water at 212 deg. F. per pound of hydrogen burnt. The amount of heat capable of becoming effective in pro ducing steam cannot therefore be greater than the difference between the total available heat and the heat thus wasted in the furnace gas. This amount is about 76 per cent, of the available heat generated by combustion of carbon, and about 78 per cent, of that generated by combustion of hydrogen. This comparison does not take into account those sources of waste which are due to imperfect combustion, but applies only to such portions of the carbon and hydrogen of fuel as are actually burnt in the furnace. In this case the comparative efficiency of these constituents of fuel in producing steam is as follows :— Combustion or Carbon. Combustion of Hydbogen. Quantity burnt, 1 lb. Equivalent evapora tion of water. at 212° at G0 Q Total heat of combustion . . Available heat Waste heat of furnace gas . . Heat units. 14,500 14,500 3,480 lbs. 15- 3'6 lbs. Effective heat ... . . 11,020 11'4 9-8 Quantity burnt, 1 lb. Equivalent evapora tion of water. at 212° at GO* Total heat of combustion . . . Latent heat of water vapour . . Available heat Waste heat of furnace gas . . . Heat units. 62,352 8,695 53,337 11,520 lbs. 64'2 11.'9 lbs. Effective heat 41,817 43'3 38 Thus the maximum evaporative efficacy of carbon and of hydrogen is, for each pound burnt, respectively equal to the conversion of about eleven and a half pounds and forty-three and a half pounds of water at 212 degrees, Fahr., into steam of the same temperature, and under the ordinary atmospheric pressure. The extent to which this efficacy is realised in the ordinary application of fuel for producing steam will depend upon the relative facilities afforded by the rate of combustion and by the construction of the boiler, for the full absorption of the effective heat from the combustion products during their passage along the flues or tubes of the boiler before being discharged into the chimney. But whatever may be the influence of these conditions in regard to evaporate effect pro duced, they do not in any degree affect the foregoing con siderations as to the maximum evaporative capabilities of the carbon and hydrogen of fuel when burnt in the manner stated, with a supply of air just twice as great as the quantity requi-