102 
ON LIQUID FUEL. 
It will be seen that water has by far the greatest capacity for heat, both in the state 
of liquid and vapour, and that a very large quantity of heat is rendered latent in the con¬ 
version of water into steam. 
In the combustion of carbon each pound requires for its conversion into carbonic acid 
gas, 2*67 pounds of oxygen, which is derived from atmospheric air ; and as this contains 
only 23 per cent, by weight of oxygen, it is necessary to supply about 12 pounds (more 
accurately 11*61 pounds) of air for every pound of carbon bjurnt. 
In the combustion of hydrogen, 8 pounds of oxygen are requisite for each pound of 
hydrogen, and to furnish this about 35 pounds (more accurately 34*78 pounds) of air 
must be supplied. 
But fuel is never burnt for raising steam in such a way that the supply of air is only 
just sufficient to furnish oxygen for the conversion of its carbon into t carbonic acid gas, 
and of its hydrogen into water vapour. In order to maintain combustion, it is necessary 
to remove the gaseous products from the furnace, as well as to supply fresh air con¬ 
tinually ; and when this is effected, as usual, by the draught of a chimney, the gaseous 
combustion products become mixed with the fresh air to some extent. The effect of 
this intermixture would be to retard the combustion of the fuel, if the amount of burnt 
air or combustion products in the atmosphere of the furnace exceeded a certain propor¬ 
tion. Consequently, it is necessary to prevent this by supplying more air than would 
suffice to furnish oxygen for combustion, so as to dilute the combustion products and 
maintain an excess of oxygen in the atmosphere immediately surrounding the fuel in 
the furnace. Careful observation has shown that in ordinary boiler furnaces the quan¬ 
tity of air requisite for this purpose amounts to as much as that requisite for effecting 
the chemical change which takes place in combustion, so that the total supply of air to 
such a furnace is usually at the rate of about 24 pounds per pound of carbon burnt, and 
about 70 pounds per pound of hydrogen burnt. 
Under ordinary circumstances, the relation between the quantities of these substances 
burnt as fuel, the total heat generated, the air supply requisite for supporting combus¬ 
tion, and the furnace gas resulting from it will be as follows:— 
Fuel. 
Quantity 
burnt. 
Air supply. 
Total heat 
generated. 
Furnace gas, 
Pound. 
Pounds. 
Heat units. 
Pounds. 
Carbon. 
1 
23*22 
14,500 
24*22 
Hydrogen. 
1 
69*56 
62,032 
70*56 
The heat generated in either case is, at the moment of combustion, transferred to the 
gaseous combustion product, and raises its temperature. In the combustion of carbon, 
the whole of the heat is effective in this way ; but in the combustion of hydrogen, a 
portion of the heat generated is consumed in determining the vaporous condition of the 
water produced, in the proportion of 9 pounds for each pound of hydrogen burnt. As 
1 pound of water at 212 J F. requires 966*1 heat units to convert it into steam of the same 
temperature, the quantity of heat which becomes latent in this way amounts to 8694*9 
heat units (9x966*1) per pound of hydrogen burnt, or 14 per cent, of the total heat of 
combustion. That portion of the heat is ineffective, either for increasing the tempera¬ 
ture of the combustion product or for producing steam in the boiler; and it must there¬ 
fore be deducted from the total heat generated, in order to ascertain the amount of heat 
available, which is as follows, compared with that generated by the combustion of 
carbon:— 
1 
Quantity 
burnt. 
Total heat 
generated. 
Latent heat 
of water 
vapour pro¬ 
duced. 
Available 
heat. 
Equivalent 
evaporation 
of water 
at 212° F. 
Carbon. 
Hydrogen .... 
Pound. 
1 
1 
Heat units. 
14,500 
62,032 
Heat units. 
8695 
Heat units. 
14,500 
53,337 
Pounds. 
15 
55 
