HEAT VALUE OF THE PROPER ADMIXTURE OF AIR.
When the air, drawn violently through the grates by the suction of the exhaust, strikes the glowing fuel, the oxygen in the air separates from the nitrogen and combines with the carbon of the coal. It has been mentioned that elements unite in certain fixed proportions. In some cases the same elements will combine in different proportions to form different kinds of products. If the supply of air is so liberal that there is abundance of oxygen for the burning fuel, the carbon will unite in the proportion of 12 parts by weight (one atom) with 32 parts by weight of oxygen (two atoms). This produces carbonic acid, an intensely hot gas, and therefore of great value in steam-making. If, however, the supply of air is restricted and the oxygen scarce, the atom of carbon is contented to grasp one atom of oxygen, and the combination is made at the rate of 12 parts by weight of carbon to 16 parts by weight of oxygen, producing carbonic oxide gas, which is not nearly so hot as carbonic acid gas. It makes a very important difference in the economical use of fuel which of these two gases is formed in the fire.
One pound of carbon uniting with oxygen to form carbonic acid gas generates 14,500 units of heat, or sufficient to raise 85 pounds of water from the tank temperature to the boiling-point. On the other hand, when one pound of carbon unites with oxygen to form carbonic oxide gas, only 4,500 heat-units are generated, or sufficient to raise 26½ pounds of water from the temperature of the tank to the boiling-point. The same quantity of fuel, it must be remembered, is used in both cases, the only difference being that less oxygen is in the fire mixture.