In all locomotive fire-boxes sufficient air for the complete combustion of the fuel, cannot be obtained through the dampers. A supplemental supply must be taken through the fire-hole door.

The reason air must be passed through the fire-hole door is that, when carbon dioxide (formed by the complete combustion of the coal at the bottom of the fire), is drawn up into the fire-box through the fire, it combines with more carbon and becomes converted into carbon monoxide, which gas, on top of the fire, will require more oxygen to burn it. This oxygen can only be obtained through the fire-hole door. As mentioned in my first article in the January issue of the Magazine, if the necessary air is not admitted, then the carbon monoxide would escape as un-burnt gases up the smoke stack and much valuable heat would be lost.

One of the surest ways of putting a fire out is to prevent air reaching it and that is simply because you stop the supply of oxygen.

Of course, if an unnecessary amount of air is admitted you will get waste of heat as explained by reason of using some of the heat produced to heat up the nitrogen in the air. This waste, of course, is not the only one, for if the fire-hole door is left open too wide, not only does the extra air drawn in require heating up, but it reduces the temperature of the gases (already heated up) as they pass to the tubes. The air and gases pass through the tubes at a great velocity and should be of a high temperature because the heat has to be transferred quickly through the tubes into the water in the boiler. If the temperature of the gases is not very much higher than the water, the transference of heat will be slow, heat thereby being lost through this slow transference. Besides this loss of heat there will be a consumption of coal without a corresponding increase of heat.

Firemen may ask, “How can we tell how much air to let in the box—first you say ‘You don't let in enough’ and then ‘You let in too much”’?

It is not difficult for a man interested in his work to determine the right quantity of air to be admitted through the fire box door. It is known, that of the substances which burn, the last to be completely consumed is the smoke, and smoke is only carbon in minute particles. A good guiding rule for firemen is:—

Sufficient air should be admitted through the fire-hole door to allow of the smoke being completely, or almost completely, consumed and no more. To belch forth clouds of black smoke for minutes on end is the sign of an extravagant and indifferent engineman; besides, it is unnecessary and is an intolerable nuisance to the public.

Some coals are almost smokeless and, in the process of burning, the fire is incandescent. When using coal of this nature it is difficult to judge when the correct quantity of air is being fed through the fire-hole door. In such cases the fire must be watched and studied.