A constant boiler feeding is the best, as it is more conducive to a uniform boiler pressure and temperature.
The fire should be fed in small charges, the fire door being kept open as little as possible, because a high temperature in the firebox is necessary to perfect combustion. If heavy charges of coal are given at once, then for some time the fire box will be cooled, and then, as the fire burns through, a fierce heat will be generated. This alternate heating and cooling is very destructive to the fire box and the tubes, as it causes an expansion and contraction that rack the joints and seams.
There are several ways of firing, each having its advocate. Upon the following points, however, there is no dispute. First, a slow combustion is the most perfect, because it produces less clinker, which saves fuel and also saves a large amount of fire cleaning and therefore of admission of cold air to the fire box. A high temperature is necessary to combustion, and the temperature of the fuel is most difficult to keep up at the sides of the fire box.
By light and frequent firing the bright fire will never be covered up, hence the temperature will be maintained. This favors an even distribution over a large surface of the fire of each shovelful of coal. But if at any point the draught is lifting the fire, and small bright pieces of fire are lifting up, it is an evidence that the fire is thinnest there or else that the bars are cleanest there. In either case, an extra amount of coal is required at that spot.
Some engineers will charge one side of the fire box lightly and then the other, this being done so as to keep up the temperature in the fire box. Others will fire first the front and then the back of the box, which answers the same purpose, but in no case should the charge be heavy.
A fireman may become so accustomed to the road and his engine, that under some conditions he may fire when he reaches certain points on the road, regulating it like clockwork.
On a trip from Philadelphia to Reading, on an engine having a Wooten fire box (whose special feature is a large fire box, which enables slow combustion), the firing was conducted as follows:
The fire was was not fed or touched until just before reaching Bridgeport, 18 miles from Philadelphia, when a thin layer of coal was spread evenly on the fire. Eleven miles were then made without opening the fire door, the next firing taking place just before reaching Phœnixville.
Ten miles were run before the next firing, which occurred just before arriving at Pottstown.
The next firing occurred at Bordenboro’, three miles from Pottstown. The remaining 8 miles were made without firing. The steam pressure did not vary more than 10 lbs. per square inch during the trip.