Radiant rays go in all directions, yet they act in the most efficient manner when striking a surface exactly at a right angle to their line of movement. The sides of a fire-box are for that reason not as efficient as the surface over the fire, and a flat surface over the fire is the best that can be had, so far as that fact alone is concerned.
When combustion is completed in a furnace, then the balance of the boiler beyond the bridge wall can be utilized for taking up heat from the gases. The most of this heat has to be absorbed by actual contact; thus by the tubes the gases are finally divided, allowing that necessary contact.
Combustion should be completed on the grates for the reason that it can be effected there at the highest temperature. When this is accomplished, the fullest benefit is had from radiant heat striking the bottom of the boiler—it is just there that the bulk of the work is done.
There must necessarily be some waste of heat by its passing up the chimney to maintain draft. It is well to have the gases, as they enter the chimney, as much below 600 deg. F. (down to near the temperature of the steam) as you can and yet maintain perfect combustion.
Every steam engine has certain well-defined sounds in action which we call noises, for want of a better term, and it is upon them and their continuance that an engineer depends for assurance that all is going well.
This remark also applies to the steam boiler, which has, so to speak, a language of its own, varying in volume from the slight whisper which announces a leaking joint to the thunder burst which terribly follows a destructive explosion. The hoarse note of the safety-valve is none the less significant because common.
The dampers and doors to the furnace and ash-pit should always be closed after the fire has been drawn, in order to keep the heat of the boiler as long as possible.
But the damper must never be entirely closed while there is fire on the grate, as explosions dangerous in their character might occur in the furnace from the accumulated gases.
Flues or tubes should often be swept, as soot, in addition to its liability to becoming charged with a corroding acid, is a non-conductor of heat, and the short time spent in cleaning them will be repaid by the saving of labor in keeping up steam. In an establishment where they used but half a ton of bituminous coal per day, the time of raising steam in the morning was fifty per cent. longer when the tubes were unswept for one week than when they were swept three times a week.
Smoke will not be seen if combustion is perfect. Good firing will abate most of the smoke.