A. In all ordinary combustion there is a definite temperature, called the ignition or kindling temperature, to which combustible substance must be heated in order that it may unite with the gas in supporting the combustion. The burning substance must not only be heated up to the kindling temperature, but it must be kept as high as this temperature, or combustion will cease.
20. Q. State why such temperature is necessary and at what place in the fire-box it is most required.
A. The center is the hottest part of the fire-box. There is a much lower temperature in the fire-box at the sides and end sheets, due to the water on the opposite sides of the sheets being of a lower temperature than the fire-box; therefore if we get as high a temperature as possible at the side and end sheets, we will increase the steam-making efficiency of the boiler. The gases which are liberated from the coal as soon as it becomes heated must attain a temperature of about 1,800 degrees Fahrenheit, known as the "temperature of ignition," before they will unite with air which must also be heated up to that point.
21. Q. How is draft created through the fire?
A. Exhaust steam escaping through the stack reduces the pressure in the smoke-box below the pressure of the atmosphere outside, therefore the air tends to force itself into the smoke-box through all openings; with everything in good condition, the easiest and largest passage for it is through the grates and other openings into the fire-box and from it through the tubes into the smoke-box and up the stack.
22. Q. Is smokeless firing practicable?
A. Yes, but it is necessary in order to obtain good results that boiler and fire-box be in good condition, coal broken to the proper firing size; then, with the hearty and intelligent co-operation of both engineer and fireman, smokeless firing is both economical and practicable.
23. Q. In what condition should the fire be in order that the best results may be obtained from the combustion of the coal?
A. The fire should be as light as the work being done by the engine will permit, evenly distributed over the grates and free from clinkers.
24. Q. How should the blower be used?