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.