The Flow Of Gases In Furnaces

328

APPENDIX IX

about 5100 cu ft; about 30 per cent of this space will be occupied by
tubes, etc., so that the net volume will be about 3570 cu ft, or a
trifle over 100 cu m. A boiler of this size should operate on a coal
consumption of about 2.5 Ib per horsepower with 20 per cent excess
air. The gases of combustion will have the following volume:

Volume of Waste Gases per Second

120 Per Cent Air Supply
140 Per Cent Air Supply

Cubic Feet
Cubic Meter
Cubic Feet
Cubic Meter

Normal ratin 150 per cent 200 per cent 300 per cent
2!
106 159 212 318
3.001 4.501 6.003 9.005
123 185 246 369
3.483 5.225 6.966 10.449

of rating . . . of rating. . . of rating . . .

The average temperature of the gases within the setting will be
about 800° C., for which the value of the factor 1+0.003672 will be
3.936. This gives the average minimum and maximum gas
volumes of 417 and 1455 cu ft, from which the average time of the
gas inside the setting will be 8.56 seconds for the minimum amount
of gas and 2.46 seconds for the maximum amount of gas. This
gives a temperature drop of from 117° to 407° C. per second.
The distance from the grate to the smoke-flue outlet is about 60 ft,
which gives an apparent average velocity of flow ranging from
7 to 24.5 ft per second.
Assuming that the combustion arch is about 6 ft above the
grate and the gases leave the fuel at a temperature of 1400° C.,
they will have an upward pressure against the arch slightly less
than 0.10 in of water, and their velocity will be about 42 ft per
second. At 200 per cent of rating the volume of gases rising from
the fuel bed will be about 212 cu ft per second, which at 1400° C.
will give a volume of 1300 cu ft. The firebox of the boiler has a
width of 15 ft and the arch will be about 12 ft deep from the front
wall, giving an area of 180 sq ft. From Bazin's formula for the
flow of water over weirs, Professor Yesmann, of the Polytechnic
Institute of Petrograd, has developed a formula which makes it
possible to approximate the depth to which the space below this
arch will be occupied by the hot gases. In reaching this arch,