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Full text of "The Flow Of Gases In Furnaces"

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82

APPLICATION OF THE LAWS OF HYDRAULICS

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(d) Vertical Distance of Grate Bars below the Hearth of the
Furnace.—This distance must be sufficient to impress the desired
velocities upon the gases in the different portions of the furnace
and firebox.
Starting with the passage of the air through the ash pit, the
volume of air required is
8.71X1.60X0.036 = 0 in3 501 per second.
In order to secure a velocity of 0 m 50 per second in the ash pit
this pit must be given an area of 1 m2 00 = 2mOXOm 50.
To impress a velocity Vi = 0 m 50 per second upon the air, a
velocity head hi=Q m 0127 is required, neglecting friction,M
from which
5i = 1.29X0.0127 = 0 mm 016 of water.
The total free area between the grate bars is equal to
1 m2 80    n    2 _.
—r— = 0 m2 60.
o
The velocity ¥2 of the air passing through the grate will be
0.501 : 0.60 = 0 m 835 per second,
from which fe = 0.036 and
B2= 1.29X0.36 = 0 mm 046 of water column.
Resistance of the Fuel Bed.—It is assumed that the depth of the
bed of burning coal upon the grate has a thickness of 150 mm and
that it is formed of three layers or tiers of coal in small pieces,
each layer having a thickness of 50 mm; therefore in passing
through the fuel bed the air will lose its velocity three times.
The velocity head required to impress upon the air the velocity
with which it passes through each layer or tier of the fuel will
be the resistance offered by that tier to the passage of the air,
and the total resistance of the bed of fuel upon the grate will be
the sum of the resistances of the number of tiers into which it is
assumed that the bed of fuel is divided.
The volume of air which will be contained in the spaces or
interstices between the pieces of coal forming the fuel bed may
be calculated as follows: the specific weight of coal is about 1.20,
^ Velocity heads required, refer to Appendix III.