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| | |Effec-|
| Height of Chimneys and Commercial Horsepower Capacity |Side| tive |Actual
Diam-+---+---+---+-----+-----+-----+-----+-----+-----+-----+-----+ of | Area | Area
eter |50 |60 |70 | 80 | 90 | 100 | 110 | 125 | 150 | 175 | 200 | Sq.|Square|Square
In. |Ft.|Ft.|Ft.| Ft. | Ft. | Ft. | Ft. | Ft. | Ft. | Ft. | Ft. | In.| Ft. | Ft.
-----+---+---+---+-----+-----+-----+-----+-----+-----+-----+-----+----+------+------
18 | 23| 25| 27| | | | | | | | | 16| .97| 1.77
21 | 35| 38| 41| | | | | | | | | 19| 1.47| 2.41
24 | 49| 54| 58| 62| | | | | | | | 22| 2.08| 3.14
27 | 65| 72| 78| 83| 87| | | | | | | 24| 2.78| 3.98
30 | 84| 92|100| 107| 113| 119| | | | | | 27| 3.58| 4.91
33 |105|115|125| 133| 141| 149| | | | | | 30| 4.48| 5.94
36 |128|141|152| 163| 173| 182| 191| | | | | 32| 5.47| 7.07
39 |154|168|183| 196| 208| 219| 229| | | | | 35| 6.57| 8.30
42 |182|200|216| 231| 245| 258| 271| 288| | | | 38| 7.76| 9.62
48 | |269|290| 311| 330| 348| 365| 389| | | | 43| 10.44| 12.57
54 | |348|376| 402| 427| 449| 472| 503| 551| | | 48| 13.51| 15.90
60 | |436|471| 503| 536| 565| 593| 632| 692| 748| | 54| 16.98| 19.64
66 | | |579| 620| 658| 694| 728| 776| 849| 918| 981| 59| 20.83| 23.76
72 | | |698| 746| 792| 835| 876| 934|1,023|1,105|1,181| 64| 25.08| 28.27
78 | | | | 885| 949| 990|1,038|1,107|1,212|1,310|1,400| 70| 29.73| 33.18
84 | | | |1,035|1,098|1,157|1,214|1,294|1,418|1,531|1,637| 75| 34.76| 38.48
90 | | | | |1,269|1,338|1,403|1,496|1,639|1,770|1,893| 80| 40.19| 44.18
96 | | | | | |1,532|1,606|1,712|1,876|2,027|2,167| 86| 46.01| 50.27
100 | | | | | | |1,760|1,865|2,043|2,197|2,359| 89| 50.11| 54.54
104 | | | | | | |1,899|2,024|2,218|2,395|2,560| 93| 54.39| 59.00
108 | | | | | | |2,051|2,190|2,399|2,591|2,770| 96| 58.83| 63.62
112 | | | | | | | |2,323|2,588|2,795|2,983| 100| 63.46| 68.42
118 | | | | | | | |2,632|2,883|3,114|3,339| 105| 70.71| 75.94
120 | | | | | | | |2,725|2,986|3,225|3,447| 107| 73.22| 78.54
124 | | | | | | | |2,915|3,193|3,449|3,687| 110| 78.31| 83.86
130 | | | | | | | |3,165|3,467|3,745|4,004| 116| 85.04| 90.76
136 | | | | | | | | |3,868|4,178|4,466| 121| 94.85|100.88
142 | | | | | | | | |4,305|4,567|4,886| 126|103.69|109.98
150 | | | | | | | | |4,719|5,097|5,448| 133|115.72|122.72
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Considering the first requirement, a circular flue is considered more
efficient than a square one, because its inside surface offers less
resistance to the passage of the gases, and there is not the likelihood
of eddies being formed. There is much difference of opinion among
engineers as to whether a stack should be narrower toward the top or
increased in size. The practice is to taper a stack toward the top,
this being done more on account of the necessity for increasing its
stability than because of the draft. Some stacks have been built,
however, with a larger inside diameter at the top than at the bottom,
with the idea of providing a greater sectional area for the passage of
the gases as their velocity is decreased. The capacity of the stack
for carrying off the products of combustion depends on the temperature
of the inside gases as compared with the temperature of the outside
air. The average temperature in stacks for power purposes ranges from
450° to 600° F., and, therefore, as there is little difference in the
travel of gases in flues between these temperatures, Table I can safely
be used in determining the diameter and height of stack for a given
capacity of power plant.
In Table I, it will be observed that the capacity of the stack is given
in horsepower, and in calculating this table it was considered that
5 pounds of coal were burned to develop 1 horsepower, this being a
high figure with the present economical systems of power generation.
Allowance has also been made, in this table, for the friction of the
gases against the side walls of the stack, it being considered that a
2-inch layer of dead air exists between the stack lining and the gases.
[Illustration: FIG. 31]
=65. Stability of Brick Chimneys.=—In considering the stability of
brick stacks, the overturning moment due to the wind must not exceed
the resisting moment of the stack to overturning about the base. For
instance, referring to Fig. 31, the pressure _p_ due to the wind acts
with the lever arm _x_ about the base of the stack, tending to overturn
it. The stack, or chimney, resists this overturning moment with its
weight _w_, acting through a lever arm _y_; if these two moments
are equal, the stack can be considered safe under the conditions
considered, though it is better to have some factor of safety, 2
usually being sufficient. An easy formula by which to determine whether
a stack is stable or not, is as follows:
_h_² × _dc_
_w_ = ------------
b
in which _w_ = weight of stack, in pounds;
_h_ = height of stack, in feet;
_d_ = mean diameter of stack, in feet;
_c_ = constant;
_b_ = width of base.
The constant _c_ varies with the shape of the stack. For a square
stack, when the wind is blowing at hurricane violence, 56 is used; for
an octagonal stack, 35; and for a round stack, 28.
To demonstrate this formula, consider a square chimney having an
average breadth of 8 feet and a width at base of 10 feet, the stack
being 100 feet high. The problem is, therefore, to find what the weight
of the stack must be in order to resist the greatest wind pressure
likely to occur. By substitution, in the formula,
100 × 100 × 8 × 56
_w_ = ------------------ = 448,000 pounds
10
With brickwork weighing about 120 pounds per cubic foot, the chimney in
question must therefore have an average thickness of somewhat more than
13 inches.
[Illustration: FIG. 32]
=66.= A good rule to follow in designing brick stacks is to make
the base at least one-tenth of the height. For stacks under 5 feet
in diameter, the walls for the first 25 feet from the top may be 8
inches, increased 4½ inches for each additional 25 feet from the top.
If the stack is more than 5 feet in diameter, the thickness at the top
should be 1½ bricks, or 12 inches, with a 4½-inch increase for each
25 feet. If the stack is less than 3 feet in diameter, the brickwork
for the first 10 feet from the top may be as little as 4½ inches; this
thickness, however, is not recommended, as the weather is likely to
penetrate such a thin wall, and sooner or later, together with the
exposure to the gases, destroy the brickwork.
=67. Construction of Brick Chimneys.=—All brick stacks must be provided
with a cast-iron or stone coping at the top, and it is usually well
to tie them in toward the base with good heavy stone band courses.
In constructing brick stacks, the brickwork should be laid up in
lime-and-cement mortar, and the bricks well covered and slid in place,
not just tapped or hit with the handle of a trowel.
All chimneys should also be provided, for a distance of at least
one-third of their height from the base, with a fire-brick lining, laid
up in fireclay, and at the bottom of this lining, where the flues from
the boiler enter the stack, cast-iron cleaning doors and frame should
be provided for removing soot that will accumulate and drop down. A
good example of a brick stack is given in Fig. 32; this stack has a
capacity of 500 horsepower, and is sufficiently stable to resist any
wind pressure.