For average conditions the atmospheric pressure may be considered 14.7
pounds per square inch, and the temperature 60 degrees Fahrenheit. For
these values and various stack temperatures K becomes:
_Temperature Stack Gases_ _Constant K_
750 .0084
700 .0081
650 .0078
600 .0075
550 .0071
500 .0067
450 .0063
400 .0058
350 .0053
Draft Losses--The intensity of the draft as determined by the above
formula is theoretical and can never be observed with a draft gauge or
any recording device. However, if the ashpit doors of the boiler are
closed and there is no perceptible leakage of air through the boiler
setting or flue, the draft measured at the stack base will be
approximately the same as the theoretical draft. The difference existing
at other times represents the pressure necessary to force the gases
through the stack against their own inertia and the friction against the
sides. This difference will increase with the velocity of the gases.
With the ashpit doors closed the volume of gases passing to the stack
are a minimum and the maximum force of draft will be shown by a gauge.
As draft measurements are taken along the path of the gases, the
readings grow less as the points at which they are taken are farther
from the stack, until in the boiler ashpit, with the ashpit doors open
for freely admitting the air, there is little or no perceptible rise in
the water of the gauge. The breeching, the boiler damper, the baffles
and the tubes, and the coal on the grates all retard the passage of the
gases, and the draft from the chimney is required to overcome the
resistance offered by the various factors. The draft at the rear of the
boiler setting where connection is made to the stack or flue may be 0.5
inch, while in the furnace directly over the fire it may not be over,
say, 0.15 inch, the difference being the draft required to overcome the
resistance offered in forcing the gases through the tubes and around the
baffling.
One of the most important factors to be considered in designing a stack
is the pressure required to force the air for combustion through the bed
of fuel on the grates. This pressure will vary with the nature of the
fuel used, and in many instances will be a large percentage of the total
draft. In the case of natural draft, its measure is found directly by
noting the draft in the furnace, for with properly designed ashpit doors
it is evident that the pressure under the grates will not differ
sensibly from atmospheric pressure.
Loss in Stack--The difference between the theoretical draft as
determined by formula (24) and the amount lost by friction in the stack
proper is the available draft, or that which the draft gauge indicates
when connected to the base of the stack. The sum of the losses of draft
in the flue, boiler and furnace must be equivalent to the available
draft, and as these quantities can be determined from record of
experiments, the problem of designing a stack becomes one of
proportioning it to produce a certain available draft.
The loss in the stack due to friction of the gases can be calculated
from the following formula:
f W² C H
[Delta]D = -------- (26)
A³
in which
[Delta]D = draft loss in inches of water,
W = weight of gas in pounds passing per second,
C = perimeter of stack in feet,
H = height of stack in feet,
f = a constant with the following values at sea level:
.0015 for steel stacks, temperature of gases 600 degrees
Fahrenheit.
.0011 for steel stacks, temperature of gases 350 degrees
Fahrenheit.
.0020 for brick or brick-lined stacks, temperature of gases
600 degrees Fahrenheit.
.0015 for brick or brick-lined stacks, temperature of gases
350 degrees Fahrenheit.
A = Area of stack in square feet.
[Illustration: 24,420 Horse-power Installation of Babcock & Wilcox
Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate
Stokers in the Quarry Street Station of the Commonwealth Edison Co.,
Chicago, Ill.]
This formula can also be used for calculating the frictional losses for
flues, in which case, C = the perimeter of the flue in feet, H = the
length of the flue in feet, the other values being the same as for
stacks.
The available draft is equal to the difference between the theoretical
draft from formula (25) and the loss from formula (26), hence:
f W² C H
d^{1} = available draft = KH - -------- (27)
A³
Table 53 gives the available draft in inches that a stack 100 feet high
will produce when serving different horse powers of boilers with the
methods of calculation for other heights.