Part 31 (1/2)

Under the same temperature conditions this chimney at an atmospheric pressure of 10 pounds per square inch (which corresponds to an altitude of about 10,000 feet above sea level) would produce a draft of,

/ 1 1 D = 052 100 10 | --- - --- | = 045 521 961 /

For use in applying this formula it is convenient to tabulate values of the product

/ 1 1 052 147|--- - -----| T T_{1}/

which ill call K, for various values of T_{1} With these values calculated for assumed atmospheric temperature and pressure (24) becoe conditions the atmospheric pressure may be considered 147 pounds per square inch, and the terees 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 forauge 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 sa at other tiases 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 volu to the stack are a e

As draft ases, 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 ad the air, there is little or no perceptible rise in the water of the gauge The breeching, the boiler darates 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 05 inch, while in the furnace directly over the fire itthe draft required to overcoh the tubes and around the baffling

One of thea stack is the pressure required to force the air for corates This pressure will vary with the nature of the fuel used, and in e of the total draft In the case of natural draft, itsthe 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 deter a stack beco it to produce a certain available draft

The loss in the stack due to friction of the gases can be calculated fro formula:

f W C H [Delta]D = -------- (26) A

in which

[Delta]D = draft loss in inches of water, W = weight of gas in pounds passing per second, C = periht of stack in feet, f = a constant with the following values at sea level: 0015 for steel stacks, terees Fahrenheit

0011 for steel stacks, terees Fahrenheit

0020 for brick or brick-lined stacks, terees Fahrenheit

0015 for brick or brick-lined stacks, terees 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 Coo, Ill]

This for the frictional losses for flues, in which case, C = the perith 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 W C H d{1} = available draft = KH - -------- (27) A

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 hts

TABLE 53