Part 30 (2/2)

Fear has at tiases, such as those containing sulphur fu surface of the boiler This feature has been carefully watched, however, and from plants in operation it would appear that in the absence of water or stea, there is no such har 32 Babcock & Wilcox Boiler Arranged for Utilizing Waste Heat fro may be Modified to Take Care of Practically any Kind of Waste Gas]

CHIMNEYS AND DRAFT

The height and dianed chimney depend upon the an of the flue, with its arrangement relative to the boiler or boilers, and the altitude of the plant above sea level There are so many factors involved that as yet there has been produced no for the stack sizes are largely empirical In this chapter a method sufficiently comprehensive and accurate to cover all practical cases will be developed and illustrated

Draft is the difference in pressure available for producing a flow of the gases If the gases within a stack be heated, each cubic foot will expand, and the weight of the expanded gas per cubic foot will be less than that of a cubic foot of the cold air outside the chimney

Therefore, the unit pressure at the stack base due to the weight of the coluas will be less than that due to a column of cold air

This difference in pressure, like the difference in head of water, will cause a flow of the gases into the base of the stack In its passage to the stack the cold air h the furnace or furnaces of the boilers connected to it, and it in turn becoas will also rise in the stack and the action will be continuous

The intensity of the draft, or difference in pressure, is usuallyan atrees Fahrenheit and the terees Fahrenheit, and, neglecting for the ases and the air, the difference between the weights of the external air and the internal flue gases per cubic foot is 0347 pound, obtained as follows:

Weight of a cubic foot of air at 62 degrees Fahrenheit = 0761 pound Weight of a cubic foot of air at 500 degrees Fahrenheit = 0414 pound ------------------------ Difference = 0347 pound

Therefore, a chih, assumed for the purpose of illustration to be suspended in the air, would have a pressure exerted on each square foot of its cross sectional area at its base of 0347 100 = 347 pounds As a cubic foot of water at 62 degrees Fahrenheit weighs 6232 pounds, an inch of water would exert a pressure of 6232 12 = 5193 pounds per square foot The 100-foot stack would, therefore, under the above temperature conditions, show a draft of 347 5193 or approximately 067 inches of water

Thethe proper proportion of stacks and flues is dependent upon the principle that if the cross sectional area of the stack is sufficiently large for the voluases to be handled, the intensity of the draft will depend directly upon the height; therefore, the method of procedure is as follows:

1st Select a stack of such height as will produce the draft required by the particular character of the fuel and the arate surface

2nd Deterases without undue frictional losses

The application of these rules follows:

Draft For for the difference in the density of the air and of the flue gases, is given by the formula:

/ 1 1 D = 052 H P |--- - -----| (24) T T_{1}/

in which

D = draft produced, ht of top of stack above grate bars in feet, P = atmospheric pressure in pounds per square inch, T = absolute atases

In this forases, it being assu froible in practice as a factor of correction is applied in using the forures and those corresponding to actual operating conditions

The force of draft at sea level (which corresponds to an atmospheric pressure of 147 pounds per square inch) produced by a chirees Fahrenheit and that of the flue gases at 500 degrees Fahrenheit is,

/ 1 1 D = 052 100 147 | --- - --- | = 067 521 961 /