Stack. The stack is one of the most important features of the front end. Many different forms and proportions of stacks have been employed but at the present time only two general types are found in use to any great extent, namely, the straight and tapered stacks.
In connection with tests conducted in the Locomotive Testing Laboratory at Purdue University, it has been found that the tapered stack gives much better draft values than the straight stack. It was also found that the effect on the draft due to minor changes of proportion, both of the stack itself and the surrounding mechanism, was least noticeable when the tapered stack was used than was the case with the straight stack. A variation of one or two inches in the diameter of the tapered stack or height of the exhaust nozzle affected the draft less than similar changes with a straight stack. For these reasons, the tapered stack was recommended in preference to the straight stack. By the term tapered stack as herein referred to, is meant a stack having its least diameter or choke 16½ inches from the bottom, and a diameter above this point increasing at the rate ot two inches for each additional foot.
The diameter of any stack designed for best results is affected by the height of the exhaust nozzle. As the nozzle is raised, the diameter of the stack must be reduced and as the nozzle is lowered, the diameter of the stack must be increased. From the facts mentioned above, it can be seen there exists a close relation between the exhaust nozzle, petticoat pipe, stack, and the diaphragm; hence a standard front end arrangement has been recommended and is presented herewith.
The best arrangement of front end apparatus is shown in Fig. 56, in which
H = height of stack above boiler shell in inches
D = diameter of shell in inches
L = length of the front end in inches
P = the distance in inches stack extends into the smoke-box
N = distance in inches from base of stack to choke
b = width of stack in inches at the base
d = diameter of stack in inches at the choke
h = distance in inches of the nozzle below the center line of smoke-box
In order to obtain the best results, H and h should be made as great as possible while the other principal dimensions should be as follows:
d = .21 D + .16 h
b = 2 d or .5 D
P = .32 D
N = .22 D
Table of Contents; Page 56; Page 59; Index