BOILERS FOR STATIONARY ENGINES.
25
we have shown that the ends of the 3 feet boiler, at 40 lbs. internalpressure, sustain 360 lbs. of longitudinal strain upon each inch of aplate a quarter ofan inch thick; whereas the same thickness of plateshave to bear, in the curvilinear direction, a strain of 720 lbs. Thisdifference of strain is a difficulty not easily overcome; and all thatwe can accomplish in this case will be to exercise a sound judgmentin Crossing the joints, in the quality of the workmanship, and in thedisti'ibution of the material. For the attainment of these objects,the following table, which exhibits the proportionate strength ofcylindrical boilers from 3 to 8 feet, may be useful:—
Bursting Pressure equivalent to the ultimate strength
Thickness of the
of the Rivetted Joints, as deduced from experiment.
Plates in decimal
34,000 lbs. to the square inch.
parts of an inch.
Feet. Inches.
3 0
•250
3 6
*291
4 0
•333
4 6
•376
5 °
S 6
450 lbs.
■416
•458
6 0
•500
6 6
•541
7 °
•583
7 6
•625
8 0
•666
Boilers of the simple form, and without internal flues, are subjectedonly to one species of strain; but those constructed with internalflues are exposed to the same tensile force which pervades thesimple form; and farther, to the force of compression, which tendsto collapse or crush the material of the internal flues.
From the existing state of our knowledge we must rest satisfiedthat the flues of ordinary boilers can be materially strengthened bythe introduction of iron hoops, but we are of opinion they shouldnever be introduced where deposits rapidly form, such as in marineboilers, &c.; for it must be borne in mind that there are two thick-nesses of material at the parts hooped, and the incrustation thatforms proves highly detrimental to the furnaces. In many caseswhere deposits have formed at the hoops the furnace-plates havebulged out very much.
Fairbairn gives a table of internal flues fitted with T-iron or angle-iron hoops. The length of the flues must be measured between therigid Supports; in an unsupported flue, as ordinarily constructed,