All About Engines

Raising Steam 65 rings shown in Fig. 29. The end of the flue section is flanged, or has a sort of lip formed upon it, and a flat ring of steel is placed between the ends of the sections. It confers stiffness on the tube, and the rivets are not exposed directly to the hot gases. While the flues must have a certain amount of stiffness to prevent them sagging, the chief dis- advantage arises from the fact that they are neces- sarily hotter than the boiler shell ; and owing to their length they press strongly against the ends of the boiler. Thus if a flue of a 30-ft. boiler is exactly the same length as the shell at the freezing point, there will be about f inch difference between them when the average Fahr, and of the shell 3500 Fahr. One method of allowing for this difference is to join up the sections of the flue by expansion rings, as shown in Fig. 30. A still more flexible flue is obtained by using corru- gated tubes illustrated by Fig. 31. The slight bend- ing which occurs—like the opening and closing of the pleating of an accordion—is then distributed over the whole length of the flue. If it is concen- trated at one point the metal is liable to become ‘‘fatigued” and to lose its elastic qualities. In some forms of Lancashire boiler—notably F Fig. 29.—Adamson’s ring Fig. 30. — Expansion ring Fig, 31.—Corrugated flue temperature of the flue is 500°