Flues. From the sectional view of the boiler illustrated in Fig. 32(See page 27 for figure and associated index) and Fig. 44, it is evident that a large part of the boiler is composed of flues or tubes. The flues give to the boiler the largest part of its heating surface. It is the flues which largely affect the life of the boiler and, therefore, the life of the locomotive, for this reason it is quite necessary to properly install and maintain them. A large amount of the repair costs is directly traceable to the flues. This is especially true in localities where water is found which causes scale to form on the flues from 1/16 to 1/2 inch in thickness, thus causing unequal expansion and contraction and overheating. These conditions cause the joints to break at the flue sheets. Cold air entering the fire-box door is another source of flue trouble. It is to these details that careful attention must be given in order to alleviate flue failures. Flues should be made of the best quality of charcoal iron, lap-welded, and subjected to severe tests before being used. They must be accurately made, perfectly round and smooth, must fill standard gauges perfectly, must be free from defects such as cracks, blisters, pits, welds, etc., and must be uniform in thickness throughout except at the weld where 2/100 of an inch additional thickness may be allowed. The present practice is to use tubes of from 2 to 21¼ inches in diameter. They vary in length from about 15 to 20 feet, the length depending on the construction of the boiler and locomotive as a whole. The tubes are supported at each end by letting them extend through the tube sheets. It is in the setting of the tubes that great care should be exercised. The tube sheets must be carefully aligned and the hole drilled through and reamed. These holes are usually made 1/16 of an inch larger in diameter than the outside diameter of the tubes. The tubes should be made not less than 1/4 nor more than 3/8 inch longer than the gauge distance over the front and back flue sheets. All back ends of tubes should be turned and beaded, and at least ten per cent of those in the front end. The number of tubes used varies according to the type and size of the locomotive but usually from 300 to 500 are employed. The flue sheets are made thicker than the other sheets of the boiler in order to give as wide a bearing surface for the tubes as possible. They are usually 5/8 inch thick. The flue sheets are braced or stayed by the flues and by diagonal braces fastened to the cylindrical shell. The bridges or metal in the flue sheets between two adjacent flues are usually made from 3/4 to 1 inch in width. The greater the width of the bridges, the greater the space between the flues; therefore, better circulation will be obtained.
Table of Contents; Page 38; Page 45; Index