LARGE LOCOMOTIVE BOILERS. - by Mn
Total Page:16
File Type:pdf, Size:1020Kb
Fsn. 190G. 165 LARGE LOCOMOTIVE BOILERS. - BY Mn. GEORGE J. CHURCHWARD,- Member of Council, OF SWISDOS. The modern locomotive problem is principally a question of boiler. The great increase in the size of boilers and in the pressures carried, which has taken place during the past few years, has necessitated the reconsideration of the principles of design which had been worked out and settled dpring many years’ experience with comparatively small boilers carrying low pressures. The high temperatures incidental to the higher pressures have required the provision of much more liberal water-spaces and better provision for circulation. Locomotive engineers Lave now apparently settled down to the use of one of two types of boiler for very large cngines, the wide fire-box extending over the frames and wheels, and the long narrow box sloping up over the axles behind the main drivers. In Great Britain the contracted loading-gauge prohibits the use of the wide fire-box type over wheels larger than 4 feet 6 inches diameter, so that it is not being used so generally as in America, where it has become practically universal. In America the great power of engines now employed renders the wide fire-box a necessity, but in Great Britain, where the coal burnt per mile is very much leas, few boilers of this kind have been built. On the Great Downloaded from pme.sagepub.com at Kungl Tekniska Hogskolan / Royal Institute of Technology on June 4, 2016 166 LARGE LOCOXOTIVE BOILEIIS. FXB. 1906. Xorthern Railway Mr. Ivatt has provided his fine “Atlantic” class with wide fire-boxes, shown by Fig. 8, Plate 22, and they are undoubtedly very successful. On the North Eastern Railway Mr. Worsdell has also designed a wide box for the boiler of his new Atlantic” type. Mr. Holden’s boiler, on the heavy suburban engine for the Great Eastern Railway, is the largest of the type yet built in this country, Fig. 3, Plate 30. For the Great Western Railway Mr. Dean designed and built some goods engines with wide fire-boxes, shown by Fig. 11, Plate 24, and the author has since designed, but not yet built, a modified form of the same type to be carried over 4 feet 6 inches wheels, Fig. 10, Plate 23. Much more experience has been gained with the wide box in America than in this country, and, so far as the author has been able to ascertain, it has been found there that the poorer coals in large quantities can be burnt with much greater facility and economy in this type than in the narrow pattern. This advantage is offset to some extent by the fact that, when standing, there is considerable waste in the wide grates as compared with the narrow, and this is, of course, serious when goods trains are kept standing about, as is often the case here. This disadvantage has been found on the Great Western Railway, but no doubt careful design and fitting of sshpans will keep this waste within bounds. A much more serious trouble has been found in the leaking of tubes in these boilers. This seems to be quite general, and the Naster Mechanics’ Association has a committee specially investigating this question with a view to finding a remedy. All methods of tube- expanding have been tried, and also much wider spacing, even up to and over 1 inch, without curing the trouble. The reduction of the depth of the fire-box, in order to get a long box sloping over the trailing wheels of coupled engines, certainly increased the trouble from leakage of stays, but the alternative of a wide fire-box entails a much heavier engine for most of the types, and then apparently tube- trouble is increased. The wide fire-box evidently requires a higher standard of skill in the fireman, for unless the grate is kept well and evenly covered, there is a tendency to have an excess of air, reducing efficiency and increasing tube-trouble. With modern high pressures Downloaded from pme.sagepub.com at Kungl Tekniska Hogskolan / Royal Institute of Technology on June 4, 2016 FER. 1906. LARGE LOOOMOTIVE BOILERS. 167 the temperature of evaporation is so much increased that the provision for circulation, which was suEcient for the lower pressures formerly used, is doubtless insuEcient. Boilers in which this provision has been made have shown a very marlred reduction in tube- and stay-troubles. It mill be noticed that, in the illustrations to this Paper, very liberal areas have been given, and this is the general tendency in America. It is probable that in the wider boxes the main mass of the fire being so much nearer the tube-plate has a bad effect on the tubes, as the intensity of the temperature at the tube-plate must necessarily be much increased. The extra width of box has enabled the tubes to be put much too near the sides of the barrel. When this is done, the water to feed up the spaces between the tubes near the back tube-plate has to be drawn almost entirely from the front of the barrel, and it is possible that in some cases the space left for this purpose is inadequate. It will probably be found that neglect of this consideration is the cause of three-fourths of the tube-trouble. In the boilers, Figs. 10, 14, and 26, an effort has been mado to provide for this upward circulation near the back tube-plate by learing a space between the tubes and barrel from top to bottom, of a sectional area equal to the combined area of the vertical spaces between the tubes at all points, with a balance to provide for the water coming back from the front of the barrel to feed the water- spaces of the fire-box. There is no doubt that the upward draught of water through the spaces between the tubes for, say, two feet from the back tube-plate is very strong indeed, and in all probability this is enough to prevent the necessary feed of water down the spaces of the fire-box unless ample ares is given, so causing stay-trouble as well as tube-trouble. By putting the clack-box for both injectors under the barrel, as shown in Figs. 10, 14, and 26, and providing an internal nozzle directing the feed back towards the fire-box, considerable assistance is probably given in feeding " solid " water back to the fire-box and hottest part of the tubes. It is generally supposed that the circulation in a locomotive boiler proceeds along the bottom of the Downloaded from pme.sagepub.com at Kungl Tekniska Hogskolan / Royal Institute of Technology on June 4, 2016 168 LABGE LOC03IOTIVE BOILERS. FEB.19CG. barrel from the front end down thc fire-box front and up the sides and back of the fire-box. The author two or three years ago fitted a number of vanes in a boiler with spindles passing through lightly packed glands to the outside, with indicators to show the dircction of the flow of water. Observations showed that the circulation was generally as stated above, but a little alteration of the firing had the effect of materially changing the direction of the currents and eveu of completely reversing them. Fig. 28, Plate 32, is snbmitted to enable the results of this experiment to be appreciated. The arrows show the different directions of the currents in the various experiments. These experiments suggested the desirability of bringing a circulating pipe from the front cf the barrel, bifurcated to each side of the fire-box at the foundation ring, but the consideration of possible danger from an outside pipe, open to the boiler, caused the experiments to be abandcned. The experiment has since becn made in America, and it is reported that great reduction of trouble with bide sheets resulted. The extended length of tubes seen in some designs of wide fire-box boilers is due to the use of 6-coupled wheels in front of the fire-box. Experience of long tubes appears to be quite satisfactory, and they certainly keep up the economical efficiency of the boiler when it is bcing forced to the limit of its capacity. In this respect the long tube fulfils the same function as that performed in boilers with shorter barrels by the Serve tube (which is favoured so much on the Continent). The ratio of diameter to length of the tube undoubtedly has a most important bearing upon the steaming qualities of the boiler and upon the efficiency of the heat absorption. This is more particularly noticeable when the boilers are being worked to the limit of their capacity. If 2-inch tubes, say, are employed in barrels 11 to 12 feet long, when the boiler is being forced the length is not sufficient to absorb the heat from the amount of gases that a 2-inch tube will pass, and overheating and waste result. The amount of tube-leaking, which is experienced with modern wide boxes in America, has brought up again the iden, that the spacing should be wider, say, 1 inch instead of 4 inch, but, from the Downloaded from pme.sagepub.com at Kungl Tekniska Hogskolan / Royal Institute of Technology on June 4, 2016 PEB. 190F. LARGE LOCOMOTIVE BOILERS. 169 investigations of a Master Mechanics’ Committee, it appears that the wider spacing does not cure the trouble. It is clearly of no use to provide wider spaces for the upward current, unless equivalent area is provided for the downcoming watcr.