328 JOURNAL Or THE ISHT. OF LOCO. ENGIXEERS. LOCOMOTIVE BLAST PIPES AND CH IMNEYS. Pr9.r red before thc Znttitution by Mr. J. MAXWELL DUNN, Graduk, London, Norember 24th, Z917. PAPER No. 56. In this paper the author feels that he is, to a certain extent, re-opening a somewhat,neglected field of investiga- tion. In the comparatively early days of railways, several experiments were conducted with a view to ascertaining the most advantageous proportions, etc., for the draught pro- ducing apparatus, but of late years, in spite of the great increase in the loads that have to be hauled and the speeds that have to be maintained, this important subject seems to have been almost entirely neglected. For the sake of clearness the paper has been divided into two parts, viz. :- I. The Blast Pipe. 2. The Chimney and Petticoat Pipe. r.-THE BLAST PIPE. The obiect of the blast pipe is to utilise the waste exhaust steam from the cylinders for the production of an artificial draught, which will cause a rapid combustion of the fuel in the firebox, and on this the good steaming of the engine depends. Its origin is somewhat obscure, for although, in 1803, Richard Trevithick built the first engine in which the exhaust steam from the cylinders was turned into the chimney, it is not generally thought that he realised the advantages attendant on the use of the steam blast, and he cannot therefore be said to have been the inventor. It is interesting to note, however, that in 1827 Hackworth’s “ Royal George ” was fitted with a blast pipe in the chimney, which was provided with a contracted orifice, Fig. I, and at the same date Stephenson was still using two ordinary exhaust steam pipes, Fig. 2. However this may be, the latter employed the blast pipe on the “ Rocket ” during the Rainhill trials, and there is no doubt that the success of this engine was due in a large measure to the use of this fitting. Downloaded from jil.sagepub.com at UNIV OF VIRGINIA on June 5, 2016 LOCOMOTIVE BLAST PIPES ASD CHIMNEYS-DUNN. 329 The action of the blast is, at speeds from about 25 m.p.h. upwards, that of a continuous jet of steam drawing the gases with it by means of the friction of its sides, although at speeds below this a plunger action takes place, as there is an appreciable interval between the exhausts. EXHAUST STEAM PIPES IN USE IN 1827 T R EVITH I CK' s STEPHE NSO N' s FIC.1. FIG. 2. In main line engines, both passenger and goods, the former is the action which predominates, and should there- fore be taken into account, while in shunting engines for use in yards, etc., the latter is the action which must be Downloaded from jil.sagepub.com at UNIV OF VIRGINIA on June 5, 2016 330 JOURNAL OF THE INST. OF LOCO. ENGINEERS. considered, although in practice a compromise between the two is usually made. The foregoing considerations influence chiefly the design of the chimney and will be dealt with under that heading. One of the most important items in the design of a blast pipe is the selection of a suitable diameter for the orifice, and in determining this it is necessary to exercise careful judgment, as it is a case of deciding between two evils. By this is ‘meant, that if the diameter of the orifice is too small, back pressure on the piston will result, together with an excessive consumption of fuel, which is drawn through the tubes in a half burnt state. On the other hand, if the nozzle is too large, the draught will be impaired and the boiler will not steam properly. In short, the diameter of the orifice should be made as large as possible, consistent with the good steaming of the boiler. It has been stated in the past, and the opinion is still held to-day by a number of locomotive engineers, that it is impossible to determine by calculation alone the most suitable diameter for a blast orifice for a given engine, and that the only method is by experiment. This idea is upheld to a certain extent by observations of actual practice, for one frequently finds two engines identical in every other respect except in the diameter of the blast orifices, which may differ by as much as half an inch. This is due, no doubt, to the varying class of coal supplied to the sheds to which the engines are allocated. According to D. K. Clark, in “ Railway Machinery,” 1855, the area of the blast orifice is regulated by the fol- lowing four items:- (I) Grate area. (2) Heating surface of the tubes. (3) Cross sectional area of the chimney. (4) Cubic capacity of the smokebox. The first two of the above are of paramount importance, but the others are generally neglected. The same authority also states that being mainly regulated by the dimensions of the boiler, the size of the orifice is practically independent of the size of the cylinders. It is also stated, by the way, that the finish of the orifice influences the results to a con- siderable degree, very superior results having been obtained from a nozzle finished to an angle on the outside, as shown in Fig. 3, leaving an edge one sixteenth of an inch thick and slightly turned over from the inside. The idea of the Downloaded from jil.sagepub.com at UNIV OF VIRGINIA on June 5, 2016 LOCOMOTIVE BLAST PIPES AND CHIMSEYS-DDNN be\.el is presumably to enable the smoke, etc., to come into contact with the steam more readily as it leaves the orifice. On the subject of the diameter of orifices, some very complete experiments were made in Germany about 20 years ago by Messrs. \.on Borries and Troske, as c1 result of which the following formula was derived :- ‘If’hcn d=dia. of blast orifice in inches, R=:irea through tubes in sq. ins., &‘=grate area in sq. ins., /SxR then tl=.xgh 1 - 2/ 8+.3R CLARK’S BLAST ORIFICE FIC.3. According to hleyer, the most suitable area for a single orifice is x/zmth of the grate area in sq. ins., and results pbtained by this rule agree closely with American practice. The area of the orifice should b.e larger or smaller accwrding ;IS the fire is shallow or deep, because if the fire is deep, a greater vacuum will be necessary to draw the air through the gr,ate and vice versa. Moles\vorth gives the diameter of the blast orifice from .35.. to .4 times that of the cylinder, but this gives rather large proportions. .4 comparison of the diameters of the cylinders and blast pipes of the six comparatively modern engines, Table I., ihows that the ratio of orifice to cylinder Downloaded from jil.sagepub.com at UNIV OF VIRGINIA on June 5, 2016 .JOURNAL OF THE INST. OF LOCO. ENGINEEBB. varies from a minimum of .z5 in the c;ise of the I>. anti IF- and G.N. tank locomotives, to a maximum of .3 in the GISC of th,e G.N. express engine. Since the conditions under which an engine has to d0 its work are wntinually varying, it is impossible to obtain a definite proportion for the diameter of the orifice, which will give the best results under all circumstanc,es, and to overcome this difficulty, what are known as variable blast pipes have been devised. Theoretially they are correct, but in practice great trouble is experienced in keeping them in working order, owing to the corrosive action of the smokc- box gases. However, if the mechanism in contact with 111: gases is left fairly loose, and not made too good R fit, it will be found that they give satisfactory results. They are ;ilso, of course, open to abuse at the hands of careless drivers, who, to save themselves trouble in ,keeping steam, will always use the smallest aperture, with consequent waste of fuel. The simplest class of variable blast pipe is that invented by the late Mr. G. Macallan of the G.E.R., to all the mmn line engines of which, the author believes, the device is fitted. It consists simply of a hinged cap having an orifice about 25 per cent. smaller in diameter than that of the fixed pipe, but with this design only one variation is obtainable. (See JOVRXAL for October, 1916.) Another device is that introduced by the Variable Blast Pipe Co.. and is shown in Fig. 4. A cone mounted on a vertical spindle was placed in the orifice, so that by raising or lowering the cone the area of the orifice could be increased or decreased at will. The spindle passed through a glanc! in the bottom of the smokebox, and was connected by 'means of cranks and rods to the weigh-bar shaft, thus enabling the area of the orifice to vary with the point of cut-off. The practice of coupling the variable gear to th.e weigh-bar shaft is, in the author's opinion, a very commendable one, ;IS. once set, it works automatically and is entirely independent of the enginemen. Experiments carried out on four differedit engines fitted with this type of bltast pipe showed an average saving of 7.61 per cent. per train mile and 8.37 per cen?. per engine mile owr four corresponding engines fitted with the ordinary pipe. In America and on the European Continent, where rectangular orifices are in use, their area is easily adjusted by the simple expedient of hinging two of the sides.