A Catechism of the Steam Engine · Chapter 11The Blast in Locomotives
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277. Q.--What is the amount of draught produced in locomotive boilers in comparison with that existing in other boilers?
A.--A good chimney of a land engine will produce a degree of exhaustion equal to from 1-1/2 to 2-1/2 inches of water. In locomotive boilers the exhaustion is in some cases equal to 12 or 13 inches of water, but from 3 to 6 inches is a more common proportion.
278. Q.--And what force of blast is necessary to produce this exhaustion?
A.--The amount varies in different engines, depending on the sectional area of the tubes and other circumstances. But on the average, it may be asserted that such a pressure of blast as will support an inch of mercury, will maintain sufficient exhaustion in the smoke box to support an inch of water; and this ratio holds whether the exhaustion is little or great. To produce an exhaustion in the smoke box, therefore, of 6 inches of water, the waste steam would require to be of sufficient pressure to support a column of 6 inches of mercury, which is equivalent to a pressure of 3 lbs. on the square inch.
279. Q.--How is the force of the blast determined?
A.--By the amount of contraction given to the mouth of the blast pipe, which is a pipe which conducts the waste steam from the cylinders and debouches at the foot of the chimney. If a strong blast be required, the mouth of this pipe requires to be correspondingly contracted, but such contraction throws a back pressure on the piston, and it is desirable to obtain the necessary draught with as little contraction of the blast pipe as possible. The blast pipe is generally a breeches pipe of which the legs join just before reaching the chimney; but it is better to join the two cylinders below, and to let a single pipe ascend to within 12 or 18 inches of the foot of the chimney. If made with too short a piece of pipe above the joining, the steam will be projected against each side of the chimney alternately, and the draught will be damaged and the chimney worn. The blast pipe should not be regularly tapered, but should be large in the body and gathered in at the mouth.
280. Q.--Is a large and high chimney conducive to strength of draught in locomotives?
A.--It has not been found to be so. A chimney of three or four times its own diameter in height appears to answer fully as well as a longer one; and it was found that when in an engine with 17 inch cylinders a chimney of 15-1/4 inches was substituted for a chimney of 17-1/2 inches, a superior performance was the result. The chimney of a locomotive should have half the area of the tubes at the ferules, which is the most contracted part, and the blast orifice should have 1/10th of the area of the chimney. The sectional area of the tubes through the ferules should be as large as possible. Tubes without ferules it is found pass one fourth more air, and tubes with ferules only at the smoke box end pass one tenth more air than when there are ferules at both ends.
281. Q.--Is the exhaustion produced by the blast as great in the fire box as in the smoke box?
A.--Experiments have been made to determine this, and in few cases has it been found to be more than about half as great as ordinary speeds; but much depends on the amount of contraction in the tubes. In an experiment made with an engine having 147 tubes of 1-3/4 inches external diameter, and 13 feet 10 inches long, and with a fire grate having an area of 9-1/2 square feet, the exhaustion at all speeds was found to be three times greater in the smoke box than in the fire box. The exhaustion in the smoke box was generally equivalent to 12 inches of water, while in the fire box it was equivalent to only 4 inches of water; showing that 4 inches were required to draw the air through the grate and 8 inches through the tubes.
282. Q.--What will be the increase of evaporation in a locomotive from a given increase of exhaustion?
A.--The rate of evaporation in a locomotive or any other boiler will vary as the quantity of air passing through the fire, and the quantity of air passing through the fire will vary nearly as the square root of the exhaustion. With four times the exhaustion, therefore, there will be about twice the evaporation, and experiment shows that this theoretical law holds with tolerable accuracy in practice.
283. Q.--But the same exhaustion will not be produced by a given strength of blast in all engines?
A.--No; engines with contracted fire grates and an inadequate sectional area of tubes, will require a stronger blast than engines of better proportions; but in any given engine the relations between the blast exhaustion and evaporation, hold which have been already defined.
284. Q.--Is the intensity of the draught under easy regulation?
A.--The intensity of the draught may easily be diminished by partially closing the damper in the chimney, and it may be increased by contracting the orifice of the blast. A variable blast pipe, the orifice of which may be enlarged or contracted at pleasure, has been much used. There are various devices for this purpose, but the best appears to be that adopted in Stephenson's engine, where a conical nozzle is moved up or down within the blast pipe, which is made somewhat larger in diameter than the base of the cone, but with a ring projecting internally, against which the base of the cone abuts when the nozzle is pushed up. When the nozzle stands at the top of the pipe the whole of the steam has to pass through it, and the intensity of the blast is increased by the increased velocity thus given to the steam; whereas when the nozzle is moved downward the steam escapes through the annular opening left between the nozzle and the pipe, as well as through the nozzle itself, and the intensity of the blast is diminished by the enlargement of the opening for the escape of the steam thus made available.
285.Q.--What is the best diameter for the tubes of locomotive boilers?
A.--Bury's locomotive with 14 inch cylinders contains 92 tubes of 2-1/8th inches external diameter, and 10 feet 6 inches long; whereas Stephenson's locomotive with 15 inch cylinders contains 150 tubes of 1-5/8ths external diameter, 13 feet 6 inches long. In Stephenson's boiler, in order that the part of the tubes next the chimney may be of any avail for the generation of steam, the draught has to be very intense, which in its turn involves a considerable expenditure of power; and it is questionable whether the increased expenditure of power upon the blast, in Stephenson's long tubed locomotives, is compensated by the increased generation of steam consequent upon the extension of the heating surface. When the tubes are small in diameter they are apt to become partially choked with pieces of coke; but an internal diameter of 1-5/8ths may be employed without inconvenience if the draught be of medium intensity.
286. Q.--Will you illustrate the relation between the length and diameter of locomotive tubes by a comparison with the proportion of flues in flue boilers?
A.--In most locomotives the velocity of the draught is such that it would require very long tubes to extract the heat from the products of combustion, if the heat were transmitted through the metal of the tubes with only the same facility as through the iron of ordinary flue boilers. The Nile steamer, with engines of 110 nominal horses power each, and with two boilers having two independent flues in each, of such dimensions as to make each flue equivalent to 55 nominal horses power, works at 62 per cent. above the nominal power, so that the actual evaporative efficacy of each flue would be equivalent to 89 actual horses power, supposing the engines to operate without expansion; but as the mean pressure in the cylinder is somewhat less than the initial pressure, the evaporative efficacy of each flue may be reckoned equivalent to 80 actual horses power. With this evaporative power there is a calorimeter of 990 square inches, or 12.3 square inches per actual horse power; whereas in Stephenson's locomotive with 150 tubes, if the evaporative power be taken at 200 cubic feet of water in the hour, which is a large supposition, the engine will be equal to 200 actual horses power. If the internal diameter of the tubes be taken at thirteen eighths of an inch, the calorimeter per actual horse power will only be 1.1136 square inches, or in other words the calorimeter in the locomotive boiler will be 11.11 times less than in the flue boiler for the same power, so that the draught in the locomotive must be 11.11 times stronger, and the ratio of the length of the tube to its diameter 11.11 times greater than in the flue boiler, supposing the heat to be transmitted with only the same facility. The flue of the Nile would require to be 35- 1/2 inches in diameter if made of the cylindrical form, and 47-3/4 feet long; the tubes of a locomotive if 1-3/8ths inch diameter would only require to be 22.19 inches long with the same velocity of draught; but as the draught is 11.11 times faster than in a flue boiler, the tubes ought to be 246.558 inches, or about 20-1/2 feet long according to this proportion. In practice, however, they are one third less than this, which reduces the heating surface from 9 to 6 square feet per actual horse power, and this length even is found to be inconvenient. It is greatly preferable therefore to increase the calorimeter, and diminish the intensity of the draught.