A Catechism of the Steam Engine Part 14

_A._--The Nile steamer, with engines of 110 horse power by Boulton and Watt, is supplied with steam by two boilers, which are, therefore, of 55 horses power each. The height of the flue winding within the boiler is 60 inches, and its mean width 16-1/2 inches, making a sectional area or calorimeter of 990 square inches, or 18 square inches per horse power of the boiler. The length of the flue is 39 ft., making the vent 25, which is the vent proper for large boilers. In the Dee and Solway steamers, by Scott and Sinclair, the calorimeter is only 9.72 square inches per horse power; in the Eagle, by Caird, 11.9; in the Thames and Medway, by Maudslay, 11.34, and in a great number of other cases it does not rise above 12 square inches per horse power; but the engines of most of these vessels are intended to operate to a certain extent expansively, and the boilers are less powerful in evaporating efficacy on that account.

263. _Q._--Then the chief difference in the proportions established by Boulton and Watt, and those followed by the other manufacturers you have mentioned is, that Boulton and Watt set a more powerful boiler to do the same work?

_A._--That is the main difference. The proportion which one part of the boiler bears to another part is very similar in the cases cited, but the proportion of boiler relatively to the size of the engine varies very materially. Thus the calorimeter _of each boiler_ of the Dee and Solway is 1296 square inches; of the Eagle, 1548 square inches; and of the Thames and Medway, 1134 square inches; and the length of flue is 57, 60, and 52 ft. in the boilers respectively, which makes the respective vents 22-1/2, 25, and 21. Taking then the boiler of the Eagle for comparison with the boiler of the Nile, as it has the same vent, it will be seen that the proportions of the two are almost identical, for 990 is to 1548 as 39 is to 60, nearly; but Messrs. Boulton and Watt would not have set a boiler like that of the Eagle to do so much work.

264. _Q._--Then the evaporating power of the boiler varies as the sectional area of the flue?

_A._--The evaporating power varies as the square root of the area of the flue, if the length of the flue remain the same; but it varies as the area simply, if the length of the flue be increased in the same proportion as its other dimensions. The evaporating power of a boiler is referable to the amount of its heating surface, and the amount of heating surface in any flue or tube is proportional to the product of the length of the tube and the square root of its sectional area, multiplied by a certain quant.i.ty that is constant for each particular form. But in similar tubes the length is proportional to the square root of the sectional area; therefore, in similar tubes, the amount of heating surface is proportional to the sectional area. On this area also depends the quant.i.ty of hot air pa.s.sing through the flue, supposing the intensity of the draught to remain unaffected, and the quant.i.ty of hot air or smoke pa.s.sing through the flue should vary in the same ratio as the quant.i.ty of surface.

265. _Q._--A boiler, therefore, to exert four times the power, should have four times the extent of heating surface, and four times the sectional area of flue for the transmission of the smoke?

_A._--Yes; and if the same form of flue is to be retained, it should be of twice the diameter and twice the length; or twice the height and width if rectangular, and twice the length. As then the diameter or square root of the area increases in the same ratio as the length, the square root of the area divided by the length ought to be a constant quant.i.ty in each type of boiler, in order that the same proportions of flue may be retained; and in wagon boilers without an internal flue, the height in inches of the flue encircling the boiler divided by the length of the flue in feet will be 1 very nearly. Instead of the square root of the area, the effective perimeter, or outline of that part of the cross section of the flue which is effective in generating steam, may be taken; and the effective perimeter divided by the length ought to be a constant quant.i.ty in similar forms of flues and with the same velocity of draught, whatever the size of the flue may be.

266. _Q._--Will this proportion alter if the form of the flue be changed?

_A._--It is clear, that with any given area of flue, to increase the perimeter by adopting a different shape is tantamount to a diminution of the length of the flue; and, if the perimeter be diminished, the length of the flue must at the same time be increased, else it will be impossible to obtain the necessary amount of heating surface. In Boulton and Watt's wagon boilers, the sectional area of the flue in square inches per square foot of heating surface is 5.4 in the two horse boiler; in the three horse it is 4.74; in the four horse, 4.35; six horse, 3.75; eight horse, 4.33; ten horse, 3.96; twelve horse, 3.63; eighteen horse, 3.17; thirty horse, 2.52; and in the forty-five horse boiler, 2.05 square inches. Taking the amount of heating surface in the 45 horse boiler at 9 square feet per horse power, we obtain 18 square inches of sectional area of flue per horse power, which is also Boulton and Watt's proportion of sectional area for marine boilers with internal flues.

267. _Q._--If to increase the perimeter of a flue is virtually to diminish the length, then a tubular boiler where the perimeter is in effect greatly extended ought to have but a short length of tube?

_A._--The flue of the Nile steamer if reduced to the cylindrical form would be 35-1/2 inches in diameter to have the same area; but it would then require to be made 47-3/4 feet long, to have the same amount of heating surface, excluding the bottom as non-effective. Supposing that with these proportions the heat is sufficiently extracted from the smoke, then every tube of a tubular boiler in which the same draught existed ought to have very nearly the same proportions.

268. _Q._--But what are the best proportions of the parts of tubular boilers relatively with one another?

_A._--The proper relative proportions of the parts of tubular boilers may easily be ascertained by a reference to the settled proportions of flue boilers; for the same general principles are operative in both cases. In the Nile steamer each boiler of 55 horse power has about 497 square feet of flue surface or 9 square feet per horse power, reckoning the total surface as effective. The area of the flue, which is rectangular is 990 square inches, therefore the area is equal to that of a tube 35-1/2 inches in diameter; and such a tube, to have a heating surface of 497 square feet, must be 53.4 feet or 640.8 inches in length. The length, therefore, of the tube, will be about 18 times its diameter, and with the same velocity of draught these proportions must obtain, whatever the absolute dimensions of the tube may be. With a calorimeter, therefore, of 18 square inches per horse power, the length of a tube 3 inches diameter must not exceed 4 feet 6 inches, since the heat will be sufficiently extracted from the smoke in this length, if the smoke only travels at the velocity due to a calorimeter of 18 square inches per horse power.

269. _Q._--Is this, then, the maximum length of flue which can be used in tubular boilers with advantage?

_A._--By no means. The tubes of tubular boilers are almost always more than 4 feet 6 inches long, but then the calorimeter is almost always less than 18 square inches per horse power--generally about two thirds of this.

Indeed, tubular boilers with a large calorimeter are not found to be so satisfactory as where the calorimeter is small, partly from the propensity of the smoke in such cases to pa.s.s through a few of the tubes instead of the whole of them, and partly from the deposit of soot which takes place when the draught is sluggish. It is a very confusing practice, however, to speak of nominal horse power in connection with boilers, since that is a quant.i.ty quite indeterminate.

EVAPORATIVE POWER OF BOILERS.

270. _Q._--The main thing after all in boilers is their evaporative powers?

_A._--The proportions of tubular boilers, as of all boilers, should obviously have reference to the evaporation required, whereas the demand upon the boiler for steam is very often reckoned contingent upon the nominal horse power of the engine; and as the nominal power of an engine is a conventional quant.i.ty by no means in uniform proportion to the actual quant.i.ty of steam consumed, perplexing complications as to the proper proportions of boilers have in consequence sprung up, to which most of the failures in that department of engineering may be imputed. It is highly expedient, therefore, in planning boilers for any particular engine, to consider exclusively the actual power required to be produced, and to apportion the capabilities of the boiler accordingly.

271. _Q._--In other words you would recommend the inquiry to be restricted to the mode of evaporating a given number of cubic feet of water in the hour, instead of embracing the problem how an engine of a given nominal power was to be supplied with steam?

_A._--I would first, as I have already stated, consider the actual power required to be produced, and then fix the amount of expansion to be adopted. If the engine had to work up to three times its nominal power, as is now common in marine engines, I should either increase correspondingly the quant.i.ty of evaporating surface in the boiler, or adopt such an amount of expansion as would increase threefold the efficacy of the steam, or combine in a modified manner both of these arrangements. Reckoning the evaporation of a cubic foot of water in the hour as equivalent to an actual horse power, and allowing a square yard or 9 square feet as the proper proportion of flue surface to evaporate a cubic foot of water in the hour, it is clear that I must either give 27 square feet of heating surface in the boiler to have a trebled power without expansion, or I must cut off the steam at one seventh of the stroke to obtain a three-fold power without increasing the quant.i.ty of heating surface. By cutting off the steam, however, at one third of the stroke, a heating surface of 13-1/2 square feet will give a threefold power, and it will usually be the most judicious course to carry the expansion as far as possible, and then to add the proportion of heating surface necessary to make good the deficiency still found to exist.

272. _Q._--But is it certain that a cubic foot of water evaporated in the hour is equivalent to an actual horse power?

_A._--An actual horse power as fixed by Watt is 33,000 lbs. raised one foot high in the minute; and in Watt's 40 horse power engine, with a 31-1/2 inch cylinder, 7 feet stroke, and making 17-1/2 strokes a minute, the effective pressure is 6.92 lbs. on the square inch clear of all deductions. Now, as a horse power is 33,000 lbs. raised one foot high, and as there are 6.92 lbs, on the square inch, it is clear that 33,000 divided by 6.92, on 4768 square inches with 6.92 lbs. on each if lifted 1 foot or 12 inches high, will also be equal to a horse power. But 4768 square inches multiplied by 12 inches in height is 57224.4 cubic inches, or 33.1 cubic feet, and this is the quant.i.ty of steam which must be expended per minute to produce an actual horse power.

273. _Q._--But are 33 cubic feet of steam expended per minute equivalent to a cubic foot of water expended in the hour?

_A._.--Not precisely, but nearly so. A cubic foot of water produces 1669 cubic feet of steam of the atmospheric density of 15 lbs. per square inch, whereas a consumption of 33 cubic feet of steam in the minute is 1980 cubic feet in the hour. In Watt's engines about one tenth was reckoned as loss in filling the waste s.p.a.ces at the top and bottom of the cylinder, making 1872 cubic feet as the quant.i.ty consumed per hour without this waste; and in modern engines the waste at the ends of the cylinder is inconsiderable.

274. _Q._--What power was generated by a cubic foot of water in the case of the Albion Mill engines when working without expansion?

_A._--In the Albion Mill engines when working without expansion, it was found that 1 lb. of water in the shape of steam raised 28,489 lbs. 1 foot high. A cubic foot of water, therefore, or 62-1/2 lbs., if consumed in the hour, would raise 1780562.5 lbs. one foot high in the hour, or would raise 29,676 lbs. one foot high in a minute; and if to this we add one tenth for waste at the ends of the cylinder, a waste which hardly exists in modern engines, we have 32,643 lbs. raised one foot high in the minute, or a horse power very nearly. In some cases the approximation appears still nearer.

Thus, in a 40 horse engine working without expansion, Watt found that .674 feet of water were evaporated from the boiler per minute, which is just a cubic foot per horse power per hour; but it is not certain in this case that the nominal and actual power were precisely identical. It will be quite safe, however, to reckon an actual horse power as producible by the evaporation of a cubic foot of water in the hour in the case of engines working without expansion; and for boiling off this quant.i.ty in flue or wagon boilers, about 8 lbs. of coal will be required and 9 square feet of flue surface.

MODERN MARINE AND LOCOMOTIVE BOILERS.

275. _Q._--These proportions appear chiefly to refer to old boilers. I wish you to state what are the proportions of modern flue and tubular marine boilers.

_A._--In modern marine boilers the area of fire grate is less than in Mr.

Watt's original boilers, where it was one square foot to nine square feet of heating surface. The heat in the furnace is consequently more intense, and a somewhat less amount of surface suffices to evaporate a cubic foot of water. In Boulton and Watt's modern flue boilers they allow for the evaporation of a cubic foot of water 8 square feet of heating surface, 70 square inches of fire grate, 13 square inches sectional area of flues, 6 square inches sectional area of chimney, 14 square inches area over furnace bridges, ratio of area of flue to area of fire grate 1 to 5.4. To evaporate a cubic foot of water per hour in tubular boilers, the proportions are-- heating surface 9 square feet, fire grate 70 square inches, sectional area of tubes 10 square inches, sectional area of back uptake 12 square inches, sectional area of front uptake 10 square inches, sectional area of chimney 7 square inches, ratio of diameter of tube to length of tube 1/28th to 1/30th, cubical content of boiler exclusive of steam chest 6.5 cubic feet, cubical content of steam chest 1.5 cubic feet.

276. _Q._--These proportions do not apply to locomotive boilers?

_A._--Not at all. In locomotive boilers the draught is maintained by the projection of the waste steam which escapes from the cylinders up the chimney, and the draught is much more powerful and the combustion much more rapid than in cases in which the combustion is maintained by the natural draught of a chimney, except indeed the chimney be of very unusual temperature and height. The proportions proper for locomotive boilers will be seen by the dimensions of a few locomotives of approved construction, which have been found to give satisfactory results in practice, and which are recorded in the following Table:

Name of Engine

Great Britain. Pallas. Snake. Sphinx.

Diameter of cylinder 18 in. 15 in. 14-1/4 in. 18 in.

Length of stroke 24 in. 20 in. 21 in. 24 in.

Diameter of driving wheel 8 ft. 6 ft. 6-1/2 ft. 5 ft.

Inside diameter of fire box 53 in. 55 in. 41-1/3 in. 44 in.

Inside width of fire box 63 in. 42 in. 43-1/4 in. 39-1/2in.

Height of fire box above bars 63 in. 52 in. 48-1/3 in. 55-1/2in.

Number of fire bars 29 ... 32 16 Thickness of fire bars 3/4 in. 1-3/4 in. 5/8 in. 1 in.

Number of Tubes 305 134 181 142 Outside diameter of tubes 2 in. 2 in. 1-7/8 in. 2-1/8 in.

Length of tubes 11 ft 3 in 10 ft 6 in 10 ft 3-1/2 in. 14 ft 3-1/4 in.

s.p.a.ce between tubes 1/2 in. 3/4 in. 1/2 in.

Inside diameter of ferules 1-9/16 in. 1-1/2 in. 1-5/16 in. 1-5/8 in.

Diameter of chimney 17 in. 15 in. 13 in. 15-1/2 in.

Diameter of blast orifice 5-1/2 in. 4-5/8 in. 4-1/2 in. 4-3/4 in.

Area of grate 21 sq. ft. 16.04 sqft 12.4 sq. ft. 10.56 sq. ft Area of air s.p.a.ce of grate 11.4 sqft 4.08 sqft 5.54 sq. ft. 5 sq. ft.

Area of tubes 5.46 sqft 2.40 sqft 2.8 sq. ft. 2.92 sq. ft.

Area though ferules 4 sq. ft. 1.64 sqft 2 sq. ft. 2.04 sq. ft.

Area of chimney 1.77 sqft 1.23 sqft .921 sq. ft. 1.31 sq. ft.

Area of blast orifice 23.76 sqin 16.8 sqin 14.18 sq. in. 17.7 sq. in.

Heating surface of tubes 1627 sqft 668.7 sqft 823 sq. ft. 864 sq. ft.

THE BLAST IN LOCOMOTIVES.

277. _Q._--What is the amount of draught produced in locomotive boilers in comparison with that existing in other boilers?