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ON STEAM BOILERS AND ENGINES.
Cotton mill boilers. — Dimensions.— Furnaces. — Flanging; boiler power; evaporative power.—To estimate the number of boilers required. --Inspection; cleaning; safety valves ; low water alarms; pipes, feed, and steam connections. —Boiler setting, with illustrations.-Engines.Economizers. — Feed water temperature. -Position of economizer.Chimney draught.—Oil consumption.—Driving.–Mechanical stokers.
THE author is indebted to Mr. W. H. Booth, of the
6 Mechanical World," and formerly of the Manchester Steam Users' Association, for the following remarks upon steam boilers, engines, &c., in connection with which he has had an extensive experience. Acknowledgment is also made to the proprietors of the above journal for the accompanying illustrations.
The boilers used to raise steam for either motive power or other purposes in the cotton trade are so universally of the Lancashire or internally-fired two-flued type, that we shall confine our remarks wholly to that type of boiler.
The making of boilers now ranks equal with, if not superior to the manufacture of steam-engines, and great care and skill are brought to bear upon their construction. The chief centres of the boiler-making trade in the cotton districts are Manchester, Hyde, Dukinfield, Blackburn, Bolton, and Preston. For some years past the standard size of Lancashire boiler has been 27 or 28 feet in length, with a diameter of 7 feet; and such a boiler, if double riveted, would be made from ths iron to stand 75 lbs. pressure, but during the three years ending December, 1883, the use of steam of 100 lbs. has become more common, and steel
has become the material of construction, while at the same time the diameter has been made 7 feet 6 inches, and the thickness of steel plates for 100 lbs. will be half an inch, and the boiler may be worked to 105 lbs. safely.
Furnaces, also, are made of steel, and should not exceed zths in thickness of plate, indeed ths is not an uncommon dimension.
Resistance to collapse is best obtained by means of the flanged seam, and in the writer's opinion it is desirable that the flanges should be turned upon a properly-shaped block by means of wooden mallets, as the flanging-machine is very severe upon plates, though, perhaps, cheaper than hand flanging. Hydraulic pressure is also suitable as a flanging agency, and preferable to a roller flanging machine. Boilers are now almost invariably made with flat ends, which require to be about ith inch thicker than the plates of the shell. These flat ends are attached to the shell by a circumferential flange or angle iron, as shown in Fig. 95, and they are stayed by means of five gusset plates at each end above the furnace tubes, and by two at the front and one at the back end below the tubes. There should be at least à distance of 10 inches between the circle of rivets attaching the tubes to the end plates and the lowest of the rivets attaching the gusset stay angle irons. If longitudinal stay rods are inserted, they should not be drawn tight, but should be slack enough to easily vibrate sideways 4 inches, and should be attached to the end plates by an inside and outside nut and washer. The distance apart of the stay bolts, which are two in number, should not exceed about 14 inches. In fact they should be placed as near together as the centre gusset will allow.
All manhole and mudhole mouthpieces, and the blocks for the attachment of the steam, safety valves, and other branches, should be of wrought iron or steel; cast iron is quite inadmissible above 75 lbs. pressure.
The strengths given as safe to adopt are, of course, on
the assumption that materials and workmanship are of the best.
With a standard 7 feet diameter boiler, there would be two furnaces 2 feet 9 inches diameter. The fire-grates would be 6 feet long, thus giving 33 square feet of grate surface. Such a boiler would be easily worked at fifteen tons per week of coal. It would burn eighteen tons comfortably, and twenty-one tons is a frequent rate of consumption, but it is not advisable to exceed twenty-five tons per week ; though at times of cleaning one of a pair of boilers, the writer has known thirty tons to be the rate of consumption in the boiler at work, but it is not economical to burn at such a rate. Two ordinary standard 7 feet 6 in. Lancashire boilers may be considered fully equal to supplying steam for 500 indicated horse-power in engines burning 24 lbs. of coal per indicated horse-power per hour, the coal consumed per week being under sixteen tons per boiler. With modern engines of good design the consumption of coal per indicated horse-power may be taken at about 21 lbs. per hour. If the boilers evaporate an average of 8 lbs. of water per pound of coal, this is equal to 20 lbs. of water per indicated horse-power per hour, and on this basis may be calculated the boiler power required. Thus, if we know that our mill of 70,000 spindles will require 900 horse-power to drive it, we may say that it will require 900 x 20 = 18,000 lbs. of water per hour. Dividing this by 8 gives 2,250 lbs. of coal, which in a week of fifty-six hours will amount to fully fifty-six tons. Reckoning to work our boilers at fifteen tons each, gives four boilers, to which it is good economy to add a spare boiler, making five in all. * In estimating the number of boilers required, the following formula may be used :—Where I. H. P. equals number of indicated horse-power required, and N equals number
1 I. H. P. of boilers, N = 1 +
This allows for a spare boiler, and easy working.