The common vertical tubular boiler still holds a prominent position, on account of its qualities of economy of floor-space and the first cost. It still also holds its bad pre-eminence as first in the list of dangerous boilers—more explosions of this type being recorded than of any other. Improvements in details in this boiler have been introduced by some makers which tend to render it less dangerous, by providing for complete circulation of the water and giving greater facilities for cleaning.
The comMOn horizontal tubular boiler has not been improved in the last ten years, except in proportions used by some makers. It remains as the most extensively used boiler in the United States, especially for moderate-sized plants. while in Europe it has never obtained much of a footing, being there considered a highly dangerous !toiler. In this country its great success has been chiefly due to its low first cost ; but it is now becoming less of a favor ite, as the water-tube boiler is coining more extensively into use.
The water-ttibe type of boiler fur land purposes has achieved an extraordinary growth during the past ten years, and it gives promise of soon being the most common form of boiler, In Europe its use is still more common than in this country, and the principal boiler exhibits at the Paris Exhibition of 1880 were of that type. Numerous modifications of OEy type have been brought out by different Makers, but the most approved form which is now adopted by several makers in tins country consists of a bank of 4-in. water-tubes, inclined at an angle of about 15°, with the horizontal surmounted by one or more horizontal water and steam drums about 36 in. in diameter. At the Philadelphia Exhibition in 1876 several water-tulte boilers were shown, but the Babcock & Wilcox was the only one of the particular variety above de scribed. This variety, however, has shown the strongest power of survival, and it is now adopted, as above said. by many makers.
In marine boilers the tendency has been to abandon a great variety of types hitherto used, and to bring into almost universal use the "Scotch" form of boiler, a plain cylindrical shell of large diameter, with two or more furnaces, leading by a vertical passage into numerous horizontal tubes. For large boilers of this type the use of the corrugated furnace-titles has become almost universal. The water-tube boiler of the general pattern used on land has not yet come into any general use at sea, although the Belleville boiler, made in France, has met success in this direction. There has, however, come into use a different type of marine water tube boiler, in which small tubes about 1 or in. in diameter are used with small drums or reservoirs, or none at all. The latter form, without drums, is known as the coil boiler. Its sole reason for existence is that it affords the largest amount of heating surface for a given bulk and weight, and is therefore used for torpedo-boats and high-speed steam launches. The other form with water-drums approaches more nearly to the land type of
water-tube boiler, and in it efforts are made to combine the desirable features of the coil boiler with the steady water-level, accessibility for repairs, and general durability of the ordinary form of water-tube boiler. Several suet) boilers are now in use on steam-yachts, and it is pro posed to use them on large ocean-going vessels, but it is too early yet to say whether any of the forms will prove permanently successful. The increase in steam pressures Carried in oc;enn vessels in recent years, up to 160 lbs. or more, makes it necessary that the Scotch form of boiler shall be built of steel plates over 1 in. in thickness. This, with its great diameter, makes it an exceedingly heavy, bulky, and costly boiler for the power it develops: and there is great need for the introduction of a new type of boiler which shall admit of the still higher pressures now desired, and he lighter and more economical of room than the present form. It is probable that some form of water-tubc boiler will soon be introduced to meet these re quirements.
The most important general change in the construction of boilers in recent years has been the almost complete substitution of soft. steel plates for the wronght-iron plates formerly used. The use of steel for steam-boilers dates kick to 1856 in England and 1862 in the United States, but it required many years to bring it into general employment. objections to it when first introduced were that it was made too high in carbon and phosphorus, the necessity for making the steel very soft then not being midertKtood, consequently cracked sheets were very eommon, and also that it was high-priced. With the introduction of the open-hearth process in Prance about 1867 and itt the United Stales in 1860, a softer grade of steel was 'nude. which, after it was learned that low phosphorus as well as low carbon was necessary for good boiler plate, became entirely successful, and better in quality than the best. boiler-iron, The im provements in steel furnaces and plant have recently greatly cheapened the cost of steel boiler plate, Sii that. it cart he 111:1110 11111011 ii?Wer cost than even ordinary grades of honer-iron, and it has therefore praetieally driven the latter out of the market. The quality of steel de sired for boiler and fire-box plates may be seen from the following specifications given by dif ferent authorities: United SIOPR NaPy.—Shell: Tensile, 58,000 to 67,000 lbs,; elongation, 22 per cent in 8 in. transverse section, 25 rwr Sin. longitudinal section. Flange: Tensile, 50.000 to 58,000 lbs.; elongation, 25 per cent in 8 in. Chemical requirements: Phosphorus, not over .035 per cent ; sulphur, not over .040 per cent. Cold-bending test : Specimen to stand being bent flat on itself. Quenching test : Steel heated to cherry red, plunged in water 82° F., and to be bent around curve one and a half times thickness of the plate.