BOILERS (ante). Those most distinctively American are the sectional or water-tube boilers. The Babcock and Wilcox boiler consists of a series of tubes inclined from the front to the rear, and connected at each end by a manifold chamber. The forward ends are connected to the steam-drum, which lies lengthwise of the boiler. The tubes and manifolds are in the fire chamber, and there are two sets of diaphragm plates, by which the bot gas, after is deflected, first downward and then upward, being made to cross the stack of tubes three times before making its exit into the chimney. The water fills the tubes and occupies the lower part of the steam-drtim. The tubes of the Hoot boiler are likewise inclined from front to rear; they are joined at the ends by triangular caps and crow-feet, and the joints are perfected by rubber gaskets. The joints are out side the fire chamber, and the steam-drum lies crosswise of the boiler. The water does not fill all the tube-space within the tire-box, nor enter the steam-drum; by this means dry steam is 'secured. while the danger of superheating is but slight, as the space reached by the water lies ii) the upper and forward part of the tire-box. The NA hating ham boiler has its tubes, connections, and steam-drum, all inclosed in the fire-box: the tubes are traversed by interior tubes, or flues, through which the hot gases are conveyed, and thus a large tire surface is secured" The Harrison boiler is made of cast-iron spber ical shells, 8 in. in external diameter, and / of an in. thick; they are east in sections, 2 or 4 spheres together, are connected by curved necks of J in. diameter, and are held together by wrought-iron bolts and caps. The joints are accurately fitted, without packing.
The water surface of a boiler is that area of metal which has water within and flame or hot gases without; at this surface the steam is generated. The arca which has hot gases without and steam within is superheating surface, at which the steam by the recep tion of heat acquires greater expansive force. The draught-area, or calorimeter, is the cross section of the area traversed by the hot gases fire, and may be taken at any point between the furnace and the chimney. Ordinarily, however, it is restricted to the space around the tubes in the water-tuhe boilers, and to the section of the flues flue-boilers. That boiler is most efficient which shows the greatest difference between' the furnace temperature and that found at the chimney, since that difference indicates the quantity of heat which has been transferred to the water in the generation of steam.
If the combustion is complete, the heat of the furnace will depend on the quantity of air furnished, that is, upon the area of the calorimeter, whence it appears that the calo rimeter should be large. But if this space be an unbroken volume, much of the hot gas may pass through without impinging against the boiler surface, and delivering its heat, whence it is desirable that the space should be divided thoroughly; and it is evident that a reduced calorimeter limy often give better results than a larger one, not properly arranged. A designer of boilers will find important tables on this point in Appleton's Cgdoixedia of lIcelthnics.
Printing is the tendency of the water in the boiler to form spray by the bursting of the steam bubbles when they come to the surface of the water, the spray going forward with the steam into the cylinder. Here it is cooled and accumulates, especially if the exhaust port is not, either by position or capacity, adequate to its discharge. Water is practically incompressible, and if a quantity of it, greater than the volume of the clear ance, is found before the piston, near the end of the stroke, it lies between the piston and the cylinder head as mischievous as a mass of metal would be in the same position. Something must yield. The crank pin may be broken, or the cylinder split, or the head burst out, and all rods and gearing will be ruined. Printing is caused by want of steam room, or of area at the surface of the water in the bodies, or by the use of dirty water. The latter cause may be cured by collecting the water in tanks, and giving it time to settle. The others may be avoided by proper construction of the boiler, by checking the steam at the throttle, or by working the engine more expansively. Any sudden removal of pressure, as the opening of the safety-valve, or of the throttle in starting, tends to produce priming, because while the water had, at the instant of the opening, a capacity for steam corresponding to the higher pressure, the.diminished pressure sets free a gush of steam that is entirely disproportioned to the ordinary conditions. Some authorities advise the insertion of a perforated plate through which the steam must pass on its way to the cylinder; the water beating against this plate is arrested, and the steam passes on more freely. In some locomotives the steani is taken by it longitudinal perforated pipe, which serves the purpose of the steam dome of usual designs. Boilers in which the steam does not circulate freely because of the disposition of the tubes, are liable to the annoyance of priming.