44

MODERN STEAM PRACTICE.

furnace Lancashire dass, the pipes passing through the water spaceto the combustion chamber plate, through which they are securelyrivetted, or clenched over. Thus a current of hot air passes throughthe tubes, mixing with the flame and gases in the combustionchamber, so that when the fires are properly attended to, as with allarrangements introduced for the prevention of this nuisance theymust be, little or no smoke will appear at the top of the chimney.The introduction of heated air into the combustion chamber afterthe smoke or gases have passed the bridge, seems mainly to keep upthe temperature of the flues, by the admixture of the oxygen of theatmosphere with the flame in the combustion chamber; this plan,where it can be conveniently applied, should always be adopted.As before stated, vertical boilers are so fitted with a series of smallair-tubes all round the fire-box, inclining downwards, thus the airfreely mixes with the live coal. In former years some personsscouted the idea of consuming the smoke after passing the bridge;the fact of our now being able to do so speaks for itself. Somemay term it gas before it has passed the bridge; but what weplainly see in the furnace we denominate smoke.

For single furnaces a very different arrangement is adopted,the smoke being consumed in the furnace: the fire-door is perfor-ated with a number of small holes 3 ^ inch in diameter, drilledclosely together. It seems impossible to give the exact number ofholes to suit all furnaces, as the same furnace, with different kindsof coal, requires more or less openings, as the case may be, and eventhe same furnace offen requires more or less air with the same kindof coal; this may be owing to the temperature of the atmosphere, orwhich way the wind is blowing; if blowing in such a direction as tofan the fire, as in the forward boilers for marine purposes, less airwill do at the furnace door. Thus it is imperative to have a greatnumber of holes, say 5 to 6 square inches for every foot of fire-gratesurface; they should be covered with a regulator, or movable disc-plate, with corresponding holes for regulating the supply; someadopt slits instead of round holes, but the latter, or jet System, is byfar the best, as it distributes the air equally amongst the gases inthe furnace. This plan necessitates regulation by the damper.Should no steam be required, or the engine not working, or evenwhen the fireman is trimming the fire, the damper can be shut, check-ing the draught for a time; the smoke remains in the furnace, or isslowly consumed there, thus preventing it issuing at the chimney top.