68

MODERN STEAM PRACTICE.

Sometimes ferrules are fitted and driven into the tubes nearest thefurnaces, but at the smoke-box end they are simply expanded, and aslight countersink left in the holes on the outside, thus forming a collarwhen the ends of the tubes are laid over. The flues and combus-tion chamber at the back of the boiler are of great importance, moreespecially with short tubes arranged and worked on the return prin-ciple. In many boilers of this dass the flame and heated gases passtoo rapidly through the boiler into the chimney, and if not fitted witha high uptake causes great waste in fuel, the flame and gases havinglittle time to act on the heating surface. The combustion chambershould be made large, so as to make the flame hang in the fluesbefore passing through the small tubes. The usual size at the topof the combustion chamber is 18 inches, and at the bottom 22 inches,from the tube-plate to the back of the chamber, this being actuallyrequired in all cases to properly expand and lay over the ends ofthe tubes. It is advisable, however, not to increase this space to anygreat extent, more especially for high pressure, as large flat surfacesare not to be desired. But for moderate pressure, 22 inches at the topand 26 inches at the bottom will tend to retard the flame and gases inthe combustion chamber. The area over the bridges is from 18 to19 square inches per nominal horse-power; thus it will be seen thatthe calorimeter of the tubes is greatly in excess of this. It must beacknowledged by all that marine boilers should have as many fur-naces as possible, bearing in mind that there is a certain size offurnace very convenient to manage, and other sizes, above or below,that are not so convenient. A good medium is a width of 2 feet9 inches, and certainly not less than 2 feet 6 inches, or greater than3 feet 3 inches, and from 6 feet to 7 feet in length, but should notexceed the Iatter, as a 7-feet furnace is quite long enough to manageproperly. But in some instances the boiler room is so limited thatlength must be substituted instead of breadth. Some makers haveeven gone to the extreme as regards the width, considering if theymake the furnace circular, a diameter of 3 feet 6 inches could beused with impunity; but experience proves the contrary, as withordinary sea water scale will form, and accumulating to any greatthickness, the furnace plates become heated and the tops comedown, even although there is a plentiful supply of water in the boiler.So as many furnaces of a medium width as can be convenientlyarranged are far better than a less number of a great width, con-sidering the more furnaces we have there will be more side surface