BOILERS. Modern marine practice is either to use the cylindrical fire-tube boiler carrying pressures of 150 to 200 pounds per square inch, or some form of water-tube boiler using pres sures of 160 to 300 pounds. W.T. (water-tube) boilers are more largely used for naval pur poses and fast passenger vessels and cargo ves sels in fresh water, cylindrical boilers for gen eral merchant service.
The substantial advantages of the cylindrical boiler are: reliability, simplicity; it is well made and generally jinderstood; it can use salty or dirty water, and it will stand hard usage with out serious loss or injury. The disadvantages are: great weight; steam cannot be gotten up or taken off quickly; it does not readily adjust itself to change of output; and heavy forced draught cannot be used.
The advantages of the water-tube boiler are: lightness; adaptability to high pressure; rapid ity of raising steam or taking it off; it is readily adjusted to sudden change of output: forced draught can be used (in nearly all types) ; and repairs or removals are more easily made. Its disadvantages: it requires great care and atten tion; it cannot use salty or dirty water or ex perience hard usage; corrosion takes place very readily; it is complicated and many types re quire a large number of mechanical attachments; and being new, it is not well understood by men who handle it and best results are not ob tained. The economy of fuel is about the same in the best of each type. For average running the cylindrical is probably the more economical.
The general form of a cylindrical boiler is the single or double ended return-tube boiler fitted with two, three, or four corrugated furnaces. Boilers vary from 9 to 20 feet in diameter, and 9 to 18 feet in length for single-ended, and 17 to 21 feet for double-ended. A cut of a single ended boiler is shown; no are the shell-plates, made in two courses of three sections each, with butt joints; Lb, corrugated furnaces, either of Fox, Purves, or Morrison patent, into which the grate and bridge wall are fitted; c, stays sup porting boiler front and combustion chamber; Weight per I. H. P. including water, 90 to 120
pounds. :Maximum coal burnt per square foot grate surface per hour about 40 pounds ordi narily 15 to 20 pounds. The efficiency of the boiler is about 70 to 75 per cent. in best con dition.
In water-tube boilers the water is contained within the tubes, and as these are relatively small great pressures can be carried and the boiler may be considerably lighter. Most types of water tube boilers have a number of steam and water DD, tube sheets with tubes, e, expanded into them; some of these are stay tubes screwed into tube sheets which they help to support. F is the combustion chamber, where the gases of com bustion are filially mixed and burned. The prod ucts of combustion pass through the tubes to the and then to the smoke pipe. The heating surface is composed of the crown sheets (top of furnaces), top and sides of combustion chamber, and tube surface, the tube surface being by far the larger portion. H, H, H are steam space stays supporting the boiler ends. The back and sides of the combustion chamber are supported by short stay bolts, I, and the top by girder stays, J. The steam space is fitted with a dry pipe which collects the steam and discharges through the stop valve to the steam pipe.
The furnace front is fitted with furnace and ash-pit doors, the ash pit being the part of the furnace below the grate. Manholes, K, are fitted to obtain access to boiler for cleaning, etc. The coverings for manholes are called manhole plates. Practically all parts of a boiler, except the grate, furnace doors, and bridge wall, are built of mild steel.
The attachments of a boiler like the one shown are: main and auxiliary stop valves in the steam pipe: check valves through which feed water enters; surface and bottom blow valves, by means of which the boiler is blown down or pumped out; two water columns to show the height of the water: pressure gauge; spring safety valve; and circulating apparatus (general ly a hydrokineter).