Brick arches are commonly used in locomotive fire-boxes to improve the Co 11 ihnstion and the distribution of the heat through the tubes. They consist of an arch of fire-brick whose front end connects with the tube sheet just below the low est tubes, and whose rear end terminates near the rear end of the fire-box a short distance below its top. The sides of this arch are com monly supported by lugs or brackets fastened to the side sheets or by water-tubes placed at in tervals underneath the arch and connecting the crown sheet and the tube sheet. Ash-pans are suspended beneath the fire-box for the purpose of catching and carrying the ashes and coals that may drop between the grate-bars; they are made of sheet steel.
The smokc-box is the chamber in the front of the boiler into which the gases of combustion flow from the tubes. There are two types of smoke-boxes in use, but the extension front shown by Figs. 7 and S is the type now almost in variably applied to new locomotives. In this box the gases issuing from the tubes either strike against the diaphragm plate and are deflected downward or pass through the netting and out of the stack which is located directly above the ex haust nozzle. The exhaust nozzle is the nozzle like casting which terminates the exhaust-steam pipes from the cylinders. The successive cylin derfuls of steam pass up through it and out of the stack above in a succession of forcible blasts which entrain the gases in the smoke-box and carry them out of the stack, thus producing a partial vacuum in the smoke-box and conse quently a suction or draught on the fire. The smarstark of a modern locomotive is usually open from top to bottom. In the older forms of locomotives the 'diamond stack' was always used, and it is still employed under special conditions.
Among the necessary fittings of a locomotive boiler besides those already described are safe ty valve. whistle, steam-gauge, water-gauge, blower, throttle valve. dry pipe, and injectors. The locations of several of these fittings on the boiler are shown by Fig. 7.
Safety valves for locomotives are of the pop type, and two are always employed, giving an aggregate area of opening of about 1 square inch to each 2 square feet of grate surface. Fig. 9 is a section through a safety valve. The valve u rests upon the seat b and is held in place by the pressure of the spiral spring d bearing down uaa the spindle e. The pressure of this spring is regulated by screwing the collar r up and down. outside of the valve-seat there is a projection f beneath which a groove 9 is cut in the casing. When the valve lifts this groove is tilled with steam which presses against the portion of the valve outside of the seat, and by taus im•reasing the effeetPte area of the valve, causes it to rise higher and remain open longer than it Ayoubd without this device. The adjustment of the
valve is so made that it will allow steam to escape until the pressure in the boiler is 4 pounds below the normal. The perforated casing or muffler h breaks the escaifing, steam into nu merous single jets and thus reduces the noise of the escaping steam.
The whistle consists of a bell (A. Fig. 10) closed at the top and sharpened at the lower edge. By opening the valve shown the steam «•capes through the narrow circular orifice B. and, enter ing the bell, sets up vibrations which cause the 'whistle.' The tone is controlled by the size of the bell and the pressure of the steam, being lower the larger the bell is, and louder the greater the steam pressure is.
Chime whistles are commonly used, and they consist usually of a bell having three compart ments, tuned respectively to the first, third, and fifth of the mu sical scale.
The blower consists of a pipe leading from the cab to the stack into which it turns up ward so that by opening a valve a jet of steam is sent up through the stack and gives 3 draught on the fire when the locomotive is stand ing still and when. consequently, the exhaust is not acting as previously described.
The throttle •alre in general use is a double seated poppet valve, shown in section by Fig. II. As will be seen, there are two valves, a and b, attached to a single stem. The upper valve, a, is the larger, and the lower valve. b, is of such diameter that it will pass through the upper va Ice seat. The steam, therefore, exerts a on the lower face of b and on the upper face of a. so that the valve is partly balanced. but the valve a being larger than the valve b. the dim a ward pressure is the greater and tends to keep the valve closed. Considerable force is required to raise the valve against this pressure. and in order that this may be exerted so as to task the as little as practicable. the lever ar rangement shown is employed. The rod from the cab is attached to the bell-crank 11, and this in turn is attached to the valve-stem E. The point of the bell-crank is provided with a slotted hole. At the start of the pull t! c length of the lever arm is about inches, while the long arm is inches. After the valve has been lifted from its seat and is free from the excess of pressure on a. the projecting horn A on the back of the bell-crank comes in contact with the bracket B, and the crank takes the position shown by the dotted line in the drawing. The end of the horn then becomes the pivot, and the length of the short arm of the lever is changed to 91/4 inches and of the long arm to about inches. The dry pipe is the steam-pipe leading from the throttle valves to the engine cylinders, as shown by Fig. 7. Injectors are used for de livering feed-water to the boiler. See INJECTOR.