. Electric traction and transmission engineering . 109. drum at the top, and to the left in the water tubes^ which areenveloped in the hot gases resulting from the combustionof the fuel. These gases ultimately pass through thedamper-zoniroWed opening near the top of the right-handenclosing brick wall, and through a breeching to the chimneyor stack. Steam is generated and confined under pressurein the upper part of the drum, and is fed through the nozzleon top to a header, whence it is conducted direct to the primemover. The capacity of a boiler is rated in horsepower STEAM STATIONSc 271 and th
. Electric traction and transmission engineering . 109. drum at the top, and to the left in the water tubes^ which areenveloped in the hot gases resulting from the combustionof the fuel. These gases ultimately pass through thedamper-zoniroWed opening near the top of the right-handenclosing brick wall, and through a breeching to the chimneyor stack. Steam is generated and confined under pressurein the upper part of the drum, and is fed through the nozzleon top to a header, whence it is conducted direct to the primemover. The capacity of a boiler is rated in horsepower STEAM STATIONSc 271 and the builder^s rating is based upon a heating surface of10 to 12 square feet per horsepower. A boiler of onehorsepower capacity is considered to be capable of allowingan evaporation of pounds per hour of water at 212° steam at atmospheric pressure, and to have an over-load capacity of 33^ per cent. If the temperature, /, ofthe feed water be less than 212°, the steam be x part dry^or the steam be superheated // F., the delivery of Fig. no. pounds of steam per hour under such conditions will re-quire a boiler of more than unit capacity, and to deliver Qpounds of steam per hour the horsepower capacity of theboiler should be Q fxr + q + Ct, - t + 32 ] horsepower, where r = latent heat of evaporation at the resultantpressure,q = heat in liquid at this pressure, andC = mean specific heat of the superheated steam. 272 TRACTION AND TRANSMISSION. The values of the various constants may be found inEngineering handbooks. The steam consumed in operating auxiharies such asfeed pumpSj vacuum pumps, and circulating pumps, rangesfrom 6 per cent to 15 per cent of that taken by the primemovers. Available boilers are limited in capacity to about2250 horsepower, and it is common to install smaller onesin batteries of two or more. 92. Feed-water Heaters. — It is undesirable to pumpcold water into a hot boiler because of excessive stresseswhich may result from wide differences in
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