The capacious coal bunkers above the boiler room with distributing apparatus similar to that boiler units here employed are of 1,050-rated horse power each and can readily be forced to three times their nominal rating which is, as in the case of all boilers based on a conven tional steaming capacity, long obsolete.
Each turbo-generator has its own bank of boilers, but the piping is so arranged that one set of spare boilers is sufficient for the entireplant. At Q will be noted an equip ment for purifying and cooling an air-blast for the artificial cooling of the turbo-generators. As the size of such generating units has in creased artificial cooling has been very gen erally adopted, the air being forced through the ventilating slots in the machines. The ca pacity of the individual boilers here noted is now a common one in plants of fair size, such boiler units having replaced very extensively those of a few hundred horse power generally in use until very recent years. In larger plants boiler units up to and'exceeding 2,000 nominal horse power have been introduced to material advantage in economy. In many plants the familiar tall stacks are replaced by rather short ones supplemented by forced draft. This ar rangement is not particularly advantageous in plants working at moderate and fairly steady load, but gives great capacity for forcing the output when abnormally heavy peak loads have to be dealt with.
As steam turbines for efficient operation de mand a rather high vacuum,28 inches to 29 inches, they are usually fitted with highly-developed surface condensers and the power required for delivering water to these and for other auxiliary purposes is considerable. One of the moot points in station design is the source of this auxiliary power, whether it should be derived from steam turbines or from electric motors operated from the generating system. Practice is about equally divided in this matter with a frequent tendency to adopt a combination of the two systems. It will be noted that the switching apparatus in this typical plant is some what intricate and occupies considerable space.
cost and generally in complication, with the size of the station, particularly if as usual the num ber of feeders is greatly increased. In view
of this switchboard apparatus and connections should be protected with the utmost care, special pains being taken to locate cables and apparatus so that injury to one may not involve others and this injunction extends to all apparatus which from its connections can conceivably cause a short-circuit. It has repeatedly hap pened that large stations have been put out of action by apparently trivial accidents oc curring at an unexpected and unprotected point.
The fundamental principles at the basis of modern station design are so to arrange the This arises from two causes, first, the abso lutely large amount of switching apparatus re quired, and second, from the fact that it is desirable and usual to install high-power switches and circuit breakers in independent concrete cubicles, from which a burn-out will not extend. As plants have increased in size the switching problem has become a very serious one. It is usual and sometimes necessary to operate all the generating units in parallel. In this case any unit which is the victim of a short-circuit may receive the aggregate output of all, so that each main protective switch has practically to be able to handle successfully the whole output of the station. In long-distance transmission plants this difficulty is often minimized by putting the high-tension trans formers in parallel only at the far end of the line, but within the station and in ordinary dis tribution plants the difficulty remains. Hence the switchboard equipment increases in relative apparatus that nothing need be done by manual labor which can better be done by mechanism, and that no heat shall be lost which can be made to serve a useful purpose. In the sta tion of Fig. 3, for example, the fuel is dealt with mechanically from the time it drops by gravity from the cars to the moment when it drops as ashes into the ash cars beneath the grates, save for such attendance as is necessary to see that the mechanical stokers are function ing properly and to lend what little aid may be needed. The steam piping is relatively short, simple and well jacketed, the surface con densers are inunediately beneath the turbines and the waste exhaust from the auxiliary tur bines goes to heat the feed water.