Automatic Stations.— The possibilities of the remote control switching system just re ferred to have borne fruit in hydraulic generat ing stations partly or wholly automatic in their performance. If the switching of a generator can conveniently be accomplished, as is the case, from a distant part of the power house with out seeing the apparatus controlled, the work ing of which is signaled back by electrical tell tales, then it is evidently possible to work a wheel gate or a main switch or any other ap paratus in precisely similar fashion from a distance of rods or miles. There have, there fore, been introduced auxiliary generating sta tions started and stopped from the main switch ing station and regularly operated without other attendance than the occasional visit of an in spector. The generators used in such case are commonly of the induction type, which do not require separate excitation by direct current and are consequently simpler to 'operate with out attendance. When desired to start the sta tion the motor-driven wheel-gate is opened by remote control of the circuit and the subsequent necessary operations of switching the generator fully into circuit are performed either by other remote control apparatus or by a drum switch thrown into operation when the generator is at speed and performing the necessary switching operations in their proper sequence. In stopping the plant the same operations are gone through with in reverse order. Most stations of this type are of relatively small power and contain but a single generator, but the system is im portant in that it enables small hydraulic priv ileges to be effectively utilized without undue cost of attendon. As the great majority of available water powers are of modest capacity economic utilization depends upon lowering the labor factor in the cost, which the generating station with remote control effectively does. It is likely to play a considerable part in the establishment of power networks utilizing, wherever available within the territory served, water powers of small capacity.
Steam-driven power stations are propor tioned very differently from hydraulic stations owing to the large space needed for the boilers and their accessories. The general layout, how ever, is quite similar, the common arrange ment being in two long parallel rooms sepa rated by a party wall, one containing the boilers and boiler equipment, the other the generators and prime movers. It is a curious fact that whereas the prune movers and generators have increased enormously in available capacity in recent years the advance in the size of boilers has been relatively very slow. It is only recently that large boiler units have come into service, despite the fact that tests upon them show very material gains in efficiency.
Boilers are rated conventionally on a basis which gives the uninitiated no notion whatever as to their real steaming capacity. The canoni cal boiler horse power has long been based on the evaporation of 34.5 pounds per hour of water from and at 212°. This nominal rating requires, when a boiler is to be applied to a prime mover, wholesale modification, on the one hand by reason of the actual steam consumption per horse power of the prime mover, and on the other hand on account of the fact that the steam is generally at high pressure and super heated. In point of fact the steam capacity of a boiler in a modern steam plant may be counted as two to three times the conventional rating so far as the actual operation of the prime mover is concerned.
But, even so, the boiler capacities have grown much more slowly than the capacities of the steam engines or turbines, the latter being almost invariably used in plants of any size. It is, however, a fact that boilers of large capacity may be counted on for con siderably higher efficiency than those of small capacity, as in many other cases, so that larger and larger units are quite certain to come into use. At present boilers of more than 1,000
horse power nominal rating are rather unusual, although units of more than double that size have been successfully used for some years past. The chief difficulty in employing very large boiler units is in adapting the setting to withstand the extremely high furnace tem peratures which result in added efficiency. It is usual, therefore, to find each large prime mover in a modern station supplied with steam by a group of four or more boilers, a number which a few years back would have been doubled.
There is a collateral advantage in thus sub dividing the boiler units, inasmuch as the boiler is the part of a steam plant which requires, probably, the most frequent repairs, and there is some advantage in having enough units for each prime mover group so that one can be cut out without seriously lowering the capacity, inasmuch as a boiler can at a pinch, particularly by help of forced draft, be driven temporarily 30 to 50 per cent above its working rate. A single boiler group of three or four units can readily be made an independent source of steam supply for its prime mover under ordinary cir cumstances. Generally, however, the steam pip ing in a modern plant is so arranged that in case of withdrawing a single boiler from serv ice its place can be filled from steam from the general supply without exceptional forcing at any point.
here shown are almost universal, as is the use of mechanical stokers for feeding the boilers, which are almost invariably of the water-tube type. This particular boiler plant delivers steam at 200 pounds' pressure and 160° F. super-heat, which is a fair example of present practice. There is a strong tendency, however, toward the use of higher pressures and super-heat in the interest of economy.
The generators in this instance are for volts, a figure convenient where there is to be local distribution beside possible feeding of transmission lines and which is besides a con venient voltage in the design of large gen erators. Smaller machines are often wound for 2,300 volts and 6,600 is rarely exceeded. The reciprocating engine has practically passed out of existence in large electric generating plants owing to the lower cost, greater com pactness and higher efficiency of the turbines, particularly at loads below the normal. The Fig. 3 shows in elevation the layout of a typical modern steam plant of fair size de signed for feeding high-tension lines. It in cludes a boiler room with typical provision for receiving and distributing the fuel, a turbine room parallel thereto containing the generators, with adjacent provision for switchboard trans formers and auxiliary apparatus, while next to the boiler room is a coal shed for the reception of fuel. This particular plant is planned for an ultimate capacity of 75,000 kva. in six 12,500 kva. generators. The location of the equipment is clearly shown in the cut. In larger plants the turbo-generators often run to 20,000 or 30,000 kilowatt units and occa sionally higher. At the present time as here shown these turbo generators are built with horizontal axis, the vertical type of a decade since being now obsolescent. Very commonly the coal shed with its track for the reception of coal cars is replaced by a storage yard equipped with automatic conveyers for the maintenance of a large fuel supply, and crushing apparatus for dealing with run-of-the-mine coal is a not uncommon accompaniment. In small plants the transformers and circuit breakers would often be installed under the switchboard gallery in stead of, as here, in an adjacent building.