Fig. 26.—Cross Section of Combined Steam- and Water-power Station at Richmond, Va.

Fig. 27.—Cross Section of Wheel House at Buchanan, Mich.

[Larger cross section] (140 kB)

The most desirable method of driving generators with vertical wheels, where the expense is not too great, is the direct mounting of each generator on the upper end of a wheel shaft (see [cut]). This method of connection not only requires a special type of generator, but may put serious limits on its speed. In general, the peripheral speed of a pressure turbine should be about 75 per cent of the theoretical velocity of water issuing under a head equal to that at which the wheel operates, in order to give the best efficiency. The rotative speeds of turbines, operating under any given head, should thus increase as their capacities and diameters decrease. Because of these principles it is the common practice, with horizontal wheels, to mount two or more on each shaft to which a generator is direct-connected in order to obtain a greater speed of rotation than could be obtained with a single wheel of their combined power. Thus, at Sault Ste. Marie the horizontal shaft on which each 400-kilowatt generator is mounted is driven at 180 revolutions per minute by four turbines under a head of about 20 feet. At Massena the head of water is 50 feet, and each 5,000 horse-power generator is driven at 150 revolutions per minute by six turbines on a horizontal shaft. Vertical turbines are sometimes mounted singly on their shafts, as was done in the hydroelectric plant at Oregon City on the Willamette River, and this practice gives speeds that are too low for direct-connected dynamos of moderate cost, unless the head of water is unusually great. At the Oregon City plant the head of water is only 40 feet, and yet a single 42-inch turbine was mounted on the vertical shaft that drives each generator.

Fig. 28.—Longitudinal Section of Buchanan, Mich., Power-house.

[Larger longitudinal section] (146 kB)

The most notable examples of direct-connected generators and vertical turbines is that at Niagara Falls, where twenty-one generators of 5,000 horse-power each are mounted at the tops of as many vertical wheel shafts in two of the four stations. Each vertical shaft in the Niagara stations is driven at 250 revolutions per minute by a pair of turbines, one above the other. The maximum head between the water in the Niagara canal and that in the tunnel which forms the tail-race is 161 feet. On ten shafts the centres of the wheel cases are 136 feet below the level of water in the canal, and no draft tubes are used.