In contrast with this figure the already mentioned Electra plant has generators of 10,000, the Santa Ana plant generators of 3,000, and the larger of the two Mill Creek plants generators of 3,500 kilowatts capacity. It should be noted that the cost of operation, as well as that of original construction, will vary materially between one large and several smaller stations of equal total capacity, the advantage as to operative cost being obviously with the one large plant.

Fig. 21.—Power-house at Great Falls, Presumpscot River.

[Larger plan] (153 kB)

All of the power-stations here considered have been equipped with water-wheels and generators operating on horizontal shafts, and this is the general practice. This arrangement brings the generators and the floor of the power-station within a few feet of the level of the tail-water. By the general use of draught tubes with turbine wheels the floors of stations are often kept twenty feet or more above the tail-water level.

Where the total available head of water is quite small, as is often the case with rivers where the volume of water is great, it is generally necessary to bring the level of the station floor down to within a few feet of the tail-water. The Birchem Bend station of the Springfield, Mass., electric system affords a good example of this sort, the floor of this station being only 2.6 feet above the ordinary level of the tail-water. At this station the difference of level between the head- and tail-water is only fourteen feet, and even with the low floor level named the top sides of the horizontal turbine wheels are covered only by 4.5 feet of water.

At the Garvin’s Falls station of the Manchester, N. H., electric system the level of the floor of the generator room is thirteen feet above the ordinary level of the Merrimac River, on the bank of which this station is located; but in this case the total head of water is about twenty-eight feet. The high water of the Merrimac in 1896, before the Garvin’s Falls station was built, reached a point 5.24 feet above its present floor level, and 18.24 feet above the ordinary level of the river at the point where the station is located.

Under the Red Bridge electric station of the Ludlow Manufacturing Company, on the Chicopee River, in Massachusetts, the tail-water is twenty feet below the level of the floor and twenty-four feet below the centres of the water-wheel and generator shafts. The difference between wheel-shaft and tail-water levels at this station is near the maximum that can be attained with horizontal pressure turbines, because a draught tube much longer than twenty-five feet does not give good results.