528 HYDRAULICS AND ITS APPLICATIONS 



and consists of a series of wooden or metal grids. These are usually 

 spaced about 1 inch apart, and in order to reduce the loss due to the 

 contraction and subsequent expansion of sections of the stream, should 

 be as narrow as is consistent with strength. The angle of inclination of 

 the strainer should be not less than 45, in order to give a large area of 

 waterway. 



Effect of Ice Formation. Although the hydraulic power plant is not 

 greatly hampered by ice formation in Great Britain and the western 

 portion of the continent of Europe, ice troubles may become serious in 

 such climates as are found in Canada and the northern portion of the 

 United States, and may necessitate some modification in the design of 

 the head-race. 



The trouble which is often experienced, particularly with frazil ice 

 (Art. 2, p. 5) due to its tendency to adhere to the racks, strainers and 

 gates of a turbine, can, however, be prevented by a slight heating of the 

 racks or turbine gates. At the Ottawa Electric Company's Power House 

 No. 1 a line of steam pipes laid above water level and against the face of 

 the rack was found to answer perfectly. In the same installation, 

 consisting of three 39-inch wheels, 30 feet head, using 100,000 cubic feet 

 of water per minute, steam was supplied by a small pipe to each of the 

 wheel housings when the unit began to lose capacity. To supply this, 

 20 tons of coal were used during four months of winter, with eleven days 

 on which frazil was bad, only occasional injection of steam being found 

 to be necessary. Electric heating of the same racks has also been tried 

 successfully, 600 amperes at *3 volts removing the ice at once from a 

 single rack bar with the air temperature at 15 Fahr. 1 These bars were 

 ^e inches thick and 18 feet long. 



In a power plant it is usually advisable that the water at the intake 

 should be covered with surface ice, as being the most effective preventa- 

 tive of the formation of both frazil and anchor ice. If there are large 

 stretches of open water above the surface ice, however, frazil is formec 

 and adheres to the lower surface, and may result in a stoppage of the 

 channel. In such a case, or when located at the foot of rapids, it is better 

 to construct a head-race of sufficient size to serve as a settling basin for 

 the ice drawn in. Even then it may sometimes be necessary to blast a 

 channel in the surface sheet. Where a long narrow canal is fed from a 

 stretch of open water the ice difficulty becomes very great. A surface 

 covering is then harmful, as encouraging the adherence of frazil. 



1 These results have been taken from "Ice Formation," by Barnes, Wiley & Son, N.Y. 

 1907. Here the whole subject is considered in detail. 



