( TURBINES 205 



of the turbine and the generator must necessarily be considered 

 together. Besides hydraulic conditions such as the head and its 

 variations, storage facilities, etc., and the limitations of the 

 turbine design, a proper selection is governed by the load factor, 

 the nature of the load, the reserve capacity, the reliability and 

 flexibility of the service, the cost and operating expenses, etc. 

 The units should be operated as near full load as possible and new 

 units should preferably be started as the load increases instead of 

 utilizing overload capacities. Where sudden overloads of con- 

 siderable magnitude come on the system for short periods it is, 

 of course, necessary to have turbine capacity sufficient to care for 

 them. Single units are never desirable except for multiple-plant 

 systems, in which case the necessary reserve can be obtained from 

 other stations. For single-plant systems the number of units 

 should preferably not be less than three or four, but above this 

 the number should be governed by the upper limit in design, con- 

 sidered both from a technical and economical standpoint. With 

 a small number of large units the first cost, the maintenance 

 charge and the necessary floor space is reduced, and the efficiency 

 is also usually better than for a larger number of smaller units. 

 The ultimate development may also influence the size, and it may 

 be found advisable to provide larger units for the initial develop- 

 ment than would otherwise have been chosen. 



In water developments by far the larger majority of installa- 

 tions are subject to wide variations in the head. In many of 

 the low-head installations the back water may bring about a 

 change in head which is beyond the capacity of one wheel or 

 runner to accommodate, and in some cases additional runners 

 must be mounted on the same shaft and cut into service at times 

 of low head. In many of the large developments this change in 

 head is the limiting feature in design of the water wheel as related 

 to the generator capacity, for in all electrical work it is essential 

 that the speed of the generator be kept constant. 



It is very generally known that the peripheral speed of a 

 water wheel bears a certain ratio to the spouting velocity of the 

 water under any given head, this ratio as a percentage varying 

 between 40 and 50 per cent for impulse turbines and between 60 

 and 80 per cent for reaction turbines. Hence the percentage 

 variation from, say, a mean of 60 per cent is only 33 above and 

 33J below for any given head. But the diameter, and conse- 



