PRACTICAL OPERATION OF DYNAMOS 221 



The economic problem is in every case to produce the best re- 

 sults at a minimum of cost and this is always a very complicated 

 problem, inasmuch as the cost of erection and maintenance of en- 

 gineering works and the value of the service rendered thereby 

 are both dependent upon minute variations of local conditions, 

 they both fluctuate from year to year with the varying stress of 

 business activity, and they both change with every improvement 

 in industrial processes. 



The first estimate of the cost of any engineering undertaking 

 is usually based upon general statistics of the cost of more or less 

 similar undertakings, and the final cost is determined by the bids 

 of contractors who have had more or less experience in the par- 

 ticular line of work and whose margin of profit must in general 

 be great enough to cover the many uncertain items that always 

 appear in any new undertaking. 



The following data on the cost of installing and operating 

 steam and electric plants may be used for making a preliminary 

 estimate of the costs of an electric station. These data are aver- 

 ages based upon the records of a large number of actual cases, 

 and the cost of a given station may depart more or less widely 

 from these figures on account of peculiar local and temporary 

 conditions. 



Cost of steam power.* The ordinates of the curve A in Fig. 

 138 give the approximate total cost per horse-power capacity, 

 including buildings, chimneys and all accessories, of boiler and 

 engine plants of various sizes, and the ordinates of curve B give 

 the coal consumption in pounds per horse-power-hour. 



The ordinates of the curves C, D, E, Fand G in Fig. 139 give 

 the costs per horse-power-year (308 days of 10 hours each) of 

 the following items : 



*The data from which the curves in Figs. 138 and 139 are plotted are taken from 

 a paper by Wm. O. Webber, The Engineer (Cleveland), Vol. 40, p. 145, February 

 2, 1903. Important data on the cost of steam power are given by C. E. Emery, 

 Trans. Am. Inst. Elec. Eng., Vol. 12, pp. 358-389, 1895 5 and by H. A. Foster, 

 Trans. Am. Inst. Elec. Eng., Vol. 14, pp. 385-421, 1897. 



