NOZZLES 285 



Thus with a head of 100 feet the wheel would develop 49 '5 horse- 

 power. 



As the nozzle area is increased, starting from zero, the power developed 

 thus increases to a maximum of 49'5 H.P. with a nozzle opening 

 a = '158 A, and then again diminishes. 



If in the above problem /be taken = '005, we get the maximum K. E. 



tvhen j = -224. The value of this K. E. = 48'1 II* foot Ibs. per second, 



so that with the same head and wheel efficiency the maximum power 

 developed would be 70'0 H.P. 



In a fire nozzle, the problem is to obtain a stream moving with as high 

 a velocity as possible, and this is evidently to be obtained by making the 



expression 1 -f k -J- K - -\- . 2 , the denominator of the fraction 



expressing the velocity, as small as possible. This is done by increasing, 





 as far as practicable, the ratio 



In the case of the supply to a Pelton wheel, a stream carrying the 

 maximum possible amount of kinetic energy is required, and as just 

 demonstrated, this can only be obtained by having a correct ratio of pipe 

 and nozzle area. With a smaller nozzle area the weight issuing per 

 second is unduly restricted, while with a greater area the loss due to 

 increased friction in the pipe, due to the greater velocity of flow, causes 

 the velocity of efflux to be reduced to such an extent as to more than 

 counterbalance the increased weight passing the nozzle. 



This fact may be of importance in the regulation of a Pelton wheel, 

 which regulates its supply of water by automatic opening or throttling at 

 the nozzle (see Art. 125). 



So long as this nozzle area has less than the critical value for the 

 particular pipe, any further opening will admit more water and a greater 

 supply of kinetic energy to the wheel. Above this point any further 

 opening of the valve admits more water, but gives a smaller supply of 

 kinetic energy to the wheel, which will consequently slow down. On the 

 other hand, with a nozzle area greater than the critical value, throttling 

 leads to an increased supply of energy being given to the wheel and to an 

 increased speed. 



For successful governing the maximum nozzle area should not exceed 

 that corresponding to the critical value, so that everywhere within the 



range of opening -, - may be positive, and the supply of energy may 



Ui l ) 



