THE JET PUMP 697 



since the total work done in pumping is proportional to h d -f- h s , 

 while this loss depends only on h d) the efficiency will increase as h 8 is 

 increased at the expense of h d . 



It follows that with a given total lift, the suction head should be 

 increased as far as possible up to about 22 feet at the expense of the 

 delivery head. This conclusion is borne out in practice. 



The efficiency of the pump is given by 



_ Q s (H d + h.) 

 Qifa-Hj 



This efficiency is of necessity low, since the action depends on the 

 mixing of two streams moving with different velocities, and hence involves 

 considerable loss by shock. E.g., in the numerical example considered 

 on p. 696, the efficiency (assuming H d = '7 h d ) is given by 



_ 7-2 (7 + 15) _ 

 12-4 X 38 



Actually, frictional losses reduce the efficiency still further, and the 

 maximum efficiency attained in practice, even with a pump placed at 

 delivery level, is about 25 per cent. This is increased to about 30 per 

 cent, where, as when used for delivering a high velocity jet of water for 

 fire purposes, the necessity for converting the kinetic energy of the jet 

 into pressure energy is absent. 



For continuous pumping and drainage operations where a fair pressure 

 supply is obtainable, and where the volume to be lifted and the working 

 head are small, the method offers the advantages of simplicity and low 

 first cost, while practically no attention is required. Unless the supply 



head is large compared with the lift, the ratio ~* is, however, very 



V 

 small often so low as J. 



The principle of the steam injector, as fitted for boiler feed purposes 

 is identical with that of the jet pump. The preceding equations, however, 

 need to be modified, since the streams of fluid on Impinging are not of 

 equal density, although they become so on condensation of the high 

 velocity steam jet. 



The principle of the jet pump has been applied in an intensifier for 

 raising the pressure of a large quantity of low pressure water by 

 means of a small supply at high pressure, the delivery pipe D leading 

 directly into the cylinder of the intensifier. While not economical 

 from an energy standpoint, the simplicity and low first cost of the 

 apparatus render it very suitable for such work where its use is only 

 occasional. 



