NEW DEVELOPMENTS IN HIGH VACUUM APPARATUS. 177 



brouglit practically to a rest in the diffuser D, thus increasing its static pressure to 

 or slightly above atmosphere. 



The efficiency of a steam air ejector depends upon the magnitudes of three 

 losses : — 



{a) The friction losses in the expansion nozzle. 



{b) The impact losses during entrainment. 



(f) The losses in the diffuser. 



Due to the development of steam turbines the nozzle design has approached 

 nearly the highest degree of efficiency possible, and the friction losses in a well- 

 designed steam nozzle are seldom more than lo per cent (see H. Frederic and 

 Kemble, Transactions of the American Society of Mechanical Engineers, 1909). 

 These losses depend mostly on the length of the nozzle and the condition of the 

 inner surface. 



The loss due to impact is influenced by the relation that the momentum of the 

 steam, plus the momentum of the air before the impact, must equal the momentum 

 of the co-mingled steam and air after impact. 



If we designate the amount of live steam to be d pounds, the amount of air 

 entrained G2 pounds, the velocity of the live steam at the outlet of the nozzles Vi, 

 the velocity of the air V2, and the velocity of the mixture Vz, we can write the fol- 

 lowing equation: — 



Gi X F? + G2 X Fi = Fi X (Gi + G2). 



The impact loss can be minimized by letting the air acquire a higher velocity 

 before combining with the steam jets. This, however, would make it necessary to 

 produce a smaller absolute pressure in the entrainment space and to provide an an- 

 nular air nozzle ahead of the same. As a result the pressure rise in the diffuser 

 would have to be greater than otherwise, and consequently the loss in the diffuser 

 will be increased. Careful experiments and calculations have shown a certain range 

 of conditions within which the sum of all the losses is a minimum. The under- 

 expansion and arrangement of an air nozzle prior to the entrainment has been 

 found to be of advantage only for very small compression ratios, which, however, are 

 not used in steam air ejectors producing high vacuum. 



The losses in the diffuser are considerably larger than those in the nozzle. They 

 range from 25 to 50 per cent and are influenced by the length and the shape of the 

 diffuser, as well as by the conditions of the inner surface. 



The ratio of compression is the ratio between the absolute pressure of the air 

 at the air intake and that at the air discharge. 



Since turbine condensers require vacua of 2-inch Hg. absolute or less, the ratio 

 of compression has to be i to 15 or more. While it is possible to obtain such a high 

 compression ratio in a single stage, carefully made experiments have shown the in- 

 advisability of using such a high compression ratio single-stage ejector for practi- 

 cal purposes. The following conclusion will illustrate why : — 



