36o 



CHAPTER XVIII 



In some recent steam turbine plants a horizontal form has been adopted, 

 and one such is indicated in Fig. 217 ; to such the term cross-current[is 

 apposite. 



FIG. 217 



In cases where a head of water is available the pump may be dispensed 

 with, and water and air removed together through a vena contracta or ejector, 

 as shown in Fig. 218, or in the more highly developed form of multiple 



jet ejector, as indicated in 



Fig. 219. 



The relative merits of the 

 systems may be briefly men- 

 tioned. 



In the " wet " system the 

 air and the water have to be 

 removed together, whereby a 

 slow movement is enjoined on 

 the pump, which soon reaches 

 very large dimensions and 

 becomes of exaggerated size 

 except for smaller plants. In 

 the " dry " system, the air 

 being removed separately, a 

 much higher pump speed can 

 be employed, and incidentally 

 a much higher volumetric 

 efficiency is obtained. 



In the counter-current sys- 

 tem the air is last in contact 

 with the water as it enters the 

 condenser ; and hence, in the 

 extreme case, may leave at the 

 temperature of the water, 

 that is to say, at the minimum 

 temperature. In a co-current 

 (parallel) condenser the lowest 

 F IG> 2 i8 temperature at which the air 



