180 ■ NEW DEVELOPMENTS IN HIGH VACUUM APPARATUS. 



slightly superheated. If less than 98 per cent saturated, the losses will rapidly in- 

 crease and the stability of operation will suffer. The presence of water in the steam 

 can easily be detected by a whistling noise and a sudden drop of pressure of the 

 steam at the pressure gauge arranged ahead of the inlet to the nozzle. If the pos- 

 sibility of obtaining wet steam exists, a steam separator should be provided. 



High superheated steam, while good for steam turbines, does not offer any ad- 

 vantages for the steam-air ejector. Its compression in the diffuser becomes diffi- 

 cult, and this counterbalances the benefit obtained from its use in the nozzles. 



The necessary minimum pressure of the live steam at the inlet to the nozzles 

 is a function of the compression ratio the ejector has to produce. The kinetic 

 energy of the steam, after expansion through the nozzle, has to be large enough 

 to compress the mixture of steam and air. The minimum steam pressure employed 

 varies from 100 to no pounds gauge. The influence of the variation in steam pres- 

 sure on a two-stage air ejector designed for 1 10 pounds gauge removing a constant 

 quantity of air against a constant back pressure is shown in Plate 72. It will be 

 noted that even low steam pressures will produce some results. 



The effect of the back pressure upon the performance is illustrated in Plate 73, 

 several curves for different live steam pressures being shown. All are based upon 

 a constant air-handling capacity. These curves also evidence the possibility of over- 

 coming a higher back pressure by an increase in the live steam pressure. 



The effect of the conditions of the air entrained is based upon the following 

 considerations. Air saturated with water vapors weighs considerably more than 

 dry air. Since we can assume that the ejector will handle the same total weight 

 whether the air is dry or saturated, the application of the Dalton Law (see Plate 

 69) will enable the determination of the respective quantities. Referring to curve 

 A in Plate 71, at 2-inch Hg. absolute, yy pounds of dry air were handled when 

 drawn directly from the atmosphere. Assume now that fully saturated air is to be 

 handled at the same absolute pressure and that this air has a temperature of 80° F. 

 The curves of Plate 69 indicate the presence of 0.67 pound of water vapor to each 

 pound of dry air in 1.66 pounds mixture. Hence the ejector would theoretically 



handle J-l— =■ 46 pounds of dry air. Careful experiments have shown the amounts 

 1.67 



actually handled to be less than the calculated theoretical values. This is partly due 

 to small globules of water suspended in the saturated air being carried into the 

 ejector. An air ejector handling a mixture containing a considerable amount of 

 vapor also has greater impact losses during entrainment and greater losses in the 

 diffuser. The weight of mixture handled by the ejector becomes less as the propor- 

 tion of vapor to dry air increases. 



Since the air withdrawn from a condenser is always saturated with water va- 

 por, the above-mentioned condition should receive careful consideration when se- 

 lecting the proper size of an air ejector for a condenser installation. 



Having thus briefly indicated the principles of operation, as well as the most 

 important factors in steam air ejector design, several types of ejectors will be de- 



