176 NEW DEVELOPMENTS IN HIGH VACUUM APPARATUS. 



amount of steam which the condenser receives. It also includes the air contained 

 in the exhaust steam. 



The air received from the condenser is always fully saturated with vapor, the 

 amount of which depends upon the temperature and absolute pressure. The rela- 

 tions between water vapor and air at different absolute pressures and temperatures 

 based on Dalton's Law are illustrated in Plate 69. 



In determining the air-handling capacity of the pump, the amount of vapor 

 should be deducted from these curves and added to the normal air leakage given 

 in Plate 68. 



In looking over Plate 69 it will be seen that at a constant absolute pressure the 

 amount of vapor decreases with the drop in temperature. It is therefore desirable 

 to remove the mixture at the lowest possible temperature in order to decrease the 

 size of air pump. This fact has been recognized and has led to the adoption of the 

 dry system in which the air and the condensate are removed separately. 



The dry system permits the removal' of the air and vapors at the lowest pos- 

 sible temperature by a dry air pump ; the condensate is taken away at a higher tem- 

 perature by either a turbine-driven or reciprocating condensate pump. Since the 

 adoption of the dry system, many developments have been made in dry air pumps, 

 the most notable being the steam air ejector, which, due to its many advantages, 

 has been extensively used during the last three years. 



The use of steam jets for the removal of air has been known for over fifty 

 years. As early as 1868 a patent was granted for a steam-jet air pump. But this 

 type of pump has never been developed to produce high vacua commercially until re- 

 cently. This was due to the lack of the correct knowledge of the properties of 

 steam and the want of interest in high vacuum. During the years 1900 to 1910 

 the properties of steam have been more accurately determined by several prominent 

 scientists who made it possible to carry out experiments for the improvement of 

 steam air ejectors. At the same time the demand for high vacuum became more 

 pronounced, due to the adoption of the steam turbine. 



The steam air ejector is a compressor in which the air to be compressed is en- 

 trained by one or more steam jets, which move through the entrainment space at 

 a very high velocity. The entrainment is made by friction. The kinetic energy, 

 originating from the steam jets, and contained in the mixture of air and steam 

 after entrainment, is transformed into pressure in a channel called the diffuser. 



Referring to Fig. 3, Plate 70, air enters through opening A into the entrain- 

 ment chamber E; live steam expands through one or more nozzles whereby its 

 static energy, due to its pressure, is transformed into kinetic energy. This trans- 

 formation causes the steam to leave the nozzle with a velocity ranging from 3,000 

 to 4,000 feet per second. The steam jets /, while passing through the entrainment 

 chamber E, entrain the air and give up part of their kinetic energy to the same. 

 The outlets of the nozzle or nozzles are arranged opposite the opening of the dif- 

 fuser, and the mixture of steam and air enters the diffuser at a somewhat lower 

 velocity than that of the steam jets leaving the nozzles. The mixture is gradually 



