182 NEW DEVELOPMENTS IN HIGH VACUUM APPARATUS. 



2. The possibility of adjusting the expansion ratio of the Hve steam from the 

 outside without changing the steam consumption. Referring to Plate 74, it will be 

 seen that the throat is located in nozzle 1 1 and that any movement of the disc to 

 or fro will not change the cross-sectional area of the throat and therefore not affect 

 the steam consumption. 



A cross-section through the Le Blanc air ejector is shown in Plate 76. It 

 has one DeLaval nozzle in the first stage, and multiple DeLaval nozzles are arranged 

 close to the inside of the diffuser walls in the second stage. It is claimed that the 

 losses due to the impact in the second stage are reduced since the mixture discharged 

 from the first stage enters the entrainment chamber of the second stage with greater 

 velocity and in a direction nearly parallel to the steam jets of the second stage. The 

 throat of the diffuser in the second stage also has an opening 2 connected with the 

 atmosphere. The object of this opening is to permit the air thus drawn in to form a 

 flexible diffuser wall. At starting and at low vacua the pressure at the diffuser 

 throat will be atmospheric. With the increase of vacuum this pressure will be low- 

 ered and the inrushing air will partly fill up the diffuser throat, thereby decreasing 

 the effective area. While this arrangement conforms with the principle of self- 

 starting as outlined above, it adds to the work of the second stage, as the air thus 

 admitted has to be compressed and ejected. 



There are also a number of other steam-air ejectors on the market, none of 

 which to the writer's knowledge have been in operation for marine service. All of 

 these have an inter-condenser between the first and second stages. The object of 

 this inter-condenser is to reduce the work of the second stage by condensing the 

 steam discharged from the first stage, thereby decreasing the total steam consump- 

 tion 30 to 40 per cent. While this scheme may look very attractive at first, a closer 

 investigation will show that there is no advantage in using an inter-condenser be- 

 tween stages in marine installations. 



On board a ship it is necessary to install a surface inter-condenser. The heat 

 contained in the steam of the first stage is entirely lost, as it is absorbed by the cir- 

 culating water. Provision has to be made for removing the condensate from the 

 inter-condenser. Circulating water has to be supplied to the inter-condenser, which 

 means additional work. The weight of the inter-condenser air ejector with its 

 necessary piping is about eight times that of the straight two-stage ejector. The 

 space required is five times as large and the arrangement of the piping is compli- 

 cated. The increased weight and the larger space required mean a slight increase 

 in propelling power. Space and weight also make it difficult to provide two or three 

 steam-air ejectors with inter-condensers working in parallel. Such an arrangement 

 can easily be made with a steam-air ejector without an inter-condenser and has the 

 advantage of increasing the flexibility and economy during light loads. 



The gain in steam consumption by using the inter-condenser is more than 

 counterbalanced by all the disadvantages mentioned. The arrangement with an in- 

 ter-condenser also lacks the simplicity which a two-stage air ejector installation 

 without inter-condenser possesses. 



