NEW DEVELOPMENTS IN HIGH VACUUM APPARATUS. 179 



vacuum ejectors used with marine-turbine condensers consist of two stages — the 

 first wStage, which is connected to the air suction of the condenser and which ex- 

 hausts into the second stage at a pressure varying from 4 to lo-inch Hg. absolute; 

 the second stage, which compresses the mixture of steam, air and vapor received 

 from the first stage to a pressure slightly above atmosphere. The steam used in both 

 stages, while passing through the ejector, gives up its energy but does not lose its 

 heat, with the exception of some small losses due to radiation. The exhaust from 

 the ejector is discharged into the feed tank. The heat in the steam is by this means 

 transmitted directly to the boiler feed with practically no loss. 



The characteristic of the performance of the steam-air ejector is illustrated in 

 Plate 71. Curve A shows the air-handling capacities of a two-stage ejector designed 

 for 2 inches absolute at different absolute pressure, when exhausting dry air and 

 when being supplied with a constant quantity of dry saturated steam of constant 

 pressure, and when exhausting against a constant back pressure of I/2 -pound gauge. 

 It will be noted that the air ejector is capable of producing an absolute pressure of 

 0.25-inch Hg. at dead end (with the air suction blanked off). The capacity in- 

 creases steadity with the increase of absolute pressure. 



The actual dry-air handling capacity of the second stage of the same ejector 

 working under similar conditions is illustrated in curve B. The steam consumption 

 of the second stage is, of course, less than the total steam consumption of the two 

 stages. 



Curve C, shown in dot-and-dash lines, represents the air-handling capacity of the 

 second stage based on the total steam consumption of the two-stage ejector. 



Comparing these curves it will be noted that at 5-inch Hg. absolute the sec- 

 ond stage alone handles as much air as both stages together, and that from this 

 point the capacity of the second stage increases over that shown in curve A. This 

 is due to the fact that, when both stages are working, the second stage also com- 

 presses the steam discharged from the first stage. One is accordingly led to the 

 conclusion that it is advisable to shut off the steam supply to the first stage at about 

 5-inch Hg. absolute to save the live steam used in the same. Curve C also indicates 

 that at about 4.5-inch Hg. absolute or more a single-stage air ejector should be 

 used, provided it can be designed self-starting. 



In connection with Plate 71, it must not be assumed that the curves indicated 

 are in every sense absolute. They depend entirely upon the ratio between the com- 

 pression ratios of the first stage and those of the second stage. The curves shown 

 are those of an ejector which has been extensively used in marine installations. 



The main factors (excluding those referring to the design) controlling the per- 

 formance of a steam-air ejector are: — 



(a) The quality of the live steam supplied. 



(b) The pressure of the live steam supplied. 



(c) The back pressure at the discharge. 



(d) The condition of air handled. 



The live steam supplied to the steam-air ejector should be dry saturated or 



