EVAPORATION 



367 



tion, although Gwynne (patent 13577, 1851) claims a low level centrifugal 

 pump for the removal of the air and condensed water jointly. The prototype 

 of the rotary pump as now used is that of Le Blanc, Fig. 227, which is often 

 termed a " hurling water " pump. The air to be evacuated enters at a, 

 together with a certain quantity of water supplied through b. The 

 removal of the air is effected by the action of the vanes c, which cut off 

 slices of water and throw them along with the air into the collector cone d, 

 whence they are discharged finally by way of e. A supplementary connection 

 is shown at the right of the figure, and through here a second vessel may 

 be evacuated up to 20 inches vacuum, since the action of the water in the 

 ejector continues in its downward passage. 

 This type of pump may be applied to a high- 

 level condenser, Fig. 212, or to a low level 

 installation, Fig. 211. In the latter case 

 the main body of water is removed by a 

 second centrifugal pump arranged on the 

 same shaft as the air pump. 



The Volume of the Air to be removed by 

 the Pump. It is impossible to calculate with 



FIG. 226 



FIG. 227 



any degree of confidence the quantity of air to be removed from the con- 

 denser. Air is introduced dissolved in the water, and is released under the 

 reduced pressure ; and, although it is known how much air water can 

 dissolve, it is not certain that the water used is saturated. Again, air 

 is introduced with the material to be evaporated, and some incondensible 

 gas is given off in the process of concentration. Finally, there is the quite 

 uncertain quantity of air introduced through leaks : certain principles of 

 interest can, however, be developed. 



The pressure in a condenser is made up of two parts : a, the pressure 

 due to the water vapour dependent on the temperature of the water ; b, that 

 due to the incondensible gases referred to as " air." Thus, with a vacuum 



