All About Engines

i82 All About Engines each pair of sheets a little air from the entrance c is trapped, the sheets forming practically water pistons, and the nozzles e and F forming an ejector. The out- let h is of such a form that the velocity of the issuing air and water is converted into a pressure just suffi- cient to overcome that of the atmosphere. Fig. 109 shows the pump connected with a self- sealing tank. The apparatus can be associated with any kind of condenser—surface condensing or jet, its sole purpose being to remove air from the con- denser and thus to improve the vacuum. They will give a vacuum of at least 28 inches when the baro- meter stands at 30 inches, and single sets have been constructed to deal with over 80,000 lb. or about 36 tons of condensed steam per hour. The method adopted by Messrs. C. A. Parsons and Co. is to use an ejector. Between the turbine and the air pump is a steam jet which creates a vacuum in the way described when the injector was explained on p. 102. This withdraws the air and uncondensable gases from the condenser, and delivers them under increased pressure to the air pump side, thereby enabling a smaller pump to be used. And since small pumps require less power to drive them, and have lighter valves and less clearances than large ones this reduction of size is a considerable advantage.