Heat ExchangersDOE-HDBK-1018/1-93HEAT EXCHANGER APPLICATIONSNormally, air ejectors consist of two suction stages. The first stage suction is located on top ofthe condenser, while the second stage suction comes from the diffuser of the first stage. Theexhaust steam from the second stage must be condensed. This is normally accomplished by anair ejector condenser that is cooled by condensate. The air ejector condenser also preheats thecondensate returning to the boiler. Two-stage air ejectors are capable of drawing vacuums to29 inches Hg.A vacuum pump may be any type of motor-driven air compressor. Its suction is attached to thecondenser, and it discharges to the atmosphere. A common type uses rotating vanes in anelliptical housing. Single-stage, rotary-vane units are used for vacuums to 28 inches Hg. Twostage units can draw vacuums to 29.7 inches Hg. The vacuum pump has an advantage over theair ejector in that it requires no source of steam for its operation. They are normally used as theinitial source of vacuum for condenser start-up.Rev. 0ME-02Page 17
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