Steam Jet Air Ejector (Turbine Vacuum System)
The efficiency of a Steam-turbine is mainly determined by the pressure of the steam at its outlet. The Turbine Surface Condenser is therefore maintained under vacuum. To enable this, the Steam Jet Ejector Vacuum System must
be capable of removing all non condensable and associated water vapour from the Surface Condenser to produce the minimum steam condensing pressure consistent with physical dimensions and heat transfer and to provide for
deaeration of the condensate.
The sources of non condensables include air leakage through all system components operating below atmospheric pressure, gases released from feedwater drains and vents admitted to the
condenser, gases released for make-up admitted to the condenser, condensate surge tank when utilized in a closed cycle and disassociation of feedwater into oxygen, hydrogen and non condensables.
The recommended design pressure of the Steam Jet Air Ejector (SJAE) System is as follows :
- For electric generation service : The design pressure is 0.035 ata (25.4 mm Hg (a)) or the condenser design pressure whichever is lower.
- For Pumps, compressors and other mechanical drives : The design pressure is that for which the condenser is designed minus 0.035 ata (25.4 mm Hg (a)).
Usually, the SJAE system is of Twin Element type i.e. of duplex design having two identical Ejectors per stage, one of which, being in operation at any one time and the other as stand-by. The Inter and After Condensers
are generally Single Element type.
The System also includes a Hogger / Start-up Ejector with Silencer for fast pre evacuation and which is in operation only till the desired vacuum has been achieved. Thereafter, the two stage holding ejectors take over.
In this field of application, combination units with Ejector and Surface Condenser coupled Liquid Ring Vacuum Pump are also becoming popular.