Air Ejectors Principle


An air ejector or steam ejector is a device which uses the motion of moving fluid (Motive Fluid) to transport another fluid (Suction fluid). It is has a wide range of application in steam ejector in boiler condenser, fresh water generator and in priming the centrifugal pump.

It works on the principle of convergent /divergent nozzle as it provides the venturi effect at the point of diffusion as the tube gets narrows at the throat the velocity of the fluid increases and because of the venturi affect it pressure decreases, vacuum will occur in the diffuser throat where the suction line will be provided.


The operating medium of an air ejector can be either high-pressure gas or liquid. This is passed through a nozzle and the pressure energy is converted into velocity energy. The high-velocity fluid aspirates the air and the non-condensable gases and the mixture is projected into a diffuser which reconverts the velocity energy into pressure energy.


Steam is suitable operating medium and is used in the steam-jet air ejector. The steam consumption is controlled by the compression ratio of the air and this factor influences the decision to adopt either single or multiple stage ejectors for a particular condition.

To meet the requirements when raising vacuum a starting ejector is provided. This is high capacity, high steam consumption ejector, of single stage and without an after condenser.

A main air ejector with standby unit is usually provided for normal operation. The heat in the operating steam is partially recovered in the condensate which flows through the inter and after coolers.