Air ejector

In thermal power plant, surface condenser is used to cool exhaust steam from turbine into condensate, which is recycled as feedwater for boiler to save working medium. When steam turns into water, its volume shrinks greatly, vacuum is formed inside condenser. Condenser increases enthalpy drop of steam at turbine outlet to benefit thermal efficiency. So, vacuum is important for the maximization of steam turbine output. However, accumulation of non-condensable gases would cause a pressure rise in condenser. To avoid this problem, air ejector is used to pull non-condensable gases out.

 

Air ejector could be driven by steam or pressurized water.

 

Steam jet air ejector is usually a two-stage vacuum system, which requires overheated steam as motive fluid. In the first stage, when motive steam passes through ejector nozzle at a high velocity, it forms negative pressure to suck steam and non-condensable gases. They are mixed, compressed and discharged into intercondenser. After cooling, non-condensable gases and steam is sucked, mixed, compressed by the second stage ejector. The mixture is finally cooled down in aftercondenser. Steam is condensed into water, while gases are released into atmosphere.

 

Water jet air ejector has the same principle with steam ejector. It receives pressurized water from jet pump to suck non-condensable. It is featured with compact structure, reliable performance, low fabrication cost, no need for inter and after condenser. To avoid backflow of water into condenser when ejector is in malfunction, non-return valve is required at the suction port. Or, an upside-down U tube with height no less than 10m is used for smaller resistance on piping system.

 

Two reasons as following would cause a temperature rise of driving water:

 

1. Suction fluid contains some steam, which would release latent heat during condensation.

2. Most of mechanical work by jet pump is transformed into heat energy of driving water.

 

Higher temeprature would increase evaporation of water in suction chamber and lower vacuum degree inside surface condenser. Supplement of low temperature driving water is necessary to keep the performance of air ejector.

 

When boiler is started up, steam turbine is warmed up slowly. More and more steam will enter surface condenser. If vacuum is not built up inside condenser, positive pressure will form and damage equipment. Vacuum is essential for the normal running up of steam turbine. In this situation, a small scale startup ejector is used.