Steam Ejectors
An Introduction To Steam Ejectors
Steam ejectors are pump-like devices that use the Venturi effect of a converging-diverging nozzle to convert the pressure energy of a motive fluid to velocity energy which creates a low pressure zone that draws in and entrains a suction fluid. The entrained suction fluid may be a gas, liquid, slurry or a dust-laden gas stream.
The Venturi effect in steam ejectors
- The Venturi effect applies to the operation of steam ejectors. Fluid under high pressure is converted into a high-velocity jet at the throat of the convergent-divergent nozzle which creates a low pressure in the steam ejector at that point.
- The low pressure draws the suction fluid into the convergent-divergent nozzle where it mixes with the motive fluid.
Steam ejectors are made of carbon and stainless steel, titanium, PTFE, carbon and other materials. Steam ejectors find wide use in vacuum pumping applications such as vapour extraction, chemical processing, evaporative cooling, vacuum distillation, vegetable oil deodorisation, vacuum refrigeration and drying.
In spite of the fact that steam ejectors have poor overall efficiency and relatively high energy consumption, they are popular in vacuum applications because of their simplicity and ease of operation. The capacity of steam ejectors is directly proportional to the weight of the motive fluid. Generally, the ratio of motive fluid to the gas pumped is high, especially under low vacuum and results in excessive demand of steam in multi-stage systems.
The overall performance of steam ejectors are sensitive to changes in operative parameters such as motive steam pressure and discharge pressure. A slight variation in operating parameters weighs heavily on the system capacity.
For suction pressure below 100mbar absolute, multi-stage steam ejectors are used, usually with condenses between the ejector stages. Condensing of motive steam greatly improves ejector set efficiency, both barometric and shell-and-tube surface condenses are used.
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