Steam ejectors find wide use in vacuum pumping applications – so called dirty application such as in Vapour extraction, Chemical processing, Evaporative Cooling, Vacuum distillation, Vegetable oil de-odourization, Vacuum Refrigeration, Drying etc. 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. Its high time now when the industry should realize the disadvantages associated with it and switch over to efficient alternatives – Dry Mechanical Vacuum Booster being one of them. Mechanical Vacuum Booster offers an efficient replacement to steam ejector, for most of the applications, as they overcome major drawbacks associated with steam ejectors. The major advantages of Mechanical Booster being :-
- Mechanical Vacuum Boosters are more energy efficient.
- Minimum of auxiliary equipment is needed; unlike for steam ejectors, which need large condensers, cooling towers, re-circulation pumps etc.
- Mechanical Vacuum Boosters are dry pumping system and don’t give rise to water and atmospheric pollution.
- Startup time for mechanical booster is very low making them ideal for Batch process operation where immediate startup and shut down is essential for energy conservation.
Apart from the above, the operating costs for mechanical vacuum systems are low, resulting in extremely short pay back period. For example, when operating in the range of 5-10 Torr the operating cost of mechanical pumping system would be about one tenth of the equivalent steam ejector system.
Steam ejectors comprise of converging – diverging nozzle through which high-pressure steam (motive fluid) is forced through. (Fig.1). The ejector nozzle converts the high-pressure head of the motive fluid into high velocity stream as it emerges from the nozzle into the suction chamber. Due to increase in velocity head, there is a drop in pressure head causing partial vacuum in the suction chamber. Pumping action occurs as the fluid / vapors present in suction chamber are entrained by the motive fluid and are carried into the diffuser, byviscous drag process.
The capacity of steam ejector 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 ejector is 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. Multi steam ejectors require inter-stage condensing as each stage adds to the pumping load for the succeeding stage and for reason of economy, condensation becomes important. The heat gained during condensation i.e. latent heat of vaporization, adds to the need for additional equipment such as re-circulation pumps, cooling towers etc. so that the same can be dissipated. In a steam ejector, steam comes in direct contact with gas/vapour pumped and many a time, this mixture of pumped vapour and water needs elaborate treatment before it can be discharged / re-used. Steam ejectors, especially multistage not only require steam generation facilities but also raise demand for auxiliary equipment such as D.M. plant for boiler feed water, condensing units, re-circulation pumps, cooling towers, effluent treatment plant etc. thereby increasing total energy consumption and maintenance costs. Steam ejectors are, therefore, no longer popular as they were once because of dramatic increase in cost of steam generation, auxiliary power and effluent treatment problems. It is for this reason many steam ejector installations have been replaced by mechanical Vacuum Pumps which use far little energy for the same service and require no additional auxiliary power, cooling tower nor give rise to effluent.