On the other side, excess motive steam pressure can lead to wastage of costly steams. So it is very much essential to maintain motive steam pressure otherwise ejector cannot give the desired performance.
This minimum pressure is called motive steam pick-up pressure. Motive steam pressure must be above a minimum pressure for stable operation. A similar problem occurs when the supply temperature of motive fluid rises above its design value which results in increased specific volume, and consequently, less steam passes through the motive nozzle.Ī steam ejector is normally designed for a motive steam pressure of 15 to 600 PSIG. If it happens, the ejector is not provided with sufficient energy to compress the process fluid to the design discharge pressure. If the pressure of the motive fluid falls below the design pressure then the nozzle will pass less amount of steam than required. A minimum pressure of the motive fluid is required to maintain a stable operation & thereby for designing a stable ejector system. The choice depends on the availability of the utility, operational feasibility, etc. Normally high- pressure steam is used as a motive fluid but compressed air or gas can also be used as the motive fluid. Motive fluid is the fluid that motivates the process fluid to draw into the ejector. For the generation of such low pressure, up to six stages ejector can be used. High-velocity motive steam entrains and mixes with the suction fluid.Ī multi-stage ejector is normally used when a generation of high vacuum is required that is normally from the atmosphere to in the range of 30 torrs to 0.05 torr. This causes the driving force to draw the suction fluid into the ejector.
In the actual scenario, the motive fluid expands to a pressure lower than the suction process fluid pressure. Velocity coming out from a motive nozzle is 3 to 4 times the Mach number. The expansion of the motive fluid through the motive nozzle causes supersonic velocities at the exit of the nozzle. In the ejector, the velocity of the motive fluid becomes very high as it expands across the converging and diverging nozzles from motive pressure to the operating pressure of process fluid. Finally, by re-compressing the mixed fluid it meets the destination pressure. As it propagates through the diverging section the kinetic energy converts into pressure energy by decreasing its velocity and increasing the pressure. Then both the fluid is mixed & flows through the diverging section consisting of a diverging nozzle. This transformation results in a low-pressure region that provides the motive force to draw the process fluid. The ejector has a converging section in which velocity increases to convert pressure energy into kinetic energy. (ρV 2)/2 = Kinetic energy per unit volume.
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