Ejector Design Calculation.pdf
the ejector is an extremely simple apparatus which has no moving parts. it does work by taking in a low-velocity flow of fluid and converting this flow to a high velocity flow. this conversion is the result of a change in the flow path, called viscosity reduction.
Ejector Design Calculation.pdf
the performance of the ejector is determined by the amount of conversion, or velocity reduction, it can achieve. velocity reduction is a function of the ratio of the secondary pressure to the primary pressure. this ratio is known as the pressure ratio. ejector performance is closely related to the pressure ratio. for example, an ejector can achieve a pressure ratio of 5 and still give good performance. however, an ejector with a pressure ratio of 3 is much less efficient.
the ejector will flow until the velocity in the primary nozzle becomes so high that the shock wave reaches the end of the mixing tube. the shock wave will cause the plug flow to be broken up and turbulent mixing will occur. the effect of this turbulent mixing is to reduce the ejector performance. if the flow in the nozzle is entirely supersonic, the shock wave will collapse and the ejector will stop working. for these reasons, and to prevent the nozzle from being eroded, the primary nozzle in an ejector has to be smaller than the mixing tube. on the other hand, if the pressure ratio is too high, the flow will be completely subsonic. this subsonic flow will have very little effect on the mixing tube, and the performance of the ejector will be poor.
the mixing tube may be of a constant diameter throughout or it may have a varying diameter. the former is the most common design. as the primary flow velocity becomes very high, the mixing tube must be of small diameter, so as to reduce the speed of the plug flow. this is the primary reason for using a varying diameter tube.