Some Problems in the Design and Operation of Jet Ejectors

Abstract

The author presents some of the more recent ejector design theories and compares them with experimental data, and he describes the operation of a few different types of ejector used at present in industry. He also gives a short survey of the relevant literature. The design theory developed in the first part of the paper is based on the momentum exchange between motive and suction fluid and the assumption that entrainment occurs at constant pressure and mixing at constant area. The equations in which the use of experimental coefficients is kept to a minimum give the motive-to-suction mass-flow ratio in terms of the pressure ratio and certain velocity ratios, as well as the required entry and mixing tube lengths and profiles. Optimization of the ejector for fixed pressure levels enabled the best ejector geometry to be determined with regard to nozzle position, mixing tube entry profile, and mixing tube length. The nozzle was in its optimum position when it reached up to the mixing tube. For this condition the optimum mixing tube length was seven tube diameters and the entry profile did not affect the operation. Although the theory is developed in more detail for an ejector using liquids the methods necessary for calculating gas ejectors and multi-phase and multi-component ejectors are outlined. The operating characteristics given in the second part of the paper emphasize the limitations of the jet pump but it is pointed out that both efficiency and compression ratio can be improved by the use of multistage designs. A particular application for a turbine condenser is discussed.