Three important avenues of perfecting multistage stream jet injectors aimed at improving their reliability, economic efficiency, and performance are presented, and examples of implementing them are given. In regard to reliability improvement, the designs of existing serially produced ejectors available from different manufacturers, in particular the Ural Turbine Works, Leningrad Metal Works, and Kharkiv Turbine Works, are analyzed. New technical solutions are developed, including external vertical ejector coolers and jet devices with a variable axial distance between the nozzle and diffuser. To reduce the ejector’s mass and dimensions, its first stage is split into two parts. Two approaches to improving the economic efficiency of the devices are considered: decreasing the working steam parameters to the injector (the temperature and pressure) and taking into account the effect that the increased ejector capacity has on the deviation of vacuum in the condenser from its normal design value. Tests of serially produced ejectors under different conditions of their operation are carried out. The ejector performance can be improved through perfecting its jet device using calculation, numerical, and experimental methods. Based on the results from visual examination and flaw detection of existing serially produced ejectors, the lines of modernizing their designs are determined, and a more accurate procedure for calculating them is developed. For elaborating the procedure, the geometrical parameters of existing serially produced ejectors and the results of their industrial tests were generalized, and their numerical modeling was carried out. The developed procedure and new design solutions have been successfully implemented in two types of ejectors. New ejectors have been installed at different thermal power plants as part of condensing and cogeneration turbines and they made it possible to obtain essential technical and economic gains. The economic gain is that the vacuum in the condenser deviates from its normal design values to a lesser extent, due to which it becomes possible to decrease excessive consumption of fuel in the steam unit’s cycle.