Multi-nozzle steam ejector in the MED-TVC desalination system

Thermal vapor compression (TVC) is an essential part that governs the overall process in the multi-effect distillation (MED) system. A slight pressure deviation from rated primary flow condition will result in the serious performance deterioration of steam ejector. Therefore, a multi-nozzle steam ejector was proposed to solve the problem of performance deterioration under multiple unstable working conditions. A computational fluid dynamics model is used to calculate the performance of the multi-nozzle ejector under different operational conditions. The effects of both nozzle combination type and nozzle exit position (NXP) on the performance of the ejector are systematically studied. The optimal five-nozzle combination type is obtained, and the ejector entrainment ratio reaches to maximum and remains unchanged when NXP value is in the range of 0 ~ 45 mm. Following this, the devised five-nozzle steam ejector is manufactured. The performance test results demonstrate that the five-nozzle ejector has strong adaptability within a certain pressure fluctuation range of working steam. When the pressure varies from 80% to 125% of the rated pressure of primary flow, the reduction of entrainment ratio for the five-nozzle ejector is kept within 7% of the design value of the rated condition. Finally, the five-nozzle ejector is designed and used in the pilot-scale MED seawater desalination system. The five-nozzle steam ejector shows relatively stable performance with variation in primary flow pressure compared with the single nozzle ejector.