Comparison of Steam, Air and Other Motive Media

A further field of application for product vapour driven ejectors are in processes where the intrusion of water has to be avoided at all costs. In addition, the use of motive media other than steam can also be energy saving. 

The design of product vapour driven jet pumps needs a detailed analysis in order to find the best degree of efficiency. The main fluid dynamic processes in a vapour- or gas-driven ejector are firstly the expansion ofthe motive medium, then the mixing of motive and suction flows and finally the compression of the mixed flow to the required discharge pressure. These processes are influenced by the geometry of the ejector, the properties of the media employed and the prevalent pressures. 

The applicability of a substance to be used as a motive medium in a vapour jet vacuum pump is also considerably influenced by its vapour pressure diagram. It must be considered that the evaporation temperature is always below a possible 

The temperature of the available cooling water and the possible condensation pressure can preclude the use of a special motive medium. Also, the variation of the vapour pressure diagram by the addition of other components must be regarded. The example of the twin component system, ethylene glycol/water, makes this point more clearly. Even small parts of water lead to a distinct decrease of the necessary cooling liquid temperature. For example, at 2 mbar and a water content of 1% condensation is possible at 35 °C, but even at 3% water content the condensation temperature has decreased to about 15 C. 

Bartscher (1990) Aufbau und Anwen-dungsmoglichkeiten von Strahlpumpen. Chemie-Technik, 19 Nr. (2), 40-45. 

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