Jet discharge pressure

The jet discharge pressure is controlled by the downstream condenser pressure. The minimum condenser pressure corresponds to the condensing pressure of steam at the condenser s vapor outlet temperature. For example, let s say that the condensing pressure of pure steam, at 120°F, is 80 mm Hg. If the condenser vapor outlet temperature is 120°F, then the lowest pressure we could expect to measure at the condenser vapor outlet would be 80 mm Hg. 

Let s further assume that the pressure drop from the jet discharge, through the condenser discharge, is 10 mm Hg. Then, the jet discharge pressure would be 90 mm Hg. Let s also say that the sonic boost is equal to 3.60. The velocity boost is assumed to be equal to 2.5. The overall compression ratio is then... 

Note that atmospheric barometric pressure is 26 in Hg (this is a chemical plant in Denver—the Mile High City). The first-stage jet gas inlet pressure is 25 mm Hg. The third-stage jet discharge pressure is 3 psig. A good rule of thumb is... 

The critical discharge pressure for each jet is determined experimentally by the manufacturer. It is usually noted on the jet specification sheet. My experience indicates that exceeding this critical jet discharge pressure by the smallest amount will force the jet out of critical flow. Or, the way 1 see it, will cause the jet to suddenly surge a few times and then lose its sonic boost. 

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