Ejector Ranges

Design ranges for single stage ejectors and use of multi stage ejectors.

Definitions:

Compression Ratio (CR) - ratio of discharge to entrained gas pressure

Pressure Ratio (PR) - ratio between the motive gas and entrained gas pressure

The feasible design range for single stage ejectors is illustrated below.

The graph is indicative only as it does not consider extreme entrainment ratios or variations in gas properties or temperatures. As a rule of thumb, CR ~ PR^0.5 at PR <15 and moderate entrainment ratios.

The thermodynamic efficiency of an ejector generally decreases with increasing PR and also with increasing CR Two stages in series are therefore considerably more efficient then a single stage when both PR and CR are high i.e. refer to the graph below (PR ~20, CR >4):

Multi stage ejectors are normally installed on vacuum ejector systems because they require high overall compression ratios. Note that the pressure ratio for the first stage ejector is very high because the entrained gas is under vacuum, thus correspondingly high compression ratios (>10) may be achieved.