Breakpoint Analysis

Breakdown of ejectors is a frequent problem, particulary on vacuum systems.  What happens in the ejector to cause this is described below. Need help? Contact Ezejector.

Refer to the graph above for a steam ejector with fixed steam and entrained gas pressures, but changing discharge back pressure.

As can be seen, the entrainment ratio remains constant when the flow regime is double-critical i.e. both the steam and the entrained gas are accelerated to sonic velocity.

When the back pressure becomes too high, the entrained gas cannot reach sonic velocity and we enter single critical flow.

The flow pattern for double and single critical flow is similar, with mixing of motive and entrained gas upstream of the shock wave.

Further increases in back pressure progressively reduce the entrained gas velocity and thus the entrainment ratio. When the break point pressure is reached, entrained gas flow reduces to zero. At this point the flow pattern collapses with the shock wave retreating from the diffuser to the motive gas nozzle outlet. As a results, the ejector discharge pressure falls and reverse flow occurs though the ejector to the entrained gas line. The ejector is now "broken" and will not re-establish the required flow regime without intervention.

The only way to "fix" the ejector is by increasing the discharge pressure it can achieve to significantly greater than the break point. This can be achieved by increasing the motive gas pressure or the entrained gas pressure - refer to graph below (the scale somewhat exaggerated for illustration). An increase in entrained gas pressure will normally occur in any case because the reverse flow will pressurise the entrained gas system. A cycle may occur when the entrained gas pressure responds by decreasing, initiating another breakdown.