ejector

An ejector is a device that uses a high-velocity jet of fluid to entrain and move another fluid, either liquid or gas, toward a discharge point. It contains no moving parts: a pressurized driving fluid (steam, compressed air, or hot liquid) enters a converging nozzle, accelerates to high velocity, and expands into a larger chamber where it meets the fluid to be moved. Friction and momentum transfer between the jet and the surrounding fluid create a partial vacuum that draws the secondary fluid in, and the mixed flow exits through a diffuser where kinetic energy converts back to pressure.

Water-jet ejectors, also called jet pumps, are common in shallow wells and sumps. A small-diameter jet of pressurized water from a surface pump enters a venturi section submerged in the well, entrains groundwater or collected water, and lifts the mixture back to the surface through a return line. This requires only one pump and one moving part (the surface pump's motor), making the system reliable in remote locations. Steam ejectors operate on the same principle but move gases: a jet of high-pressure steam draws non-condensable gases, air, or vapor from an enclosed space into a condenser where the steam condenses, removing the evacuated gases.

Ejectors appear everywhere vacuum is needed without a mechanical pump: in distillation columns they maintain partial vacuum, in refrigeration systems they remove trace air from the condenser, and in surface condensers of steam turbines they pull air from the steam space. Their capacity depends directly on the pressure and mass flow of the driving fluid; a larger or hotter steam supply increases suction lift. Performance degrades when the driving fluid loses pressure or when the discharge backpressure rises; both reduce the jet velocity and entrainment ratio.

Variants and selection

Single-stage ejectors handle light vacuum or small gas volumes; multi-stage units cascade the output of one ejector as the driving fluid for the next, achieving lower absolute pressures. Air-operated ejectors replace steam in facilities without steam supply, though they require larger volume flow rates for equivalent lift. The driving fluid and the entrained fluid must be compatible: water-jet pumps cannot lift hot or corrosive liquids, while steam ejectors must discharge to a condenser that can handle the condensate volume.

The name reflects the device's core action: it ejects, or forcefully expels, its driving fluid into a mixing chamber where the kinetic energy does the work of moving a second fluid. Industrial practice rates ejectors by their suction lift (how far below atmospheric pressure they pull), their capacity in gallons per minute or cubic feet per hour, and the minimum driving pressure required. Because they produce no discharge valve noise and run silently, they appear in applications where quiet operation matters; however, their efficiency is lower than that of mechanical pumps, so they suit applications where reliability and simplicity outweigh energy cost.