Eductor

An eductor, also known as an ejector or ejector pump, is a device that uses a high-velocity jet of motive fluid to entrain and transport a second fluid, creating flow without movable mechanical parts. Eductors are widely used to generate vacuum, transfer liquids, or mix chemicals in a simple, compact form.

Principle: The motive fluid passes through a converging-diverging nozzle, increasing velocity and reducing static pressure in the mixing chamber. The low pressure draws in the surrounding fluid (the entrained liquid or gas). The two streams join and slow down in a diffuser, raising the mixture to discharge pressure.

Construction: A typical eductor has a motive-fluid inlet, a nozzle, a mixing chamber, an entrainment inlet, and a discharge diffuser. There are steam-jet eductors, air-jet eductors, and water eductors, among others. Most have no moving parts; performance is dominated by the nozzle design and the backpressure of the receiving system.

Performance and considerations: The entrainment ratio ER is the mass flow of entrained fluid divided by the motive-fluid flow. Eductors require adequate motive pressure; their suction lift is limited by the pressure differential and backpressure. Viscosity, density, and surface tension affect efficiency. Cavitation and backflow are potential issues.

Applications: used for vacuum generation in tanks and process lines, liquid transfer and drainage, chemical mixing, spill cleanup, ventilation and fume control, and other industrial processes where a simple, low-maintenance pump with no moving parts is advantageous.

Advantages and limitations: advantages include simplicity, ruggedness, and lack of moving parts; availability of motive fluid; ability to operate in hazardous environments. Limitations include limited suction lift, sensitivity to backpressure, dependency on motive fluid supply, and lower efficiency compared to mechanical pumps for some tasks.

Related concepts include ejectors and Venturi nozzles.