C-P Systems
What Is an Eductor (Ejector)? | Process Engineering Glossary
What Is an Eductor (Ejector)?
An eductor, also called an ejector, is a device that uses the momentum of a high-velocity motive fluid, typically steam, air, or liquid, passing through a nozzle to entrain and move a secondary fluid, creating suction without any moving mechanical parts. As the motive fluid accelerates through a narrowing nozzle, it creates a low-pressure zone that draws in the secondary fluid, and the two streams then mix and are discharged together at an intermediate pressure.
Because eductors have no moving parts, they offer notable reliability advantages over mechanical pumps or compressors in certain applications, though they generally operate at lower efficiency and are best suited to services where their specific advantages outweigh this efficiency trade-off.
Applications of Eductor
Vacuum Generation
Steam or air-driven ejectors are commonly used to generate vacuum conditions for process equipment, particularly in services where the reliability of having no moving parts is valued over the efficiency of a mechanical vacuum pump.
Chemical Mixing and Dilution
Eductors are used to draw in and mix a secondary chemical or dilution stream using the motive force of a primary flowing stream, providing simple, reliable mixing without a separate powered mixer.
Tank Mixing and Agitation
Liquid eductors installed within storage tanks can provide mixing and agitation by recirculating tank contents through the eductor nozzle, avoiding the need for a separate mechanical agitator in some applications.
Benefits of Knowing Eductor
Provides mechanically simple, reliable operation. With no moving parts, eductors offer inherent reliability advantages, particularly valuable in services where mechanical pump or compressor maintenance would be difficult or costly.
Well suited to certain hazardous or corrosive services. The absence of moving parts and shaft seals makes eductors attractive for handling hazardous or corrosive fluids where sealing a mechanical device would be challenging.
Relatively low capital cost for appropriate applications. Eductors are often less expensive than equivalent mechanical equipment for applications where their performance characteristics are well suited to the service.
Limitations to Consider
Lower efficiency than mechanical alternatives. Eductors generally operate at lower thermodynamic efficiency than mechanical pumps or compressors, making them less suitable where energy efficiency is a primary concern.
Performance is sensitive to motive fluid conditions. Eductor performance depends significantly on maintaining adequate motive fluid pressure and flow, and deviations from design conditions can significantly degrade actual performance.
Limited turndown compared to some mechanical alternatives. Eductors typically have a more limited effective operating range than variable-speed mechanical equipment, making them less flexible for services with widely varying flow requirements.
Eductor FAQ
How does an eductor relate to Bernoulli’s equation principles?
Eductor operation is a direct practical application of Bernoulli’s equation, since the pressure drop created by accelerating motive fluid through the nozzle is exactly the phenomenon Bernoulli’s principle describes, relevant also to broader equipment sizing.
Why might an eductor be selected over a can pump for a specific service?
While a can pump offers sealless operation through an enclosed motor design, an eductor achieves a similar absence of dynamic seals through an entirely different, mechanically simpler mechanism with no moving parts at all, sometimes favored for chemical dosing system applications.
How does eductor performance relate to critical flow behavior?
Eductor motive nozzle performance often involves critical flow conditions, where the motive fluid reaches choked flow through the nozzle, a consideration grounded in the same fluid mechanics used in control valve sizing and relief valve sizing.
Outbound Links
About C-P Systems
SETTING THE STANDARD FOR CHEMICAL ENGINEERING FIRMS EVERYWHERE
Through unmatched professionalism, knowledge and experience, we set the industry bar for chemical engineering firms. With decades of chemical plant engineering and piping design experience, our team of licensed engineers can handle any project scope.