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What Is an Eductor / Ejector? | Process Engineering Glossary
What Is an Eductor / Ejector?
In piping engineering and process engineering, an eductor or ejector is a device that uses the kinetic energy of a high-velocity motive fluid to entrain and transport a second fluid, with no moving parts. The motive fluid passes through a converging nozzle, accelerates to high velocity, and creates a low-pressure zone that draws in the suction fluid. Both fluids then mix and discharge together through a diffuser at an intermediate pressure.
The two terms are used interchangeably in most process plant contexts. Some engineers distinguish them by motive fluid type: eductors use liquid as the motive, while ejectors use steam or gas. In practice, both names describe the same operating principle.
Applications of Eductors and Ejectors
Vacuum Distillation
Multi-stage steam jet ejectors create and maintain the vacuum in crude oil vacuum distillation columns in refineries. The vacuum reduces the column operating temperature below the thermal cracking threshold for heavy crude fractions, producing valuable gas oil products without coking or degradation.
Priming of Centrifugal Pumps
Eductors prime large centrifugal pumps by evacuating air from the pump casing before startup. The eductor connects to the pump suction casing and draws air out, creating a partial vacuum that allows liquid to rise into the casing from the suction sump. This method suits pumps installed above the liquid surface where self-priming is not possible.
Chemical Injection and Mixing
Eductors inject concentrated chemicals into pipelines and tanks by using the process fluid as the motive to entrain and dilute the chemical addition. This approach provides mixing energy at the injection point without a separate agitator, reducing equipment count and maintenance requirements in chemical dosing applications.
Refrigeration Systems
Steam jet refrigeration systems use high-pressure steam as the motive fluid to entrain and compress water vapour evaporating from a chilled water surface at low pressure. The ejector replaces the mechanical compressor in conventional refrigeration cycles. These systems suit applications where low-grade waste steam is available and where the simplicity of no moving parts offers operational advantages.
Benefits of Eductors and Ejectors
No Moving Parts
The absence of rotating components eliminates mechanical wear, seal failures, and lubrication requirements. Eductors and ejectors therefore require very little routine maintenance compared to centrifugal or positive displacement pumps and compressors. This reliability advantage is particularly valuable in remote, hazardous, or difficult-to-access locations.
Handles Difficult Fluids
Eductors move fluids that would rapidly damage conventional pumps, including abrasive slurries, corrosive liquids, two-phase mixtures, and solids-laden streams. The simple internal geometry with no tight clearances accommodates particles and viscous materials that would block or erode a centrifugal impeller.
Simple Installation and Low Cost
Eductors and ejectors are compact, lightweight, and require only pipe connections to install. They need no foundations, no electrical supply, no motor, and no coupling alignment. Capital cost is therefore very low relative to a mechanical pump or compressor of comparable suction capacity.
Limitations to Consider
Low Efficiency
Eductors and ejectors have inherently low thermodynamic efficiency compared to mechanical compressors and pumps. A significant fraction of the motive fluid energy transfers to the suction fluid, but much is lost in the mixing process. For applications requiring large suction flows relative to the motive flow, the motive fluid consumption becomes disproportionately high and a mechanical compressor is more economical.
Fixed Operating Point
The performance of an ejector at a given motive pressure follows a specific operating curve. Operating conditions that differ significantly from the design point reduce efficiency sharply and can cause the ejector to lose its suction entirely if the discharge pressure rises above the critical value for the device. Consequently, eductors and ejectors suit applications with relatively stable operating conditions rather than those requiring wide flow range control.
Motive Fluid Contamination
Because the motive fluid mixes with the suction fluid in the device, the two streams combine in the discharge. This is acceptable in many applications but creates contamination problems where the motive fluid and suction fluid must remain separate. Steam jet ejectors introduce steam into the suction vapour, producing a mixed discharge that requires condensation and separation before the process stream can proceed. In liquid jet applications, the motive water or liquid blends with the suction stream at the discharge.
Eductor / Ejector FAQ
What is the difference between an eductor and an ejector? In most process plant usage the terms are interchangeable. Where a distinction is made, an eductor uses a liquid as the motive fluid and an ejector uses steam or a gas. Both operate on the same Venturi principle: a high-velocity motive fluid creates a low-pressure zone that entrains a suction fluid, and the combined stream discharges at an intermediate pressure through a diffuser. Neither device has moving parts.
How does a steam jet ejector create vacuum? High-pressure steam accelerates through a converging nozzle to very high velocity, creating a low-pressure zone at the nozzle exit. This low pressure is below the suction pressure of the vapour stream from the vacuum vessel. As a result, vapour flows from the vessel into the ejector suction chamber. The steam and vapour mix and discharge through the diffuser at a pressure above the vacuum but below the steam inlet pressure. Multi-stage arrangements achieve progressively deeper vacuum by connecting ejectors in series with intercondensers between stages.
When should an ejector be used instead of a mechanical compressor? An ejector suits applications where a high-pressure motive fluid is already available, where the suction flow is modest relative to the motive consumption, where no electrical power is available, where the fluid is abrasive or would damage rotating equipment, or where minimal maintenance is essential. A mechanical compressor is more appropriate where suction flow is large, where efficiency is critical, where the discharge pressure must be precisely controlled, or where the motive fluid cannot be allowed to contaminate the process stream.
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