What Is an Ejector? Steam Jet Vacuum

An ejector (also called a steam jet ejector or jet pump) is a device that uses the energy of a high-pressure motive fluid—typically steam—to entrain and compress a low-pressure gas or vapor. Ejectors create vacuum conditions in process equipment without using mechanical moving parts. They are widely used in refineries, petrochemical plants, and power stations for vacuum distillation, evaporation, and gas removal.

The ejector operates on the Venturi effect: the motive steam accelerates through a converging nozzle, creating a high-velocity jet that entrains the suction gas. The combined stream then decelerates in a diffuser, converting velocity energy into pressure energy and compressing the suction gas to a higher discharge pressure.

How an Ejector Works

Component	Function
Motive nozzle	Accelerates motive steam to supersonic velocity, creating a low-pressure zone
Suction chamber	Low-pressure area where process gas is drawn in by the high-velocity steam jet
Mixing section	Motive steam and suction gas mix, transferring momentum from steam to gas
Diffuser (diverging section)	Converts kinetic energy to pressure energy, compressing the mixture
Discharge	Combined stream exits at intermediate pressure to condenser or next stage

Ejector Types and Configurations

Configuration	Vacuum Level	Application
Single-stage ejector	Down to ~100 mbar abs	Moderate vacuum, simple applications
Two-stage ejector with intercondenser	Down to ~15 mbar abs	Vacuum distillation columns
Three-stage ejector system	Down to ~3 mbar abs	Deep vacuum for molecular distillation
Four-stage or more	Down to ~0.5 mbar abs	Ultra-high vacuum applications
Hybrid (ejector + liquid ring pump)	Variable	Energy-efficient vacuum systems

Each stage discharges into an intercondenser that removes condensable vapors (steam and process condensate), reducing the load on the next ejector stage.

Key Design Parameters

Parameter	Typical Specification
Motive steam pressure	5-15 barg (saturated or slightly superheated)
Suction pressure	0.5-500 mbar abs (depending on stages)
Discharge pressure	Atmospheric or to condenser operating pressure
Motive steam consumption	10-100 kg steam per kg of suction gas (varies with compression ratio)
Materials	Carbon steel body, stainless steel nozzle and diffuser
Design standard	HEI Standards for Steam Jet Vacuum Systems
Turndown	Limited—ejectors are essentially fixed-capacity devices

Common Mistake

Ejectors have very limited turndown capability. Operating an ejector significantly below its design suction load can cause unstable operation (hunting or surging), where the ejector oscillates between pulling vacuum and breaking vacuum. Always size ejectors for the actual operating load, not excessive design margins, and consider using multiple parallel ejectors for systems with variable loads.

Ejector Advantages and Limitations

Advantage	Limitation
No moving parts—high reliability	High steam consumption (energy cost)
Low capital cost	Fixed operating point (limited turndown)
Can handle corrosive or dirty gases	Requires large condensing water supply
Simple installation and maintenance	Noise generation at discharge

Ejectors are connected to the process piping system through flanged nozzles and require motive steam supply piping sized per the pipe specification. Steam quality is critical—wet steam erodes the motive nozzle, degrading performance over time.

Read the full guide to oil and gas equipment