Ejector vs. Eductor (What's the "diff" anyway?) By John Mathews, PE - Chief Applications Engineer, Croll Reynolds, Inc. Frequently, people will send us an inquiry for a "steam jet eductor vacuum system." This is a misnomer. Although the terms "ejector" and "eductor" are frequently interchanged, strictly speaking, they are not the same thing. What's the difference? Both are "jet pumps" that operate in accordance with the well-known Bernoulli equation. A high pressure/ low velocity motive fluid is converted into a lower pressure/ higher velocity jet through a nozzle. Jetting past a suction connection in the head of the device will entrain a secondary low pressure fluid. In said head (nothing more than a pipe tee really), some of the kinetic energy of the motive fluid is transferred almost instantaneously to the suction fluid until the mixture achieves a uniform velocity at the low pressure. The velocity of this mixture is then converted into an intermediate pressure (i.e. lower than the motive pressure, but higher than the suction pressure) in the throat or venturi of the device. The mixture's kinetic energy is converted back to pressure energy. This balance could also be done considering momentum transfer. Where they start to differ is in the motive fluid: • •
An ejector uses a gas phase motive, usually steam or air. An eductor uses a liquid phase motive, usually water.
Where else do they differ? •
•
•
•
•
Motive nozzle: The motive nozzle in a gas jet ejector is almost always a "converging-diverging" nozzle. The gas velocity leaving the nozzle can be as high as Mach 2 (~2000 ft/sec). The motive nozzle for a liquid jet eductor is almost always just a "converging" nozzle. The liquid velocity leaving it seldom exceeds 20 ft/sec. Suction head size: The head of a gas jet ejector can be (but not always is) a tee 1-2 pipe sizes larger than the suction connection size. The head of a liquid jet eductor is a tee 2-4 pipe sizes larger than the suction connection size. It almost always longer than a "standard" tee of that size. Throat bore size: In a gas jet ejector, it is practice to limit the throat bore size to 0.5 - 0.55 X the discharge diameter. In a liquid jet eductor, this can be as much as 0.75 X the discharge diameter. Throat velocity: In a gas jet ejector, this can approach Mach 1 (1000 ft/sec) depending on the compression ratio. This helps to explain why these units are loud. In a liquid jet eductor, it's usually 10 ft/sec or less. Liquid jet eductors are usually quiet. Throat length: In a gas jet ejector, the overall throat length (end of suction head to outside face of discharge flange) is usually between 6.5 and 8.5 X the discharge diameter. (Multinozzle units may be a bit shorter.) In a liquid jet eductor, this number is usually between 4 and 5 X the discharge diameter.
Overall, though, if you see a unit such as this, it really is hard to determine what it is. Both look the same from the outside. You probably have to look at the motive line and/or the nameplate. In terms of what Croll Reynolds manufactures:
Ejectors (Note: Most of our equipment falls into the "ejector" category.): • • • • • • •
Single and multi-stage vacuum systems Thermocompressors of all sorts "BL" series NCG scavenging ejectors and turbine gland ejectors (small pressure differential operating near atmospheric pressure) Siphon ejectors (steam motive pumping water) Atmospheric air ejectors (used to extend the range of liquid ring vacuum pumps) Gas jet compressors Sulfur pit ejectors
The last two are used primarily by the petrochemical industry, where the word "eductor" is engrained. Try as we might, there probably is no changing this.
Eductors: • • • •
AQUA-DUCTORs (liquid motive with liquid suction, sometimes used for solids transfer) AQUA-VACTORs (liquid motive handling air producing vacuum) Circulators (in-tank mixing eductors)\ Jet venturi scrubbers
It is possible to combine both devices in vacuum service. Although seldom done today, there are instances where a water jet eductor has served as the final pumping stage and direct contact condenser for one or two high vacuum steam ejectors. Water jet eductors, rather than the customary liquid ring vacuum pump, have also been used to back an atmospheric air ejector, Some things we produce are neither "fish nor fowl." For example a "jet heater" is not a jet at all but simply a steam/water mixer without the pressure recovery that is characteristic of a jet pump. Other examples would be some of our desuperheaters and high-energy venturi scrubbers. In conclusion, there's no need to fret over the differences. If you say "eductor" and mean "ejector" or vice-versa, we'll still know what you're looking for; we just wanted to
set the record straight. Little-known fact: "Eductor" was once a trademarked name which has come into generic use, much like the word "Band Aid."
