How does an eductor work?

Basically the eductor, often referred to as a Venturi pump, is a simple device that converts pressure to velocity when a fluid (liquid or gas) is forced through a restriction in an enclosed line.  This creates a partial vacuum in the eductor immediately after the orifice.  It uses fluid mechanics to obtain work from energy stored in the motive fluid.  Because the reduced pressure at the restriction creates a partial vacuum, this device can be used, among other applications, to extract samples from pipes, vent stacks, or ambient air or even to mix samples. (Click here for more complete discussion.)

Can I use one eductor to draw samples from multiple points?

An eductor often has the ability to deliver more flow than is required to meet flow rate or time delay for one system.  The flow capacity can be divided among two or more sample lines/analyzers.  Multiple sample lines can be drawn through a sample switching system or sinultaneously through multiple analyzers/sensors.

Can I draw a sample from a point that is at reduced pressure, i.e.  less than atmospheric pressure?

Because an eductor creates a partial vacuum at the sample intake, these devices can be used to draw samples that are at reduced pressure.  Eductors are commonly used to draw stack samples for environmental analysis and to sample furnace exhaust to control combustion efficiency.

Can I use a motive pressure at less than the supply pressure?

The Motiv-Air-Torr can operate at a range of pressures, depending on sample flow requirements.  For example, if a flow rate of 1 LPM is required for a sampling device the motive pressure can be reduced to 5 psig, reducing utility usage while meeting time delay requirement. (view performance curves)

Where can I use an eductor?

There are innumerable applications for this simple technology, from environmental sampling to gas cooking stoves to large scale dredging operations.  The small mini eductors (or mini aspirators) that Jacobs Process Analytics offers are routinely used for:

• Sampling: In process analytics for sampling process gases or aspirating stack gases for continuous analysis or for discrete sampling for lab analysis.  The eductor can also be configured to return the sample to the process.
• Mixing or dilution: The mini-eductor can be used to mix samples in a specific controlled ratio.  Any pressurized gas (air, nitrogen, argon, natural gas, … ) can be used as the motive force that creates the vacuum flow.  Our standard off-the-shelf eductors meet the majority of applications.  However, we also make customized eductors to suit your specific flow requirements.
• Vacuum: Create a vacuum for evacuating vessels or lines.
• Venting: Remove noxious vapors from a chemical process or vessel.

Can I use process or sample stream as the motive force to drive the eductor?

A process stream which is at elevated pressure, e.g. natural gas lines, can be used to drive the eductor.  The sample can be diluted at a fixed ratio or returned to the process, depending on the requirements. View typical installation designs.

I need to return the sample to a flare header.  Can I still use an eductor if there is a backpressure? 

An eductor is used to draw samples from remote, positive  or negative pressure sample points and are either vented, returned to the process or sent to a flare.  Often there is a backpressure at the exhaust.  Compensation for this condition requires that the motive pressure be increased.  Below are curves that illustrate flow capacities vs motive pressure at different back pressures

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Backpressure/flow rate curves: The above illustrates the sample or vacuum flow rate vs motive force for three different back pressures at the exhaust. This data is useful when it is necessary to return the sample to the process or to send it to a flare header. The flow rates are shown for 5 psig (blue) backpressure at the exhaust, 10 psig (red) backpressure and 15 psig (green) backpressure. These curves shift significantly higher when an elevated sample pressure exists (see below).

This plot illustrates the effect of an elevated sample pressure while experiencing a 15 psig backpressure at the exhaust (atmospheric sample pressure, red; 5 psig elevated sample pressure, blue).  The sample flow rate is significantly increased with the elevated sample pressure.

What is the difference between an eductor and an aspirator?

These terms are often used interchangeably.  Other terms used for the same device are Venturi pump, Bermoulli vacuum pump, jet pump, mini-eductor, mini-aspirator, …

What are the advantages of using an eductor over using a mechanical pump?

An eductor is a simple device that has many advantages over a mechanical pump.

1. Capital cost:  The mini-eductor is an inexpensive device to purchase and install. Compare this to the cost of a mechanical pump, especially if it must be explosion proof or made of an exotic material.  An eductor is a fraction of the cost.
2. Installation cost: Installation requires only an air line and a regulator.  Mechanical pumps necessitate installation of conduit, wiring and dedicated circuitry.  Mechanical pumps also occupy much more space, which can be a serious consideration for sample conditioning system enclosures.
3. Compact:  Our eductor is built into a tubing Tee with compression fittings and/or threaded ends as integral part of the device.  The mini-eductor (or mini-aspirator, Venturi pump) is easily integrated into any sample system.  No additional fittings are required.
4. Safety:  The Bernoulli vacuum pump or mini-eductor is an intrinsically safe device.
5. Reliability:  Eductors or aspirators are virtually maintenance-free.  Installation of a mechanical pump automatically reduces the reliability of any system.  Moving parts wear, reducing efficiency and capability, and must be serviced periodically.

Can I get a mini-eductor in materials other than stainless steel?

Materials of construction

• The standard materials of construction for off-the-shelf eductors are 316 SS and Teflon.
• Several other metals are also offered (hastalloy, titanium, 316 L, monel, …) depending on process make-up and conditions.
• For certain low flow requirements ruby or sapphire is utilized in our eductor orifices.

Sizes

• Off-the-shelf metal eductors are made from ¼” compression fitting union Tees.
• Many other fittings are also available including a combination of compression fittings and NPT threadings, e.g. ¼” NPT male run Tee, male NPT branch ( 1/8” & ¼” NPT), 3/8” union Tee, …  Give us your requirements.