This application involves removing gases at a continuous rate from an area while maintaining the pressure at a stable level. These applications often involve removing gases or fumes that are continuously recurring. An example would be removing smoke from a welding or machining area. This process also could be used for injecting oxygen into a liquid stream. If the gases being removed have undesirable characteristics, it is possible in some cases to neutralize them by using a reactive motive fluid.
Evacuation using an Eductor
This application involves pulling gases (using jet pumps) from a defined volume by pumping the tank down from a starting pressure to a final lower pressure. It is generally sized by determining the amount of time it takes to reduce the pressure in the vessel to the desired final pressure. Examples of this application would be reducing the pressure in a reaction vessel to purge it of detrimental gases, or removing steam from a vessel before opening it to increase operator safety. A variation of this type of application is the use of eductors to prime piping or a system with liquid. Priming evacuations may be used to bring the level of liquid up to pump level to avoid the pump being started dry or to establish a siphon.
Jacoby Tarbox eductor models for liquids pumping gases are: ML, MLE; for gases pumping gases: SG, HG. These models can be used for both exhausting and evacuation. See the following specifications tables for operating parameters.
Other models of eductors are available for specific applications. In some cases, these specialty eductors can pump a suction volume up to 50 times the motive volume. Most jet pumps are available in sizes from 1/2" through 3" in a wide variety of materials. Expedited deliveries are possible if needed. Units are available in sizes up to 12".
Eductors operate on the basic principles of flow dynamics. This involves taking a high pressure motive stream and accelerating it through a tapered nozzle to increase the velocity of the fluid. Gas Motives are compressible fluids and are put through a converging-diverging nozzle. The gas can exceed the speed of sound. This fluid is then carried on through a secondary chamber where the friction between the molecules of it and a secondary gas (generally referred to as the suction fluid) causes this secondary gas to be pumped. These fluids are intimately mixed together and discharged from the eductor.
There are three connections common to all eductors:
This connection is where the power for the eductor is generated, by increasing the velocity of the motive fluid. The jet pump nozzle in this section is manufactured to take advantage of the physical properties of the motive fluid. Eductors with liquid motives use a converging nozzle, as liquids are not generally compressible. Eductors with gas motives utilize converging-diverging nozzles to achieve maximum benefit from the compressibility of the gas. All Jacoby Tarbox nozzles for eductors have smooth flow paths. Flow paths with rough surfaces cause eductors to operate less efficiently.
This connection of the eductor is where the pumping action of the eductor takes place. The motive fluid passes through the suction chamber, entraining the suction gas as it passes. The friction between the fluids at the interface of the motive fluid causes the chamber to be evacuated as the gas in the chamber is removed. This allows the pressure in the suction vessel to push additional flow into the suction connection of the eductor. The high velocity of the motive stream in this section of the eductor directs the combined fluid toward the discharge section of the eductor.
As the motive fluid entrains the suction gas, part of the kinetic energy of the motive fluid is imparted to the suction gas. This allows the resulting mixture to discharge at an intermediate pressure. The percentage of the motive pressure that will be recovered is dependent upon the percentage of motive flow to suction flow and the amount of vacuum at the suction connection.
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This application involves pulling gases (using jet pumps) from a defined volume by pumping the tank down from a starting pressure to a final lower pressure. It is generally sized by determining the amount of time it takes to reduce the pressure in the vessel to the desired final pressure. Examples of this application would be reducing the pressure in a reaction vessel to purge it of detrimental gases, or removing steam from a vessel before opening it to increase operator safety. A variation of this type of application is the use of eductors to prime piping or a system with liquid. Priming evacuations may be used to bring the level of liquid up to pump level to avoid the pump being started dry or to establish a siphon.
Liquid Eductors