Ejectors and eductors are used to lift, pump, mix and agitate liquids, granular solids and slurries. They are sometimes called aspirators, Venturi pumps, or jet pump ejectors. Because eductors are non-mechanical, they do not include moving parts or require lubrication. Because they do not require electricity, eductors can be used in hazardous environments. Eductors use the kinetic energy from a stream of compressed gas or liquid. The movement of this stream through a tapered tube and Venturi nozzle increases the pressure of the flow. The subsequent discharge of the stream then creates an area of low pressure that draws in the contents of the suction chamber. Mixing occurs in the diffuser and the motive fluid is released through an exhaust port. Often, eductors are made of polytetrafluoroethylene (PTFE), a fluorinated thermoplastic with outstanding chemical resistance and excellent lubricity. Eductor nozzles are usually made of stainless steel.
There are two basic technologies for eductors which use compressed gas: exhausting and evacuating. Exhausting eductors remove the gas at a continuous rate while maintaining a stable pressure. These devices are often used to remove smoke from welding areas and to inject oxygen into liquid streams. In some cases, the introduction of a reactive, motive fluid can be used to remove a gas’s undesirable characteristics. Evacuating eductors remove gases from a vessel by decreasing the tank’s volume. Sizing involves a determination of the amount of time required to reduce the pressure to a specified lower level. For both types of gas-based eductors, specifications include motive media, motive pressure, maximum pressure, and outlet pressure. For eductors that use compressed liquid, specifications include motive media, motive pressure, pressure recovery and maximum suction lift.
Eductors are used widely in petrochemical, material processing, and power generation applications. They often conform to ASME/ANSI B16.5 standards, a set of guidelines from the American Society of Mechanical Engineers (ASME) and the American National Standards Institute (ANSI).