Eductor

Eductor-jet pump

An eductor-jet pump or aspirator can be used for draining areas which may contain combustible fluids (which could ignite if exposed to the workings of a standard electric or internal combustion powered pump) or high levels of debris which could damage screws or blades in conventional pump designs.

A source of pressurized fluid (e.g. a firehose) is connected to a chamber which is open on one end, and leads to an exhaust hose on the other end. The pressurized fluid is forced through nozzles (called eductor jets) mounted axially on the inside of the pump chamber, pointed in the direction of the exhaust hose. The passage of the pressurized fluid through the chamber and into the exhaust hose creates a suction on the open end of the chamber (see Venturi effect), such that any fluid the pump chamber has been submerged in will be drawn into the chamber and thence into the exhaust hose along with the fluid from the eductor jet nozzles.

There are three connections common to all jet pumps.

  1. Eductor 'motive' connection (top): This connection is where the power for the eductor is generated, by increasing the velocity of the motive fluid. The eductor nozzle in this section takes 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. Preferably, eductor nozzles have smooth flow paths manufactured and controlled to the tightest economical tolerances. Flow paths with sudden steps or roughness on these high velocity surfaces cause turbulence, which makes the flawed eductor nozzle operate less efficiently.
  2. Suction connection (side): 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 fluid as it passes. The friction between the fluids causes the chamber to be evacuated. This allows pressure in the suction vessel to push additional fluid into the suction connection of the eductor. The high velocity of the motive stream in this section of the eductor directs the combined fluids toward the outlet section of the eductor.
  3. Discharge connection (bottom): As the motive fluid entrains the suction fluid, part of the kinetic energy of the motive fluid is imparted to the suction fluid. This allows the resulting mixture to discharge at an intermediate pressure. The percentage of the motive pressure that can be recovered is dependent upon the ratio of motive flow to suction flow and the amount of suction pressure pulled on the suction port. The mixture then passes through the diverging taper that converts the kinetic energy back to pressure. The combined fluid then leaves the outlet.

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