EDUCTOR AND EJECTOR – in shipbuilding, industrial (How to distinguish, calculation, selection, and purchasing)

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At the beginning, there’s no need to fret over the differences that the different 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. 

Eductors/Ejectors are liquid Jet pumps that use water or other liquids or gas under pressure to create a pumping action. In standard eductors the drive pressure liquid discharges through a single nozzle to start the pumping action. Ejectors are self priming and can handle liquid and solid mixtures. There are no moving parts to break down or wear out. Eductors are available with flanged or threaded connections, for portable use or for permanent installation. Eductor is used to pump the liquids from chain lockers, bilges, cargo holds, ballast tank or any space that has to be pumped dry.

Depend on purpose and requirement, the eductor position can be variate as below:

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Installation purposes for:

• Stripping of ballast/cargo tanks

• Stripping during tank cleaning

• Emergency stripping

• Stripping of chain lockers, steering engine rooms, cofferdams, auxiliary pump rooms, sewage tanks

• Stripping of liquids with large particles (ejectors with wide body)

• Removal of gasses

• Use in an explosive atmosphere

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CRITERIA FOR SELECTION OF PROPER SIZED EJECTORS:

• Installation purpose

• Required suction capacity 

The entrained flow-suction flow is less than drive flow-motive flow (about 50-80% of drive flow)

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• Estimated suction lift

• Estimated discharge height

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• Driving pressure/flow

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Ex: From the Main fire pump (9.9 bar), the pressure drop to 8.5 at driving line

In initial design, the Driving pressure is Triple (X3) of Discharge pressure or (recovered Head or Head pressure at end of Eductor).

Refer to Ex-Vendor:

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We can choose the eductor type and design the Driving pressure

Refer to the design file:

eductor design

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In case I (The recovered discharge head is 0.5 and the Entrained flow ratio is 0.2). Meanwhile case II (The Entrained flow is up to 0.8 but the Discharge head will reduce to 0.3 or driving head)

• Liquid type ( for material selection )

• Flange type ( for assembly ability )

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