An Eductor is a device that works on the venturi effect for pump operation. It is a type of jet pump used to pump out liquid/sediments where the suction head is too low for the normal pump to operate.
Based on the venturi effect and Bernoulli principle here primary fluid medium ( water / Same fluid ) is used to create a vacuum to suck in the secondary air, gas or liquid.
Because of its simple construction and no moving parts, it’s used in hazardous areas, ships, and industries involving extraction of the high volume of debris.
The pump consists of a suction chamber, a nozzle input, suction point, and discharge point. Here high-pressure low-velocity fluid is converted to low-pressure high-velocity fluid. Thus producing the pressure difference required for suction.
Some of the advantages of the Eductor pump that make them unique are; no moving parts, low cost, self primed, can be used against corrosive materials, simple construction and are idle for hazardous areas or fluid exposed to risks of explosion.
While the diameter of the Educator assembly ( mainly suction chamber ) depends much on the volumetric flow rate of the working fluid. The diameter of the nozzle assembly depends on the suction requirement, pressure drop, and motive fluid pressure.
Advantage of Using Eductor Over Regular Pumps
There are clearly some clear advantages of using eductor that makes them suitable for pumping operation under certain conditions. The most prominent and common reasons are its low operation cost, scalability, and ability to allow vacuum-assisted pump operation.
One can operate as much as 150 eductor pump at a time under varying vacuum conditions. They came out quite handy in stripping operations in tanks, deep wells and sumps.
It is one of few pumps with no vacuum limit and can work with medium containing metal/solid contaminants; that would otherwise damage a regular pump. Further, it suits well over low to very high viscous fluids and does not require a prime mover or electricity to operate.
This is why eductor pumps are used for soil stabilization, in chocked drainage systems, drilling operations, cleaning operations and various other on site applications.
Reason Behind Suction In Eductor Pumps
An educator pump works on the Bernoulli principle; which states that the speed of the fluid in a medium increases simultaneously with a decrease in pressure. This can be easily explained by using conservation of mechanical energy.
This remains constant through out the flow and can be written as,
Thus an increase in pressure results in a reduction in velocity or vice versa. Now since the fluid flows at a specific rate; thus by using the equation of continuity, we get P1V1A1 = P2V2A2 = P3V3A3.
Again, as the quantity of fluid flowing the rough the pipe depends on the product of its velocity and cross-section area ( Q = A x V ); Here P1= P3, V1 = V3, and A1 = A3.
Now for the Eductor pump where A1 is more than A2 ( Nozzle Assembly ), here V1 is less than V2 and by using the venturi effect we get P2 < Patm < P1.
Since the pressure inside the suction chamber is less than that of the atmosphere. Thus the secondary fluid is forced into the chamber producing the required pumping action.
Here the nozzle tip converts the high-pressure low-velocity fluid flow into low-pressure high-velocity fluid flow. In the process, the pressure difference inside the chamber and atmosphere creates a vacuum responsible for pumping action.
Construction Features And Its Assembly
An eductor is a simple construction with a cast/alloy/stainless steel body, a motive fluid nozzle assembly, converging inlet nozzle, screw, Gasket O-ring, suction line, suction chamber, diffuser, and discharge.
The construction of the pump is such that a tapered jet or nozzle is located inside the suction chamber. It is located as such the fluid leaves out axially outwards towards the exit or diffuser outlet.
Just below this nozzle lies a suction pipe used to suck in any form of fluid needed to be extracted or pumped out of the system.
The flow side is connected to the motive or driving fluid. Because of the continuous flow of driving fluid and venturi effect the pump takes suction and discharge it through the outlet via a diffuser to obtain a better pump efficiency.
Difference Between An Eductor And An Ejector
1. Eductor Use water or liquid as its motive fluid.
1. Ejector ideally use steam or Air as their motive fluid.
2. The motive fluid nozzle is of converging type.
2. The motive fluid nozzle is of converging-diverging type.
3. Bigger suction head
3. Smaller suction head
4. Low operating velocity ( 10 ft/sec for motive liquid )
4. High Operating velocity ( 1000 ft/sec for motive fluid )
5. No limit on bore size.
5. Throat bore size is limited to 0.5 to 0.55 X discharge diameter.
6. Silent operation
6. Not as silent as Eductors
7. High compression ratio
7. Low compression ratio
What Is An Eductor Used For?
There are a ton of applications for Eductors in various industries where they want a low-cost alternative for the centrifugal pumps.
They are used for handling solid waste in granular form or slurries, mixing of liquids, create vacuum, agitate liquid and pumping operations.
Thanks to their low cost and having no moving parts they are widely used as the main pump in hazardous areas.
Further on the ship, they are used as the priming pump for centrifugal pumps, as stripping pump in ballast management, In vacuum toilets, In generating a vacuum in a freshwater system, pumping out slurries and residue in oil tanker.
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