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Centrifugal Pump
Centrifugal Pump :
A centrifugal pump is a mechanical device designed to move a fluid by means of the transfer of rotational energy from one or more driven rotors, called impellers.
Fluid enters the rapidly rotating impeller along its axis and is cast out by centrifugal force along its circumference through the impeller’s vane tips.
The action of the impeller increases the fluid’s velocity and pressure and also directs it towards the pump outlet. The pump casing is specially designed to constrict the fluid from the pump inlet, direct it into the impeller and then slow and control the fluid before discharge.
Centrifugal pump designs offer simple and low cost solutions to most low pressure, high capacity pumping applications involving low viscosity fluids such as water, solvents, chemicals and light oils. Typical applications involve water supply and circulation, irrigation, and the transfer of chemicals in petrochemical plants. Positive displacement pumps are preferred for applications involving highly viscous fluids such as thick oils and slurries, especially at high pressures, for complex feeds such as emulsions, foodstuffs or biological fluids, and when accurate dosing is required.
Air Operated Pump
Air Operated Diaphragm Pump :
An Air-Operated Double-Diaphragm pump (AOD) is a type of positive displacement (PD) pump.
It consists of two pumping chambers that are alternately filled and discharged by the movement of flexible diaphragms. Compressed air is alternately fed to, and vented from, air chambers on the opposite sides of the diaphragms to create the pumping action.
AOD pumps are generally used for transfer applications and can handle a wide variety of feeds including sludges, slurries, abrasive and shear sensitive fluids. Although they are robust, reliable and easy to maintain they can be noisy, prone to icing, and are limited to low pressure applications.
How does an AOD pump work?
An AOD pump moves fluid by the alternate and repeated back-and-forth movement (strokes) of two flexible membranes or diaphragms mounted on a common shaft (Figure 1). Although diaphragm pumps can be driven directly, with the shaft connected directly to a motor, an AOD pump uses compressed air with a sophisticated distribution system directing the air alternately to two air chambers on the opposite sides of the diaphragms. Figure 1 shows a complete cycle of an AOD pump:
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The compressed air control system directs air to air chamber A.
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The shaft moves to the left, and the movement of the diaphragms increases the volume of pumping chamber B and, at the same time, reduces the volume of pumping chamber A (Figure 1a). Valves on either side of the pumping chambers ensure that fluid flow through the pump only occurs in one direction: from inlet to discharge. The effect is to draw fluid from the inlet into chamber B, and discharge it from chamber A.
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As the shaft completes the stroke, the air distribution system vents air chamber A and directs the compressed air to air chamber B.
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The shaft now moves to the right, reducing the volume of pumping chamber B and, at the same time, expanding the volume of pumping chamber A (Figure 1b). Fluid is now discharged from chamber B, and drawn into chamber A.
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As the shaft completes the stroke, the air distribution system vents air chamber B and the cycle repeats when compressed air is directed to air chamber A again.
The action of an AOD pump creates pulses in the discharge and this must be taken into account with any downstream plant design. Vibrations arising from the pulsing discharge can also be a source of noise, as can the constant venting of compressed air by the distribution system. There can also be problems with the build-up of ice around the air vent or within the air distribution system.
Despite these potential problems, AOD pumps are robust, reliable, easy to maintain, and relatively low cost. They tend to be general workhorse devices and are typically used for transfer applications in a wide variety of industries.
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