Introduction:
In the field of fluid mechanics, pumps play a crucial role in transferring fluids from one place to another. There are different types of pumps available, and they can be classified based on their working principle. One way to classify pumps is based on whether they are positive displacement pumps or dynamic (centrifugal) pumps.

Positive Displacement Pumps:
Positive displacement pumps are designed to displace a fixed amount of fluid with each stroke or rotation. These pumps create a vacuum or low-pressure area at the inlet, which allows the fluid to enter. Then, they use mechanical force to push the fluid out through the outlet. Positive displacement pumps are known for their ability to deliver a constant flow rate regardless of the discharge pressure.

Reciprocating Pump:
A reciprocating pump is a type of positive displacement pump. It consists of a piston or plunger that moves back and forth inside a cylinder. As the piston moves away from the cylinder, it creates a vacuum that allows the fluid to enter the pump. When the piston moves towards the cylinder, it forces the fluid out through the discharge valve. Reciprocating pumps are commonly used in applications requiring high pressure and low flow rates.

Vane Pump:
A vane pump is another type of positive displacement pump. It consists of a rotor with vanes that slide in and out of slots in the rotor. As the rotor rotates, the vanes trap the fluid and carry it around the pump casing. The fluid is then discharged through the outlet. Vane pumps are known for their smooth operation and ability to handle thin liquids.

Lobe Pump:
A lobe pump is also a positive displacement pump. It consists of two lobes (rotors) that rotate in opposite directions within a casing. The fluid is trapped between the lobes and the casing, and it is carried around the pump. Lobe pumps are commonly used for high-viscosity fluids and in applications requiring gentle handling of the fluid.

Centrifugal Pump:
A centrifugal pump, on the other hand, is a dynamic pump. It uses the centrifugal force generated by a rotating impeller to move the fluid. The impeller accelerates the fluid radially outward, creating a vacuum at the center. The fluid enters the pump through the suction inlet and is discharged through the outlet. Centrifugal pumps are known for their ability to deliver high flow rates but are not suitable for applications requiring constant flow rates.

Conclusion:
In conclusion, the centrifugal pump is not a positive displacement pump. While reciprocating pumps, vane pumps, and lobe pumps are all positive displacement pumps that deliver a fixed amount of fluid per stroke or rotation, centrifugal pumps operate based on dynamic principles using centrifugal force.