Introduction:
In the field of fluid mechanics and pump technology, different types of pumps are used for various applications. Positive displacement pumps and centrifugal pumps are two main categories of pumps. Positive displacement pumps operate by trapping a fixed amount of fluid in a chamber and then forcing it into the discharge pipe. On the other hand, centrifugal pumps use centrifugal force to increase the fluid velocity and transfer energy to the fluid, pushing it radially outward.

Explanation:
Reciprocating Pump:
A reciprocating pump is a positive displacement pump that uses a piston or a diaphragm to displace fluid. It operates by converting rotary motion into reciprocating motion, creating a suction effect to draw in fluid and then forcing it out under pressure. This type of pump is commonly used in applications where high pressure is required, such as in hydraulic systems or high-pressure cleaning.

Centrifugal Pump:
A centrifugal pump is not a positive displacement pump. It operates by converting mechanical energy into kinetic energy and then into potential energy. The fluid enters the pump through the center, called the eye, and is accelerated by the rotating impeller. The kinetic energy is then converted to pressure energy as the fluid exits the pump through the volute casing. Centrifugal pumps are widely used for low to medium pressure applications, such as water supply, irrigation, and heating systems.

Vane Pump:
A vane pump is a positive displacement pump that uses sliding vanes to create a seal and move fluid. It consists of a rotor with vanes that slide in and out of slots in the rotor. As the rotor turns, the vanes create expanding and contracting chambers, resulting in the displacement of fluid. Vane pumps are commonly used in applications requiring a steady flow rate, such as hydraulic systems, automotive power steering, and fuel transfer.

Lobe Pump:
A lobe pump is a positive displacement pump that uses two or more lobes (rotors) to move fluid. The lobes mesh together and create chambers that trap and transport fluid from the suction side to the discharge side. Lobe pumps are known for their gentle handling of shear-sensitive fluids and are used in applications such as food processing, pharmaceuticals, and wastewater treatment.

Conclusion:
Among the given options, the centrifugal pump is not a positive displacement pump. Centrifugal pumps operate based on the principle of centrifugal force, which is different from the mechanism used in positive displacement pumps like reciprocating, vane, and lobe pumps.