Arrangement of Rotary Pumps in Descending Order of Specific Speed at Their Best Efficiency

Introduction:
The specific speed of a pump is a dimensionless number that represents the speed at which a pump operates most efficiently. It is a key parameter used to compare the performance of different types of pumps. In this question, we are given four types of rotary pumps - positive displacement, centrifugal, and axial - and we need to arrange them in descending order of specific speed at their best efficiency.

Explanation:
To determine the arrangement of these pumps, we need to understand the characteristics and operating principles of each type.

Positive Displacement Pump:
- Positive displacement pumps have a fixed displacement volume per cycle, which means they deliver a constant flow rate regardless of the pressure.
- These pumps operate by trapping a specific volume of fluid and then displacing it into the discharge pipe.
- The specific speed of positive displacement pumps is relatively low because they operate at low speeds and deliver a constant volume of fluid.

Centrifugal Pump:
- Centrifugal pumps use centrifugal force to increase the fluid's velocity and transfer it to the discharge pipe.
- These pumps have a high flow rate but a low-pressure rise capability.
- The specific speed of centrifugal pumps is higher than positive displacement pumps because they operate at higher speeds and deliver a larger volume of fluid.

Axial Pump:
- Axial pumps use rotating propellers or impellers to create a flow in the axial direction.
- These pumps are typically used in applications where a large volume of fluid needs to be moved at relatively low pressures.
- The specific speed of axial pumps is higher than both positive displacement and centrifugal pumps because they operate at higher speeds and deliver a larger volume of fluid.

Arrangement of Pumps:
Based on the explanations above, we can arrange the pumps in descending order of specific speed at their best efficiency as follows:

Axial Pump: Axial pumps have the highest specific speed among the given options because they operate at high speeds and deliver a large volume of fluid efficiently.

Centrifugal Pump: Centrifugal pumps have a higher specific speed compared to positive displacement pumps because they operate at higher speeds and deliver a larger volume of fluid.

Positive Displacement Pump: Positive displacement pumps have the lowest specific speed among the given options because they operate at low speeds and deliver a constant volume of fluid.

Therefore, the correct arrangement of the pumps in descending order of specific speed at their best efficiency is option 'C' - Axial, Centrifugal, Positive Displacement.

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