Differences between centrifugal pumps and rotor pumps

1. Working principle:
Centrifugal pumps: Use the centrifugal force generated by the rotating impeller to transport fluid. The impeller rotates at high speed, causing the fluid to be thrown from the center of the impeller (inlet) to the outer edge of the impeller (outlet), obtaining kinetic energy and pressure energy. The fluid speed decreases in the pump casing, and part of the kinetic energy is converted into pressure energy.

 

Rotor pumps: A type of positive displacement pump. One or more rotating rotors in the pump casing form a closed working chamber with the inner wall of the pump casing. When the rotor rotates, the volume of the working chamber changes periodically (the volume on the suction side increases to produce a vacuum to inhale the fluid, and the volume on the discharge side decreases to squeeze the fluid out), thereby forcing the fluid to be transported. Common rotor types include gears, screws, cams, vanes, roots, etc.

 

2. Characteristics of flow and pressure:

Centrifugal pumps: The flow rate changes significantly with the outlet pressure (lift). As the pressure increases, the flow rate decreases (following the characteristic curve of the pump). The theoretical flow rate is proportional to the speed. The pressure (lift) generated is proportional to the impeller diameter and the square of the speed. At the same speed, the flow rate is variable and the head is variable.

 

Rotor pump: The flow rate is relatively stable and is little affected by the change of outlet pressure (ignoring the effect of leakage). The theoretical flow rate depends on the geometric size of the rotor, the speed and the volume change of the working chamber, which is proportional to the speed. The pressure generated mainly depends on the resistance (back pressure) of the outlet pipeline, which can be very high in theory (limited by the structural strength of the pump and the driving power). At the same speed, the flow rate is basically constant and the pressure is variable.

 

3. Viscosity adaptability:

Centrifugal pump: The efficiency is very sensitive to the viscosity of the fluid. As the viscosity increases, the internal friction loss (disc friction loss, etc.) increases sharply, resulting in a significant decrease in efficiency, and the flow rate and head will also decrease. Suitable for conveying low-viscosity fluids (such as water, light oil, etc.).

 

Rotor pump: The efficiency is relatively less affected by viscosity. When conveying high-viscosity fluids (such as heavy oil, asphalt, syrup, polymer melt, etc.), the efficiency decrease is much smaller than that of centrifugal pumps, and the efficiency may even increase within a certain range (due to reduced leakage). Very suitable for conveying medium and high viscosity fluids.

 

4. Self-priming ability:

Centrifugal pumps: Generally do not have self-priming ability (except for specially designed self-priming centrifugal pumps). The pump housing and suction pipe need to be filled with liquid (priming the pump) before starting, otherwise it will not work properly.

 

Rotor pumps: Most types (such as gear pumps, screw pumps, and vane pumps) have good self-priming ability and can suck and discharge air or gas-liquid mixtures. Priming the pump is usually not required before starting (unless the inlet position is higher than the pump body).

 

5. Efficiency:

Centrifugal pumps: The efficiency is usually very high near the design operating point (best efficiency point BEP). However, when the flow rate or head deviates far from the design operating conditions, the efficiency will drop sharply. Sensitive to viscosity.

 

Rotor pumps: The efficiency curve is relatively flat, and the efficiency does not change much in a wide range of flow and pressure. The efficiency performance is better than that of centrifugal pumps at medium and high viscosities. However, the presence of internal gaps will cause a certain amount of volume loss (leakage), and the efficiency is usually slightly lower than the efficiency of high-efficiency centrifugal pumps of the same specifications at the BEP point.