Centrifugal vs. Positive Displacement Pumps: What’s the Difference?

Centrifugal pumps and positive displacement pumps are two major categories of pumps, each with unique characteristics, operating principles, and applications. Understanding the differences between these two types of pumps is essential for selecting the right pump for specific industrial, commercial, or residential applications. Here’s a detailed comparison of centrifugal and positive displacement pumps:

1. Operating Principle

Centrifugal Pumps

• How It Works: Centrifugal pumps operate by converting rotational kinetic energy into hydrodynamic energy. The pump’s impeller rotates, creating a centrifugal force that pushes the liquid outward from the center of the impeller to the outer edge of the pump casing. As the fluid moves outward, it gains velocity and pressure, resulting in continuous flow.
• Flow Generation: Centrifugal pumps create flow by imparting velocity to the liquid, relying on the centrifugal force generated by the rotating impeller.

Positive Displacement Pumps

• How It Works: Positive displacement (PD) pumps move fluids by trapping a fixed volume of liquid in a chamber and then mechanically forcing it out of the pump. This process repeats in cycles, delivering a consistent, fixed volume of fluid with each stroke or rotation.
• Flow Generation: PD pumps displace fluid in discrete, consistent amounts. The flow is independent of pressure, as the pump moves a fixed amount of fluid for every cycle.

2. Flow Characteristics

Centrifugal Pumps

• Flow Rate: The flow rate of a centrifugal pump varies with the pressure in the system. As the resistance or pressure increases, the flow rate decreases.
• Non-Linear Flow: The performance curve of a centrifugal pump is non-linear, meaning the flow rate and pressure are interdependent. When head (pressure) increases, the flow decreases.
• Best for Continuous Flow: Centrifugal pumps are ideal for applications where a continuous, smooth flow is needed, especially in low-viscosity fluids.

Positive Displacement Pumps

• Flow Rate: PD pumps deliver a constant flow rate regardless of the pressure, as long as the pump speed remains constant. The flow rate is proportional to the pump speed.
• Linear Flow: The flow rate is consistent across a range of pressures, making PD pumps ideal for applications requiring precise dosing or metering.
• Best for Variable Flow: PD pumps excel in applications where fluid flow needs to remain constant even under varying pressure conditions, such as in high-viscosity or shear-sensitive fluids.

3. Pressure Handling

Centrifugal Pumps

• Pressure Dependent: Centrifugal pumps are designed for low to moderate pressure applications. The pressure they can generate depends on the speed of the impeller and the system resistance (head).
• Best for Low-Pressure Applications: Centrifugal pumps are best suited for applications where lower pressure is required, such as water circulation or irrigation.

Positive Displacement Pumps

• High Pressure Capable: PD pumps can handle high-pressure applications effectively. Because they generate flow independent of the pressure, they can work against high-resistance systems without losing efficiency.
• Best for High-Pressure Applications: PD pumps are commonly used in high-pressure applications such as hydraulic systems, fuel injection, and viscous fluid handling.

4. Fluid Viscosity Handling

Centrifugal Pumps

• Low Viscosity Fluids: Centrifugal pumps are ideal for pumping low-viscosity fluids like water, thin oils, or alcohols. High-viscosity fluids reduce the efficiency of centrifugal pumps and can cause them to lose prime or struggle to maintain flow.
• Not Suitable for High-Viscosity Fluids: High-viscosity fluids can cause the pump to underperform, as centrifugal pumps rely on the fluid’s velocity for flow generation.

Positive Displacement Pumps

• High Viscosity Fluids: PD pumps excel at moving high-viscosity fluids, such as honey, molasses, slurries, or thick oils. They provide consistent flow regardless of the fluid’s viscosity.
• Handles a Wide Range of Viscosities: Positive displacement pumps are versatile and can handle a wide range of fluid viscosities without significant performance loss.

5. Efficiency

Centrifugal Pumps

• Efficiency at Constant Flow: Centrifugal pumps are most efficient when operating at their best efficiency point (BEP), which is the point where the pump operates with minimal energy consumption and maximum flow at a given head.
• Efficiency Drops at High Pressure: When operated far from their BEP (especially at high pressure), centrifugal pumps become less efficient, using more energy for less output.

Positive Displacement Pumps

• High Efficiency Across a Range of Conditions: PD pumps maintain high efficiency across a wide range of pressures and flow rates. Since their output is not dependent on pressure, they tend to be more energy-efficient in high-pressure applications compared to centrifugal pumps.

6. Self-Priming Capability

Centrifugal Pumps

• Generally Not Self-Priming: Standard centrifugal pumps are not self-priming, meaning they require the pump casing to be filled with liquid before starting. If air enters the system, it can lose prime and stop pumping.
• Special Designs Required: Some centrifugal pumps are designed as self-priming, but these are specialized and more expensive than standard centrifugal models.

Positive Displacement Pumps

• Self-Priming: PD pumps are generally self-priming, meaning they can pump air and create suction to draw liquid into the pump. This makes them more suitable for applications where the pump may need to operate in situations where priming can be difficult or air may enter the system.

7. Shear Sensitivity

Centrifugal Pumps

• High Shear: Centrifugal pumps generate shear forces that can damage shear-sensitive fluids, such as certain food products, emulsions, or biological fluids. The impeller’s rotation can break down or alter the fluid structure.
• Not Ideal for Delicate Fluids: Because they are high-shear, centrifugal pumps are not ideal for applications where maintaining fluid integrity is critical.

Positive Displacement Pumps

• Low Shear: PD pumps exert much lower shear forces, making them ideal for handling shear-sensitive fluids. This is important in industries like food and beverage, pharmaceuticals, and chemicals, where preserving product integrity is crucial.
• Ideal for Delicate Fluids: PD pumps are preferred for applications requiring gentle fluid handling, such as pumping creams, yogurt, or emulsions.

8. Application Range

Centrifugal Pumps

• Common Applications:
  • Water supply and circulation.
  • HVAC systems.
  • Irrigation systems.
  • Fire protection.
  • Low-viscosity chemical transfer.
  • Municipal water systems.
• Best for High Flow, Low Pressure: Centrifugal pumps are ideal for high-flow applications with low to moderate pressure requirements.

Positive Displacement Pumps

• Common Applications:
  • High-viscosity fluid transfer (oils, syrups, slurries).
  • Metering and dosing (chemicals, pharmaceuticals).
  • Hydraulic systems.
  • Food and beverage processing (yogurt, honey, dough).
  • High-pressure systems (fuel injection, oil transfer).
• Best for Low Flow, High Pressure: PD pumps are ideal for high-pressure applications where flow rates need to be precise and constant, even in variable pressure conditions.

Summary of Differences:

Conclusion:

The choice between centrifugal and positive displacement pumps depends on the specific requirements of the application. Centrifugal pumps are ideal for high-flow, low-viscosity, and lower-pressure applications, while positive displacement pumps excel in applications requiring precise flow control, high pressure, and handling of high-viscosity or shear-sensitive fluids.