How To Select Different Types of Rotary Pumps Based on Viscosity

Rotary pumps are high-efficiency positive displacement pumps that precisely transfer fluid by dynamically adjusting the volume between the rotor and pump body. Renowned for their stable flow, minimal pressure pulsation, and robust self-priming capabilities, they are indispensable workhorses in critical sectors including petrochemical processing, food and pharmaceutical manufacturing, wastewater treatment, and hydraulic systems. Ideal for handling high-viscosity media, abrasive particle-containing fluids, and shear-sensitive materials, these pumps ensure reliable performance even in the most demanding industrial environments.

Rotor Type and Viscosity Matching Principles

1. Low-viscosity fluids (<1000 mPa·s): 

Small-sized rotors (such as rotors #0 or #1) should be selected. Their small surface area and low torque reduce excessive disturbance to the low-viscosity fluid.

For example:

Rotor #0: Suitable for the range of 15 mPa·s to 1000 mPa·s

Rotor #1: Suitable for samples of 30-100 mPa·s when matched at 12 r/min

2. Medium to high viscosity fluids (1000-100000 mPa·s) 

require large rotors (such as rotor No. 3 or No. 4) to provide greater torque to overcome fluid resistance.

3. Non-Newtonian fluids

Rotor selection must be based on shear characteristics:

Thixotropic fluids: Avoid interrupting the test and prevent structural damage.

Pseudoplastic fluids: Require adjustment based on rotational speed, e.g., rotor #3 + 30 r/min.

Rotor Structure Selection Criteria:

Coaxial Cylindrical Rotor: Suitable for medium to low viscosity homogeneous liquids, large contact area, high stress sensitivity.

Parallel Plate/Conical Plate Rotor: Suitable for high viscosity or particulate samples, adjustable spacing suppresses secondary flow.

Turbine Rotor: Designed specifically for low viscosity fluids, compact structure.

Operating Precautions:

Filling volume should be controlled to 2/3 of the container to avoid residue at the bottom affecting the liquid surface level.

The rotor immersion angle should be level with the viscometer marking line. Stop the test if the rotor vibrates.

Regularly maintain the rotor connecting shaft using a dedicated lubricant.

How to determine the viscosity of a fluid?

1. Definition and Classification of Viscosity Viscosity is a measure of the resistance to fluid flow. It is divided into dynamic viscosity (unit: Pa·s) and kinematic viscosity (unit: m²/s). Dynamic viscosity directly reflects the viscosity of the fluid, while kinematic viscosity is often used in fluid dynamics calculations.

2. Judgment Methods

Laboratory Measurements:

Capillary Method: Calculates viscosity by measuring the flow velocity of the fluid in a capillary tube.

Rotational Viscometer: Measures viscosity by measuring the rotational resistance of a rotor in the fluid; suitable for fluids of varying viscosities.

Falling Ball Method: Calculates viscosity by measuring the settling velocity of a small ball in the fluid.

Everyday judgment: Compare the viscosity of fluids by touch; for example, syrup is thicker than milk. Observe the flow rate of fluids; high-viscosity fluids (such as honey) flow slowly, while low-viscosity fluids (such as water) flow quickly.

3. Relationship between Fluid Type and Viscosity

Newtonian fluids: Constant viscosity, such as water and oil.

Non-Newtonian fluids: Viscosity varies with shear force, such as tomato sauce (pseudoplastic fluid) and starch solution (thick fluid).

4. The Effect of Temperature on Viscosity

Liquid viscosity generally decreases with increasing temperature (e.g., honey flows more easily after heating). Gas viscosity, on the other hand, increases with increasing temperature (e.g., warm air is more viscous). Using these methods, the viscosity and properties of a fluid can be accurately determined.

How does fluid viscosity affect pipe resistance?

**Basic Relationship:** Fluid viscosity (μ) is the core factor in flow resistance. High-viscosity fluids generate greater internal frictional resistance.

According to Newton's law of viscosity, shear stress τ = μ·(dv/dy) directly reflects the proportional relationship between viscosity and resistance.

**Differences between Laminar and Turbulent Flow:**

Laminar Flow: Resistance is entirely dominated by viscosity. Pressure loss Δp = 32μLv/d² (Poiseuille equation), showing a linear relationship with viscosity.

Turbulent Flow: Inertial forces are enhanced, but viscosity still affects resistance through the friction coefficient, and pressure loss is proportional to the square of the flow velocity.

**Engineering Impacts:** High-viscosity fluids significantly increase pump energy consumption and reduce centrifugal pump efficiency, but may increase the flow rate of positive displacement pumps. Changes in viscosity alter the Reynolds number (Re = ρvd/μ), thus affecting the flow state (laminar/turbulent).

**Temperature and Pressure:** Increased temperature decreases liquid viscosity (e.g., oils), while gas viscosity increases with increasing temperature. The effect of pressure on viscosity is usually negligible.

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For example:

Rotor No. 3 + 30 r/min: Scalable to 36-132 mPa·s

Rotor No. 4 + 6 r/min: Suitable for fluids above 10000 mPa·s