Centrifuge Working Principle: Classification, Rotor Types, and Standard Operation

The centrifuge working principle is one of the most important concepts in modern laboratory and medical testing applications. From blood separation and molecular diagnostics to nucleic acid purification and cell analysis, laboratory centrifuges play a critical role in improving sample processing efficiency and experimental accuracy.

Understanding the centrifuge working principle helps laboratory professionals choose the correct rotor, speed, and centrifugation method for different applications. In this guide, we will explain how a centrifuge works, the different types of laboratory centrifuges, common rotor classifications, and standard centrifuge operation procedures.

1.Centrifuge Working Principle and Basic Theory

The centrifuge working principle is based on the generation of centrifugal force through high-speed rotation. Different particles experience different sedimentation rates depending on their density, size, and distance from the rotor center.

Under natural conditions, particles suspended in a liquid settle slowly due to gravity. However, this process is far too slow for time-sensitive medical testing. Centrifugation technology leverages the powerful centrifugal force generated by the high-speed rotation of the centrifuge rotor to accelerate the sedimentation of particles. This enables the separation of liquid from solid particles, or of two immiscible liquids with different densities.

When the external force acting on an object is less than the centripetal force required for its circular motion, the object will move away from the center of the circle. This phenomenon where an object moves away from the center is defined as centrifugal phenomenon, also known as centrifugal motion. Centrifugal motion occurs due to the disappearance or insufficiency of centripetal force.

Centrifugation works on the principle that any object moving in a circular path at a constant angular velocity is subjected to an outward centrifugal force.

The magnitude of centrifugal force (Fc) equals the product of the centrifugal acceleration ω2 r and the particle mass m, expressed as:

In the formula, ω stands for the angular velocity of rotation, N is the rotational speed of the rotor per minute, r refers to the centrifugal radius, and m represents mass.

Relative centrifugal force refers to the multiple of the Earth's gravity exerted on particles in a centrifugal force field, with the unit of g.

Core Concept Distinction: RPM vs RCF

In practical laboratory applications, understanding the centrifuge working principle is essential for selecting the correct RCF value and achieving reliable separation results.

 RPM: Revolutions Per Minute, which only describes how fast the rotor spins.

RCF (Relative Centrifugal Force): The actual force acting on the sample. It is proportional to the square of the rotational speed and the centrifugal radius.In standardized experimental operations, RCF (usually expressed as ×g) must be taken as the standard.

2.Types of Laboratory Centrifuges and Their Applications

 Laboratory centrifuges are generally divided into three major categories according to their maximum rotational speed:

1.Low-speed centrifuge

It is a type of centrifuge routinely used in clinical laboratories. Its maximum rotating speed is within 4000 r/min, with a relative centrifugal force of up to 2810×g. It adopts solid-liquid sedimentation separation. In clinical laboratories, it is mainly applied to separate plasma and serum from whole blood, as well as formed elements in urine, pleural and peritoneal fluid, cerebrospinal fluid and other specimens.

2.High-speed Centrifuge

Its maximum rotating speed reaches 20,000 r/min with a maximum relative centrifugal force of 17,000×g, adopting solid-liquid sedimentation separation.It is mainly used for PCR molecular biological detection, such as the extraction of DNA and RNA from serum samples of patients infected with hepatitis B virus and hepatitis C virus.To avoid the denaturation and inactivation of enzymes and biomolecules caused by temperature rise during high-speed centrifugation, some high-speed centrifuges are equipped with a refrigeration system, also known as high-speed refrigerated centrifuges.

3.Ultracentrifuge (> 30,000 r/min)

The maximum rotational speed ranges from 50,000 to 80,000 r/min, with a maximum relative centrifugal force of 510,000×g. It adopts differential sedimentation separation and density gradient zone separation. It is mainly applied in scientific research for the isolation of subcellular organelles, as well as the separation of viruses, nucleic acids, proteins and polysaccharides in samples.

Drive System (Motor):

It is the core component of a centrifuge. Early centrifuges mostly adopted series-excited motors, while modern medical centrifuges generally use brushless DC motors or variable-frequency motors to achieve precise speed control. It consists of two parts: the stator and the rotor.

A series-excited motor features a high starting torque; its speed drops sharply as the load increases, and it runs at an extremely high speed under no-load conditions. Do not remove the centrifuge rotor during use unless there are special operational requirements.

Rotor and Centrifuge Tubes:

Take the most common angled rotor as an example. It is usually made of aluminum alloy or titanium alloy, featuring a flat-top conical main body. A shaft hole in the center connects and fixes it to the motor shaft. The rotor is equipped with 6 to 12 symmetrical inclined holes for placing centrifuge tubes, with an angle ranging from 14° to 40°.

Common types of centrifuge rotors include angled rotor, swing-out rotor, zonal rotor, vertical rotor and continuous-flow rotor.

Control System:

Microcomputer precisely controls the speed, time, temperature and acceleration/deceleration curves.

Temperature Control System:

Standard configuration for refrigerated models to ensure the biological activity of samples.It consists of four parts: compressor, condenser, capillary tube and evaporator, adopting a fully enclosed refrigeration compressor.

Safety Protection System:

It includes door interlock, rotor identification, unbalance detection and other functions to ensure operational safety.

3.Centrifuge Rotor Types and Centrifugation Methods

Different centrifuge rotor designs directly influence the centrifuge working principle in specific laboratory applications, especially in cell separation, protein purification, and molecular biology experiments.

1. Fixed-angle rotor

The centrifuge tube forms a fixed angle of 14°~40° with the rotating shaft. Particles slide down the outer wall of the tube to the bottom.Advantages: low center of gravity, high speed resistance, short sedimentation path. Suitable for collecting cell pellets.

2. Swing-out rotor

It stands vertically at rest and swings to a horizontal position (90°) during operation. Particles settle straight down along the axis to the center of the tube bottom. Advantages: The sediment spreads evenly flat at the tube bottom for easy collection; it is an excellent tool for density gradient centrifugation.

3. Analysis of Common Centrifugation Methods

Differential Centrifugation: Also known as fractional centrifugation. According to the difference in sedimentation velocity of separated substances, centrifugation is carried out step by step with different rotational speeds and durations. This method is mainly used to separate particles with large differences in size and density.

Density Gradient Centrifugation: Also known as zonal centrifugation.It is divided into rate zonal centrifugation and isopycnic zonal centrifugation. This method is mainly applied to particles with little difference in sedimentation velocity. The sample is placed in an inert gradient medium for centrifugal sedimentation or sedimentation equilibrium. Under a certain centrifugal force, particles are distributed to specific positions in the gradient liquid to form distinct zones for separation. It is commonly used for isolating PBMC and other samples.

Centrifuges are the cornerstone of medical laboratory testing and life science research. For every laboratory technician and researcher, proficiently mastering the working principles of centrifuges and properly selecting separation methods is not only a commitment to experimental data accuracy, but also a respect for occupational safety. May this guide serve as a reliable assistant in your daily work, ensuring every centrifugation operation is precise and error-free.

Understanding the centrifuge working principle is essential for safe and efficient laboratory operation. From selecting the correct centrifuge rotor to choosing appropriate RPM and RCF settings, every factor directly affects sample separation quality and experimental accuracy.

Modern laboratory centrifuges are widely used in blood separation, molecular diagnostics, biological research, and clinical testing. By mastering proper centrifuge operation procedures and maintenance methods, laboratory professionals can improve both safety and testing reliability.

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