How fast does the particle settling rate in a clean room

A Clean Room (clean room) is a special environment designed to control pollutants such as particles and harmful gases in the air. It is widely used in pharmaceuticals, microElectronics, aerospace and other fields. The particle settling rate is a key parameter in clean room management, which directly affects the maintenance of cleanliness and the quality of products. This article will analyze the particle settling rate in a clean room in detail through examples, calculation formulas and international standards.

The importance of particle settling rate

The particle settling rate determines how quickly particles in the air settle to the surface, thereby affecting the cleanliness of the clean room. According to the ISO 14644-1:2015 standard, the air cleanliness level of a modular clean room is closely related to the size and number of particles in the air. For example, iso 14644-1 stipulates that for ISO5 clean rooms, the number of particles larger than 0.5 microns per cubic meter of air shall not exceed 3520. If the particle settling rate is too fast, it will form contamination on the surface of equipment and products, affecting product quality.

Calculation formula for particle settling velocity

The particle settling velocity can be calculated using the Stokes formula. Assuming the particle is spherical, its settling velocity ( v ) can be calculated by the following formula:

Where:

• ( rp ) is the particle density in kg/m³

• ( rf ) is the fluid (air) density in kg/m³

• ( g ) is the acceleration due to gravity, which is about 9.81 m/s²

• ( r) is the particle radius in m

• ( eta ) is the dynamic viscosity of the fluid (air) in Pa·s

For example, for a particle with a diameter of 1 micron (( 1 μm= 1 × 10^{-6} m )), assuming the particle density is 2500 kg/m³, the air density is 1.2 kg/m³, and the air viscosity is ( 1.81 ✖️ 10^{-5} ) Pa·s, by calculation, it can be concluded that the settling velocity is about 0.003 m/s.

Practical application of particle settling velocity

In practical applications, the particle settling velocity affects the design and operation of clean rooms. For example, in a 100 square meter clean room, assuming that the ceiling height is 3 meters, if the air contains an average of 1,000 particles with a diameter of 0.5 microns per cubic meter, the number of particles that settle to the ground every day can be calculated by the particle settling velocity. If the settling velocity is 0.001 m/s, about 0.0864 particles will settle per square meter per day, which is a very critical data for production environments with high cleanliness requirements.

Particle settling control in international standards

The international standard ISO 14644-3:2019 specifies the test methods for clean rooms and related controlled environments, including the measurement method of particle settling velocity. The standard recommends the use of equipment such as settling plates and light scattering meters to measure particle settling velocity to ensure the accuracy of the data. For example, the settling plate method requires that the settling particles be collected in a fixed area within a specific time (usually 1 hour), and the settling velocity is calculated by analyzing the number of particles and the area.

Control strategy for particle settling velocity

In order to control the particle settling velocity in the clean room, air purification Equipment such as high efficiency filters (HEPA) and ultra-high efficiency filters (ULPA) are usually used. In addition, optimizing air flow design, maintaining a positive pressure environment, and regular cleaning and maintenance are also effective control strategies. According to the ISO 14644-4:2001 standard, the design and construction of the modular clean room should ensure the effectiveness of the air flow pattern and the filtration system to minimize particle settling.

The particle settling velocity is of great significance in clean room management and directly affects the maintenance of cleanliness and product quality. Through scientific calculations and effective control strategies, it can be ensured that the clean room environment meets international standards and meets production needs.

List of relevant standards and specifications

•  ISO 14644-1:2015 Air cleanliness levels for clean rooms and related controlled environments

•  ISO 14644-3:2019 Test methods for clean rooms and related controlled environments

•  iso 14644-4:2001 Design, construction and initial start-up of clean rooms and related controlled environments

•  ISO 14698-1:2003 Biocontamination control for clean rooms and related controlled environments

By deeply understanding and following these standards, the particle settling rate of clean rooms can be managed more scientifically and effectively to ensure the efficiency and safety of the production environment.