Cleanroom Airflow Principles and Their Applications

Main Approaches for Controlling Particulate Contamination in Cleanrooms:

1. Control pollution sources and reduce the amount of pollution generated.

2. Quickly and effectively remove pollution that has already occurred indoors.

3. Effectively prevent external pollution from entering (or effectively prevent indoor pollution from escaping outdoors).

Concepts in Fluid Mechanics:

1. Steady flow and unsteady flow (based on whether kinematic elements like velocity and acceleration have a time factor).

2. Gradually varied flow (streamlines are basically equidistant and nearly straight) and rapidly varied flow (based on the shape of streamlines). Gradually varied flow includes uniform flow (the limiting case of gradually varied flow) and non-uniform flow.

3. Laminar flow and turbulent flow (based on the presence or absence of mass exchange).

I. Principle of Non-Unidirectional Flow Cleanrooms

Main characteristic: The flow cross-section of the air changes from the inlet to the outlet. Also known as turbulent flow.
Flow state essence: Rapidly varied flow, turbulent flow, steady flow.

(4) Side Supply Air: Generally uses side supply, with return air on the same side lower down. This form is suitable for workshops with low ceiling heights, often used in the renovation of old factories. Side supply creates many vortices indoors, and the cleanliness can only reach Class 10,000, but the project cost is low.

The working principle of non-unidirectional flow cleanrooms is: when a stream of clean air is supplied from the air supply outlet into the room, it rapidly diffuses and mixes in all directions. Simultaneously, roughly the same volume of air is exhausted through the return air outlets. This clean air stream dilutes the polluted indoor air, reducing the original high particle concentration until equilibrium is reached. Therefore, the faster and more uniform the airflow diffusion, the better the dilution effect.
The principle of non-unidirectional flow cleanrooms is dilution.
According to their working principle, the achievable cleanliness for non-unidirectional flow cleanrooms is generally Class 1,000 to 10,000 (ISO 6-7). If designed well, it can reach Class 100 (ISO 5) in some cases. Generally, the higher the indoor air change rate, the higher the achievable cleanliness.

II. Laminar Flow Cleanrooms

The air supply surface of a laminar flow cleanroom is covered with HEPA filters. The entire supply air surface acts as a large air supply outlet. After pressure equalization and flow distribution by the plenum and HEPA filters, the airflow streamlines from the supply outlet to the return outlet are parallel to each other, filling the entire room cross-section and moving forward at a uniform velocity, like a large piston, pushing the original polluted indoor air into the return outlets, thereby purifying the indoor air. Because the airflow streamlines are always parallel, with no vortices, laminar flow is also called unidirectional flow cleanrooms. Based on the airflow organization pattern, they are classified into vertical unidirectional flow cleanrooms and horizontal unidirectional flow cleanrooms.

1. Vertical Unidirectional Flow Cleanrooms
(1) Ceiling fully covered with HEPA filters for supply air, full floor grilles for return air.
Advantage: Can obtain a uniform downward unidirectional airflow, resulting in strong self-cleaning capability and the ability to achieve the highest cleanliness levels.
Disadvantage: Ceiling structure is more complex, cost and maintenance expenses are high, and HEPA filter leak sealing is more difficult.
(2) Side-mounted HEPA filters with ceiling damping layer for supply air, full floor grilles for return air.
(3) Both ceiling and side-mounted HEPA filters for supply air, full floor grilles for return air.
(4) Full ceiling supply air, return air on both sides lower down.

2. Horizontal Unidirectional Flow Cleanrooms
(1) Supply air wall fully covered with HEPA filters for horizontal supply, full wall return air.
(2) "Tunnel" type unidirectional flow supply.

Basic Principle
In a unidirectional flow cleanroom, the clean airflow is not one or several streams but fills the entire room cross-section. Therefore, this type of cleanroom does not rely on mixing but on a "pushing" effect to discharge the dirty indoor air through the entire cross-section to the outside, thereby achieving the purpose of purifying the indoor air.

3. Main Characteristics
(1) The air velocity is relatively uniform across the entire room cross-section.
(2) At least within the working zone, the streamlines are parallel. Parallel streamlines means the time-averaged streamlines are parallel to each other.
(3) At least within the working zone, there are no vortices.

4. Three Characteristic Indicators for Unidirectional Flow Cleanrooms
(Streamline Parallelism, Non-Unidirectional Flow Index, Lower Limit Velocity)

1. Streamline Parallelism
   The role of parallel streamlines is to ensure that particles emitted from a source do not propagate perpendicular to the flow direction. If the range of such propagation is within the allowable limit, then a slight inclination of the streamlines should be permissible. Thus, for unidirectional flow cleanrooms, there is an allowable streamline parallelism.
   The working zone is where the main process occurs, so the cleanliness level is defined by the particle concentration in the working zone. China's regulation for the cleanroom working zone is: from 0.8m to 1.5m above the floor. Because the workbench height might be as low as 0.75m, it is more strictly defined here as 0.75m.
   (1) Fluctuating Velocity
   U = ū + Δu
   u --- Instantaneous velocity;
   ū --- Time-averaged velocity;
   Δu --- Fluctuating velocity.
   (2) Non-Unidirectional Flow Index

   βu --- Non-Unidirectional Flow Index;
   ui --- Velocity at each measurement point;
   ū --- Time-averaged velocity;
   n --- Number of measurement points.
   For unidirectional flow cleanrooms, this value should not exceed 0.25; otherwise, it indicates poor performance of the unidirectional flow cleanroom.

2. Lower Limit Velocity
   (1) Role of Airflow Velocity in Unidirectional Flow Cleanrooms

   1. Multi-directional dispersion of polluted airflow:

   2. Polluted airflow in the same direction as supply airflow:

   3. Polluted airflow opposite to the supply airflow:

   4. Entire room contamination:
      (2) Controlling Multi-directional Pollution
      For the human body, when the air velocity reaches 0.22 m/s, at a distance of 0.3m from the body, the pollution concentration has already dropped below Class 5 (ISO 5) / Class 100 levels.
      (3) Controlling Co-flow Pollution
      Based on previous analysis, if particles are released at a height of 1.8m, and it is desired that they do not exceed that person's work area and enter an adjacent worker's area (maximum distance from the center of the body to either side is 0.5m), thereby causing contamination, then the supply air velocity should not be less than 0.3 m/s.
      (4) Controlling Reverse-flow Pollution
      Vertical Unidirectional Flow Cleanrooms
      The main reverse flow pollution in vertical unidirectional flow cleanrooms is the rising airflow from heat sources. Therefore, the necessary supply air velocity should be determined based on the heat source parameters.
      Horizontal Unidirectional Flow Cleanrooms
      The main reverse flow pollution in horizontal unidirectional flow cleanrooms is the secondary airflow caused by people walking. Therefore, the necessary supply air velocity should be determined based on the velocity of this secondary airflow.
      (5) Meeting Appropriate Self-Cleaning Time
      In the case of entire room contamination, it is necessary to rapidly clean the indoor air within a suitable time, which is greatly related to the airflow velocity in the cleanroom. Higher airflow velocity means stronger self-cleaning capability and shorter self-cleaning time, but is less economical. Research has found that when the airflow velocity exceeds about 0.25 m/s, the self-cleaning time basically stabilizes at around one minute. Further increasing the velocity has a negligible effect on shortening the self-cleaning time.
      (6) Comprehensive Analysis
      The required air velocities for controlling the four types of pollution discussed above are not significantly different. Based on comprehensive analysis, the lower limit velocities can be categorized into three grades.