Laminar flow clean room

The inlet surface of the laminar flow clean room is covered with high-efficiency filters, and the entire air supply surface is a large air supply outlet. The air flow of the supply air passes through the pressure equalization effect of the static pressure box and the high-efficiency filter, and the air flow lines from the supply outlet to the return air are parallel to each other, filling the entire room section. It advances forward at a uniform speed, like a large piston, discharging the indoor polluted air into the return outlet, thus achieving the purpose of purifying the indoor air. Due to the fact that the streamline of the airflow is always parallel and there is no vortex, laminar flow is also known as parallel flow cleanroom. According to the airflow organization form, it can be divided into vertical laminar flow clean room and horizontal straight laminar flow clean room.

1. Vertical unidirectional flow clean room
(1) The ceiling is covered with high-efficiency filters for air supply, and the entire floor grille provides return air
This is a typical vertical unidirectional flow clean room, as shown in the figure.
Principle of purification work shed
The advantage is that it can obtain uniform downward unidirectional airflow, thus having strong self-cleaning ability and achieving the highest level of cleanliness.
The disadvantages are: the roof structure is complex, the cost and maintenance expenses are high, and it is difficult to plug the high-efficiency filter.
(2) Side mounted high-efficiency filter with ceiling damping layer for air supply, and full floor grille for air return
(3) Ceiling and side dual cloth high-efficiency filters for air supply, full floor grille for air return
(4) Full ceiling air supply, with downward return air on both sides
2. Horizontal unidirectional flow clean shed
(1) The air supply wall is covered with high-efficiency filters for horizontal air supply, and the entire wall surface returns air
(2) Tunnel style unidirectional airflow

Basic principles
In a unidirectional flow clean room, the clean airflow is not one or several, but fills the entire room section. Therefore, this type of clean room does not rely on mixing, but on pushing out the indoor dirty air along the entire section to the outside, thus achieving the purpose of purifying indoor air.

3. Main features
(1) The flow velocity is relatively uniform across the entire cross-section of the room.
(2) At least parallel flow lines within the workspace. Parallel streamlines refer to time averaged streamlines that are parallel to each other.
(3) At least there are no eddies in the workspace.

4. Three characteristic indicators of unidirectional flow clean room
(Parallel streamline, non unidirectional flow, lower limit wind speed)
1. Parallelism of streamline
The function of parallel streamlines is to ensure that the dust particles emitted by the dust source do not propagate perpendicular to the flow direction. If the spread range is within the allowable range, a slight inclination of the streamline should also be allowed. In this way, there is an allowed parallelism of flow lines for unidirectional flow cleanrooms.
The workspace is the main area where processes are carried out, so the cleanliness level is expressed by the dust concentration in the workspace. The regulations for clean room work areas in our country are: 0.8-1.5m from the ground up. As the workbench surface may be as low as 0.75m, it is more strictly set at 0.75m here.
(1) Pulsating velocity
U=ū+△ u
U – instantaneous flow velocity;
ū – hourly average speed;
△ u – pulsating velocity.
(2) Non unidirectional flowability
Formula
β u – Non unidirectional flowability;
UI – Speed of each measuring point;
ū – hourly average speed;
N – Number of measurement points.
For unidirectional flow cleanrooms, it should not exceed 0.25, otherwise it indicates poor performance of the unidirectional flow cleanroom.

3. Lower limit wind speed
(1) The role of airflow velocity in unidirectional flow clean room
1) Polluted airflow spreads in multiple directions:
2) Polluted airflow and supply airflow are in the same direction:
3) Pollution airflow and supply airflow are reversed:
4) The entire room is contaminated:
(2) Control multidimensional pollution
For the human body, when the wind speed reaches 0.22m/s, the pollution concentration has decreased to below level 5 (100) at a distance of 0.3m from the human body.
(3) Control co directional pollution
According to the previous analysis, if the dust particles emitted from a height of 1.8m do not exceed the working range of the person and enter the range of adjacent workers (the maximum distance from the center to both sides of the human body is 0.5m), causing pollution, the air supply speed should not be less than 0.3m/s.
(4) Control reverse pollution
Vertical unidirectional flow clean room
The main reverse pollution of a vertical unidirectional flow clean room is the airflow rising from the heat source, therefore, its necessary air supply speed should be determined based on the parameters of the heat source.
Horizontal unidirectional flow clean shed
The main reverse pollution of a horizontal unidirectional flow clean room is the secondary airflow caused by human walking, therefore, the necessary air supply speed should be determined based on the velocity of the secondary airflow.
(5) Meet the appropriate self purification time
When the entire room is contaminated, it is important to be able to quickly purify the indoor air at an appropriate time, which is closely related to the airflow velocity in the cleanroom. High airflow speed, strong self-cleaning ability, short self-cleaning time, but not economical. Research has found that after an airflow velocity of approximately 0.25m/s, the self purification time tends to be consistent, reaching about one minute. Increasing the velocity further reduces the effect of self purification time.
(6) Comprehensive analysis
The wind speeds required for controlling pollution in the four aspects discussed above are not significantly different. Based on comprehensive analysis, the lower limit wind speeds are divided into three levels.
Principle of purification work shed
Airflow organization and air supply in clean rooms of the pharmaceutical industry
The “Code for Design of Cleanroom” stipulates that:
Levels 1-4 should have vertical unidirectional flow
Level 5 should have vertical unidirectional flow or horizontal unidirectional flow
6-9 levels are suitable for non unidirectional flow