Two core features of a clean room（1）

2023-09-18

original

Marya

Clean Room, also known as a clean room or purification room. It is the foundation of pollution control, and without a clean room, it is impossible to mass produce pollution sensitive parts. In FED-STD-2, a clean room is defined as a room with air filtration, distribution, optimization, construction materials, and devices, where specific operating procedures are established to control the concentration of suspended particles in the air, thereby achieving appropriate levels of particle cleanliness.

Clean Room, also known as a clean room or purification room. It is the foundation of pollution control, and without a clean room, it is impossible to mass produce pollution sensitive parts. In FED-STD-2, a clean room is defined as a room with air filtration, distribution, optimization, construction materials, and devices, where specific operating procedures are established to control the concentration of suspended particles in the air, thereby achieving appropriate levels of particle cleanliness.
To achieve good cleaning results in a clean room, not only should reasonable air conditioning purification measures be emphasized, but also corresponding measures should be taken by the process, architecture, and other professions: not only should there be reasonable design, but also careful construction and installation that comply with regulations, as well as correct use of the clean room and scientific maintenance and management.
In order to achieve good results in clean rooms, there have been many literature explanations from different perspectives both domestically and internationally. In fact, it is difficult to achieve ideal cooperation between different professions, and designers find it difficult to grasp the quality of construction and installation, as well as the usage and management situation, especially the latter. In terms of clean room purification measures, many designers or construction companies often do not pay enough attention to their necessary conditions, resulting in unsatisfactory cleaning effects. This article only briefly discusses the two necessary conditions for achieving cleanliness requirements in clean room purification measures.

1. Air supply cleanliness

To ensure that the cleanliness of the air supply meets the requirements, the key is the performance and installation of the final filter in the purification system.
The final filter of the purification system generally adopts high-efficiency or sub high-efficiency filters. According to the Chinese standard, the efficiency of high-efficiency filters is divided into four levels: Class A is ≥ 99.9%, Class B is ≥ 99.9%, Class C is ≥ 99.999%, and Class D is (for ≥ 0.1) μm particles) ≥ 99.999% (also known as ultra high efficiency filters); The sub high-efficiency filter is (for ≥ 0.5) μm particles) 95-99.9%. The higher the efficiency, the more expensive the price of the filter. So when choosing a filter, it should be based on both meeting the cleanliness requirements of the air supply and considering economic rationality. From the perspective of cleanliness requirements, the principle is to choose low-performance filters for low-level cleanrooms and high-performance filters for high-level cleanrooms.
Generally speaking, a high and medium efficiency filter can be selected for a 1 million level filter; Below 10000 levels, sub high efficiency or Class A high efficiency filters can be selected; Class B filters are selected for grades 10000 to 100; Class C filters are selected for grades 100 to 1. There seem to be two types of filters to choose from for each cleanliness level here. The choice of high-performance or low-performance filters depends on the specific situation: when environmental pollution is severe, the indoor exhaust proportion is large, or the clean room is particularly important and requires a large safety factor, in either or both of these cases, a higher type of filter should be selected; On the contrary, lower performance filters can be selected. For 0.1 μ For clean rooms that require control of particles, regardless of the concentration of particles being controlled, a D-type filter should be selected. The above is only from the perspective of filters. In fact, to choose a good filter, it is necessary to comprehensively consider the characteristics of the clean room, filter, and purification system.
To ensure the cleanliness of the air supply, only qualified filters are not enough, and it is also necessary to ensure:
a. Do not damage the filter during transportation and installation;
b. Tight installation. To achieve the goal of 'a', construction and installation personnel are required to be well trained, with knowledge of installing purification systems and proficient installation skills. Otherwise, it will be difficult to ensure that the filter is not damaged, and there are profound lessons to be learned in this regard.
Secondly, the issue of installation tightness mainly depends on the quality of the installation structure type. The design manual generally recommends that for a single filter, an open installation type should be used, so that even if leakage occurs, it will not leak into the room; The use of finished high-efficiency air supply vents ensures easy tightness. In recent years, liquid tank sealing and negative pressure sealing have been commonly used for air filters with multiple filters.
The sealing of the liquid tank must ensure that the joint of the liquid tank is tight and the overall framework is on the same level. Negative pressure sealing is to keep the outer periphery of the joint between the filter and the static pressure box and the frame in a negative pressure state, similar to the surface mounted type, even if it leaks, it will not leak into the room. In fact, as long as the installation frame is flat and the contact between the filter end face and the installation frame is even, it should be said that it is not difficult for any installation type to achieve the installation tightness requirements of the filter.

2. Air distribution

The airflow organization of a clean room is different from that of a general air-conditioned room. It requires the cleanest air to be delivered to the operating area first, and its function is to limit and reduce pollution to the processed products. Therefore, when designing air flow organization, these principles should be considered: minimize eddy currents as much as possible to avoid bringing pollution outside the working area into the working area; Try to prevent the secondary flying of dust to reduce the chance of dust pollution on the workpiece; The airflow in the work area should be as uniform as possible, and the wind speed should meet the process and hygiene requirements. When the airflow flows towards the return air outlet, it is necessary to effectively remove the dust in the air. Choose different air supply and return methods based on different cleanliness requirements.
Different air flow organizations have their own characteristics and scope:
1)Vertical unidirectional flow: It can obtain uniform downward airflow, which is convenient for process equipment layout, has strong self-cleaning ability, and can simplify common facilities such as personal purification facilities. In addition, the four air supply methods also have their own advantages and disadvantages: fully distributed high-efficiency filters have advantages such as low resistance and long filter replacement cycle, but the roof structure is complex and the cost is high; The advantages and disadvantages of side layout high-efficiency filter top feeding, full orifice plate top feeding, and full layout high-efficiency filter top feeding are opposite. When the system is not running continuously, the inner surface of the orifice plate is prone to dust accumulation and poor maintenance, which has some impact on cleanliness; Dense diffuser top feeding is only suitable for large clean rooms above 4m in height due to the need for a mixing layer, and its characteristics are similar to that of full orifice plate top feeding; The return air method of evenly arranging return air outlets at the bottom of the opposite sides of the grid and the opposite sides of the wall is only applicable to clean rooms with a clear spacing of less than 6m on both sides; The lower part of a single wall with a return air outlet is only suitable for clean rooms with small wall spacing (such as ≤<2-3m).
2)Horizontal unidirectional flow: Only achieve a cleanliness level of 100 in the first working area. As the air flows to the other side, the dust concentration gradually increases, so it is only suitable for clean rooms with different cleanliness requirements in the same room process; Local placement of high-efficiency filters on the air supply wall can reduce the amount of high-efficiency filters used and save initial investment compared to full horizontal placement, but there are vortices in local areas.
3)Turbulent airflow: The characteristics of orifice plate top delivery and dense diffuser top delivery are the same as before: the advantages of side delivery are easy to arrange pipelines, no need for technical interlayer, low cost, and conducive to the renovation of old factory buildings. The disadvantage is that the wind speed in the working area is high, and the dust concentration on the downwind side is higher than that on the upwind side; High efficiency filter air outlet top delivery has the advantages of simple system, no pipeline after the high efficiency filter, and direct delivery of clean air flow to the working area. However, the diffusion of clean air flow is slow, and the air flow in the working area is relatively uniform; However, when multiple air vents are evenly arranged or high-efficiency filter air vents with diffusion plates are used, the airflow in the working area can also be more uniform; However, in non continuous operation of the system, the diffusion plate is prone to dust accumulation.
The above discussion belongs to a relatively ideal state and is also recommended by relevant national regulations, standards, or design manuals. In practical engineering, due to objective conditions or subjective reasons of the designer, the airflow organization has not been designed properly. Common examples include: vertical unidirectional flow using the lower return air of adjacent walls on both sides, local level 100 using up supply and up return air (i.e. no vertical curtain is added below the local air supply outlet), turbulent flow type clean rooms using high-efficiency filters, such as top supply and up return air or single side down return air (with a large distance between the walls), etc. These airflow organization methods have been measured, Most of its cleanliness does not meet the design requirements. Due to the current regulations on empty or static acceptance, some clean rooms of this type can barely reach the designed cleanliness level in empty or static conditions, but their anti pollution interference ability is very low. Once the clean room enters the working state, it cannot meet the requirements. The correct airflow organization should be set up in local areas with curtains hanging down to the height of the work area, and upward feeding and upward returning should not be used for 100000 levels. Also, most factories currently produce high-efficiency air supply outlets with diffusion plates, which are only decorative orifice plates and do not have the function of diffusing airflow. Designers and users should pay special attention to this.

Two core features of a clean room（2）

Maintenance structural characteristics of cleanroom