Cleanliness meets the required conditions
First, the air cleanliness
To ensure that the cleanliness of the air supply meets the requirements, the key is to clean the performance and installation of the final filter of the system. The final stage filter of the purification system generally uses a high efficiency filter or a sub-high efficiency filter. According to China's standards, high-efficiency filter efficiency is divided into four grades: Class A is ≥99.9%, Class B is ≥99.9%, Class C is ≥99.999%, Class D is (for ≥0.1μm particles)≥99.999% (also known as super High-efficiency filter); sub-high efficiency filter is (for ≥0.5μm particles) 95~99.9%. The higher the efficiency, the more expensive the filter is. Therefore, when selecting a filter, it should be done in order to meet the requirements of air supply cleanliness and economic consideration. In view of the cleanliness requirements, the low-grade clean room is selected as a low-performance filter, and the high-grade clean room is selected as a high-performance filter. Generally speaking: 1 million grades can choose high-efficiency filter; below 10,000 grades can use sub-efficient or A-class high-efficiency filter; 10,000-100 grades use B-type filter; 100~1-class use C-type filter . Here each cleanliness level seems to have two types of filters to choose from, whether high performance or low performance, depending on the specific situation: when the environment is seriously polluted, or the proportion of indoor exhaust is large, or the clean room It is especially important that a large safety factor is required. In either or both cases, a higher type of filter is selected; instead, a lower performance filter can be selected. For a clean room where 0.1 μm particles are required to be controlled, a Class D filter should be selected regardless of the controlled particle concentration. The above mentioned only from the filter point of view, in fact, to choose the filter should also fully consider the clean room characteristics, filter characteristics and purification system characteristics.
To ensure the cleanliness of the air supply, only qualified filters are not enough, but also to ensure that: a. transportation, installation process does not damage the filter; b. tight installation. To achieve the first, the construction and installation personnel are required to be well trained. They must have knowledge of installing the purification system and have more skilled installation skills, otherwise it will be difficult to ensure that the filter will not be damaged. Secondly, the installation tightness problem depends mainly on the quality of the installation structure. The design manual is generally recommended: for a single filter, the surface is installed, so that even if leakage occurs, it will not leak into the room; use the finished product efficient air supply port, tightness It is also easier to get a guarantee.
Second, airflow organization
The airflow organization of the clean room is different from that of a general air-conditioned room. It requires that the cleanest air be sent to the operating site first. Its function is to limit and reduce the pollution of the workpiece. To this end, these principles should be considered when designing airflow organization: minimize eddy currents, avoid bringing pollution outside the work area into the work area; try to prevent secondary dust from flying to reduce the chance of dust contamination of the workpiece; airflow in the work area As uniform as possible, and the wind speed should meet the requirements of process and hygiene. When the airflow flows to the air return, the dust in the air can be effectively carried away. According to different cleanliness requirements, choose different delivery and return air modes.
Different airflow organizations have their own characteristics and scope:
(1) Vertical unidirectional flow: Both can obtain uniform downward airflow, facilitate process equipment layout, strong self-cleaning ability, and simplify common facilities such as personal purification facilities, etc. The four air supply modes also have their own advantages and disadvantages: The high-efficiency filter has the advantages of low resistance, long filter replacement period, etc., but the ceiling structure is complicated and the cost is high; the side cloth high-efficiency filter top feed, the full-hole plate top feed and the full-blown high-efficiency filter top feed are opposite to each other. In the case of non-continuous operation of the whole orifice plate, the inner surface of the orifice plate is easy to accumulate dust, and the maintenance is not good, which has some influence on the cleanliness; the dense diffuser is topped, because it needs a mixed layer, so it is only suitable for 4m. The above-mentioned tall clean room has the same characteristics as the full-hole plate top-loading; the return air mode for uniformly arranging the return air outlets on the plates on the opposite sides and the opposite sides of the wall is only applicable to the clean room with a net spacing of less than 6 m on both sides; The return air outlet of the lower wall of the side wall is only suitable for clean rooms with small wall spacing (such as ≤<2~3m).
(2) Horizontal unidirectional flow: only achieves 100 cleanliness in the first working area, and the dust concentration gradually increases during the flow of air to the other side, so it is only applicable to the same room process with different cleanliness requirements. Clean room; the air supply wall partial high-efficiency filter is more horizontal than the full-size filter to reduce the amount of high-efficiency filter, saving initial investment, but there are eddy currents in the local area.
(3) turbulent flow: the characteristics of the orifice feeding and dense diffuser top feeding are the same as the above: the advantages of side feeding are easy to arrange the pipeline, no technical interlayer is required, and the cost is low, which is beneficial to the transformation of the old factory building. The disadvantage is the working area. The wind speed is relatively high, and the leeward side has higher dust concentration than the windward side; the high-efficiency filter tuyere has the advantages of simple system, high efficiency filter, no pipeline, clean airflow directly to the working area, etc., but the clean airflow spreads slowly, the working area airflow It is more uniform; however, when a plurality of tuyères are evenly arranged or a high-efficiency filter tuyere with a diffusing plate is used, the airflow in the working area can be made uniform; however, in the case of non-continuous operation of the system, the diffusing plate is easy to accumulate dust.
The above discussion is all in an ideal state and is also recommended by the relevant national standards, standards or design manuals. In actual engineering, it may be limited by objective conditions, or the airflow organization is not designed due to the subjective reasons of the designer. Commonly, the vertical unidirectional flow adopts the return air from the lower side of the adjacent side walls, and the local 100-level adopts the upper air return. (ie, there is no drooping curtain under the local air supply port), the turbulent flow type clean room adopts high-efficiency filter air outlet top to send back or single side back (larger spacing to the wall), etc. These airflow organization methods are measured and cleaned. Most of them do not meet the design requirements. Due to the empty or static acceptance of the current regulations, individual such clean rooms barely reach the design cleanliness level in the open or static state, but the anti-pollution interference ability is very low, and once the clean room enters the working state, the requirements are not met. The correct area of ​​the airflow organization should be drooped to the height of the work area, and the 100,000 class should not be used to send the upper back. There are also high-efficiency air supply openings with diffuser plates produced by most factories at present, and the diffusing plates are only decorative orifice plates and do not act as a diffusion airflow.
Third, the amount of air supply or wind speed
Sufficient ventilation is to dilute and eliminate indoor polluted air. According to different cleanliness requirements, when the clean room height is high, the number of air changes is appropriately increased. Among them, the ventilation capacity of the 1 million-class clean room is considered according to the high-efficiency purification system, and the rest are considered according to the high-efficiency purification system; when the 100,000-class clean room high-efficiency filter is concentrated in the machine room or the system end adopts the sub-high efficiency filter, Appropriately increase the number of ventilations by 10-20%.
For the above recommended ventilation values, it is considered that the unidirectional flow clean room passes through the room section and the wind speed is low, and the turbulent flow type clean room is the recommended value with a sufficient safety factor. Vertical unidirectional flow ≥ 0.25m / s, horizontal unidirectional flow ≥ 0.35m / s, detection of cleanliness in the open state or static, although can meet the requirements, but the pollution resistance is poor, once the indoors into the working state, cleanliness It may not meet the requirements. Such examples are not individual. At the same time, there are no fans in China's ventilator series that are more suitable for purification systems. Generally, designers often do not accurately calculate the air resistance of the system, or do not notice. Whether the selected fan is in a favorable working point on the characteristic curve, so that the air volume or wind speed will not reach the design value shortly after the system is put into operation. The US Federal Standard (FS209A~B) has been stipulated before October 27, 1987: the airflow rate of the unidirectional flow clean room through the clean room section is usually maintained at 9Oft/min (0.45m/s), without interference throughout the room. Under the condition that the speed unevenness is within 20%, any significant decrease in airflow speed will increase the possibility of pollution between the self-cleaning time and the working position (all except the dust concentration after the FS209C was issued in October 1987). The parameter indicators are not specified). To this end, the current design value of the unidirectional flow velocity in China is appropriately improved, and the effect is still relatively good. The turbulent flow clean room is the recommended value with a sufficient safety factor, but many designers are still not assured. When doing the specific design, the ventilation capacity of the 100,000-class clean room will be increased to 20~25 times/h, 10,000-plus. Up to 30~40 times/h, 1000 level increased to 60~70 times/h. This not only increases the equipment capacity, increases the initial investment, but also increases the maintenance and management costs in the future. In fact, there is no need to do this. do. In the preparation of China's air clean technical measures [7], more than 100 clean rooms in China have been investigated and tested. Many clean rooms are still tested under dynamic conditions. The results show that: 100,000 levels ≥ 10 times / h, 10,000 ≥ 20 times / h, 1000 level ≥ 50 times / h ventilation can meet the requirements. The US Federal Standard (FS2O9A~B) stipulates that non-unidirectional flow clean room (100,000 class, 10,000 class), room height 8~l2ft (2.44~3.66m), usually consider the whole room to be ventilated at least every 3 minutes ( That is 20 times / h). Therefore, the design specification has already considered a large margin factor, and the designer can safely use the recommended value of the ventilation.
Fourth, static pressure difference
Maintaining a certain positive pressure in the clean room is one of the essential conditions to ensure that the clean room is not or less contaminated to maintain the design cleanliness level. Even in a vacuum chamber, it must have a positive pressure in an adjacent room or suite that is not below its cleanliness level, and the cleanliness of the vacuum chamber can be maintained.
The positive pressure value of the clean room refers to the value of the indoor static pressure greater than the outdoor static pressure when the doors and windows are all closed. It is achieved by the method that the air supply volume of the purification system is greater than the amount of return air and the amount of exhaust air. In order to ensure the positive pressure value of the clean room, the air supply, return air and exhaust fan are preferably interlocked. When the system is turned on, the blower is started first, then the return fan and the exhaust fan are started. When the system is shut down, the exhaust fan is first turned off, and then the fan and blower are turned off. To prevent the clean room from being contaminated when the system is turned on and off.
The amount of air required to maintain a positive pressure in a clean room is primarily determined by the tightness of the maintenance structure. In the early stage of clean room construction in China, due to the poor airtightness of the retaining structure, it is necessary to maintain the positive air pressure of ≥5Pa for 2~6 times/h. The current airtightness of the maintenance structure has been greatly improved, and the same positive pressure is maintained as long as 1 ~2 times / h air supply; maintain ≥ 1OPa as long as 2 ~ 3 times / h air supply.
China's design specifications stipulate that the static pressure difference between clean rooms and clean areas and non-clean areas should be no less than 0.5mmH2O (~5Pa), and the static pressure difference between clean area and outdoor should be no less than 1.OmmH2O (~l0Pa) ). The value seems to be low for three reasons: 1. Positive pressure is the ability of the clean room to suppress indoor air pollution through the gap between the door and window, or to reduce the amount of pollutants in the room to a minimum when opening the door and window for a short time, the positive pressure It means the strength of the ability to suppress pollution, of course, the greater the positive pressure, the better (more on this later). 2. The air volume required for positive pressure is limited. The air volume required for positive pressure of 5Pa and positive pressure of 10Pa is only about 1 time/h. Why not do it? Obviously, the positive pressure value is lower than l0Pa. 3. US Federal Standard (FS209A~B) stipulates that when all entrances and exits are closed, the minimum positive pressure difference between the clean room and any adjacent low-cleanness areas is 0.05 inches of water (12.5Pa), which has been Adopted by the state. However, the positive pressure value of the clean room is not as large as possible. According to the actual engineering test, when the positive pressure value is ≥30Pa, it is more difficult to open the door. When the positive pressure value is ≥50~70Pa, the door and window gap will emit a whistle. The weak or some people with a discomfort do not feel uncomfortable. However, the relevant norms or standards of many countries in China and abroad do not specify the upper limit of positive pressure. As a result, many units only seek the lower limit to meet the requirements, regardless of the upper limit, which is the actual cleanliness encountered by the author. The positive pressure value in the chamber is as high as 100 Pa or more, which causes very bad results. In fact, adjusting positive pressure is not a difficult thing. It is entirely possible to control it within a certain range.
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