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Clean room to obtain good clean effect, not only to focus on taking reasonable air conditioning purification measures, but also requires the process, construction and other professional to take corresponding measures: not only to have a reasonable design, but also carefully in line with the specifications of the construction and installation, and the correct use of clean room and scientific maintenance and management. In order to make clean room obtain good effect, there are many domestic literatures from different angles to elaborate. In fact, it is difficult for different professions to coordinate ideally, and it is difficult for designers to grasp the construction and installation quality as well as the use and management, especially the latter. As far as clean room purification measures are concerned, many designers, or construction parties, often do not pay enough attention to the necessary conditions, resulting in poor clean effect. In this paper, only the four necessary conditions to meet the requirements of cleanliness in clean room purification measures are briefly discussed.
First, 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 of the purification system. The final filter of the purification system generally adopts high efficiency filter or sub-high efficiency filter. According to the standard of our country, the efficiency of HEPA filter is divided into four grades: A class is ≥99.9%, B class is ≥99.9%, C class is ≥99.999%, D class is ≥99.999%(for ≥0.1μm particles) (also known as ultra-high efficiency filter); The sub-high efficiency filter is 95~99.9% (for ≥0.5μm particles). The more efficient the filter, the more expensive it is. So the choice of filter should be in line with both to meet the requirements of air cleanliness, and to consider economic rationality. From the requirements of cleanliness, the principle is to choose low performance filters in low-grade clean rooms and high performance filters in high-grade clean rooms. Generally speaking: 1 million grade can choose high school filter; Less than 10,000 class can choose sub-high efficiency or class A high efficiency filter; Class B filter is selected for class 10000 ~100; Class C filter is selected for class 100~1. Here each cleanliness level seems to have two types of filters to choose from, choose high performance and low performance, depends on the specific situation: when the environment pollution is serious, or indoor exhaust air ratio is large, or the cleanroom is particularly important, need bigger safety coefficient, in one of those or circumstances, to choose high class of filter; On the contrary, you can choose a lower performance filter. For the 0.1μm particles required to control the clean room, regardless of the particle concentration control to choose the D type filter. The above mentioned only from the filter point of view, in fact, to choose a good filter to 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. B. Tight installation. To achieve the A, the construction and installation personnel are required to be well trained, both with the knowledge of the installation of the purification system and with more skilled installation skills, otherwise it will be difficult to ensure that the filter is not damaged, which is a deep lesson. Secondly, the installation tightness problem, mainly depends on the advantages and disadvantages of the installation structure type, design manual generally recommended: for a single filter using the type of clear installation, so that even if leakage occurs, it will not leak to the room; The use of finished high efficiency air outlet, tightness is relatively easy to be guaranteed. For the tuyere of multiple filters, liquid tank seal and negative pressure seal are often used in recent years. The tank seal must ensure that the tank joint is tight and the whole frame is on the same level. The type is the same even if the leakage will not leak into the room. In fact, as long as the installation frame is flat, the filter end face is in uniform contact with the installation frame, and any installation type makes the filter meet the requirements of installation tightness. It should be not difficult.
Two, air distribution:
The air distribution of a clean room is different from that of a general air-conditioned room. It requires the cleanest air to be sent to the operating part first. Its role is to limit and reduce the pollution of processed materials. To this end, in the design of air distribution should consider these principles: minimize vortex, to avoid the pollution outside the work area into the work area; Try to prevent the second dust flying to reduce the chance of dust pollution to the workpiece; The air flow in the working area should be as uniform as possible, and the wind speed should meet the requirements of the process and hygiene. When the air flow to the return air outlet, the dust in the air can be effectively taken away. According to different requirements of cleanliness, choose different ways of sending and returning air. Different airflow structures have their own characteristics and range:
Vertical uniflow: 1, a uniform air flow down, can be obtained for process equipment layout, self-purification ability strong, can simplify personal purification facilities and other common facilities such as common advantages, and their respective advantages and disadvantages of four air supply way: with high efficiency filter has the advantages of low resistance, change the filter cycle is long, but the roof structure is complex, high cost; The advantages and disadvantages of top feeding of side distributed HEPA filter and full orifice plate are opposite to those of full distributed HEPA filter. When the top feeding of full orifice plate is in discontinuous operation of the system, dust is easy to accumulate on the inner surface of orifice plate, which is not well maintained and has some influence on cleanliness. The top feed of dense diffuser requires a mixing layer, so it is only suitable for tall clean rooms above 4m, and its characteristics are similar to the top feed of full hole plate. For the relative two sides of the grid plate and the lower part of the relative two sides of the wall evenly arranged return air outlet return air mode is only applicable to the clean room on both sides of the net spacing is less than 6m; The return air outlet at the lower part of the unilateral wall is only suitable for the clean room with small wall spacing (such as ≤<2~3m).
2, horizontal unidirectional flow: only in the first working area to reach level 100 cleanliness, when the air flow to the other side of the process containing dust concentration gradually increased, so it is only suitable for the same room process has different cleanliness requirements of the clean room; The local distribution of the HEPA filter on the air supply wall can reduce the consumption of the HEPA filter and save the initial investment compared with the full horizontal distribution, but there is eddy current in the local area.
3. Turbulence type airflow: the characteristics of orifice plate top feeding and dense diffuser top feeding are the same as above: the advantages of side feeding are easy to pipe layout,
No technical interlayer, low cost, is conducive to the reconstruction of the old workshop, the disadvantage is that the wind speed of the working area is larger, the leeward side of the dust concentration is higher than the windward side; High-efficiency filter tuyere top feeding has the advantages of simple system, no pipe after the high-efficiency filter, and clean air directly reaches the working area, but the clean air diffuses slowly, and the air flow in the working area is uniform. However, when a plurality of tuyere are evenly arranged or high efficiency filter tuyere with diffusion plate is used, the air flow in the working area can also be more uniform. But in the case of discontinuous operation of the system, the diffusion plate is easy to accumulate dust. The above discussion belongs to the ideal state, and it is also recommended by the relevant national norms, standards or design manuals. In practical engineering, air distribution is not designed well due to objective conditions or subjective reasons of designers, which are common. Vertical uniflow take on both sides of the adjacent wall bottom return air, the local best level by giving last time (that is, the local air diffuser without sagging under the curtain around), turbulence model send you back to the clean room with high efficiency filter tuyers top or unilateral next time (on the wall the wide spacing), such as the air distribution mode, checked by actual measured its cleanliness most of them can not meet the design requirements. Due to the current specifications of empty or static acceptance, individual such clean rooms in empty or static state barely reach the design level of cleanliness, but the ability to resist pollution interference is very low, once the clean room into the working state will not meet the requirements. Correct air distribution in local areas should be set up curtain drooping to the height of the working area, 100,000 level should not be used to send up and back. As well as most of the efficient air supply outlets with diffusion plates produced by factories, the diffusion plates are only decorative orifice plates and do not play the role of diffusion airflow. Designers and users should pay special attention to this.
Three, air supply volume or wind speed:
Adequate ventilation is to dilute and eliminate indoor air pollution. According to different cleanliness requirements, when the clean room height is high, the number of ventilation should be increased appropriately. Among them, the ventilation volume of the 1 million grade clean room is considered according to the high school efficiency purification system, and the rest are considered according to the high efficiency purification system; When HEPA filters of class 100,000 clean rooms are centrally arranged in the machine room or subHEPA filters are adopted at the end of the system, the ventilation times can be appropriately increased by 10-20%. For the above recommended ventilation volume, the author believes that the wind speed of one-way flow clean room through the room section is low, and the turbulence type clean room is the recommended value with a more sufficient safety factor. Vertical unidirectional flow ≥0.25m/s, horizontal unidirectional flow ≥0.35m/s, in the empty state or static detection of cleanliness, although it can meet the requirements, but the anti-pollution ability is poor, once the room into the working state, cleanliness may not meet the requirements, this kind of instance is not an individual; At the same time, there is no suitable for purification system in our country's fan series with blower, the general designer and often accurate calculation has been made to the system air resistance, or did not notice the selection of fan is advantageous in the characteristic curve of working point, makes the system shortly after put into operation, air or wind speed less than design value. The United States federal standard (FS209A~B) has been so stipulated before October 27, 1987; Uniflow cleanroom clean room section of air velocity are usually in a 9 oft/min (0.45 m/s), under the condition of the room without interference, the speed is not uniformity in less than 20%, any obvious decline in air velocity will increase the net work time and location between pollution affects the possibility of (FS209C issued in October 1987 after all parameters than the dust concentration indexes if there are no relevant provisions). For this reason, the designer thinks that it is appropriate to improve the current domestic design value of one-way flow speed. The South Unit of China Construction Corporation has done this in the actual project, and the effect is relatively good. Type turbulence clean room is to have the full recommended value of safe coefficient, but many designers are still not trust, when applied to the design of specific level 100000 clean room ventilation rate will increase to 20 ~ 25 times/h, 10000 level increased to 30 ~ 40 times/h, 1000 level increased to 60 ~ 70 times/h, not only increased the equipment capacity, increased the initial investment, also increases the maintenance costs in the future, in fact it is not necessary to do the same. In the preparation of China's air clean technical measures have been investigated and measured more than 100 domestic clean rooms, many clean rooms or in the state detection, the results show that: 100,000 class ≥10 times /h, 10 thousand ≥20 times /h, 1000 class ≥50 times /h ventilation can meet the requirements. The United States federal standard (FS2O9A~B) provides: non-unidirectional flow clean room (100,000 class, 10,000 class), room height 8~l2ft(2.44~3.66m), usually consider the whole room at least every 3 minutes ventilation (20 times /h). Therefore, the larger coefficient of affluence has been taken into account in the design specification, and the designer can safely choose the recommended value according to the required ventilation volume.
IV. 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 free from or less polluted and to maintain the design cleanliness level. Even if it is a negative pressure clean room, it must have no less than its cleanliness level of adjacent rooms or suites to maintain a certain positive pressure, the cleanliness of a negative pressure clean room can be maintained. Positive pressure value of clean room refers to the value that indoor static pressure is greater than outdoor static pressure when doors and Windows are all closed. It is through the purification system air supply volume is greater than the return air volume and exhaust air volume to achieve the method. In order to ensure the positive pressure value of the clean room, the supply air, the return air and the exhaust fan are best interlocked. When the system is opened, the supply fan is started first, and then the return fan and the exhaust fan are started. When the system is shut down, the exhaust fan should be turned off first, and then the return fan and the air supply fan should be turned off to prevent the clean room from being polluted when the system is opened and closed. The air volume required to maintain the positive pressure in the clean room is mainly determined according to the airtightness of the maintenance structure. In the early stage of clean room construction in China, due to the poor airtightness of the envelope structure, it is necessary to maintain a positive pressure of ≥5Pa to send air volume of 2~6 times /h; At present, the airtightness of the maintenance structure has been greatly improved, maintaining the same positive pressure as long as 1~2 times /h air supply; Maintain ≥ 1OPA as long as 2~3 times /h air supply. China's design specifications: different grades of clean room and clean area and non-clean area between the static pressure difference should not be less than 0.5mmH2O(~5Pa), clean area and outdoor static pressure difference should not be less than 1 ommH2O (~ L0Pa). Positive pressure is the ability of the clean room to inhibit indoor air pollution through the gaps in the doors and Windows, or the ability to reduce pollutants into the room to a minimum when the doors and Windows are opened in a short time. The positive pressure size means the strength of the ability to inhibit pollution, of course, not the greater the positive pressure, the better (will be discussed later). 2. The air volume required by positive pressure is limited. The air volume required by positive pressure of 5Pa and positive pressure of 10Pa is only about 1 times /h, so why not do it? Obviously, the lower limit of positive pressure is L0pA. 3. The United States federal standard (FS209A~B) states that when all entrances and entrances are closed, the minimum positive pressure difference between a clean room and any adjacent area of low cleanliness is 0.05 inches of water column (12.5Pa), which has been adopted in many countries. But the positive pressure value of the clean room is not the greater the better, according to the unit of more than 30 years of actual engineering testing shows that: when the positive pressure value ≥30Pa, it is more difficult to open the door, close the door slightly do not pay attention to, bang! Scary a jump; When positive pressure value ≥50~70Pa, door and window gap will issue whistle sound, the weak or have some kind of unsuitable person feeling. However, the relevant specifications or standards of many countries at home and abroad do not stipulate the upper limit of positive pressure. As a result, many units only ask for the lower limit to meet the requirements. No matter how much the upper limit is, the positive pressure value in the actual clean room encountered by designers is as high as 100Pa or more, which causes very bad effect. In fact, it is not very difficult to adjust the positive pressure, it is completely possible to control it within a certain range. It has been reported that some countries in Eastern Europe set the positive pressure value as 1-3mmH20(about 10-30Pa), and we believe that this range is more appropriate. For a clean room, the above four necessary conditions must be met at the same time, in order to ensure that the cleanliness meets the requirements and has the ability to resist pollution interference.
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