Clean Room, also known as clean room or clean room. It is the foundation of pollution control. Without clean rooms, pollution-sensitive parts cannot be mass-produced. In FED-STD-2, a clean room is defined as a room equipped with air filtration, distribution, optimization, construction materials, and devices in which specific, regulated operating procedures are used to control the concentration of airborne particulates in order to achieve an appropriate particulate cleanliness level.
Clean room to obtain a good clean effect, not only to take reasonable air conditioning purification measures, but also require technology, 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, as well as the correct use of clean room and scientific maintenance management. In order to make the clean room get good results, there have been a lot of literature at home and abroad from different perspectives. In fact, it is difficult to achieve ideal coordination between different professions, and it is difficult for designers to grasp the quality of construction and installation as well as the use and management, especially the latter. In terms of clean room purification measures, many designers, or construction parties, often do not pay enough attention to the necessary conditions, resulting in unsatisfactory cleaning effect. This paper only discusses the four necessary conditions of cleanliness in clean room purification measures.
First, air supply cleanliness
In order to ensure that the air supply cleanliness meets the requirements, the key is the performance and installation of the final stage filter of the purification system.
The final filter of the purification system is usually high efficiency filter or sub-high efficiency filter. According to our standard [1], the efficiency of high efficiency filter 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); Subhepa filter [2] is 95~99.9% (for ≥0.5μm particles). The more efficient the filter, the more expensive it will be. Therefore, the selection of filters should be in line with both to meet the requirements of air supply cleanliness, but also to consider economic and reasonable. Considering the cleanliness requirements, low performance filter is selected for low level clean room and high performance filter is selected for high level clean room. Generally speaking :1 million grade can choose high school filter; Sub-high efficiency or class A high efficiency filter can be used for less than 10,000 grade; Class B filter is selected for 10,000 ~100 grade; Class C filters are selected for grade 100~1. Here each cleanliness level seems to have two types of filters to choose from, choose high performance or low performance, depending on the specific situation: when the environmental pollution is serious, or indoor exhaust proportion is large, or the clean room is particularly important, the need for a large safety factor, in these or one of the cases, are to choose a high class of filter; Otherwise, lower performance filters can be selected. For the clean room requiring control of 0.1μm particles, Class D filters should be selected regardless of the concentration of controlled particles. The above mentioned only from the filter point of view, in fact, to choose a good filter but also a comprehensive consideration of 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 ensure that :a. Transportation, installation process does not damage the filter; b. The installation is tight. To achieve the first a, the construction and installation personnel are required to be well-trained, have the knowledge of installing the purification system, and have more skilled installation skills. Otherwise, it will be difficult to ensure that the filter is not damaged. There is a profound lesson in this aspect. Secondly, the installation tightness problem, mainly depends on the advantages and disadvantages of the installation structure, the design manual generally recommends: for a single filter with open type, so that even if there is leakage, it will not leak into the room; The high efficiency air outlet is adopted, and the tightness is easy to be guaranteed. For the air of multiple filters, liquid tank seal and negative pressure seal are often used in recent years. Tank sealing must ensure that the tank joints are tight and the whole frame is at the same level. Negative pressure seal is to make the filter and the plenum and the outer perimeter of the frame joint in a state of negative pressure, and the open type even if leakage will not leak into the room. In fact, as long as the installation frame is smooth, the filter end face and the installation frame contact evenly, any installation type to make the filter meet the installation tightness requirements should be not difficult.
Two, air distribution
The airflow organization of clean room is different from that of general air conditioning 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, these principles should be considered in the design of airflow organization: minimize eddy current, avoid contamination from outside the work area into the work area; Try to prevent dust secondary flying to reduce dust pollution opportunities to the workpiece; The airflow in the working area should be as even as possible, and the wind speed should meet the process and health requirements. When the airflow flows to the return air outlet, the dust in the air should be effectively taken away. According to different cleanliness requirements, choose different air delivery and return modes.
Different air distribution has its own characteristics and ranges:
1) Vertical one-way flow: can obtain uniform downward airflow, easy to arrange process equipment, strong self-purification capacity, can simplify the personal purification facilities and other common facilities and other common advantages, four air supply methods have their own advantages and disadvantages: full high efficiency filter has low resistance, long filter replacement cycle advantages, but the ceiling structure is complex, high cost; The advantages and disadvantages of side cloth high efficiency filter top feeding, full hole plate top feeding and full cloth high efficiency filter top feeding are opposite, the full hole plate top feeding in the system non-continuous operation, the inner surface of the hole plate is easy to accumulate dust, poor maintenance, some influence on cleanliness; Dense diffuser top feeding, because a section of mixed layer is needed, so it is only suitable for 4m tall clean rooms, whose characteristics are similar to that of full orifice plate top feeding; For the plate on both sides of the grating and the lower part of the wall on both sides of the uniform arrangement of the return air outlet is only suitable for clean rooms with a net spacing of less than 6m on both sides; The lower side wall cloth return air outlet is only suitable for clean rooms with small wall spacing (such as ≤<2~3m).
2) Horizontal unidirectional flow: only reach level 100 cleanliness in the first working area, when the air flows to the other side in the process of dust concentration gradually increased, so it is only suitable for the same room process with different cleanliness requirements of clean rooms; Compared with full horizontal feeding, local distribution of high efficiency filter can reduce the consumption of high efficiency filter and save the initial investment, but there is eddy current in local area.
3) Turbulence type air flow: the characteristics of orify-plate ejection and dense diffuser ejection are the same as above: the advantages of side ejection are easy to arrange pipes, no technical interlayer is required, low cost, and conducive to the reconstruction of old factory buildings. The disadvantages are that the wind speed in the working area is larger, and the leeward side has higher dust concentration than the windward side; High efficiency filter tuyere top delivery has the advantages of simple system, no pipeline after high efficiency filter, clean air directly to the work area, but the clean air diffusion is slow, the work area air flow is more uniform; However, when a number of tuyere are evenly arranged or the tuyere with diffusion plate is used, the air flow in the working area can 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 is also recommended by the relevant national norms, standards or design manuals. In the actual project, or limited by objective conditions, or due to the subjective reasons of the designer, the airflow organization is not designed well. Common ones are: vertical one-way flow takes the lower return air of the adjacent two sides of the wall, local hundred adopts the upper return (that is, the local air supply port is not added under the sagging curtain), and the turbulence type clean room adopts the top of the high efficiency filter tuyair to send back or the unilateral lower return (to the wall distance is larger), etc. Most of these air distribution methods are not up to the design requirements after being measured. As the current specification [6] stipulates empty state or static acceptance, individual clean rooms of this kind barely reach the designed cleanliness level in empty state or static state, but the anti-pollution interference ability is very low, and once the clean room enters the working state, the requirements cannot be reached. The local area of the correct air distribution should be set up to hang down to the height of the working area, and the 100000 level should not be used to send back. At present, most of the high-efficiency air supply ports with diffusion plates produced by factories are only decorative orifice plates without diffusion airflow, so designers and users should pay special attention to them.
Three, air supply volume or wind speed
Adequate ventilation volume is to dilute and eliminate indoor polluted air. According to different cleanliness requirements, when the clean room is higher, the number of ventilation should be increased appropriately. Among them, the ventilation volume of 1,000,000 grade clean rooms is considered as high school and primary purification system [7], and the rest are considered as high efficiency purification system. When 100,000-class clean room HEPA filters are centrally arranged in the machine room or subhePA filters are used at the end of the system, the air exchange times can be appropriately increased by 10-20%.
For the above recommended value of ventilation volume, the author believes that the wind speed through the section of unidirectional flow clean room is low, and turbulence clean room is the recommended value with 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 can meet the requirements, but the anti-pollution ability is poor, once indoor into the working state, cleanliness may not meet the requirements, such cases are not individual; Meanwhile, there are not yet fans suitable for the purification system in our country's fan series. Generally designers have not accurately calculated system air resistance, or do not pay attention to whether the selected fan is at more favorable working point on the characteristic curve. That makes the system not reach the design value soon after it is put into operation. The United States Federal standard (FS209A~B) has been so stipulated before October 27, 1987: the airflow velocity of unidirectional flow clean room through clean room section is usually maintained at 9Oft/min(0.45m/s). Under the condition of no interference in the whole room, the velocity ununiformity is within 20%. Any significant decrease in airflow velocity increases the likelihood of contamination effects between the self-cleaning time and the working position (all parameters other than dust concentration are not specified after the FS209C was promulgated in October 1987). Therefore, I think it is appropriate to improve the current domestic design value of unidirectional flow velocity, our unit in the actual project to do so, the effect is relatively good. Turbulence type clean room is the recommended value of sufficient safety factor, but many designers are still not assured, when doing the specific design will be 100,000 level clean room ventilation increased to 20~25 times /h, 10,000 level increased to 30~40 times /h, 1000 level increased to 60~70 times /h, so that not only increase the equipment capacity, increase the initial investment, It also increases the cost of future maintenance and management, which is unnecessary. In the preparation of our air cleaning technology measures [7], more than 100 domestic clean rooms have been investigated and measured, many clean rooms are still under dynamic detection, the results show that: 100,000 grade ≥10 times /h, 10,000 ≥20 times /h, 1000 grade ≥50 times /h ventilation can meet the requirements. The United States federal standard (FS2O9A~B) stipulates: non-unidirectional flow clean room (class 100,000, class 10,000), room height 8~l2ft(2.44~3.66m), usually consider the whole room at least once every 3 minutes (i.e. 20 times /h). Therefore, the provisions of the design specification [6] have taken into account the larger affluence coefficient, and the designer can safely choose according to the recommended value of ventilation in Table 1.
Four, static pressure difference
The maintenance of a certain positive pressure in the clean room is one of the essential conditions to ensure that the clean room is not contaminated or less, 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 to maintain a certain positive pressure in the adjacent room or suite, so that the cleanliness of the negative pressure clean room can be maintained.
The positive pressure value of clean room refers to the value of indoor static pressure greater than outdoor static pressure when the doors and Windows are all closed. It is achieved through the purification system of air supply volume is greater than the return air volume and exhaust air volume. In order to ensure the positive pressure value of the clean room, the supply air, return air and exhaust fan are best interlocked. When the system is opened, the supply fan is started first, and then the return fan and exhaust fan are started. When the system is shut down, turn off the exhaust fan first, and then turn off the return fan and the supply fan 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. Because of the poor tightness of the retaining structure in the early construction of clean room in our country, 2~6 times /h air supply is necessary to maintain positive pressure ≥5Pa. 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 volume; Maintain ≥1OPa as long as 2~3 times /h air supply.
Our design specification [6] stipulates that the static pressure difference between different levels of clean rooms, clean areas and non-clean areas should not be less than 0.5mmH2O(~5Pa), and the static pressure difference between clean areas and outdoor should not be less than 1.OmmH2O(~l0Pa). The author thinks that the value seems low, there are three reasons :1. Positive pressure is the ability of clean room to inhibit indoor air pollution through the gaps in doors and Windows, or the ability to reduce pollutants into the room to a minimum when opening doors and Windows in a short time. Positive pressure size indicates 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 difference between the air volume required by 5Pa positive pressure and 10Pa positive pressure is only about 1 time /h. Obviously, l0Pa is better for the low limit of positive pressure value. 3. United States Federal Standards (FS209A~B) specify a minimum positive pressure difference of 0.05 inches of water (12.5Pa) between clean rooms and any adjacent low-cleanliness areas when all entrances and exits are closed. This value is adopted in many countries.
But the positive pressure value of the clean room is not the bigger the better, according to the actual engineering testing of the unit for more than 30 years shows that: when the positive pressure value is more than 30Pa, it is more difficult to open the door, close the door slightly do not pay attention to, bang! A fright; When the positive pressure value is more than 50~70Pa, the door and window gaps will emit whistles, the infirm or have some kind of discomfort. However, the relevant norms or standards of many countries at home and abroad have not stipulated the upper limit value of positive pressure. As a result, many units only seek the lower limit value to meet the requirements, no matter how much the upper limit value is reached, the positive pressure value in the actual clean room encountered by the author is up to more than 100Pa, resulting in a very bad effect. In fact, it is not a difficult thing to adjust the positive pressure, it is entirely possible to control it within a certain range. It has been introduced that the positive pressure value of a country in Eastern Europe is defined as 1-3mmH20(about 10~30Pa), the author thinks 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 due ability to resist pollution interference.