The necessary conditions for the cleanliness of clean room
Clean room clean to obtain good results, not only to take reasonable measures and requirements of air purification, technology, construction and other major measures: not only to have reasonable design, but also well meet the installation specifications of the construction and use of clean room, the correct and scientific maintenance management. In order to get a good effect in the clean room, many domestic and foreign literatures have been expounded from different angles. In fact, it is difficult to achieve the ideal cooperation between different professions, and it is difficult for designers to grasp the quality of construction and installation and use and management, especially the latter. The cleaning room cleaning measures, many designers, or the construction side, often on the necessary conditions for not paying enough attention, resulting in clean effect is not ideal. Four necessary conditions for cleaning and cleaning to achieve cleanliness.
A, air cleanliness:
To ensure the air cleanliness requirements, is the key to the performance of the filter and the installation of final purification system. General use of HEPA filter or sub HEPA filter purification system for final filter. According to the Chinese standard high filter efficiency is divided into four grades: a class of more than 99.9%, the B class is greater than or equal to 99.9%, C is more than 99.999%, class D (of more than 0.1 M particles) = 99.999% (or filter); efficient filter (of more than 0.5 m particles) 95~99.9%. The higher the efficiency, the more expensive the filter. So the choice of filter should be not only to meet the air cleanliness requirements, but also consider the economic and reasonable. Considering the cleanliness requirements, with low levels of clean room with low performance filter, high level cleanroom with high performance filter principle. Generally speaking: 1 million can choose high school effect filter; lower than the level of can choose a high efficiency or a class of high efficiency filter; 10 thousand ~100 class B class filter selection; 100~1 class choose C filter. Here each grade seems to have two kinds of filters to choose, choose high performance or low performance, depending on the specific circumstances: when the serious environmental pollution, indoor air discharge or a large proportion, or the clean room is particularly important, requires a larger safety factor, in the case of one or both. To choose a high class filter; but can be low performance filter. For 0.1 M particle control clean room, regardless of the concentration of particles to control the selection of D filter. From the above point of view only from the filter, in fact, to choose a good filter should also take into account the characteristics of clean room, filter characteristics and purification system characteristics.
To ensure the cleanliness of the air filter, only qualified is not enough, but also to ensure the A. transport, the installation process does not damage the tightness of B. filter. To do the A, requires the construction and installation personnel should be trained with regularity, installation of purification system of knowledge, but also have the installation skills more skilled, otherwise it will be difficult to ensure that no damage to the filter, this is a profound lesson. Secondly, tightness of installation, mainly depends on the structural mode of installation quality, design manual is generally recommended: for the single filter besurface mounted type, so even if the leakage occurs, will not leak into the room; the finished HEPA, tightness is also relatively easy to be guaranteed. The wind - multiple filters, in recent years, often using liquid sealing and vacuum sealing. Tank seal must ensure liquid tight joints and the whole frame are in the same horizontal plane. Negative pressure seal is to make the filter and the static pressure box and frame joints in the outer periphery of the negative pressure state, and with the same type of installation, even if leakage will not leak into the room. In fact, as long as the installation of the formation of the framework, the end face of the filter and the installation of the framework of a uniform contact, any type of installation so that the filter to meet the installation requirements should not be difficult to say.
Two, air flow organization:
Clean room air flow organization and the general air conditioning room is different, it requires the most clean air to the operation of the first place, its role is to limit and reduce the pollution of the processing. Therefore, in the design of airflow organization should consider these principles: minimize eddy current, avoid working outside the area pollution into the work area; to prevent dust flying two times to reduce dust pollution on the chance of the workpiece; the work area to flow as uniform as possible, and the wind speed to meet the technical and health requirements, when the gas flow return air flow to make the dust in the air can be effectively removed. According to the cleanness of the different requirements, choose different supply and return air way.
Air distribution is different, each have their own characteristics and scope: 1): vertical unidirectional flow can be obtained even downward airflow, easy process and equipment layout, self purification ability, can be simplified to purify people of common facilities and other common advantages, four kindssupply modes and their respective advantages and disadvantages: with high efficiency filter has the advantages of resistance low, replace the filter cycle is long, but the roof structure is complex, high cost and high efficiency filter cloth; side top, top plate to send and send with the advantages and disadvantages of the HEPA filter top sent instead, which all orifice top supplying continuous operation in the system of non perforated plate inner surface, easy dust, maintenance is not good. Some impact on cleanliness; intensive diffuser ejector, due to the need of a mixed layer, so only applies to more than 4m high Dajie room, its characteristics and the top plate to send similar relative to the grille plate on both sides; And relatively evenly arranged on both sides of the wall bottom return air return way only for clean room on both sides of the spacing of less than 6m; the lower part of the cloth single wall return air only for smaller to the wall distance (such as <2~3m) of the clean room. 2) level: unidirectional flow only reached 100 in the first grade of cleanliness of work area, when the dust laden air flow process on the other side of the concentration gradually increased, so only for the same room process have different clean room cleanliness requirements; send windwall local cloth filter is covered with horizontal supply can reduce the dosage of HEPA filter at the beginning, saving investment, but the local eddy current. 3) turbulence airflow: orifice plate ejector and dense diffuser ejector characteristics with the same advantages: easy side supply pipeline arrangement, without technical dissection, low cost, is conducive to the transformation of the old plant, the disadvantage is the wind speed of working area is large, the downwind side dust concentration higher than the windward side; high efficiency filter the air ejector has the advantages of simple system, high efficiency filter, clean air pipe directly to the work area and other advantages, but the slow diffusion of clean air, the work area flow is uniform; but when the uniform arrangement of a plurality of outlet or diffuser plate with high efficiency air filters, also makes the work flow is uniform; but in the system of non continuous operation condition, easy dust diffusion plate.
All of the above statements are in the ideal state, but also recommended by the relevant national norms, standards or design manual. In the actual project or limited by the objective conditions, or because the designer subjective reasons but not design good air distribution, common are: vertical unidirectional flow on both sides of the wall adjacent to take lower return air, and sent back by local 100 (local outlet without sagging under the curtain), turbulent type clean room with air filter the top sent back next time (on the larger or unilateral wall spacing), the airflow through the test, the cleanliness of most of them fail to meet the design requirements. Due to the current specification empty state or static acceptance, this kind of clean room for individual barely reached the cleanliness level of design in the empty state or static, but the anti pollution interference ability is very low, once the clean room into the work of the state is not up to the requirements. The local airflow right should be set down to curtain around the work area height, level 100 thousand should not be sent back by. There are currently most of the factory production with a diffuser plate of high efficiency air outlet, the diffuser plate is only decorative hole plate and can not afford the proliferation of air flow, please designers and users should pay special attention to.
Three, to send wind or wind speed:
The ventilation volume is sufficient for dilution and removal of indoor air pollution, according to the different requirements of the cleanliness of clean room, when the height is relatively high, increasing the number of appropriate. Among them, class 1 million clean room ventilation is in high efficiency purification system, the other according to the efficient purification system; when the class 100 thousand clean room are arranged in the room or high efficiency filter system at the end of the sub filter, may be appropriate to increase the number of 10-20%.
For the ventilation of the recommended amount of numerical I think: unidirectional flow clean room by room section of lower wind speed, turbulent type clean room is a sufficient safety factor recommended value。 Vertical unidirectional flow is larger than 0。25m/s, the level of unidirectional flow is larger than 0。35m/s, detection of cleanliness in the empty state or static state, although can meet the requirements, but the anti pollution ability is poor, once the room into the working state, cleanliness, they may not be up to the requirements, this kind of case is not belong to the individual; at the same time, China's fan series not suitable for air purification machine system, the general designer is often not accurate calculation of air resistance system, or failed to pay attention to the working point of fan is relatively favorable characteristic curve, which makes the system shortly after put into operation, air or wind speed can not reach the design value。 The United States federal standard (FS209A~B) until October 27, 1987 this provision: unidirectional flow clean room is usually maintained at 9Oft/min speed through the air clean room section (0。45m/s), no interference in the whole room conditions, the velocity uniformity in gas flow velocity is less than 20%, the possibility of any significant decline will increase the impact of pollution between the self purification time and work location。
To this end, the appropriate increase in the current design of one-way flow velocity of the value of our unit in the actual project in accordance with this done, the effect is still relatively good。 Turbulent type clean room is a full safety factor of the recommended values, but many designers still not assured, in the specific design of ventilation will be increased to 20~25 class 100 thousand clean room /h, 10 thousand grade increased to 30~40 times /h, 1000 grade increased to 60~70 /h, do not only increase the equipment capacity, increase the initial investment, maintenance management fees also increased day after, actually there is no need to do so。 In the preparation of clean technology measures of Air China has investigated more than and 100 domestic clean room by many clean room or in the dynamic test, the results showed that: 100 thousand level is more than 10 times /h, 10 thousand /h, 1000 times more than 20 more than 50 times the air volume can meet the requirements of /h。 The United States federal standard (FS2O9A~B) provisions: non unidirectional clean room (100 thousand, 10 thousand), 8~l2ft (2。44~3。66m), high room is usually considered the whole room ventilation once at least every 3 minutes (20 /h)。 Therefore, the design specification has taken into account the greater richness factor。
Four, static pressure difference:
It is one of the essential conditions for the clean room to maintain a certain degree of positive pressure to ensure that the clean room is not subject to or less pollution, and to maintain the cleanliness of the design. Even the negative pressure clean room, it must be not less than the cleanliness level of adjacent rooms or apartments to maintain a positive pressure, negative pressure clean room cleanliness can be maintained.
Clean room pressure value refers to the door and window in the closed state, the indoor static pressure is greater than the value of outdoor static pressure. It is achieved by the method of purification system of air supply and exhaust air to the air volume is greater than. In order to ensure clean room pressure value, supply air and return air and exhaust fan interlock system when opening the best, start the blower, and then start the return fan and exhaust fan system; first off the exhaust fan, then close the air blower and the air machine, to prevent contamination in clean room system open and close.
To maintain the air pressure in the clean room is mainly based on the maintenance of the structure to determine the tightness. Our clean room building envelope closed early because of less than 5Pa to maintain positive pressure to 2~6 /h air tightness; the maintenance structure has been greatly improved, maintaining the same positive pressure as long as the 1~2 /h air; maintain more than 1OPa as long as 2~3 /h air.
The design code of our country stipulates that the static pressure difference between the clean room of different grades and the clean area and the non clean area should not be less than 0.5mmH2O (~5Pa), the static pressure difference between the clean area and the outside should not be less than 1.OmmH2O (~l0Pa). This value seems low, there are three reasons: 1 positive pressure clean room is said through inhibition capacity gaps of doors and windows of indoor air pollution, or open the doors and windows in a short time when the pollutants into indoor capacity reduced to the minimum, the size of that positive pressure to curb pollution ability, but not the positive pressure better. 2 positive pressure air flow required, 5Pa positive pressure and 10Pa positive pressure ventilation required only about 1 /h, Why not?? lower limit value of l0Pa more superior obviously positive pressure. 3 United States federal standard (FS209A~B) requirements: when all the entrance closes, between the low cleanliness of clean room with any area adjacent to the lowest positive pressure difference of 0.05 inches of water (12.5Pa), this value has been adopted by many countries.
But the clean room pressure value is not the bigger the better, according to the actual engineering test: when the pressure value is greater than or equal to 30Pa, it is difficult to open the door, close the door a little attention, bam! Scary; when the pressure value is greater than or equal to 50~70Pa, the gap of the door and window will be issued a whistle, the infirm or a certain indication of discomfort. But the relevant norms or standards at home and abroad in many countries are not provided positive pressure limits, this results in many units only limit to meet the requirements, regardless of the upper limit to how much, the positive pressure is higher than 100Pa in the actual clean room I encountered in a very bad effect. In fact, it is not difficult to adjust the pressure, it is possible to control it within a certain range.
For a clean room, the above four conditions must be met at the same time, in order to ensure the cleanliness of the requirements and have the ability to resist pollution interference.