1. Principle of vertical unidirectional flow clean room
Vertical unidirectional flow is one of the most widely used airflow patterns in high-grade cleanrooms. In the clean room, the high-efficiency air filter (or ultra-high efficiency air filter) is arranged on the ceiling or the side. From the air supply port to the return air inlet, there is almost no change in the cross section of the airflow, plus the air supply static pressure box and the high efficiency filter. The pressure equalization effect makes the flow velocity on the cross section of the whole chamber relatively uniform, and at least in the working area, the flow line is unidirectionally parallel and there is no eddy current. The clean airflow is not one or a few strands, but full of the whole room section. Therefore, the clean room does not rely on the mixing and dilution, but pushes the dirty air in the room to the outside through the entire section to purify the indoor air. the goal of. The air returns to the circulating fan through the raised floor, thereby forming a vertical unidirectional flow pattern that is sent back and forth. When the width of the room is less than or equal to 6 m, the air flow pattern of the upper return side may be formed at the lower part of the side wall, which is basically a vertical one-way flow type.
2. Three elements of one-way flow
In addition to satisfying that the airflow fills the clean room or the clean area, the unidirectional flow has three elements: airflow velocity, airflow non-uniformity, and airflow parallelism. And there are strict requirements for the three elements.
(1) Airflow speed
The principle of choice of air velocity should be such that the entire airflow flows evenly without a stagnation zone. The airflow speed should be able to resist the interference of personnel activities and thermal rise. For the cross-section wind speed Vd of the cross section of the vertical unidirectional flow clean room, refer to the following values:
a) no one in the room or often in an unmanned state of 0.1 to 0.15 m/s;
b) General conditions ISO1 ~ 4 0.25 - 0.35m / s, ISO5 level 0.2 ~ 0.35m / s;
c) when there is a rising hot gas flow (flow rate is Vr), Vd>Vr;
d) consider personnel comfort Vd <0.5m / s;
e) When the process has special requirements, it is determined according to the process requirements.
(2) Unevenness of airflow speed
At the beginning of the development of clean technology, the unevenness of airflow is very strict. The US FS209B stipulates that the unevenness is within ±20%. This provision does not state whether the speed is the reference value and is the measured average of the design wind speed or the section wind speed. It is generally considered that the quasi-value is the average wind speed measured in the section. After the US FS209, the requirement for airflow unevenness was removed. This is obviously an understanding from long-term practice. From the long-term test of the clean room, this requirement is basically not achieved. We know that the outlet airflow is equal to the core of the main section of the jet, the airflow speed is equal, and the streamlines are parallel, and other parts are impossible. Due to the uneven density of the filter media, the pressure of the filter air supply static pressure tank is uneven, and the wind speed of the adjacent filter vents and the surface of the filter due to the windless zone formed by the adjacent high-efficiency filter frame. The wind speed is different.
3, the parallelism of the streamline
Various specifications state that one element of a unidirectional flow is that the airflow is parallel. This point has been adhered to so far. However, practice has proved that this is completely unnecessary. The eddy current zone is not formed as long as the flow lines (trace) in the entire airflow do not intersect. The dust particles are removed along the traces (streamlines) without being trapped or diverted to other traces.
Under what circumstances does the two streamlines form eddy currents? There is no conclusion about this. The local resistance loss of the dilatant tube in fluid mechanics is estimated to be the smallest when the diffusion angle is 5-8o. This means that when the diffusion angle is 5-8o, only the loss of the flow velocity distribution is formed, and there is substantially no loss of the vortex zone. Therefore, theoretically, when the angle between the two streamlines is less than 5-8o, no eddy current will be formed; thus it is inferred that as long as the outflow of the air supply port, the adjacent streamline expansion angle is less than 5-8o, the whole airflow follows a single trend. The flow can still form a unidirectional flow. In addition to meeting the requirements of the three characteristics of the one-way flow clean room, it should also meet the requirements of the Cleanroom Construction and Acceptance Code (50591-2010). The return air velocity should not exceed 2.0m/s, and the return air inlet height and The ratio of the clean room width should be greater than or equal to 0.1. In general applications, the return air opening is set below 0.8m below the lower limit of the working area.
4, vertical one-way flow clean room new technology
(1) Clean room with full ceiling and wind returning on both sides
Conventional vertical unidirectional flow clean rooms mostly use ceiling-filled high-efficiency filters for air supply and full floor grilles for return air. The clean room is characterized by a uniform downward unidirectional parallel airflow and thus a high self-cleaning capability to achieve the highest level of cleanliness. Not only can the process equipment be arranged arbitrarily, but also the personal purification facility can be simplified. However, for the return of the full floor grille, the floor gives a poor visual appearance, walking (especially female) and placed objects have an unstable feeling, and small things are easy to fall under the floor. Therefore, this kind of floor is not conducive to the promotion of vertical unidirectional flow clean room. To this end, it is necessary to improve this type of return air, and there is a clean room in which the wind is returned to the sides of the full ceiling. The full-ceiling air supply is opposite to the two sides of the wall. The return air-type clean room is a variant of the traditional vertical unidirectional flow clean room, also known as the quasi-unidirectional flow clean room. This method of sending and returning air can cause a good airflow pattern in the room, and most of the indoor area (including the working area of ​​the person) is in the mainstream area, and the required cleanliness index can be achieved under a small air supply volume. Obviously, it is an ideal airflow.
5. Comparison of economics of vertical unidirectional flow clean room
Numerous studies at home and abroad have shown that the cleanliness of clean rooms is mainly determined by the reasonable airflow organization and efficiency of high efficiency filters. For airflow, unidirectional flow is undoubtedly the best airflow pattern because it achieves a high level of cleanliness. Of course, low-level cleanliness can be used for non-unidirectional flow. The typical vertical unidirectional flow lines are parallel, and the path of dust particles at the same height is the same and the shortest. Of course the vertical unidirectional flow is the optimum air flow. But for the ceiling is covered with high-efficiency filter air supply, full floor grille return air system. However, this system also has obvious disadvantages, except for the poor visual effects mentioned above; due to the use of the grille floor. This kind of floor is generally made of cast aluminum, plastic, steel, etc., and the cost is relatively high. The use of the grille floor not only consumes space but also costs about 30% to 50% of the cost of interior decoration of the one-way flow clean room. The system is expensive and is not conducive to the promotion of vertical unidirectional flow clean rooms. The clean room with the return air on both sides of the full ceiling is overcoming the above shortcomings, so the clean room is popular because of its relatively low cost and better flow pattern.
6, system use occasions
Because the vertical unidirectional flow clean room is mainly used for high cleanliness grades. Whether it is an industrial clean room for the control of inanimate particles or a biological clean room for controlling living micro-materials (bacteria) and dead micro-materials (dust), this system is applicable, but In order to obtain the ideal airflow distribution, the room width is less than 6 meters.
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