Clean room refers to the removal of particles, harmful air, bacteria and other pollutants in the air within a certain space range, and the indoor temperature, cleanliness, indoor pressure, air velocity and air flow distribution, noise, vibration and lighting, static electricity control within a certain range of needs, and give a specially designed room. That is to say, regardless of the changes in the external air conditions, the room can maintain the characteristics of cleanliness, temperature, humidity and pressure required by the original set. The main function of the clean room is to control the cleanliness and temperature and humidity of the atmosphere in contact with the product (such as silicon chips, etc.), so that the product can be produced and manufactured in a good environmental space, which we call the clean room. According to international practice, the dust-free purification level is mainly specified according to the number of particles whose diameter per cubic meter of air is greater than the division standard. In other words, the so-called dust-free is not 100% without a little dust, but controlled in a very small amount of units.Of course, the particles that meet the dust standards in this standard are already small and small compared to our common dust, but for optical structures, even a little bit of dust will have a very large negative impact, so in the production of optical structure products, dust-free is an inevitable requirement... The number of motes with a particle size of less than 0.3 microns per cubic meter is controlled below 3,500, which has reached the international dust-free standard A level. The dust-free standard of chip-level production and processing has higher requirements for dust than grade A, and such high standards are mainly used in the production of some higher-grade chips. The number of motes of 0.5uM and below is strictly controlled within 1000 per cubic inch, which is commonly known as the 1K level in the industry.
Air: Clean rooms (facilities) that have been built and are ready for use. It has all the relevant services and features. However, there is no equipment operated by operators in the facility. Static: A clean room (facility) with complete functions, proper settings, and can be used or used according to the settings, but there are no operators in the facility.
Dynamic: clean room in normal use, perfect service functions, equipment and personnel; If needed, you can do your normal work.
Unidirectional flow (laminar flow) cleanroom principle In the clean room, from the air supply port to the return air outlet, there is almost no change in the section of the air flow through the way, coupled with the equalization and current sharing of the inlet static pressure box and the high efficiency filter, the flow rate on the section of the whole room is relatively uniform, and at least in the work area, the flow line is one-way parallel, and there is no vortex. These are the three characteristics of unidirectional flow cleanrooms. The streamlines here are unidirectional parallel, which means that the time-average streamlines are parallel to each other and have a single direction. The flow state of a unidirectional flow cleanroom is turbulent from the Reynolds number, so the so-called laminar flow in a laminar flow cleanroom and laminar flow in fluid mechanics are completely different concepts. Therefore, the term laminar flow clean room is not appropriate, and some foreign standards and articles also point out this, such as the British standard BS-5295 has defined the so-called laminar flow clean room as a one-way flow clean room, but the habit of care is indicated in parentheses as laminar flow.The former Federal German standard VDl-2083 used the term "non-turbulent displacement flow" and pointed out in the note that the concept of laminar flow was only used at that time to distinguish the disordered (disordered) flow cleanroom that predates the laminar flow clean room, and the exact meaning of "laminar flow system" is not layered flow, but displacement flow of turbulent flow, and the concept mentioned in this chapter is the gradual flow of turbulent flow. In fluid mechanics, this flow state can also be called parallel flow or unidirectional flow.In 1977, China chose the term "parallel flow" in the "Technical Measures for Clean Air", and after the term, it was also pointed out that it was customary to call it "laminar flow"... In a one-way flow clean room, the clean air flow is not one or several strands, but fills the whole room section, so this clean room does not rely on the mixing and dilution of the indoor dirty air by the clean air flow, but relies on the clean air flow to exhaust the indoor dirty air along the entire section to the outside, so as to achieve the purpose of purifying the indoor air. Therefore, some people in the former Federal Republic of Germany called the air flow of a one-way flow cleanroom "piston flow" and "horizontal push flow", and the former Soviet Union called it "squeezed weak air jet". Clean air is like an air piston, along the "cylinder" of the room, forward (down) push, and so that the dust particles can only enter forward (down), without returning, and squeeze the original high dust concentration of air out of the room. In unidirectional flow cleanrooms and unidirectional flow purification plants, reverse airflow along the wall and under the lap of the two filters can be found. This air flow will transmit the pollution from below to the upper part and then send it down, destroying the state of the "piston flow" described above, which is extremely harmful. For local purification equipment with openings to the outside world, such as clean workbenches, this air flow will lead to external polluted air flow, and the design must reduce the invalid area occupied by the filter frame, and try to make the wall of the space as close as possible to the effective superimposed air section of the filter. From the above analysis, it can be considered that there are two important prerequisites for ensuring the characteristics of unidirectional flow cleanrooms (high cleanliness and rapid self-purification recovery ability): (1) the cleanliness of the incoming flow; (2) Piston flow of incoming flow;
For the cleanliness of the incoming flow, it is not a problem for the one-way flow cleanroom that supplies air through the high-efficiency filter or above, and for the "piston flow" situation of the incoming flow, it needs to be analyzed ~ step by step.
According to the principle of fluid mechanics, the incoming flow conditions will have a direct and important impact on the future flow conditions, and the outlet air flow of the supply air surface is also the incoming flow of the work area. The turbulence of the incoming flow is large, which will affect the unidirectional flow characteristics of the following; If the incoming flow is not filled with the flow section, it will affect whether the "piston flow" can be formed later and the speed of formation; The flow is not filled with the flow section is also a factor in the flow movement. Therefore, under the condition of using high-efficiency filters, the necessary condition to ensure the characteristics of the unidirectional flow cleanroom is "piston flow", and the necessary condition of "piston flow" is to fill the flow cross-section with incoming flow. However, it is also unrealistic for a room rather than a section of duct to have the air flow completely fill the flow section from start to finish. Initially, it was indeed thought that this unidirectional parallel air flow should fill the entire cleanroom, but this was technically and economically unfavorable. With the development of understanding, it has been pointed out that this unidirectional flow is only "the meaning of all air flowing at the same speed along parallel flow lines in a limited field", and it is believed that it can greatly suppress the occurrence of vortices. The unidirectional flow characteristics of the clean air through the whole space dominate such a room, also known as a one-way flow clean room.That is to say, it is not required to have one-way parallel streamlines, uniform speed, no eddy current in the whole room, but only requires one-way parallel flow characteristics to dominate in this room, such as the one-way flow in the entire workspace floor of the space, this room is a one-way flow clean room. The high cleanliness achieved by the unidirectional flow clean room in the relevant standards and measures introduced earlier refers to the cleanliness achieved in the work area. Therefore, the densely arranged streamlined diffuser ejection and all-well plate ejection are also used as a means to achieve vertical unidirectional flow... The principle of turbulence
Clean rooms are divided according to their air flow state, mainly divided into turbulent (non-unidirectional flow) clean rooms, unidirectional flow clean rooms and radial flow clean rooms (also known as sagittal flow clean rooms), this section mainly discusses turbulent clean rooms. As pointed out earlier, no matter what kind of clean room, it is a turbulent flow state. So why aren't they all called turbulent cleanrooms?
The name of turbulence clean room is borrowed from Japanese, but the Japanese turbulence is the meaning of turbulence, and the term turbulence officially adopted in China in the "Air Clean Technical Measures" is both turbulent and different from the proper noun of fluid mechanics. Internationally, it is customary to call this clean room a non-unidirectional flow clean room. The so-called turbulent clean room generally includes the following various forms of air supply:
(1) High efficiency filter top feeding (with diffusion plate and without diffusion plate
(2) Streamlined diffuser jacking
(3) Local well plate top feeding
(4) Side delivery Clean rooms with these forms of air supply, judging by Reynolds numbers, obviously belong to turbulent flow regimes, but it is not appropriate to call them turbulent clean rooms. The air flow is sent down along the entire ceiling, the indoor speed reaches more than 0.25m/s, this indoor flow state is also turbulence, but it will be clear later, although these two air supply methods cause indoor turbulence flow, but in terms of its principle of purifying indoor air, it is fundamentally different, the latter belongs to the purification principle of unidirectional flow or parallel flow clean room. Therefore, if the former is called a turbulent cleanroom, the latter should also be included. The main feature of the turbulent clean room is that the flow section of the air flow from the incoming flow to the outflow (from the air supply port to the return air outlet) is varied, and the clean room section is much larger than the air supply port section, so it cannot form a uniform speed air flow in the whole room section or the working area section of the whole room. Therefore, the flow lines after the air supply port have a large or larger angle with each other, the radius of curvature is very small, and the air flow cannot flow in a single direction in the room, it will collide with each other, and there will be reflux and vortex. This determines that the essence of the flow state of the turbulent cleanroom is: mutation flow; Non-uniform flow. This is more accurate and comprehensive than turbulence to describe turbulent cleanrooms. The turbulence is mainly determined by the Reynolds number, that is, it is mainly affected by the flow rate, but if a high-efficiency filter is used to feed the air, even if the flow rate is very low, the above results will be produced, because it is a sudden flow and a non-uniform flow. Therefore, in this case, there is not only mixing between the flow layers due to turbulent flow, but also large reflux and vortex mixing that occur in the whole room. Therefore, to summarize, the principle of turbulence clean room is: when a clean air flow is sent into the room from the air supply port, it quickly spreads and mixes around, and at the same time discharges almost the same amount of air flow from the return air outlet, this clean air flow dilutes the indoor polluted air, dilutes the original indoor air with a high dust concentration, and reaches equilibrium. So the faster and more uniform the airflow spreads, the better the dilution effect. Therefore, the principle of turbulent cleanroom is dilution.
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Time :2022-10-17
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