Air distribution in non-unidirectional flow clean rooms and its main forms:
1. Basic Principles of Non-Unidirectional Flow Clean Room
Non-unidirectional flow clean room continuously dilutes indoor air by air supply, and gradually discharges indoor pollutants to achieve balance. In order to ensure a good dilution effect, the most important thing is that the indoor air flow diffuses as fast as possible. When a clean air flow is sent into the room from the air supply port, it can diffuse and mix rapidly around, and the air flow is discharged from the indoor air return port. By using the mixed dilution function of the clean air flow, the air with high dust concentration in the room is diluted, so that the pollutants produced by the indoor pollution source can be evenly diffused and discharged outdoors in time, thus reducing the indoor pollution. The concentration of dust in the air makes the indoor cleanliness meet the requirements.
2. Characteristic index of non-unidirectional flow clean room
Similar to the one-way flow clean room, the performance of the non-one-way flow clean room can also be reflected by the performance index of the non-one-way flow clean room. There are three main characteristics of non-unidirectional flow clean rooms:
(1) Frequency of ventilation
The function of ventilation frequency is to ensure that there is enough clean air to dilute. Frequency of ventilation = clean room air supply/clean room volume
(2) Air distribution
The function of air distribution is to ensure uniform air supply and return, and to give full play to the dilution of clean air. Therefore, a single air outlet is required to have sufficient diffusion effect; the whole clean room air outlet layout should be uniform, as far as possible to increase the number of air outlets, in order to reduce eddy current and airflow cycles. The air distribution in non-unidirectional flow clean room is analyzed by measuring the flow field streamline, and there is no quantitative standard. Non-unidirectional flow cleanroom can only achieve the cleanliness below ISO level 6 (1000), in order to achieve ISO level 5 (100) or higher cleanliness, it is necessary to use unidirectional flow cleanroom.
(3) Self-purification time
The self-cleaning time of non-unidirectional flow clean room refers to the time required for the room to be reduced from a certain pollution state to a certain clean state. The self-cleaning time of non-unidirectional flow clean room reflects the ability of clean room to recover from contaminated state to normal state, so the shorter the self-cleaning time, the better. The self-cleaning time of non-unidirectional flow clean rooms is generally not more than 30 minutes.
3. Main Forms of Air Distribution in Non-Unidirectional Flow Clean Room
Non-unidirectional flow clean rooms can be divided into different air distribution forms according to the structure and location of air supply and return air outlets. The main air distribution forms and characteristics of common non-unidirectional flow clean rooms are as follows:
(1) Top feeding of diffuser. Its form is simple and suitable for small area clean room.
(2) top feeding with diffusive tuyere filter. It enlarges the scope of the clean air flow in the top feeding of the filter, and has one more chance of dust accumulation at the outlet than that in the top feeding of the filter. After long-term non-operation, the diffusive air outlet should be wiped clean before running.
(3) Side delivery of tuyere. This air distribution is simpler than the top feeding of the filter, and is especially suitable for rooms without roof space. The air duct is located in the roof of the corridor, which is suitable for the clean room with general air conditioning requirements.
(4) Oblique tuyere delivery. Used for room length insufficient side delivery or not allow reflux occasions.
In addition to the above non-unidirectional flow air distribution, there are also local orifice top-feeding air distribution, local damper top-feeding air distribution and double-wall return top-feeding air distribution.
The air distribution of non-unidirectional flow clean room is most typical in top-feeding and side-down return air. The air outlet of the top feeding high efficiency air filter with orifice plate diffuser is helpful for the diffusion of clean air into the clean room. Under the air outlet of the high efficiency air filter, it is located in the center of the so-called main air supply area, and its cleanliness is generally significantly higher than that of the surrounding area. The so-called surrounding area refers to the part of the space covered by the contaminated air and the expanding cross-section of the airflow continuously after the air supply enters the room. The cleanliness of the areas where the air supply air does not cover between the adjacent air outlets and the corner of the room will be lower. There are different requirements for cleanliness in different areas of the clean room, so the first two kinds of flow patterns are often combined together. The high demand parts are unidirectional flow, while the other parts of the room are non-unidirectional flow, which is called mixed flow pattern.
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