Clean room filter configuration
The filter is a versatile product, and the High efficiency filter
used is the same regardless of the grade of clean room. Once the quality of the filter is guaranteed, the ventilation parameters (number of air changes or average wind speed) are the key factors in determining the quality of the clean project. Table 4-1 shows some of the factors that should be considered when designing a clean room. The “airflow form”, “average wind speed” and “air exchange times” indicators in the table are derived from the ISO standard, and the columns “end filter efficiency” and “filter factory inspection method” are added by the author.
The “average wind speed” in Table 4-1 is only for the unidirectional flow clean room, and the average wind speed of this clean room must be pressed against the diffusion speed of the dust. The “number of air changes” in the table is for a conventional clean room with a height of about 3 m. This parameter is usually only meaningful for non-unidirectional flow and mixed flow clean rooms.
At present, the high-efficiency filters used in high-end clean rooms in foreign countries have all been scanned and tested. Due to historical reasons, some cleanrooms with low requirements still use filters tested by the traditional total efficiency method. Due to the different requirements of specific projects and the differences in design concepts, the designer will choose different ventilation times and average wind speeds. The number of air changes is small, investment is low, and operating costs are low, but there is a risk in cleanliness. In recent years, the number of air changes in domestic design clean rooms has increased. Energy-saving? Reliable? The benevolent sees the benevolence, the wise sees the wisdom, the merchant sees the business. There is a “300,000 class” in the domestic pharmaceutical industry GMP regulations. The 300,000-level environment is equivalent to the calm sea and the scenic area with excellent environmental quality. To get 300,000 grades, in most cases it is not necessary to install a terminal high-efficiency filter. It is ok to use some filters (efficiency level F8kilO) in the air conditioning system. “Improving the filter efficiency of the filter can reduce the number of air changes.” This is a terrible idea. The main source of dust in cleanrooms is people and equipment, compared to much less dust entering the chamber through high-efficiency filters. If the ventilation parameters are the same, use 99.9999% filter and 99.9r7% filter, the cleanliness of the clean room will not be much different.
The unidirectional flow clean room is also called the "laminar flow" or "uniform flow" clean room. The average wind speed of this clean room is crucial because of the free diffusion rate of dust. 15—0.2m/s, if the wind speed can't hold the dust diffusion speed, use the better filter is also no good. The non-unidirectional flow clean room is also called the "turbulent flow" clean room, and another image called the dilution type dilutiontype) clean room. The average wind speed of such a clean room is far lower than the dust diffusion speed, and the air supply only serves as a dilution. Because of the relatively large number of operators in such clean rooms, the amount of dust generated in equipment and production processes is large, so the efficiency of the filter is high. It is no longer the determinant of cleanliness, because the efficiency of the worst high-efficiency filter is also 99.9r7%, which is sufficient to meet the dust concentration requirements in the dilution chamber. If you have a good hand and are willing to buy a better filter, then it is another matter. There has been a view in China that the “U-efficient” filter (filtering efficiency of t>0.5ttm particles ≥95%) can be selected for the 10 million clean room. The reason is that the sub-high efficiency filter is cheap and the resistance is small.
As far as the price of the filter is concerned, the sub-high efficiency filter of the same size on the market is not much cheaper than the high efficiency filter. When the sub-high efficiency filter is selected, the number of air exchanges in the clean room should be significantly increased. For this reason, the number of air-conditioning systems should be increased, the number of air supply ports (and the number of filters) should be increased, and the increase in investment is far greater than that of high efficiency. The difference between the price and the sub-high efficiency filter.
In terms of resistance, the initial resistance of the clean room end filter is not so important, because no matter what kind of filter, the final resistance is set at 500-600Pa. The designer should consider that the ventilation system is still working properly when the filter approaches the final resistance, and the difference in the initial resistance is really irrelevant. In the production process, the high-efficiency filter is tested on a performance-by-stage basis, while the sub-high-efficiency filter is only subjected to a small amount of sampling. The appearance of the two may be the same, but the quality is significantly different. Using a sub-high efficiency filter in a clean room will only cost you money.