2.0 Cleanroom Design High Efficiency Filters

2.0 Cleanroom Design High Efficiency Filters

Keywords : cleanroom, high efficiency filter, air supply, high efficiency air filter, air filter   

1 Introduction: This article simply analyzes the flow - resistance characteristics of high-efficiency air filters and uses examples to illustrate the author's experience with the selection of high-efficiency air filters. At the same time, the calculation of the supply air volume of the clean room system puts forward its own empirical algorithm. The author's point of view is for reference only.

   According to GB50073-2001 , “ Design Code for Clean House ”, “The air flow rate of the air filter should be less than or equal to the rated air volume. The resistance and efficiency of an efficient ( sub-efficient, ultra-high efficiency ) air filter set in the same clean area should be close to ” . At the same time, it was pointed out that “ the air volume in the clean room should be taken from the following three major values: 1.1 to ensure the air cleanliness level of the amount of air supply. 1.2 According to the heat, wet load calculation to determine the amount of air supply. 1.3 to the clean room The amount of fresh air supplied. ” From this, it can be seen that the selection of the filter and the calculation of the air supply volume ( air change times ) in the air conditioning system are of great significance.

   In the actual design process, the choice of filter should generally take into account the investment and life of the filter, the designer chooses the actual air flow of the filter is less than its own rated air flow, the specific method is in accordance with a filter A rated air flow ratio ( eg, 60%) is used to select a certain type of filter. It is considered that as long as the filter is smaller than the rated air flow, the imbalance between them can be achieved by installing a regulating valve, thus failing to check the resistance of the filter. Failure to bring the resistance of a high-efficiency air filter in the same clean area close to each other causes troubles in adjusting the air volume and affects the service life of the filter. Under normal circumstances, it is considered that the number of air changes in the clean room is larger than the air volume determined by the calculation of the heat and moisture load ( 8 to 10 times per hour for ordinary air conditioners ) , and the air is supplied from the clean room. The amount of choice is generally based on the level of cleanliness, according to the clean room to ensure different levels of use of different air changes, without checking the heat, wet load calculation to determine the amount of air supply, such calculations exist the problem is that a purification The air conditioning system can only have one air supply status point, when different levels of clean rooms ( such as 10,000 and 100,000 or even thousands of clean rooms are in one system ) or rooms with large differences in indoor load are in one system ( such as locker rooms and when the room cooling load between the difference between the major operating times or even 10 times), can not determine a correct air condition point, resulting in some cold room, while some rooms hot.

2 High efficiency filter selection calculation:

   Due to the different characteristics of the clean room, the selection of a high-efficiency filter must first be determined according to the requirements of the clean room level, the degree of sterility, temperature and humidity, fire resistance, and corrosion protection. If Class 100 or below select Class A or Class B , Grade 100 or above will require Class C filters; under high temperature and high humidity conditions, filters for metal separators and metal frames are preferred; plastic separators are preferred for corrosion protection Filters with plastic frames; with fire protection requirements, all materials of the filter should be non-flammable and so on. This article does not consider these issues, only consider the choice of the impact of conventional high-efficiency filters ( filtering ≥ 0.5μm dominated ) and stroke.

   In practice conventional filters refers to classes A and B defined in the standard HEPA filter, which is commonly used in clean room design efficiency filter, generally lower than or equal to 100,000 can be selected from the class A; 10 000 ~ 100 Grade B can be selected . Any conventional high efficiency filter, i.e., can calculate the efficiency ≥0.5μm 0.9999 microparticles 5 according to "9."

   First clear about the relationship between air flow efficiency filter by its resistance, proved high efficiency filter resistance to flow is non-linear relationship, but in the 'general amount could not be for a long time than the rated wind resistance and the air flow passing through the relationship as approximately as Linear relationship, the error is not big ' . Table 1 is a Beijing Tongchuang filter factory production efficiency filter specification sheet, FIG. 1 is produced by the plant efficiency filter resistance characteristic, it can be seen from Figure 1 the characteristic curve characteristics of different filter sizes which are different resistance We can obtain the resistance according to the corresponding resistance curve of the selected filter type . To facilitate the selection, we can treat the curve in Figure 1 and use the air flow rate of each model to remove feature size of the filter (i.e., high efficiency filter volume V = H × W × D) , and then re-fit curve of FIG. 2 as, it can be seen from the figure two trend curves overlap, as long as the feature size Choose the right one ( you can consider the resistance of different models of filter structure, because the filter's real filter area is smaller than the filter's cross-sectional area, there is an effective area ratio, and the small size of the filter effective area is relatively large, and the large model of small ) , the two curves will be more closely approximated, and the corresponding coefficients of the two quasi-harmonic equations are also not much different.