What’s the relationship between HEPA filters and folds?
1. Resistance characteristics of filter materials
At present, high efficiency air filtration materials include glass fiber filter paper, electret polypropylene, PTFE, etc. (Fan storage, 2001), among which glass fiber filter paper stable performance, reasonable price, is the mainstream high efficiency air filtration materials, and other filtration materials or expensive or unstable performance, have not been widely used. Figure 1 shows the resistance performance test results of the two filter materials selected in this experiment. It can be seen that the resistance of imported filter paper A is significantly lower than that of domestic filter paper B.
2. Effect of pleat spacing on resistance
In the case of a certain size of the HEPA filter, reduce the fold spacing of the filter material, can increase the filter material area of the filter, reduce the filter speed, reduce the resistance of air flow through the filter material. However, with the decrease of pleat spacing, the airflow channel will also become smaller, which will increase the energy loss (resistance) of airflow flow in the airway. Therefore, there is a suitable pleat spacing to minimize the total drag of the filter. For this reason, this paper tests the resistance of three sizes of flat-panel compact HEPA filters with different filter materials and different pleat spacing at 1000 m³.h-1 air volume, and the results are shown in Table 1.
It can be seen from the data in Table 1 that within the scope of this experiment, there are different optimal pleats spacing between two kinds of filter materials and three kinds of common specifications of flat-panel pleated HEPA filters. And the two kinds of filter media have the same law, with the increase of fold depth, the optimal fold spacing also increases correspondingly. The structural parameters with the lowest resistance are shown in Table 2.
The data in Table 2 are different from the structural parameters reported in the literature (Xu Xiaohao, 2005). This also shows that the filter with the same structure form should have different optimal structural parameters by using different filtering materials. Hepa filters with partitions are another common type of filter. Table 3 shows the measured results of filter resistance of two sizes made of filter material B. It can be seen from the data in Table 3 that the diaphragm HEPA filter with a depth of 150 mm has the lowest resistance when the pleat spacing is 4.8 mm(the corrugated height of the separator is 2.4 mm). For a HEPA filter with bulkhead at a depth of 292 mm, the optimal pleat spacing is 5.4 mm(corrugated height of bulkhead is 2.7 mm).
It can be seen that no matter the tightly pleated type or the high efficiency air filter with a separator, when the filter material pleated depth is determined, there is the best filter material pleated spacing. With the increase of filter fold depth, the optimal fold spacing also increases. Different filter media have different optimal structure form.
3. Effect of fold depth on resistance
In the case of no strict requirements on the depth of the filter size, increasing the depth of the filter material fold can also effectively increase the filter material area, reduce the resistance of air flow through the filter material. The increase of filter material fold depth will also lead to the increase of friction resistance in the airflow channel, therefore, there is also a most reasonable filter material fold depth of the lowest resistance.
FIG. 2 shows the resistance curve of a group of flat-panel pleated HEPA filters. High efficiency air filter end size is 610mm ×610mm, pleat spacing is 3. 3mm, using imported filter material A, depth is 50 mm, 60 mm, 69 mm, 80 mm and 90 mm, in 850m³.h-1, 1000 m³. It is clear from FIG. 2 that for a fixed pleat spacing, there is an optimal filter material pleat depth that provides the lowest filter resistance.
The resistance curve of a high efficiency air filter with a clapboard made of domestic filter material B. In the depth range of commonly used HEPA filters with partitions (120~320mm), increasing the filter depth can effectively reduce the filter resistance. When the filter depth is small (120 mm), this effect is greater; When the filter depth is larger (292 mm), the drag reduction caused by increasing the depth is not obvious.
It can be seen that for the fixed filter material pleat spacing, there should be the best filter material pleat depth that makes the filter resistance lowest. In order to reduce the filter resistance, can increase the filter material fold depth, but at the same time must consider adjusting the filter material fold spacing.
4. Influence of pleated shape on resistance
Usually, one side of the corrugated divider stands against the bottom of the filter material fold, and one side exposes the filter material fold of 5mm. Its width is 5~8mm larger than the depth of the filter material fold, forming a rectangular flow channel. By reducing the width of the corrugated separator, the bottom of the filter material fold can be formed into V-shaped shapes of different sizes through a specific manufacturing process. The two airflow channels are shown in Figure 4.
The dimensions of the HEPA filter with partition board used in this experiment are 610mm×610mm×292mm, and the corrugated height of partition board is 3.8mm. The resistance measured under the air volume of 1700m³.h-1 is shown in Figure 5.
Obviously, when the fold number and fold depth of the filter material at the same time, the V-shaped section flow channel filter, compared with the rectangular section flow channel filter, the filter media area is slightly smaller (rectangular flow channel, that is, d=0, the filter media area of the filter is 23.9 m²; d=30mm V profile flow channel filter, filter material area is 23.6 m².). But according to the measured situation of resistance, the resistance of the filter is lower. That is, the flow channel with V-shaped profile can obtain lower filter resistance with smaller filter material area. When d=15 mm, the filter media area is 23.608 m²; When d=40mm, the filter material area is 23.602 m², it can be considered that the filter area is basically the same, so the filter material resistance is basically the same, then the filter resistance difference (12Pa) is basically the structural resistance of the difference, visible, V-shaped section of the airflow channel is a lower resistance of the airflow channel form. Hepa filters with slanted corrugated plates and partitions not only increase the filtration area, but also actually adopt a better airflow channel form.