Structural Design of HEPA Filter
With a certain amount of filtration efficiency with rated flow, the structural design of HEPA filter at present is how to obtain the minimum pressure drop with the corrugation angle (for the type with isolator), the corrugation height (for the type with i
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Structural Design of HEPA Filter
With a certain amount of filtration efficiency with rated flow, the structural design of HEPA filter at present is how to obtain the minimum pressure drop with the corrugation angle (for the type with isolator), the corrugation height (for the type with isolator) or the line height (for the type without isolator), and the passage depth (for both types).
5.1
Flow State in the Passage of HEPA Filter
For the flow state in the passage (inlet and outlet) of ideal HEPA filter with isolator as shown in Fig. 5.1, Cheng Daiyun obtained the calculation result about the flow distribution in the passage on the basis of Bernoulli equation and modified momentum conservation equation, which is shown in Fig. 5.2 and Table 5.1 [1]. The conclusion obtained is that under the condition of low initial velocity, high corrugation, and high pressure drop, the flow in the passage is close to laminar flow with linear distribution of velocity (Re < 2,000), i.e., x v1 v0 1 L v2 v0
x L
(5.1) (5.2)
Otherwise, the nonlinear distribution will be obvious. Experiment with Laser Doppler velocimeter has proved the above conclusion, which is shown in Fig. 5.3 [1]. In terms of outlet passage, the linear correlation coefficient of velocity distribution is more than 0.99.
Z. Xu, Fundamentals of Air Cleaning Technology and Its Application in Cleanrooms, DOI 10.1007/978-3-642-39374-7_5, © Springer-Verlag Berlin Heidelberg 2014
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5 Structural Design of HEPA Filter
Fig. 5.1 Model for passage of air filter
Fig. 5.2 Relationship between the velocity and distance in the passage
Table 5.1 Calculation result about the velocity distribution in passage of air filter with low initial velocity (m/s) and high corrugation (several millimeters)
x/L 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
v1/v0 0.899 0.800 0.701 0.603 0.505 0.407 0.308 0.207 0.105
5.2 Total Pressure Drop of HEPA Filter
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Fig. 5.3 Measured axial distribution of velocity along the outlet passage
5.2
Total Pressure Drop of HEPA Filter
According to Chap. 4, the total pressure drop of air filter can be expressed as: ΔP ¼ΔP1 þ ΔP2ð1Þ þ ΔP2ð2Þ þ C C ¼ ΔP3 þ ΔP4
(5.3)
where ΔP1 is the pressure drop of filter media; ΔP2(1) is the frictional resistance of inlet passage; ΔP2(2) is the frictional resistance of outlet passage; C is the local resistance of both inlet and outlet passages; ΔP3 is the local resistance of inlet passage; ΔP4 is the local resistance of outlet passage. The core purpose of structural design is to calculate the pressure drop. Foreign scholar has proposed other kind of the expression for the total pressure drop [2]. It did not reflect the influence of the flow rate, so the meaning is not clear. But in Chap. 4 we know that ΔP2 is related to the n power of the velocity, where the exponent n should be determined by experiment. It reflects the property, but it is not convenient for calculation. From the above theoretical and experimental analysis for the flow state in the air passage, the flow velocity is approximately linear.
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