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Title: Dynamic filtration resistance of fibrous filter media used for general ventilation and clean air-conditioning
Authors: Zhang, Wanyi
Degree: Ph.D.
Issue Date: 2021
Abstract: In recent years, the pursuit of people for better indoor air quality and the cleanliness requirement for the operating environments in factories have brought explosive growth to the market of air purifiers and clean air-conditioning systems. At present, the most effective method commonly used to separate particulate matters from air is to filter air with fibrous filter media. The performance parameters of a fibrous filter medium include filtration efficiency, resistance and dust loading capacity. These parameters would change gradually with the deposition of particles into the fibrous filter medium. The growth of filters' resistance can directly lead to an increase in energy consumption and a decrease in supply air volume for a ventilation system. Consequently, the dynamic filtration resistance of fibrous filter media directly determines the dust loading capacity, energy consumption and service life of air filters. However, there has been a lack of widely accepted theoretical models for the dynamic filtration resistance of fibrous media in the open literature. It has been widely accepted that a dust loading procedure may be divided into three stages: the depth filtration, transition stage and surface filtration. However, for fibrous media used for general ventilation and cleaning air-conditioning, only the depth filtration and/or the surface filtration are usually experienced. Therefore, the research work reported in this Thesis focuses mainly on the dynamic filtration resistance models for fibrous media during depth filtration and surface filtration using both theoretical modeling and experimental approaches, so as to improve dust loading capacity for Polytetrafluoroethylene (PTFE) high efficiency particulate air (HEPA) media. In the research work reported in this Thesis, firstly, the influences of filtration velocity, air flow humidity and aerosol concentration on the dynamic resistance of fibrous filter media was studied experimentally. The experimental results showed that, in the filtration velocity range of 2~10 cm/s which was commonly used for general ventilation, the influence of an increase in filtration velocity on the dynamic resistance of fibrous filter media was not significant. However, an increase over 5% in relative humidity (RH) of passing air flow would lead to a significant decrease in resistance in a short time period when the media was loaded with polydisperse KCl aerosols. The resistance growth trend for a medium loaded with KCl aerosols at an air RH close to the deliquescence point of the KCl aerosol was similar to that loaded with a liquid aerosol. But polydisperse SiO2 aerosols loaded into the media were not sensitive to the changes in air flow RH. In addition, the aerosol concentration had no significant influence on the dynamic resistance of a filter medium at the same filtration velocity. Secondly, a layered resistance model by incorporating the mass concentration distribution of deposited particles into Bergman model was proposed to predict the depth filtration resistance for fibrous media. Experiments for five-stacked glass fiber media loaded with four monodisperse SiO2 aerosols and polydisperse SiO2 aerosols were conducted to investigate the distribution of deposited particles. The resistance growths obtained respectively from experiments and model prediction showed good agreement when the modification of critical loaded dust mass was applied. To theoretically evaluate the distribution of deposited particles inside a loaded medium, the D-model was established by dividing a filter medium into unit layers and applying the definition of single fiber efficiency to calculating the mass of particles captured by each layer. The variation trends in loaded dust mass predicted using the numerical solutions for the D-model agreed well with those obtained from the dust loading experiments.
Thirdly, dust cake resistance models for both monodisperse aerosols and polydisperse aerosols were respectively developed by calculating the shielding effect of particles based on the kinetic theory. To monitor the changes in thickness and porosity for dust cakes during their formation, a laser scanning porosity measuring system was established and its accuracy and reproducibility demonstrated. When using this system to monitor the formation process of a dust cake, it was found that the thickness growth rate for a dust cake was gradually slowed down with an increase in loaded dust mass and finally leveled so that the dust cake porosity was decreased linearly. The results of dust loading experiments for PTFE membranes using monodisperse SiO2 with different geometric mean diameters and polydisperse SiO2 with different particle size distributions suggested that, for monodisperse aerosols, the smaller the average diameter was, the faster the filtration resistance on the surface of PTFE films grew, and the dust layer porosity was higher under the same resistance growth. For polydisperse aerosols, the rate of increase in filtration resistance of the PTFE membrane and the porosities of dust cakes were decreased significantly with an increase in geometric standard deviation. Finally, based on the above three parts of completed work, an experimental evaluation method for dust loading performances for HEPA filter media was developed from three aspects: the applicability of experimental aerosol in terms of particle size and humidity sensitivity, experimental setups and procedures, and the evaluation method of energy consumption. This method was used to compare the dust loading performances when using an H14 PTFE medium with those when using an H14 glass fiber medium. Scanning electronic microscope (SEM) photos were taken after the two media were loaded to having a resistance of 900 Pa at 5.3 cm/s. The comparison results showed that although the PTFE medium had a 50% lower initial resistance, its resistance growth rate was far greater than that for the glass fiber medium. To remove the application limit of PTFE media due to their poorer dust loading performances, composite PTFE media were developed by replacing the windward base materials of PTFE media. Three electret filter media of three different filtration efficiencies were used as the windward materials to make composite PTFE media. The average resistances during dust loading for these composite PTFE media were experimentally obtained and compared. Results show that the dust loading performances of the above three PTFE media were significantly improved after compositing, all being better than that of a glass fiber HEPA medium. Among them, "PTFE + medium efficiency electret" had the best energy consumption performance when using an eligible experimental aerosol.
Subjects: Filters and filtration
Air conditioning -- Equipment and supplies
Ventilation -- Equipment and supplies
Hong Kong Polytechnic University -- Dissertations
Pages: xxix, 242 pages : color illustrations
Appears in Collections:Thesis

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