Development of composite filters with high efficiency, low pressure drop, and high holding capacity PM_2.5 filtration

Highlights:

• A composite media (CM) has high efficiency and holding capacity but low pressure drop.

• CM required 8 and 3 times less power than HEPA glass fiber and PTFE to load 2 g m⁻² PM_2.5.

• Theory shows the successively loss of charge of CM led to a uniform deposition of PM_2.5.

• Current CM is a perfect depth filtration filter and should be applied to HVAC system.

Abstract

Many efforts are being made to develop filters with high efficiency and high holding capacity but remaining a low pressure drop. A two-layer composite filter to achieve the goal was developed, in which the first layer was a charged coarse fibers to provide large void space for particle loading and the second was a thin layer of charged melt-blown with finer fibers to enhance the overall efficiency. Experimental results showed that although the new composite media had a lower initial efficiency than the other two HEPA filters (PTFE and glass fiber filter), its figure of merit (FOM) was the highest. Besides, the composite media had a better holding capacity for PM_2.5 than the other two. At a fixed mass load, i.e., 2 g m⁻², the PTFE (ΔP = 380 Pa) and glass fiber (ΔP = 165 Pa) required around 7.6 and 3.3 times more power, respectively, than the composite media (ΔP = 50 Pa). Due to the low charge level of the coarse fiber layer and the fine fiber diameter of the melt-blown layer, resulted in no efficiency reduction along the loading process. Theoretical analysis showed that the charge shielding and the loss of efficiency in the successive top-down layers were timely compensated by the efficiency enhancement caused by the loading effects, which made the composite media a much uniform deposition of PM_2.5 in layers. This was the main reason resulting in the high holding capacity and low pressure drop of the current composite media which acted like a perfect depth filtration media.

Introduction

Many large cities, e.g. Beijing and Shijiazhuang of China, Delhi, Mumbai and Kanpur of India, are being exposed to extremely high PM_2.5 concentrations [1], [2]. The high concentration PM_2.5 can transport into indoor environment through infiltration or air exchange from windows or ventilation system. Providing clean air for indoor environments by filtration through HVAC systems and indoor air cleaners (IAC) is in high demand [3]. Although simply increasing filter thickness and solidity can enhance the efficiency, it will lead to an increase of pressure drop and energy consumption [4]. There has been a lot of effort made to develop filters having low pressure drop (low energy consumption) while remaining high efficiency and high holding capacity.

Particles loaded on the superficial surface of filter or inside the filter will also contribute to the pressure drop as they acted as flow obstacles. Roth et al. [5] found that a dirty filter can account for up to 20% of system pressure drop in commercial building HVAC systems. Nassif [6] modeled the effects of the pressure drop of dirty filters in a constant-speed fan HVAC system and reported that the fan and air-conditioning system need to run longer to meet the same cooling load, resulting in an annual increase in fan energy consumption and cooling energy use of up to 5% and 9%, respectively. The total energy for the fan and cooling in the commercial building HVAC in the US was estimated to be 1.3% and 2.5%, respectively, of the total primary energy consumption [7]. Therefore, this amount of extra energy use is non-negligible and efforts should be made to improve the filter design to save energy. Realizing the importance of sustainability, ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) as well as EUROVENT (Europe’s Industry Association for Indoor Climate, Process Cooling, and Food Cold Chain Technologies) have made a big push to develop classifications of energy efficiency for air filters [8], [9]. However, in the current methods, relatively coarse dusts were used in the filter loading/aging tests [10], where energy consumption could be much underestimated compared to loading with fine particles. It was found that a threefold holding capacity could be achieved when loading with monodisperse 0.6 μm particles compared with 0.2 μm particles when the pressure drop was increased from 0.8 to 2 in-H₂O [11]. To understand the loading characteristics of using more realistic polydisperse fine particles, particles mimicking PM_2.5 size distribution (mass medium aerodynamic diameter, MMAD ∼ 0.3–0.7 μm [12], [13] were produced to load several HVAC filters [3]. Results confirmed that a lower holding capacity was seen in the fine particle loading than those with coarse particles. To be mentioned, Tang et al. [3] was one of the few researches showing filter loading performance against PM_2.5 distribution. A similar result was also reported by Hinds and Kadrichu [14]. They found that loading glass fiber filters with fine NaCl (MMAD = 0.81 μm) increased the air resistance much faster than that of coarser aluminium oxide (MMAD = 7.5 μm) and Arizona dusts (MMAD = 2.8 μm). In addition, Wang et al. [15] was the first discovered that there existed a relationship between the pressure drop growth curves and dust concentration in the loading. The dusts tested were A1 and A3 grades (Powder Technology Inc., Burnsville, MN, USA), regarded as micron to super-micron particles. They found the pressure drop growth decreases as the dust concentration increases. They also developed empirical equations to describe the loading curves.

In the above, the effects of particle size on loading performance were discussed. However, when focusing on fibrous media, there still remains many critical factors which can influence the pressure drop growth during the loading, including filter structure, packing density, fiber diameter, fiber charge, etc. According to Brown [11], the key points to achieve the high efficiency and high holding capacity include: apply charges to the fibers; particle deposition patterns favor a lower increase of pressure drop; formation of dendrites is minimized and delayed or their collapse rates are increased; and the filters perform a depth filtration. For example, the media with an open structure (or low packing fraction) was found to have a threefold increase of holding capacity when reducing the packing density from 0.32 to 0.07 [11]. Besides, Sae-Lim et al. [16] conducted a theoretical calculation and found that a media with an increasing packing density progressively towards the outlet side could have a quadruple service life than the media with uniform packing. However, no experimental data were available to validate and support the theoretical results. On the aspect of fiber diameter, at a constant packing density, the finer fiber normally results in a higher layer efficiency and the particles are collected in a small volume in the leading layers, causing a greater tendency to clog the upper layers of the filter. One extreme example is PTFE filter which is normally made with very thin thickness but due to the very small pore, it can capture most of incoming particles on its surface, namely surface filtration. For coarse dusts collection, PTFE filter could perform perfectly as the collected coarse dusts form dust cake on the surface quickly and the cake will not contribute too much pressure drop due to the large size of dusts and larger void space formed within the cake [11]. However, cake formed by fine ambient particles is anticipated to create much higher pressure drop thus it is meaningful to investigate the holding capacity of PTFE for PM_2.5 to clarify this issue. In comparison, when the pore size of the filter media is larger, the incoming particles can penetrate and get collected throughout the medium. This is called depth filtration. Additionally, filter media with pleated shape can dramatically decrease the pressure drop compared with flat filter when loaded with same mass of dust particles [17].

In addition to the arrangement of packing density and packing structure to increase holding capacity, electret filter media, where charges added to the fibres increases the filtration efficiency without increasing pressure drop, are especially suited and being widely applied in HVAC and IAC to save energy [4], [18]. However, the significant efficiency reduction during the loading process due to the shielding of fibre charge is much concerned. In the authors’ previous work [3], a composite media consisting an electret layer on the top followed by a mechanical nanofiber layer was developed and found to have not only a reduced efficiency reduction but also low pressure drop along the loading. However, whether the composite media having a better holding capacity than other traditional media, e.g., HEPA filters made of PTFE and glass fiber, remains unknown. Besides, it is very meaningful to modify the composite media to see if the reduction efficiency during the loading could be further minimized.

In this study, the initial size fractional penetration for particles ranging 30–500 nm of a modified composite media based on Tang et al. [3] was investigated and compared with that of the traditional PTFE and glass fiber HEPA media. The figure of merit (FOM), used to determine the filter performance were investigated to confirm the advantage of the new composite media. For the loading tests, systematic experiments with different face velocities and loading concentrations of mimicked PM_2.5 were conducted for all three media to compare their performances and differences. Besides, depositions mechanisms were analyzed fundamentally by single fiber and single layer theory to understand the loading characteristics. The final goal is to apply the findings to provide an optimized composite media to achieve energy efficiency and long service life of PM_2.5 removal for indoor air purification applications.