Manuscript number: INA-22-05-271
Title: HEPA filters for airliner cabins: State of the art and future development by Zhang et al.
Comments:
This study experimentally evaluated the efficiency and dust holding capacity (DHC) of 23 HEPA filters used in different commercial airliners. The results showed that the efficiencies for 0.3-0.5 m particles, pressure drops and DHCs of these HEPA filters ranged 98.405-99.998%, 133.6-412.0 Pa and 32.2-37.0 g/m2, respectively. The authors analyzed and discussed the effects of pleat designs and filter geometries on the filter performances. Besides, the feasibility, in terms of lower pressure drop and high efficiency, of using nanofiber media in the airliner HEPA filters was discussed. The paper is easy to follow and the topic should be of interests to the readers of Indoor Air. However, there are a number of scientific issues to be clarified to improve the current paper.

Major comments
1. The airliner HEPA filters have different geometries and how were they installed and tested in the standard wind tunnel, especially for the prism and cylinder filters?
2. The major challenge in the evaluation of HEPA filter in a standard test rig is that a sufficient high concentration of challenge particles is needed to determine the high efficiency (low concentration downstream the filter) of the HEPA filters. It is required to show the details of the particle generator and particle counter. It is desirable to show 1-2 cases of measured upstream and downstream particle concentration of the filter and how to calculate the efficiency. Based on the knowledge of the reviewer, the current generator and particle counter are not suitable and not qualified to determine the high efficiency of HEPA filters. More specifically, the concentration limit of the TSI 9306V2 is too low (~200 #/c.c.) to determine the efficiency of HEPA filters.
3. The use of A2 fine dusts is not representative of PM10 as they have a size distribution from submicron to ~100 micrometer. The growth rate of pressure drop with loading A2 dusts would be much lower than that of PM10 particles.
4. Neutralization was not applied for the challenge particles, which would lead to an overprediction of the filter efficiency.
5. The use of data of nanofibers from literature to compare with that of glass fiber in HEPA filters is not suitable. First, the specifications of the nanofiber and glass fiber, including thickness, basic weight, porosity, etc., and the efficiency evaluation methods amongst the literature are different. The authors can compare them only based on the two media have the same bases and/or conduct by the same laboratory. Nanofibers usually need coarse fiber layer as the substrate, but the slip effect is much reduced after adding the coarse fiber layer.

Minor comments

1. Abstract: The determination of DHC for flat sheet filter media is straightforward. Was the measured DHC of 32.2-37.0 g/m2 in this study based on flat sheet media or superficial area of the filter box? As mentioned earlier, the description of nanofibers may be reexamined and rewritten.
2. Method 2.1: For the same filter media, filtration efficiency increases and pressure drop reduces with reducing face velocity which is the results of flow rate and media area. The comparison shown in Table 1 could not directly extract which filter media is the best. The authors stated the V-shape filters allow to contain more filter media (area) in each filter, however, again due to various different parameters amongst these filters, it’s not easy to conclude a generalized performance for these filters.
3. Method 2.2, line 146, more details for the experimental system, including the make and model of the instruments. How the filters with different geometries are clamped in the wind tunnel?
4. Method 2.2, line 174, the weighing was for whole filter or filter media?
5. Table 2, the major filtration mechanism by nanofiber is interception, thus causing surface filtration. They would not be good candidates to have high DHC, thus not applicable for a HEPA filter with good DHC.
6. Fig. 3, should be five ranges of applicable flow rates?
7. Results 3.1.1, line 285, too many different parameters amongst the 23 HEPA filters leading to unclear comparisons. Line 291, pleat density of 152-231 pleats/m was regarded to be low by the authors. Could the authors provide a classification and justification? Lines 305-306, sentence is not clear. Lines 308-310, the optimal design of pleat density has been published (Chen et al., 1995, Aerosol Sci. Technol. 23, 579–590.). Line 315, why?
8. Results 3.4.2, line 437, is 1000 Pa usually used as the final pressure drop?