Determination of key parameters (air exchange rate, penetration factor and deposition rate) for selecting residential air cleaners under different window airtightness levels

Highlights

• A method to calculate P, k and a by PM_2.5 mass concentration was proposed.

• Recommended values of P, k and a for air cleaner design and selection were developed.

• Relationship between air cleaner effective area and CADR was quantified and discussed.

Abstract

Healthy indoor air quality is a basic standard for good living environment, and air cleaners are commonly used in residential applications, especially in China, to control indoor PM_2.5 pollutions from outdoors. An accurate cleaner selection method will help keep satisfying indoor air quality, as well as reducing cost and materials of equipment. Three key parameters, namely, air exchange rate (a), particle penetration factor (P) and deposition rate (k), have been suggested as direct influence on air cleaner selection when pollutants are coming from building infiltration. In relevant standards, however, there is no method that can link the three key parameters to various airtightness levels of external windows, which may often exist in real applications. This study, therefore, has proposed a calculation method for deciding a, P and k, based on monitored indoor and outdoor PM_2.5 mass concentrations, with given recommended design values under different external window airtightness levels. Results showed that both air exchange rates and penetration factors were significantly influenced by the airtightness level of windows, and both their values were found to be different from the recommended values in the current Chinese standard, providing evidence to support future standard revision and update. Meanwhile, deposition rates showed good agreement with the recommended value in the current Chinese standard (i.e. 0.2h⁻¹). Additionally, with increased window airtightness level, the required clean air delivery rate (CADR) of air cleaners showed a downward trend, which means an air cleaner with smaller size, lower energy consumption and less material.

Introduction

In current society, people are spending over 90 % time inside buildings. Therefore, providing a healthy indoor air quality is essential for developing livable and sustainable buildings (Martins & Carrilho da Graça, 2018; Salonen et al., 2013; Sharmin et al., 2014). In the past few decades, China has experienced a rapid development in economy and urbanization. However, the issue of atmosphere pollution has also become serious, and the impact of fine particulate matter (PM_2.5) on people’s health conditions has captured great attention of researchers (Chan & Yao, 2008; Chatzidiakou, Mumovic, & Summerfield, 2012; Kong et al., 2020; Roelofsen, 2018). PM_2.5 refers to those airborne particulate matters with aerodynamic diameters smaller than 2.5 μm, which is inhalable by human to affect their health (Fang et al., 2020; Pateraki et al., 2020; Velasco & Rastan, 2015).

Under serious outdoor PM_2.5 pollution conditions, people usually choose to close all external windows to prevent outdoor PM_2.5 going into indoors. However, much evidence has proven that outdoor PM_2.5 can still penetrate into indoors by infiltration through existing cracks on building facades, especially around windows (Liu, Liu, Zhang, & Guan, 2019; Stapleton & Ruiz-Rudolph, 2018; Tran, Alleman, Coddeville, & Galloo, 2017). To obtain healthy indoor air quality, air cleaners are being widely used in China to tackle high level of PM_2.5 pollution outdoors (Deng et al., 2017; Liu et al., 2017; Zhang et al., 2011; Zhao, Wang, Yin, Yu, & Ding, 2016). Existing studies have revealed that when all external windows are closed, the removal capacity of air cleaners to PM_2.5 is greatly influenced by its clean air delivery rate (CADR), room dimensions (such as the height and area), room air exchange rate (a), kinetic characteristics of PM_2.5 (penetration factor (P) and deposition rate (k)) and outdoor PM_2.5 pollution level (Jin, Yang, Du, & Yang, 2016; Shaughnessy & Sextro, 2006; Xu, 2019). Namely, all these parameters are needed when selecting appropriate air cleaners for specific applications.

To guide design and selection of air cleaners, the American Household Appliance Manufacturers Association (AHAM) has proposed a standard, namely, ANSI/AHAM AC-1-2006. In this standard, the relationship between air cleaners’ effective area and CADR has been established, with other parameters described as influential factors. In this relationship, the room height is dependent on the building under investigation and the outdoor PM_2.5 mass concentration is mainly determined by local atmospheric pollution level. The remaining three parameters, i.e. P, k and a, were often set as constant values for all buildings. For example, under natural ventilation conditions, a = 1.0h⁻¹, P = 1.0 and k = 0.2h⁻¹, which were recommended by ANSI/AHAM AC-1-2006. The Chinese standard, GB/T 18801-2015, was developed in 2015, according to the American standard ANSI/AHAM AC-1-2006. For buildings in China (mostly under infiltration condition), the three parameters were recommended as a = 0.6h⁻¹, P = 0.8 and k = 0.2h⁻¹. This method of using constant values for these three parameters has been adopted by standards of other countries as well, such as Canada (NRCC-54013) and Japan (JEM 1467-2013).

In real buildings, however, most of these parameters were influenced significantly by the airtightness level of external windows (Lai, 2003; Provan & Younger, 1986; Wallace, 1996; Younes, Abishdid, & Bitsuamlak, 2012). For a, many studies have justified that it is indirectly proportion to the airtightness level of windows (Cui, Cohen, Stabat, & Marchio, 2015; Deng, Zhuangbo, Fang, & Cao, 2018; Kiwan et al., 2013; Montoya, Pastor, & Planas, 2011; Younes et al., 2012), as well as P (Chen, Zhao, Zhou, Jiang, & Tan, 2012; Li & Chen, 2003; Tian et al., 2009). For k, however, sufficient evidence is available showing that there is no significant impact from airtightness level of windows (El Hamdani, Limam, Abadie, & Bendou, 2008; Lai, 2003; Zhao & Wu, 2007). Therefore, it seems to be reasonable to use a constant value defined in the standard for k when selecting air cleaners. However, if the same method is also used for a and P, the selected air cleaner may not be appropriate for buildings with different airtightness levels of external windows.

To quantifiable identify the impact of this assumption, this paper has introduced a study calculated P, a, as well as k (to decide a practical constant value for k) by indoor and outdoor PM_2.5 mass concentrations, under different airtightness levels of external windows. In the study, both indoor and outdoor PM_2.5 mass concentrations from five rooms with different airtightness levels of external windows have been monitored for calculation. Based on the quantified impact, recommended values were provided to guide selection of air cleaners more specifically to the application.