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MERV Filter Models for Aerobiological Applications
Abstract and Figures
In this article a modified single fiber model of filtration is described and used to generate filter performance curves that can be fit to MERV data in the 0.3-10.0 micron size range and that can be extended down to the size range of viruses. Coupled with the summary of logmean diameters of airborne microorganism included here, these models will enable estimation of filtration rates for viruses and bacteria.
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... Using a filter efficiency per size-class as per the simple and useful definition in [17], perform the filter clogging computation done in Section 2 for a polydisperse aerosol (you will have to choose a MERV filter class). What do you observe as far as the indoor distribution is concerned? ...
... measurements), enabling the use of a continuous function between known points. Such procedures were used for the particle size dependency of filter efficiency in [17] or to fit the pressure drop and efficiency functions used in Section 2.1 of Chapter 2. ...
... 17 shows the results obtained. On the left, the water height in the tank is plotted: after an overshoot in the vicinity of ∼ 150 [s] followed by a few oscillations, it stabilises at the set value of 1 [m]. ...
On one end of the scientific literature, profusion of works about solving partial differential equations exist, however often with a somewhat unfamiliar mathematical formalism. On the other end, volumes dealing with building physics may be either technical about HVAC or rather generic about the equations to be used.
Having struggled quite a bit in the past years with the practical implementation of numerical methods in this field, it appeared that putting together the recipes used in a modern programming language could be of interest.
Hence the parti pris in this book is to show the link between the governing equations and how to solve them, aiming at a practical use (i.e. ”how to make things work”). It can be seen as a toolbox for simulation engineering, a basis for the illustration of theory or a kick-start for the study of more complex problems.
This manual is composed of three chapters, with gradual increase in difficulty:
• Chapter 1 succinctly explains the fundamentals of the numerical methods used.
• Chapter 2 shows applications of these methods to heat transfer in phase change materials, PID control, indoor air quality and geothermal heat pumps.
• Chapter 3 deals with coupled problems and minimisation. Applications to polydispersed aerosols in enclosures, heat and mass transfer in walls and parameter fitting for transient problems are proposed.
... The filter efficiency is determined by imposing the filter efficiency curve over the particle size distribution (Lee and Liu, 2012) (Kowalski and Bahnfleth, 2002). The example in Figure 4 uses a normal distribution to the sizes of influenza virus bearing particles measured in airplanes, daycare centers and health centers (Yang et al., 2018). ...
From pandemic to seasonal, the COVID-19 pandemic experience suggests many common respiratory infections rather than likely having a fomite etiology as previously thought, are primarily caused by the inhalation of infectious aerosols shed by ill persons during coughing and normal breathing and talking. Given this new understanding, the good news is that, unlike indoor-sourced noxious and irritating gases that can only be mitigated practically by diluting them with outdoor air ventilation, the indoor infectious aerosol illness transmission route can be addressed by circulating already conditioned air through commonplace commercial filters. Given that infectious aerosols released from the breath of occupants were practically an unknown vector of respiratory disease in the healthcare community for many decades, understandably HVAC regulations have not addressed this issue yet. However, this is about to change. To further this new end, this paper develops the formulae needed to set conditioned air recirculation rates through such filters for design infectious aerosol emission and inhalation rates, HID values, exposure times and occupancies, and target significantly lower than currently normal airborne infection reproduction rates. The analysis extends the equations previously developed for group inhalation of infectious aerosols to develop equations predicting the number of infections likely to occur from this inhalation and the rate of disease spread (reproduction). The governing equations provided and exemplified use group exposures since the number of infections (reproduction number) is group based. Examples using the equations provided are given for many different settings and two case study findings are compared with their predictions. Some settings such as the typical office are shown to already have a relatively low infection reproduction rate. Alternatively, others such as a typical school classroom or a longer commercial air flight require increased filtered ventilation air flows to yield a similarly low reproduction rate. The formulae and their application will be of interest to government and industry health and HVAC standard setting bodies.
... Filtration has a crucial role in controlling air quality and indoor safety, from outside air pollution to indoor airborne contaminants, like aerosols, especially when dealing with recirculation systems between spaces (Kowalski and Bahnfleth, 2002). The SARS-CoV-2 has a diameter of 100nm, becoming suitable for rapid airborne spread, but also stopped by efficient sub-HEPA filters . ...
The COVID-19 pandemic, through governmental stay-at-home orders, forced rapid changes to social human behavior and interrelations, targeting the work environments to protect workers and users. Rapidly, global organizations, US associations, and professionals stepped in to mitigate the virus's spread in buildings' living and work environments. The institutions proposed new air system HVAC settings without efficiency concerns, such as improved flow rates and filtering for irradiation, humidity, and temperature. Current literature consensually predicted an increase in energy consumption due to new measures to control the SARS-CoV-2 spread. The research team assumed the effort of validating the prior published outcomes, applied to US standardized high-rise office buildings, as defined and set by the key entities in the field, by resorting to a methodology based on software energy analysis. The study compares a standard high-rise office building energy consumption, and CO2 emissions and operations costs in nine US climate zones — from 0 to 8, south to north latitudes, respectively —, assessed in specifically the most populated cities, between the previous and post COVID-19 scenarios. The outcomes clarify the gathered knowledge, explaining that climate zones above mixed-humid type (4) tend to increase relative energy use intensity by 21.72%, but below that threshold the zones decrease relative energy use intensity by 11.92%.
... a Baseline values represent conditions for a typical mid-rise multifamily building built between 1960 and 2010 in Boston, MA with greater than average leakage b In CONTAM, the MERV 4, 8, and 12 filter ratings were associated with 10, 66, and 97.5% removal efficiencies of PM 2.5 , respectively, based on research from Kowalski et al.[66] c Local cooking exhaust assumed to operate for entire duration of all cooking events cooking exhaust settings: on/off). The Boston University Shared Computing Cluster (a heterogeneous Linux cluster) was used to minimize simulation time by running multiple cases in parallel. ...
While residential energy and ventilation standards aim to improve the energy performance and indoor air quality (IAQ) of homes, their combined impact across diverse residential activities and housing environments has not been well-established. This study demonstrates the insights that a recently-developed, freely-available coupled IAQ-energy modeling platform can provide regarding the energy and IAQ trade-offs of weatherization (i.e., sealing and insulation) and ventilation retrofits in multifamily housing across varied indoor occupant activity and mechanical ventilation scenarios in Boston, MA. Overall, it was found that combined weatherization and improved ventilation recommended by design standards could lead to both energy savings and IAQ-related benefits; however, ventilation standards may not be sufficient to protect against IAQ disbenefits for residents exposed to strong indoor sources (e.g., heavy cooking or smoking) and could lead to net increases in energy costs (e.g., due to the addition of continuous outdoor air ventilation). The modeling platform employed in this study is flexible and can be applied to a wide range of building typologies, retrofits, climates, and indoor occupant activities; therefore, it stands as a valuable tool for identifying cost-effective interventions that meet both energy efficiency and ventilation standards and improve IAQ across diverse housing populations.
... The pump was sized for 60 ft hd (179.3 kPa) at design flow. Filter single-pass efficiency was set at 0.40, an approximate value for a MERV 8 filter assuming a 1µm particle size (Kowalski and Bahnfleth 2002). ...
Many microbes and viruses have been identified that cause disease transmission through an airborne or surface‐associated route. Prominent examples of diseases that are transmitted by these routes include influenza, the measles, tuberculosis, the common cold, and COVID‐19. A common feature among these diseases is that the etiologic agents that are responsible for their transmission are highly susceptible to inactivation by exposure to UV‐C radiation. UV‐based systems have been demonstrated to be effective for control of these pathogens and the diseases they cause. Many configurations of systems for disinfection of air can be found, including upper‐room, lower‐room, in‐duct, and in‐room mobile devices. Implementation of these systems requires that human exposure be controlled to below well‐defined safety limits, while the air or surface is subjected to a relevant UV‐C dose. Far UV‐C radiation has been shown to be effective for inactivation of airborne pathogens, while demonstrating far less potential for damage to human skin and eye tissues than conventional UV‐C radiation; this combination of characteristics offers the potential for development of UV‐based germicidal applications in occupied spaces. Safety aspects of UV‐C systems are reviewed in detail, including current and proposed guidance for human exposure. Ozone formation is another health‐related concern for these systems; the physics and chemistry involved in photochemical formation of ozone are also presented. Tools that are used for quantification of UV dose–response behavior among aerosolized pathogens are reviewed, along with genomic models that are used to predict UV dose–response behavior for these pathogens. Testing and validation methods are also reviewed. Models that are used to simulate the dynamics of indoor air quality (IAQ) are developed and illustrated. Also included is information to address the application of UV‐C radiation for disinfection of surfaces.
The spread of the coronavirus SARS-CoV-2 affects the health of people and the economy worldwide. As air transmits the virus, heating, ventilation and air-conditioning (HVAC) systems in buildings, enclosed spaces and public transport play a significant role in limiting the transmission of airborne pathogens at the expenses of increased energy consumption and possibly reduced thermal comfort. On the other hand, liquid desiccant technology could be adopted as an air scrubber to increase indoor air quality and inactivate pathogens through temperature and humidity control, making them less favourable to the growth, proliferation and infectivity of microorganisms. The objectives of this study are to review the role of HVAC in airborne viral transmission, estimate its energy penalty associated with the adoption of HVAC for transmission reduction and understand the potential of liquid desiccant technology. Factors affecting the inactivation of pathogens by liquid desiccant solutions and possible modifications to increase their heat and mass transfer and sanitising characteristics are also described, followed by an economic evaluation. It is concluded that the liquid desiccant technology could be beneficial in buildings (requiring humidity control or moisture removal in particular when viruses are likely to present) or in high-footfall enclosed spaces (during virus outbreaks).
Germicidal ultraviolet light (UV) systems have been widely used in the field of healthcare to help disinfect equipment, surfaces, and air supply systems. Applications include disinfection of whole rooms (including operating rooms), walls and floors, medical equipment, and cooling coils. UV is a highly effective and predictable technology to ensure sterilization of almost any types of surfaces and to eliminate microbial growth. Public health agencies such as the Centers for Disease Control and Prevention in the USA (CDC) recommend UV use as an effective technology to disrupt pathogen transmission in building ventilation systems. The current rarity of UV disinfection stems from the fact that it has been erroneously assumed that air filters are sufficient to provide sterilized air. The last 25 years of data has shown that this is far from reality. When dealing with sub-micron bio-contaminants in the size range of 0.1–0.4 micron, even the best filtration technologies fail to stop them all. HEPA filter challenged with a concentration of one million viable particles per cubic meter at a flow rate of 1000 m³/h could allow as much as 500,000 particles every hour to go through. During the course of a single day, a total of 12 million bio-viable particles will penetrate the filter and contaminate the aseptic zone. Those uncaptured bio-contaminants particulates can be rendered innocuous by a proper use of UV germicidal irradiation (UVGI) technology. This chapter presents an overview of how germicidal UV can be used to sterilize a wide spectrum of microorganisms (e.g., bacteria, mold spores, and viruses) in air stream as well as on contaminated wall and objects.
Ultraviolet Germicidal Irradiation (UVGI) of cooling coils is done to control biofouling that can increase their flow resistance and decrease their heat transfer coefficient. UVGI is also applied in air-handling units to improve indoor air quality (IAQ) by deactivating airborne microorganisms. A typical coil cleaning application delivers a smaller UV dose than an air treatment system, but should provide some collateral air treatment benefit. To date, this effect has not been studied. In this investigation, the benefit of air treatment provided by a cooling coil irradiation system is estimated via simulations employing a subset of the DOE Commercial Reference Buildings library. Benefits were quantified in terms of changes in appropriate measures for each building type: work-loss days (WLD) for office buildings, disability adjusted life years (DALY) for schools, and hospital acquired infections (HAI) for healthcare facilities. UVGI sized for coil cleaning results in a 0% to 7% average reduction in the measure of interest for each building type. This reduction is negatively related to the average outdoor air fraction in each building type, i.e., incremental benefit of UVGI decreases with increasing outdoor air fraction. Combining WLDs with US Gross National Income to monetize annual savings for office buildings yields $0.22/m² - $3.11/m² for Small, $0.12/m² - $0.91/m² for Medium, and $0.01/m² - $0.23/m² for Large types. Combining DALYs with US Gross National Income to monetize savings for Primary and Secondary Schools results in a wide range: $0.00/m² to $0.62/m² due to the large range of values one might reasonably assign to a DALY. In hospitals, reduction in airborne HAIs resulted in estimated savings of $0.05/m² to $0.40/m².
The filtration of airborne particulates has been studied extensively and removal efficiencies can be adequately predicted from theory or from catalog data. The filtrations of airborne microorganisms, however, has not been specifically addressed by theory and has seen limited empirical study. This paper addresses the variety of factors that may cause microbial filtration efficiency to deviate from predicted values based on particulate size alone. A model is developed to incorporate those factors likely to have significant impact, namely, aspect ratios and lognormal size distributions. This model is then challenged with a database of known airborne pathogens and allergens for which these parameters have been established. Results suggest existing filtration models are accurate within reason for the prediction of filtration efficiencies of airborne bacteria and spores, provided logmean diameters are used. Implications for the use of filtration in health care facilities are discussed.
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Filtration Association. http://www.nafahq.org/Position%20Statement.pdf.
Engineering Solutions to Indoor Air Problems
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