Proceedings: Building Simulation 2007 COMPARISON OF THREE IAQ CALCULATION METHODS Chris Muller1 and W. Brad M. Stanley1 1Purafil, Inc., 2654 Weaver Way Doraville, Georgia 30340 USA that can be used to aid in contaminant-based design of ABSTRACT building ventilation systems based on concentration Calculating contaminant concentrations in or the targets and emission information using mass balance required ventilation for a space has been a difficult and calculations. Corresponding ventilation rates can be confusing part in the application of the IAQ Procedure calculated for each contaminant of concern. of ANSI/ASHRAE Standard 62.1-2004; Ventilation One is the Indoor Air Quality Design Tool (IAQDT) for Acceptable Indoor Air Quality. Appendix D of that can be used to calculate transient concentrations of ASHRAE Standard 62 presents one method for contaminants based on the HVAC system performing these calculations, but it is limited to the configuration and operation (Walton 2003). It differs steady-state analysis of a single zone. More recently, from the mass balance model in Appendix D of two software tools have been developed by the United Standard 62.1 in that it does not assume steady States National Institute of Standards and Technology conditions to exist. (NIST) to facilitate these calculations and to include transient effects. One is the Indoor Air Quality Design The other modeling tool developed by NIST is a Tool (IAQDT) that was developed to aid in multizone indoor air quality and ventilation analysis contaminant-based design of ventilation systems, such computer program called CONTAM (Anon.). It is as when using the IAQ Procedure. The IAQDT differs designed to help one determine airflows, contaminant from Appendix D in that it does not assume steady concentrations and personal exposure. state conditions to exist. The other is CONTAM, a A comparison of a sample office space using these multizone airflow and indoor air quality and three approaches will be performed. This comparison ventilation analysis computer program designed to is intended to highlight similarities and differences in help predict airflows and contaminant concentrations the resulting contaminant concentrations in the space, in multizone building systems. possible reasons for the differences between each This paper reports on the application of all three model, and the appropriateness of each model for use methods to a single zone, showing how the results in office building scenarios. obtained by each method may be similar, with exceptions occurring when transient effects are ASHRAE Standard 62.1 Mathematical Model important. The paper also evaluates difficulties that The formulas provided in Appendix D of Standard arise from applying a single zone mass balance model 62.1 take into account (among other things) the to a multiple zone system. Application and amount of outdoor air, contaminant generation rate(s), appropriateness of each method for modeling indoor outdoor contaminant concentrations, filter locations air quality and when using the IAQ Procedure will be and efficiencies, ventilation effectiveness, supply air discussed. circulation rate and the fraction recirculated. The schematic of a representative system is shown in KEYWORDS Figure 1. Design, Modeling, Indoor Air Quality, ASHRAE The variables in this model are defined as follows: Standard 62, Indoor Air Quality Procedure Vo = volumetric flow of outdoor air, from ambient INTRODUCTION into air handling system (AHS); The American Society of Heating, Refrigerating, and Vr = volumetric flow of return air, from the zone to Air-Conditioning Engineers (ASHRAE) Standard the AHS and ambient air; 62.1-2004 “Ventilation for Acceptable Indoor Air Vs = volumetric flow of supply air, from the AHS; Quality,” (ASHRAE 2004) provides formulas in Appendix D for calculating the space contaminant R = recirculation flow factor; concentrations of a zone for application with the Fr = flow reduction factor, used with variable air standard’s Indoor Air Quality (IAQ) Procedure. These volume (VAV) systems; equations are based on a single-zone mass balance under steady state conditions. They have been used for Co = contaminant concentration in the outdoor air; many years to calculate contaminant concentrations in Cs = contaminant concentration in the occupied zone; a zone and compare them to guideline levels. Ef = filter efficiency for contaminant; The U.S. National Institute of Standards and Technology (NIST) has developed two software tools Ev = ventilation effectiveness; - 1369 - Proceedings: Building Simulation 2007 N = contaminant generation rate in the zone; In addition to the mass flows described above, the following variables are used for each contaminant of A, B = filter locations in the recirculation air and the concern: supply (mixed) air, respectively. c = contaminant concentration in the outdoor air, This model is intended to model one zone at a time. In o kg/kg; the simplest form, the user would use constant contaminant generation rates and filter removal cs = contaminant concentration in the mixed supply efficiencies, and the mechanical system properties to air, kg/kg; determine the total concentration in the space or the c = contaminant concentration in the zone and amount of outside air or recirculation air needed. For z return air; kg/kg; the purposes here, the concentration of the contaminant in the space will be calculated and ηv = filter efficiency for the ventilation air stream; compared to a guideline level. An example of those η = filter efficiency for the recirculation air stream; formulas (Eq. 1) is shown below for the setup modeled t in this work. ηs = filter efficiency for the supply air stream; • Filter Location: “B” ηc = filter efficiency for the air cleaner; • HVAC flow type: constant • Outdoor air flow type: constant G = contaminant generation rate in the zone; • Space contaminant concentration (Cs): R = contaminant removal (sink) coefficient in the zone. N + EvVo (1− E f )Co Cs = (1) For each contaminant we calculate the concentration in Ev (Vo + RVr E f ) the fully mixed zone and the supply flow by simultaneously solving Eq. 2 and 3. These are based Indoor Air Quality Design Tool (IAQDT) on a mass balance for the contaminant concentration on the supply flow over system #1 and a mass balance The mathematical model behind the IAQDT is based for the rate of change of contaminant concentrations in on mass balances of a single-zone system. This system the zone over system #2. These equations are solved (or zone) over which the balances are written is shown throughout a day from midnight (00:00) to midnight in Figure 2. It takes into account the amount of outside (24:00) until steady state is reached, i.e. the air, recirculation, total supply air and infiltration to the contaminant concentration at the end of the day equals space as well as the amount of exhaust, return air and the contaminant concentration at the beginning of the exfiltration from the space. Air cleaning technologies day. These calculated concentrations throughout the applied in the HVAC system or in the space can be day are saved and compared to limit concentrations for modeled if efficiencies and flows are determined. further reporting. The zone model allows for the following airflows: mc&&ss=− m v(11ηη v)() c o +− m & t t c z (2) m& i = infiltration flow – direct from ambient into the zone, ⎛⎞ρρzzVVzz ⎜⎟+++++mmmmRcm&&&&ccηη r x e∑ z,t −− & s()1 s c s,ttt = c z,−Δ ++ mcG & io ∑ ⎝⎠ΔΔtt m& e = exfiltration flow – from the zone to ambient, (3) = exhaust flow – from the zone to ambient, m CONTAM Version 2.4 & x = air cleaner flow – within the zone through a m& c CONTAM (or CONTAMW) is a multizone indoor air filter, quality and ventilation analysis computer program also developed by NIST and is designed to help predict the m& r = return flow – from the zone to the air handling system, following items. m& s = supply flow – from the air handling system 1. Airflows: infiltration, exfiltration, and room-to-room to the zone, airflows in building systems driven by mechanical means, wind pressures acting on the exterior of the = spill flow – from the air hamdling system to m& u building, and buoyancy effects induced by the ambient, indoor and outdoor air temperature difference. = recirculation flow – from the return to the m& t 2. Contaminant Concentrations: the dispersal of supply side of the air handling system, airborne contaminants transported by these airflows; = ventilation flow – from ambient to the transformed by a variety of processes including m& v supply side of the air handling system, chemical and radio-chemical transformation, adsorption and desorption to building materials, -or- if there is no air handling system, from filtration, and deposition to building surfaces, etc.; ambient directly to the zone. and generated by a variety of source mechanisms. - 1370 - Proceedings: Building Simulation 2007 3. Personal exposure: the predictions of exposure of RESULTS occupants to airborne contaminants for eventual risk assessment. Results from each model are shown in Table 4. CONTAM can be useful in a variety of applications. Standard 62.1 versus the IAQDT Its ability to calculate building airflows is useful to The differences between the resulting indoor air assess the adequacy of ventilation rates in a building, concentrations using the Standard 62.1 model and the to determine the variation in ventilation rates over time IAQDT were greatest for ammonia and carbon and the distribution of ventilation air within a building, monoxide which are not controlled by the air cleaning and to estimate the impact of envelope air tightening system. Figure 3 provides an example showing efforts on infiltration rates. The prediction of concentration versus time data for ammonia and sulfur contaminant concentrations can be used to determine dioxide as derived by the IAQDT. the indoor air quality performance of a building before it is constructed and occupied, to investigate the Since the ammonia is not being removed by the air impacts of various design decisions related to cleaning system and has an internal source (building ventilation system design and building material occupants), it is able to build up during the day.