Predicted and Measured Air Exchange Rates

s part of an ongoing indoor air quality (IAQ) modeling study, air exchange rates of ten Kuwait residential buildings were measured according to the American Society for Testing and Materials (ASTM) E741-83 standard which is a tracer gas decay technique. In this study, SF 6 was used as the tracer gas. Air exchange rates were predicted based on the characteristics of the HVAC system, openings (such as windows), leakage areas, pertaining average wind speed, average indoor/outdoor temperature difference and wind and stack coefficients. When the coefficients recommended in the literature were used, predicted air exchange rates consistently were higher by about 34% than those obtained from tracer measurements. Therefore, a modified procedure has been proposed to accurately predict the air exchange rate of tightly controlled residential buildings of this region. Normally, IAQ models are based on a mass balance equation involving outdoor and indoor concentrations, sink, source and air exchange rate terms. An important element of these models is the air change rate, which is defined as the ratio of the net volumetric flow rate entering (or leaving) the volume of the building. Therefore, it is equivalent to the reciprocal of the mean residence time (or space velocity), which is the usual concept used in traditional chemical reactor theory. The air exchange rate (also called the air change rate) is normally symbolized by ACH and is a measurement of how much fresh outdoor air replaces indoor air in a given time period. It is measured in units of air changes per hour and equals the ratio of the hourly indoor air volume replaced by outdoor air to the total indoor volume. Therefore, a high ACH means a large volume of outdoor air comes in and replaces the indoor air over time. In an accompanying detailed study, indoor and outdoor concentrations of certain volatile organic compounds (VOCs) have been measured in the residential areas of Kuwait with the ultimate aim of developing a mathematical IAQ model. 1 The model requires, among other factors, an accurate estimate of air exchange rate (ACH). The air exchange rate of a building cannot be estimated based on the building’s construction or age or from a simple visual inspection. It is possible only when a detailed quantification of the leakage sites and their magnitude area are made. Then the air exchange rate of a building can be calculated in a straightforward manner, given the location and leakage functions for every opening in the building envelope the wind and stack coefficients over the building envelope, and any mechanical ventilation airflow rates. Although these inputs are normally available, the studies involving experimental verifications are scarce. This means that the reliability of these calculations is unknown. Further, it is also uncertain whether these techniques can be used confidently for buildings that may have different physical natures— such as those in Kuwait and the Arabian Peninsula. The buildings in these locations are relatively new and are designed to cope with harsh conditions, such as extreme temperature and dust storms. The aim of this work is to measure air exchange rates for some typical Kuwaiti residential buildings and compare them with the predictions. The ultimate goal is to obtain a modified predictive technique.