Development and Testing of Air Exchange Rate Methods to Improve Cost Effectiveness While Maintaining Accuracy — United States Air Force Center for Engineering and the Environment
Background. One of the important factors that determine whether vapors that intrude into buildings will adversely impact human health is the magnitude of the air exchange rates (AERs) for those building. AERs are defined as the flow rate of air through a building divided by its volume. For buildings with high AERs, vapors that intrude into those building will more rapidly exchange with the outdoor air, thereby tending to reduce indoor vapor concentrations, thus lessening the risk to human health. For this study, alternative methods for determining AERs have been developed and compared against more traditional methods.
Objectives. The primary objective for undertaking this work was to see if simple, cost-effective AER methods could be developed that are comparable in accuracy to more standard methods that are commonly used. This paper focuses on model development, application, and comparison with other methods. A protocol for cost-effectively applying these techniques is developed.
Approach. Starting from basic mass-balance principles, alternative AER algorithms were developed. The methods accommodated multiple tracer releases (helium and sulfur hexafluoride), instantaneous and continuous tracer releases, and alternative tracer collection techniques. The various AER methods were solved either analytically, using least squares, or using alternative finite different approaches. While the basics of some of the techniques have been developed before this study, many of the techniques developed were different from previously documented approaches. For example, while a method exists for determining AERs using instantaneous tracer releases, for this study three least squares techniques for instantaneous releases were developed, each from slightly different assumptions, such that comparisons between methods help to provide confidence in the methods’ predictive results. Extensive analytical testing was completed before the methods were applied to case studies. The case studies consisted of applications using commercial buildings on United States Air Force (USAF) installations. The buildings chosen were small enough that a single AER, properly calculated, represented a whole-building average. Tests were completed at buildings on four installations. The methods were applied in such a way that the newly developed methods could be directly comparable to the more standard methods. This often meant that several tests were ongoing simultaneously so that site conditions could be comparable for each test. A protocol was developed that provides a straightforward and cost effective methodology for accurately determining AERs.
Results/Lessons Learned. Overall, the results of the standard methods and the newly developed methods compared well against each other. Specific details of the comparisons are provided in the presentation. It was determined that the methods developed were very cost effective when compared to standard methods.