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Design and Optimization of UVGI Air Disinfection Systems

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Design and Optimization of UVGI Air Disinfection Systems The Pennsylvania State University The Graduate School College of Engineering Design and Optimization of UVGI Air Disinfection Systems A Thesis in Architectural Engineering by Wladyslaw Jan Kowalski Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2001 We approve the thesis of W. J. Kowalski Date of Signature ______________________________________ ______________ William P. Bahnfleth Associate Professor of Architectural Engineering Thesis Adviser Chair of Committee ______________________________________ ______________ Thomas S. Whittam Professor of Biology ______________________________________ ______________ Richard G. Mistrick Associate Professor of Architectural Engineering _______________________________________ ______________ Stanley A. Mumma Professor of Architectural Engineering _______________________________________ ______________ Richard A. Behr Professor of Architectural Engineering Head of the Architectural Engineering Department iii ABSTRACT Mathematical models of the response of populations of microorganisms exposed to ultraviolet germicidal irradiation (UVGI) are developed that include two-stage response curves and shoulder effects. Models are used to develop a C++ computer program that is capable of predicting the performance of UVGI air disinfection systems. The algorithms are based on models for 1) the intensity field of UVGI lamps, 2) the intensity field due to UVGI reflective enclosures, and 3) the kill rate of microorganisms to UVGI exposure as they pass through the modeled intensity field. The validity of the UVGI lamp model is established by comparison with lamp photosensor data. The validity of the overall predictive model is established by comparison of predictions with laboratory bioassays for two species of airborne pathogens – Serratia marcescens and Bacillus subtilis. First stage rate constants, second stage rate constants, and the defining shoulder parameters are determined for Aspergillus niger and Rhizopus nigricans based on bioassay data, and it is shown how predictions using only single stage rate constants can deviate significantly from predictions using the complete survival curve. A dimensional analysis of UVGI systems identifies nine dimensionless parameters responsible for determining the effectiveness of any rectangular UVGI system. A factorial analysis of the dimensionless parameters based on data output by the program identifies the most critical parameters and the inter-relationships that determine UVGI system effectiveness. Response surfaces are generated using program output to illustrate the inter-relationships of the dimensionless parameters. The optimum values of the dimensionless parameters are summarized that result in optimized performance. Economic optimization is demonstrated by a series of examples that calculate life cycle costs, and principles of economic optimization are summarized. Conclusions are presented that will produce more energy-efficient and effective designs and a proposed model for improved UVGI systems is presented. iv TABLE OF CONTENTS LIST OF FIGURES ... .. .................................................................................................... vii LIST OF TABLES..... .. .................................................................................................... ix Acknowledgements .. .. .................................................................................................... x Chapter 1. Introduction..................................................................................................... 1 Chapter 2. Literature Review............................................................................................. 3 2.1. History of UVGI ............................................................................................ 3 2.2. UVGI Systems Today ................................................................................... 4 2.3. Airborne Pathogens ...................................................................................... 6 2.4. UVGI Rate Constant Studies ......................................................................... 7 2.5. Relative Humidity Effects............................................................................... 7 2.6. Air Temperature Effects................................................................................. 8 2.7. Shoulder Effects ........................................................................................... 8 2.8. Second Stage Rate Constant......................................................................... 9 2.9. Radiation View Factors ................................................................................. 9 2.10. Ventilation Systems.................................................................................... 10 2.11. Design Guidelines and Criteria for Air Disinfection .......................................... 10 2.12. New Technologies and Materials .................................................................. 10 Chapter 3. Objectives and Scope...................................................................................... 12 3.1. Objectives .................................................................................................... 12 3.2. Scope... ...................................................................................................... 12 Chapter 4. Mathematical Modeling of UVGI........................................................................ 14 4.1. Lamp Intensity Field...................................................................................... 14 4.2. Reflected Intensity Field ................................................................................ 17 4.3. Inter-Reflected Intensity Field......................................................................... 20 Chapter 5. Mathematical Modeling of Microbial Response................................................... 24 5.1. Modeling Microbial Decay.............................................................................. 24 5.2. Two-Stage Survival Curves ............................................................................. 26 5.3. The Shoulder................................................................................................ 28 5.4. The Complete Microbial Decay Model............................................................. 33 Chapter 6. The UVX Computer Program............................................................................. 35 6.1. Description of the Program ............................................................................ 35 6.2. Operation of the Program............................................................................... 36 6.2. Program Input and Output.............................................................................. 44 Chapter 7. Model Validation ............................................................................................. 45 v 7.1. Lamp Model Validation.................................................................................. 45 7.2. Bioassays for Serratia Marcescens ................................................................ 47 7.3. Bioassays for Bacillus subtilis ....................................................................... 49 7.4. Shoulder and Second Stage Model Validation ................................................. 50 7.4.1 Bioassays for Aspergillus niger .................................................................... 52 7.4.2 Bioassays for Rhizopus nigricans ................................................................. 56 Chapter 8. Dimensional Analysis of UVGI Systems............................................................ 60 Chapter 9. Factorial Analysis of UVGI Systems ................................................................. 66 9.1. The 2k Factorial Screening Analysis............................................................... 66 9.2. RSM Analysis of the Central Composite Design .............................................. 69 Chapter 10. Optimization of Performance and Economics ................................................... 74 10.1. Performance Optimization............................................................................ 74 10.2. Economic Optimization ............................................................................... 81 10.2.1. Life Cycle Cost of UVGI Systems .............................................................. 81 10.2.2. Basic Cost Relationships of UVGI System Components .............................. 84 10.2.3. Annual Costs of UVGI Systems................................................................. 86 10.2.4. Economic Optimization of UVGI Systems .................................................. 90 10.2.5. Economic Optimization UVGI Recirculation Systems .................................. 94 10.2.5. Performance of Multiple Lamp and Axial Configurations ............................... 103 Chapter 11. Summary and Conclusions............................................................................. 106 REFERENCES ........ ..................................................................................................... 111 Appendix A: Airborne Pathogen Database......................................................................... 119 Appendix B: UVGI Rate Constants ................................................................................... 124 Appendix C: UVGI Units of Intensity and Dose..................................................................
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