Performance of Rotary Enthalpy Exchangers

PERFORMANCE OF ROTARY ENTHALPY EXCHANGERS by GUNNAR STIESCH A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE (Mechanical Engineering) at the UNIVERSITY OF WISCONSIN-MADISON 1994 ABSTRACT Rotary regenerative heat and mass exchangers allow energy savings in the heating and cooling of ventilated buildings by recovering energy from the exhaust air and transferring it to the supply air stream. In this study the adsorption isotherms and the specific heat capacity of a desiccant used in a commercially available enthalpy exchanger are investigated experimentally, and the measured property data are used to simulate the regenerator performance and to analyze the device in terms of both energy recovery and economic profitability. Based on numerical solutions for the mechanism of combined heat and mass transfer obtained with the computer program MOSHMX for various operating conditions, a computationally simple model is developed that estimates the performance of the particular enthalpy exchanger and also of a comparable sensible heat exchanger as a function of the air inlet conditions and the matrix rotation speed. The model is built into the transient simulation program TRNSYS, and annual regenerator performance simulations are executed. The integrated energy savings over this period are determined for the case of a ventilation system for a 200 people office building (approx. 2 m3/s) for three different locations in the United States, each representing a different climate. Life cycle savings that take into account the initial cost of the space-conditioning system as well as the operating savings achieved by the regenerator are evaluated for both the enthalpy exchanger and the sensible heat exchanger over a system life time of 15 years. The present worth of the accumulated savings ranges from $ 28,000 to $ 38,000 for the enthalpy exchanger and from $ 7,000 to $ 24,000 for the sensible heat exchanger. The enthalpy exchanger results in greater payoffs in all locations, but its advantage is most significant in a warm and humid climate where the sensible heat exchanger performs poorly. ii ACKNOWLEDGMENTS Many thanks are owed for the completion of this work and I would like to express my gratitude towards those who contributed to making it possible. Professor Sanford Klein invited me to the Solar Lab, and during the past 16 months he was not only my major - and a great - academic advisor who constantly helped me find the answers to any kind of engineering problem, but he was also an exceptional teacher who made his students actually have fun in the two thermodynamics classes he taught. Professor John Mitchell also contributed substantially to the progress of this project, especially towards the end, when his knowledge of all kinds of HVAC-systems was of great value. Thanks to Professor William Beckman, the director of the Solar Lab, for his efforts in finding a sponsor for the second part of this project and for cultivating such an extraordinary environment as the Solar Lab, where it is a pleasure to work in. The German Academic Exchange Service provided me with the opportunity of attending graduate school in the United States by funding the one-year exchange program between the universities of Hannover and Madison which was initiated by Dipl.-Ing. Frank- Detlef Drake. The third semester of my studies were funded by the University of Wisconsin- Madison Graduate School. I would also like to thank Tom James and Randy Dahmen from the Carnes Company for providing samples of their products as well as information about it. Special thanks go to all my friends in the Lab and in the UW-Sailing Team. Meeting with you at places other than the university helped making my stay in America an exceptional fun and rewarding experience. Finally, I would like to thank my family and my friends back home in Germany. Your visits, letters and phone calls were always greatly appreciated. iii TABLE OF CONTENTS Abstract ii Acknowledgments iii List of Figures vii List of Tables x Nomenclature xi 1 Introduction 1 1.1 Rotary Regenerators 3 1.2 Performance Characteristics 4 2 Adsorption Fundamentals 10 2.1 Physical Adsorption 10 2.2 Adsorbent Classification 11 2.3 Potential Theory of Adsorption 14 3 Adsorption Isotherm Measurement - Water Vapor 16 3.1 Experimental Apparatus and Procedures 16 3.1.1 Desiccant Samples 16 3.1.2 Experimental Setup 17 3.2 Experimental Data and Analysis 20 3.2.1 Maximum Adsorption Capacity 20 3.2.2 Equilibrium Adsorption Isotherms 22 3.2.3 Polanyi-Approximation 24 3.2.4 Experimental Uncertainties 25 iv 4 Heat- and Mass Transfer in Rotary Regenerators 29 4.1 Mathematical Model Formulation 29 4.2 Equilibrium Theory 32 4.3 Simplified Solution for the Case of Maximum Enthalpy Exchange 35 4.4 Simplified Solution for Intermediate and High Rotation Speeds 37 5 Instantaneous Regenerator Performance in Various Operating Conditions 42 5.1 Comparison Between Polymer and Silica Gel Matrices 43 5.1.1 Minimum Rotation Speed 44 5.1.2 Effectiveness and Outlet Properties 53 5.1.3 Conclusions and Design Guidelines 56 5.2 Heat- and Mass Transfer Effectiveness - Catalog Data and Calculated Values 57 5.3 The Enthalpy Exchanger in Cooling Systems 59 5.4 The Enthalpy Exchanger in Heating Systems 63 6 Annual Performance Simulations 67 6.1 The Simulation Program 68 6.1.1 Heating Mode 69 6.1.2 Cooling Mode 74 6.1.3 The TRNSYS Program 75 6.1.4 Practical Considerations and Constraints 76 6.2 Simulation Results 77 6.2.1 Energy Recovery in the Heating Mode 78 6.2.2 Energy Recovery in the Cooling Mode 81 6.3 Economic Analysis 84 6.3.1 Installation Cost 85 v 6.3.2 Annual Operating Savings 88 6.3.3 Life Cycle Savings 90 7 Adsorption Measurement - Volatile Organic Compounds 92 7.1 Gas Chromatography and Frontal Analysis Technique 93 7.2 System Components 96 7.2.1 Gas Chromatograph 97 7.2.2 Flame Ionization Detector (FID) 98 7.2.3 Data Acquisition 100 7.2.4 Gas Supply and Flowmeter 101 7.2.5 Matrix Sample 102 7.3 Test Procedure 103 7.3.1 Preparations 103 7.3.2 Run Execution 105 7.3.3 Data Analysis 106 7.4 Results 108 7.5 Experimental Uncertainties 111 8 Recommendations and Conclusions 112 8.1 Conclusions 112 8.2 Unresolved Issues 114 Appendix A: EES Program for Calculation of Minimum Rotation Speed 116 Appendix B: Listing of TRNSYS Subroutines, Types 70, 71, 72 118 Appendix C: Sample TRNSYS Deck 138 References 140 vi LIST OF FIGURES Page Figure 1.1 Matrix of a Rotary Regenerator 3 Figure 1.2 Matrix Structure 4 Figure 1.3 Air-Conditioning System with Regenerative Recovery Unit 5 Figure 1.4 Air Cooling Using a Sensible Heat Exchanger 6 Figure 1.5 Air Cooling Using an Enthalpy Exchanger 6 Figure 1.6 Air Heating Using a Sensible Heat Exchanger 7 Figure 1.7 Air Heating Using an Enthalpy Exchanger 7 Figure 1.8 Enthalpy Exchanger Outlets for Various Lewis Numbers 9 Figure 2.1 Classification of Adsorption Isotherms 13 Figure 3.1 Experimental Setup 18 Figure 3.2 Maximum Adsorption Capacity and Desorption Velocity 21 Figure 3.3 Equilibrium Adsorption Isotherms 23 Figure 3.4 Experimental Data and Polanyi Curve Fit 25 Figure 4.1 Temperature Effectiveness: MOSHMX Output and Curve Fit 40 Figure 4.2 EX Outlet States for Various Rotation Speeds: MOSHMX Output and Curve Fit 41 Figure 5.1 Water Based Polymer: Adsorption Capacity vs. Temperature 45 Figure 5.2 Silica Gel: Adsorption Capacity vs.Temperature 45 Figure 5.3 Water Based Polymer: Adsorption Capacity vs. Humidity Ratio 46 Figure 5.4 Silica Gel: Adsorption Capacity vs. Humidity Ratio 46 vii Figure 5.5 Van’t Hoff Plot for Constant Adsorption Potentials 48 Figure 5.6 Cross Sectional Dimensions of the Investigated Matrix 53 Figure 5.7 Heat and Mass Transfer Effectivenesses of the Water-Based Polymer Coated Matrix 58 Figure 5.8 Single-Zone Cooling with Reheat. No Regenerator 60 Figure 5.9 Air-Conditioning System with Exhaust Air Mixing 61 Figure 5.10 Single-Zone Cooling with Reheat, Exhaust Air Mixing and Sensible HX 62 Figure 5.11 Total Energy Recovery of HX and EX in a Cooling System 63 Figure 5.12 Schematic of a Heating System 64 Figure 5.13 Psychrometric Chart for Heating System 64 Figure 5.14 Total Energy Recovery of HX and EX in a Heating System 66 Figure 6.1 HX Operation with Exhaust Condensation and Complete Evaporation into the Supply Stream 69 Figure 6.2 HX Operation In Excess Water Conditions 70 Figure 6.3 Schematic of Subroutine Type 71 71 Figure 6.4 EX Operation without Excess Water 72 Figure 6.5 EX Operation in Excess Water Conditions 73 Figure 6.6 Schematic of Subroutine Type 70 73 Figure 6.7 TRNSYS Information Flow Diagram 75 Figure 7.1 Schematic of a Gas Chromatograph System 93 Figure 7.2 Idealized Frontal Chromatogram 94 Figure 7.3 Diffuse Frontal Chromatogram 95 Figure 7.4 Experimental System 96 Figure 7.5 Pneumatic Gas Stream Selection Valve 97 viii Figure 7.6 Flame Ionization Detector 98 Figure 7.7 FID Sensitivity Optimization 99 Figure 7.8 Method for Area Analysis 108 Figure 7.9 Propane Chromatogram. No Sample 109 Figure 7.10 Propane Chromatogram. Desiccant Sample 109 Figure 7.11 Propane Chromatogram. Activated Carbon 110 Figure 7.12 Toluene Chromatogram. Desiccant Sample 111 ix LIST OF TABLES Page Table 3.1 Constants for Calculation of Humidity Above Saturated Salt Solutions 19 Table 3.2 Adsorption Capacity as a Function of Temperature and Relative Humidity 22 Table 5.1 Properties of Examined Air Inlet States 50 Table 5.2 Derivatives and Wave Speeds for Inlet Pair 1 51 Table 5.3 Derivatives and Wave Speeds for Inlet Pair 2 51 Table 5.4 Derivatives and Wave Speeds for

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