PRINCIPLES OF HEATING, VENTILATION AND AIR CONDITIONING with Worked Examples

Transcription

1 PRINCIPLES OF HEATING, VENTILATION AND AIR CONDITIONING with Worked Examples

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3 PRINCIPLES OF HEATING, VENTILATION AND AIR CONDITIONING with Worked Examples Nihal E Wijeysundera World Scientific NEW JERSEY LONDON SINGAPORE BEIJING SHANGHAI HONG KONG TAIPEI CHENNAI TOKYO

4 Published by British Library Cataloguing-in-Publication Data PRINCIPLES OF HEATING, VENTILATION AND AIR CONDITIONING WITH WORKED EXAMPLES system now known or to be invented, without written permission from the publisher.

5 To my grandchildren Emiko Chrisanthi, Sunil Hitoshi, Isabella Anjali, Amali Satomi, and Helina Maya v

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7 Preface Courses in Heating, Ventilation and Air Conditioning (HVAC) are usually offered in departments of mechanical engineering, civil engineering, architecture and building science. This book is written mainly with the interests of students and instructors in these departments in mind. However, a significant part of the contents may be used in courses such as, thermal systems and heat transfer, especially the worked examples. Practicing engineers could use this book to clarify the fundamental principles behind various design procedures recommended in professional handbooks. A number of professional societies like the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) publish comprehensive handbooks and design guides for use by HVAC engineers. These handbooks are updated regularly to include the most recent design procedures, developed through sponsored research projects. One of the main challenges for instructors in HVAC courses is to distill the materials available in professional handbooks, to a concise form to be included in regular undergraduate courses. This is often a time consuming task because the handbooks are intended for practicing engineers. This book tries to make the task easier for instructors by presenting the material in a directly useable format. For students the contents should appear as extensions and applications of the material covered in basic courses on thermodynamics, heat transfer and fluid mechanics. Every effort is made to include simple derivations for most of the design parameters used in practice, without making the mathematical details unduly complicated. For instance, in chapter 9 a simple onedimensional thermal network approach is used to derive the fenestration design parameter called the solar heat gain coefficient (SHGC). Likewise, in chapter 10 a lumped-capacity transient thermal model is vii

8 viii Preface used to clarify the physical meaning of the radiant time series (RTS), and its application in cooling load estimation. In chapter 9 a vector approach is introduced to analyze complex three-dimensional geometrical design problems. These situations are encountered in computing incident angles of solar beams on inclined surfaces, and in determining the effectiveness of shading devices like overhangs. Included in this book are the most up-to-date empirical models available in the ASHRAE Handbook Fundamentals, that are relevant for design. In particular, in chapter 9, for computing the solar radiation absorption and transmission in building envelopes, the latest two-parameter model is used to estimate the clear-sky radiation at different locations. In design oriented courses such as HVAC, it is important for students to understand the fundamentals behind the recommended design procedures. Comprehensive worked examples provide an ideal means to present design concepts in a practically useful manner. With this objective in mind, about 15 worked examples are included in each chapter, carefully chosen to expose students to diverse design situations encountered in HVAC practice. Computations required in worked examples illustrating basic principles are performed using a calculator. Worked examples involving more realistic design situations are done using MATLAB programs, included in the book. At the end of each chapter there are additional problems for which numerical answers are provided. The format of the worked examples and problems is a novel feature of this book. For instructors, this should provide a useful source for problems to be included in courses, tutorials and examinations. MATLAB programming is now taught routinely in most engineering and science courses. Therefore a number of MATLAB codes, for solving HVAC design problems requiring extensive computations, are also included. Computer codes are included for the following applications: (i) computation of psychrometric properties, (ii) design of cooling towers, (iii) design of wet-coil heat exchangers, (iv) computation of hourly diffuse and direct solar radiation intensities, (v) computation of sol-air

9 Preface ix temperature, (vi) estimation of hourly cooling load due to people, lights, roofs, and walls, (vii) design of overhangs, and (viii) design of duct and pipe systems. I wish to thank ASHRAE for granting permission to extract representative design data from the ASHRAE Handbook Fundamentals, for inclusion in this book. I was fortunate to have had the opportunity to teach a number of courses in refrigeration, air conditioning, and thermal systems at the Department of Mechanical Engineering, National University of Singapore (NUS). The notes developed for these courses provided the framework and much of the material for this book. I am thankful to my colleagues in the energy and bio-thermal division at NUS, with whom I shared the teaching of these courses, for many valuable discussions on HVAC systems. I am thankful to Dr. Raisul Islam for fruitful discussions on a number of practical design aspects of chiller systems and their energy efficiency. I wish to thank Dr. A. H. Jahangeer for providing valuable technical support on many occasions and Mr. Mahipala D. Fernando for fruitful discussions on heat pump systems. Thanks are due to my sons Duminda and Harindra, and my daughtersin-law Sindhu and Sophia, for their constant encouragement. Finally, my heartfelt thanks are given to my wife Kamani for her encouragement and generous support towards the completion of this project. Nihal. E. Wijeysundera

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11 Preface Contents Chapter 1 Introduction to Heating, Ventilation and Air Conditioning An Overview of HVAC Systems Some Optional Designs of HVAC Systems HVAC system using air as the energy transport medium HVAC system using water as the energy transport medium HVAC system using water and air as energy transport media Packaged and unitary systems Reversible heat pumps for heating and cooling Overview of HVAC Design Procedure Aims and Organization of the Book 13 References 15 Chapter 2 Heat Transfer Principles Introduction Modes of Heat Transfer One-dimensional Steady Heat Conduction Thermal Network Analogy Thermal resistances in series Overall heat transfer coefficient Thermal resistances in parallel Boundary conditions General Form of Fourier's Law Cylindrical systems Conduction with Internal Heat Generation Convection Heat Transfer Forced convection heat transfer Correlations for the heat transfer coefficient Natural convection heat transfer Radiation Heat Transfer Spectrum of electromagnetic radiation Black surface Emissive power of real surfaces Emissivity of a gray surface 37 vii xi

12 xii Contents Absorption, transmission and reflection Kirchhoff's law of radiation Radiation exchange between two black surfaces Radiation exchange between a black surface and a gray surface Radiation exchange between two gray surfaces Radiation exchange between a curved surface and a flat surface Worked Examples 43 Problems 59 References 62 Chapter 3 Refrigeration Cycles for Air Conditioning Applications Introduction Carnot Refrigeration Cycle Using a Vapor Standard Vapor Compression Cycle Analysis of the Standard Vapor Compression Cycle Actual Vapor Compression Cycle Modifications to the Standard Vapor Compression Cycle Two-stage compression with flash intercooling Two-stage compression with two evaporators Refrigerants for Vapor Compression Systems Vapor Compression Systems for Air Conditioning Applications Window-unit air conditioners Central air conditioning systems using chilled water Compressors of water chillers Reversible heat pump systems Vapor Absorption Refrigeration Cycles Three-heat-reservoir model Analysis of Actual Absorption Cycles Equilibrium of water LiBr mixtures Worked Examples 89 Problems 114 References 117 Chapter 4 Psychrometric Principles Introduction Mixtures of Air and Water Vapor Properties of Air Water Mixtures Relative humidity, humidity ratio and degree of saturation Enthalpy of moist air Specific volume of moist air Adiabatic saturation and wet-bulb temperature Measurement of wet-bulb temperature The Psychrometric Chart Constant dry-bulb temperature lines 130

13 Contents xiii Saturation curve and constant relative humidity lines Constant wet-bulb temperature lines Constant specific volume lines Enthalpy moisture protractor Sensible heat ratio protractor Worked Examples 136 Problems 153 References 156 Appendix A4.1 - MATLAB Code for Psychrometric Properties 157 Chapter 5 Psychrometric Processes for Heating and Air Conditioning Introduction Basic Psychrometric Processes Mixing of two moist air streams Sensible heating or cooling Dehumidification by cooling Humidification of air Evaporative cooling Space condition line Applications of Psychrometric Processes Single-zone Air Conditioning Systems Summer air conditioning systems Summer air conditioning systems with reheat Summer air conditioning systems with bypass paths Winter air conditioning systems Air conditioning systems using evaporative cooling Multi-zone Air Conditioning Systems Multi-zone systems with reheat Dual-duct multi-zone air conditioning systems Variable air volume (VAV) systems Worked Examples 185 Problems 212 References 216 Chapter 6 Direct-Contact Transfer Processes and Equipment Introduction Review of Mass Transfer Principles Steady mass diffusion through a plane wall Steady convection mass transfer Simplified Model for Simultaneous Heat and Mass Transfer Air Washers or Humidifiers Analysis of air washers Efficiency and number of transfer units (NTU) Cooling Towers 229

14 xiv Contents Analysis of cooling towers Enthalpy potential based model for cooling towers Approach and range of cooling towers Property Relations for Moist Air and Water Worked Examples 235 Problems 258 References 260 Appendix A6.1 - MATLAB Code for Cooling Tower Design 261 Chapter 7 Heat Exchangers and Cooling Coils Introduction Design Analysis of Dry-Coil Heat Exchangers Some common types of heat exchangers Analysis of counter-flow heat exchangers The LMTD method The effectiveness NTU method Evaporators and condensers Cross-flow heat exchangers Efficiency of extended surfaces Overall heat transfer coefficient for finned tubes Wet-Coil Heat Exchangers or Cooling Coils Physical processes in wet-coils Analysis of wet-coil heat exchangers Numerical model for wet-coils Worked Examples 290 Problems 331 References 335 Appendix A7.1 - MATLAB Code for Design of Chilled Water Coils 335 Chapter 8 Steady Heat and Moisture Transfer Processes in Buildings Introduction Steady Heat Transfer through Multi-Layered Structures Parallel path method Isothermal plane method Zone method Radiation heat transfer coefficient Heat transfer in gas filled cavities Steady Heat Transfer through Fenestrations Windows and doors Overall heat transfer coefficient Below Grade Heat Transfer in Buildings Heat transfer through basement walls Heat transfer through basement floors Heat transfer through surfaces at grade level 355

15 Contents xv 8.5 Infiltration in Buildings Heating load due to infiltration Infiltration air flow rates Estimation of infiltration flow rates Moisture Transport in Building Structures Fick's law Worked Examples 363 Problems 388 References 392 Chapter 9 Solar Radiation Transfer Through Building Envelopes Introduction Fundamentals of Solar Radiation Beam and diffuse solar radiation Direction of beam radiation Angle of incidence of beam radiation on a surface Total radiation incident on an inclined surface Clear-sky model of direct and diffuse solar radiation Absorption of Solar Radiation by an Opaque Surface Transmission and Absorption of Solar Radiation Effective properties of a single layer Transmittance of a multi-layered fenestration Radiation absorption in multi-layered fenestrations Overall Energy Transfer through Fenestrations Shading of Surfaces from Solar Radiation Worked Examples 419 Problems 443 References 446 Chapter 10 Cooling and Heating Load Calculations Introduction Outdoor Design Conditions Thermal Comfort and Indoor Design Conditions Heat transfer from the human body Indoor design conditions Indoor air quality Internal Heat Sources in Buildings Heat gain from people Heat gain from lighting Heat gain from equipment Transient Effects in Building Energy Transfer Transient heat conduction through walls Heat gain by a thin surface Cooling Load Calculation Methods 468

16 xvi Contents Heat balance method (HBM) Radiant time series (RTS) method Application of the RTS method and the CTS method Heating Load Calculation Methods Worked Examples 479 Problems 507 References 513 Appendix A MATLAB Code for Cooling Load due to People 514 Appendix A MATLAB Code for Cooling Load due to Wall Conduction 515 Appendix A MATLAB Code for Cooling Load due to Windows 519 Appendix A MATLAB Code for Shading of Windows 523 Chapter 11 Air Distribution Systems Introduction Total Pressure Distribution Pressure Loss in Duct Networks Pressure loss in straight ducts Pressure loss in fittings Total pressure loss in duct sections Air Distribution Fans Axial flow and centrifugal fans Fan characteristics Fan laws Fan Duct Network Interaction Design Methods for Duct Systems Equal friction method Static regain method Optimization of Duct Systems Air Distribution in Zones Air flow from diffusers Air diffusion performance index Design aspects of air distribution systems Worked Examples 557 Problems 582 References 585 Appendix A MATLAB Code for Pressure Loss in Circular Ducts 586 Appendix A MATLAB Code for Equal Friction Design Method 587 Appendix A MATLAB Code for Static Regain Design Method 588 Chapter 12 Water Distribution Systems Introduction Energy Equation for Hydronic Systems Head Losses in Hydronic Systems Friction head loss in pipes 593

17 Contents xvii Dynamic head loss in fittings Pump Characteristics System Pump Interaction and Flow Control Design of Water Distribution Systems Direct-return and reverse-return systems Design of pipe networks Worked Examples 604 Problems 625 References 630 Appendix A MATLAB Code for Head Loss in Pipe-Sections 630 Chapter 13 Building Energy Estimating and Modeling Methods Introduction Degree Day Method for Estimating Energy Use Bin Method for Estimating Energy Use Generation of bin data Applications of the bin method Cycling of furnaces Air-source heat pumps Cooling towers Variable occupancy rates Simulation Methods for Estimating Energy Use Central HVAC systems Simulation of multi-chiller systems Simulation of water-loop heat pump system (WLHPS) Worked Examples 656 Problems 685 References 690 Appendix A MATLAB Code for Bin Data and Degree Days 691 Index 693