Ventilation Systems
Design and performance
Edited by Hazim B. Awbi Ventilation Systems
Properly designed ventilation systems for cooling, heating and cleaning the outdoor air supplied to buildings are essential for maintaining good indoor air quality as well as for reducing a building’s energy consumption. Modern developments in ventilation science need to be well understood and effectively applied to ensure that buildings are ventilated as efficiently as possible. Ventilation Systems provides up-to-date knowledge based on the experience of internationally recognised experts to deal with current and future ventilation requirements in buildings. Presenting the most recent developments in ventilation research and its applications, this book offers a comprehensive reference to the subject, covering the fundamentals as well as more advanced topics. It is a unique publication as it covers the subject rigorously in a way needed by researchers but has a practical flavour that will be of value to a wide range of building professionals.
Hazim B. Awbi is Professor of Building Environmental Science at the University of Reading, UK, and founder of the Indoor Environment and Energy Research Group (IEERG). Also available from Taylor & Francis
Ventilation of Buildings 2nd edition H. Awbi Hb: ISBN 978–0–415–27055–3 Pb: ISBN 978–0–415–27056–1
Building Services Engineering 5th edition D. Chadderton Hb: ISBN 978–0–415–41354–1 Pb: ISBN 978–0–415–41355–8
Tropical Urban Heat Islands N. H. Wong et al. Hb: ISBN 978–0–415–41104–2
Heat and Mass Transfer in Buildings K. Moss Hb: ISBN 978–0–415–40907–0 Pb: ISBN 978–0–415–40908–7
Energy Management and Operating Costs in Buildings K. Moss Hb: ISBN 978–0–415–35391–5 Pb: ISBN 978–0–415–35392–2
Renewable Energy Resources 2nd edition J. Twidell and T. Weir Hb: ISBN 978–0–419–25320–4 Pb: ISBN 978–0–419–25330–3
Mechanics of Fluids 8th edition B. Massey and J. Ward Smith Hb: ISBN 978–0–415–36205–4 Pb: ISBN 978–0–415–36206–1
Housing and Asthma S. Howieson Hb: ISBN 978–0–415–33645–1 Pb: ISBN 978–0–415–33646–8
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Design and performance
Edited by Hazim B. Awbi First published 2008 by Taylor & Francis 2 Park Square, Milton Park, Abingdon, Oxon OX14 4RN Simultaneously published in the USA and Canada by Taylor & Francis 270 Madison Ave, New York, NY 10016, USA This edition published in the Taylor & Francis e-Library, 2007. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” Taylor & Francis is an imprint of the Taylor & Francis Group, an informa business © 2008 Taylor and Francis, editorial material; individual chapters, the contributors All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any efforts or omissions that may be made. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data Ventilation systems: design and performance / edited by Hazim B. Awbi. p. cm. Includes bibliographical references and index. ISBN-13: 978-0-419-21700-8 (hardback : alk. paper) 1. Ventilation. I. Awbi, H. B. (Hazim B.), 1945– TH7658.V455 2007 697.9 2–dc22 2006100008
ISBN 0-203-93689-2 Master e-book ISBN
ISBN10: 0–419–21700–2 (hbk) ISBN10: 0–203–93689–2 (ebk) ISBN13: 978–0–419–21700–8 (hbk) ISBN13: 978–0–203–93689–4 (ebk) Contents
List of contributors viii Preface ix Acknowledgements xi
1 Airflow, heat and mass transfer in enclosures 1 YUGUO LI 1.1 Introduction 1 1.2 Transport phenomena in buildings 2 1.3 General governing equations of airflow, heat and mass transfer 8 1.4 Turbulence and its modeling 26 1.5 Jets, plumes and gravity currents 41 1.6 Solution multiplicity of building airflows 45 1.7 Experimental methods 51 References 56 Nomenclature 58
2 Ventilation and indoor environmental quality 62 BJARNE OLESEN, PHILO BLUYSSEN AND CLAUDE-ALAIN ROULET 2.1 Introduction 62 2.2 Indoor environmental quality 63 2.3 Indoor air quality 64 2.4 Thermal comfort 78 2.5 Indoor environment and performance 95 References 99
3 Energy implications of indoor environment control 105 CLAUDE-ALAIN ROULET
3.1 Introduction 105 3.2 Energy flow in buildings 106 vi Contents
3.3 Assessing energy flows 116 3.4 Energy and indoor environment quality 121 3.5 Strategies for HVAC systems and components 137 3.6 Heat recovery 140 3.7 Effect of ventilation strategies on the energy use 146 3.8 Summary 150 Notes 151 References 151 Nomenclature 153
4 Modeling of ventilation airflow 155 JAMES AXLEY AND PETER V. NIELSEN
4.1 Introduction 155 4.2 Microscopic methods 156 4.3 Macroscopic methods 189 References 249 Nomenclature for macroscopic methods 260
5 Air distribution: system design 264 PETER V. NIELSEN AND HAZIM B. AWBI
5.1 Introduction 264 5.2 The room and the occupied zone 265
5.3 The qo– To relationship for the design of air distribution systems in rooms 267 5.4 Design of mixing ventilation 268 5.5 Design of displacement ventilation 276 5.6 Design of high momentum displacement ventilation 284 5.7 Design of vertical ventilation 288 5.8 Comparison between different air distribution systems 292 References 297
6 Characteristics of mechanical ventilation systems 300 CLAUDE-ALAIN ROULET
6.1 Introduction 300 6.2 Types of ventilation systems 300 6.3 Temperature and humidity control with mechanical ventilation (air-conditioning) 303 6.4 Components of air-handling units 306 Contents vii
6.5 Airflow rate measurements in ventilation systems 324 6.6 Summary 342 References 343 Nomenclature 344
7 Characteristics of natural and hybrid ventilation systems 345 PER HEISELBERG
7.1 Introduction 345 7.2 Ventilation concepts 349 7.3 System solutions and characteristics 352 7.4 Ventilation components 365 7.5 Control strategies 374 7.6 Examples 379 References 398
8 Measurement and visualization of air movements 400 CLAUDE-ALAIN ROULET
8.1 Introduction 400 8.2 Air velocity measurement 400 8.3 Measuring air tightness 407 8.4 Visualization of air movement 415 8.5 Age of air and air change efficiency 418 8.6 Mapping the age of air in rooms 428 8.7 Summary 435 References 436 Nomenclature 437
Index 439 Contributors
Hazim B. Awbi, Ph.D. is Professor of Building Environmental Science in the School of Construction Management and Engineering, University of Reading, UK, and founder of the Indoor Environment and Energy Research Group (IEERG). His research is in ventilation, room air movement, Computational Fluid Dynamics (CFD) and heat transfer in buildings. James Axley, Ph.D. is Professor at the School of Architecture and the School of Forestry and Environmental Studies, Yale University, USA and his research is on the development of theory and computational tools for building thermal, airflow, and air quality simulation and design analysis. Philomena Bluyssen, Ph.D. is a Research Scientist at TNO, Netherlands and has published extensively on indoor air quality. Per Heiselberg, Ph.D. is Professor in the Department of Civil Engineering and Head of Hybrid Ventilation Centre at Aalborg University, Denmark. Yuguo Li, Ph.D. is Professor in the Department of Mechanical Engineer- ing, University of Hong Kong and his research is in natural ventilation, CFD, bio-aerosols and engineering control of respiratory infection. Peter V. Nielsen, Ph.D., FASHRAE is Professor at Aalborg University, Denmark and Honorary Professor at the University of Hong Kong. His research is in room air movement and Computational Fluid Dynamics (CFD). He was awarded the John Rydberg Gold Medal in 2004. Bjarne W. Olesen is Professor and Head of the International Centre for Indoor Environment and Energy, Department of Mechanical Engineer- ing, Technical University of Denmark. The Centre is one of the world’s leading research centres in indoor environment, people’s health, com- fort and productivity. Claude-Alain Roulet, Ph.D., is Adjunct Professor at the EPFL (Swiss Federal Institute of Technology, Lausanne), Switzerland and private consultant in building physics and indoor environment quality. Preface
This book is authored by eight distinguished researchers in ventilation and indoor air quality from five countries. It is a follow-on from the success- ful book Ventilation of Buildings, which is authored by the title editor. The new title draws from the vast experience of the eight authors in the field, includes their knowledge of the subject and presents the results from extensive international research programmes involving the authors as well as results from the work of other researchers. The book deals with the applications of ventilation science in buildings. Buildings are responsible for a large proportion of a country’s total energy consumption and a large part of this is used in ventilation, i.e. heating, cooling and cleaning of outdoor air supplied to buildings. Properly designed ventilation systems are essential for maintaining good indoor air quality, which is necessary for a productive building as well as for reducing a build- ing’s energy consumption. To achieve these aims, it is essential that modern development in ventilation science is well understood and effectively applied by those involved in building and system design and maintenance. This book aims to provide the building professionals with up-to-date knowledge based on the experience of internationally recognised experts to enable them implementing current and future ventilation requirements in buildings. The book covers the fundamentals as well as the more advanced topics to cater for a wide range of readers. This unique publication covers the subject rigorously in a way needed by researchers and, at the same time, has a practical flavour and therefore should appeal to a wide range of building professionals. The book offers a comprehensive reference for researchers, designers, architects and specifiers of ventilation systems in buildings. Chapter 1 presents the fundamental principles and physics of the air- flow and heat transfer phenomena that occur within buildings. The basic fluid flow and heat transfer concepts and their analyses are presented with worked examples giving particular emphasis to the flow in enclosures. Chapter 2 presents the latest knowledge on human requirements for ther- mal comfort and air quality indoors and the impact of these on ventilation rates. The results and arguments presented in Chapter 2 show that there is x Preface a tendency for specifying higher fresh air supply rates in buildings than is recommended by most current ventilation guidelines. This will undoubtedly have a large impact on energy usage, which will require a proper assessment of the energy flow for ventilation to mitigate the impact. Chapter 3 describes methods used for assessing the energy flow in buildings and ventilation systems. It presents guidelines for improving the energy performance of buildings without compromising the indoor environment. Chapter 4 intro- duces the modeling of airflow into and within buildings by describing two categories of models that are commonly used nowadays: the macroscopic and the microscopic approaches. Whereas the macroscopic methods are based on modeling the air flow in buildings including their heating, ven- tilating and air-conditioning (HVAC) systems as collection of finite-sized control volumes, the microscopic methods, which are better known as com- putational fluid dynamics (CFD) models, on the other hand are based on the continuum approach that provides detailed descriptions of the flow, heat and mass transport processes within and outside the building. Chapter 5 deals with the characteristics of different types of air distribution systems, including new methods that have recently been developed, and the methods used for selecting and designing these for mechanically ventilated building enclosures. In Chapter 6, the types of HVAC systems are characterised, and the methods used for assessing the components of such systems are described, including the measurement techniques that are used to assess their performance. Chapter 7 describes the characteristics and performance of natural and hybrid ventilation systems and their components. Such systems are finding wider applications in modern buildings as, if properly designed, these can provide good indoor environment at lower energy consumption than conventional mechanical systems. Examples of buildings using hybrid systems are also presented. Finally, Chapter 8 describes various techniques that are applied in ventilation and room air movement measurements and airflow visualization. Such techniques are very useful for setting-up, com- missioning and maintaining ventilation systems as well as estimating the airflow through the building envelop.
Hazim B. Awbi Reading, UK, 2007 Acknowledgements
The following figures and tables are reproduced with permission:
Table 2.3 ‘Smoking free spaces in commercial buildings’ according to ASHRAE 62.1, CR 1752, and EN15251. © American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., www.ashrae.org Figure 2.5 (a) de Dear, R. and Brager, G. S. (1998) ‘Developing an adap- tive model of thermal comfort and preference’, ASHRAE Transactions, 104(1a): 145–167. © American Society of Heating, Refrigerating and Air- Conditioning Engineers, Inc., www.ashrae.org Figure 5.25 Nielsen, P. V., Topp, C., Snnichsen, M. Andersen, H. (2005). ‘Air distribution in rooms generated by a textile terminal – comparison with mixing ventilation and displacement ventilation’. ASHRAE Transactions 111 (Part 1): 733–739. © American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., www.ashrae.org
Chapter 1 Airflow, heat and mass transfer in enclosures
Yuguo Li
1.1 Introduction Airflow and transport phenomena play an important role in air quality, thermal comfort and energy consumption in buildings. Advances in air- flow control in buildings in the past four decades have made it possible to design and evaluate building ventilation not only qualitatively but in many situations also quantitatively. In recent years, a broad range of practical ventilation problems have been investigated by the application of computa- tional fluid dynamics (CFD) and advanced airflow measurement methods. This chapter describes the fundamental principles of airflow, heat and mass transfer phenomena that take place in buildings. The need of empha- sizing multi-disciplinary nature is noted here. Much of the basic theory and concepts on airflow, heat and mass transfer are described in clas- sical textbooks of heat transfer and fluid mechanics, with new develop- ments reported in journals such as Journal of Heat Transfer, Journal of Fluid Mechanics and International Journal of Heat and Mass Transfer. Historically, the concepts and technologies developed in other engineering disciplines have also been successfully applied and extended to ventila- tion application. Examples include the application of the residence time concept (Danckwerts, 1952) developed in chemical engineering to venti- lation efficiency (Sandberg, 1981) and the application of CFD originally developed for the aerospace industry (Nielsen, 1974). Contribution of the ventilation community to fluid mechanics and heat and mass trans- fer has also been evident, such as the development of non-isothermal jets (Koestel, 1955). The ultimate goal of an in-depth understanding of fluid mechanics in building airflow is to provide engineers effective and efficient design and analysis tools. Either experiments or numerical predictions (which may be considered as numerical experiments) can only provide data, but no conclusions. It is important that the fundamental principles can be applied in analysing the data from either experiments or CFD and also drawing conclusions from data. The quality of data is crucial for drawing any good or new conclusions. Effective and accurate methods for obtaining the required 2 Yuguo Li data are important parts of any airflow study, which again require good understanding of the fundamental principles. Airflow problems in buildings can be treated at various levels of theoretical rigour depending on the specific task and physical complexity of the problem.
1.2 Transport phenomena in buildings
1.2.1 Basic concepts Fluid flow and transport phenomena are inherently associated with build- ings as the primary function of buildings is to create an adequate indoor environment for the occupants or equipment therein. It is now established that the velocity, turbulence, temperature and humidity are all important thermo-fluid parameters for thermal comfort (Fanger, 1970; Fanger et al., 1998), and some other quantities such as the composition of air, particle contents, odours etc. are all important parameters for indoor air quality (Spengler et al., 2001). One of the basic fluid flow and transport problems in buildings is in the design of air distribution systems. Figures 1.1–1.3 show sketches of three different ventilation systems, i.e. mixing ventilation (Figure 1.1), displacement ventilation (Figure 1.2) and kitchen local exhaust ventilation using a range hood (Figure 1.3). The three systems involve a broad range of airflow, heat and mass transfer phenomena, including wall jets (Figure 1.1), thermal plumes (Figures 1.2 and 1.3), gravity currents (Figure 1.2), natural convection along vertical walls (Figure 1.3), heat transfer and pollutant transport (all). The airflow in these systems may be analysed using two different approaches. Let us take displacement ventilation as an example.
Figure 1.1 A sketch of air distribution pattern in a room ventilated by a mixing system. Airflow, heat and mass transfer in enclosures 3
Figure 1.2 A sketch of air distribution pattern in a room ventilated by a displacement system.
Figure 1.3 A sketch of airflow pattern through a kitchen local exhaust ventilation using a range hood.
• if our interest is to ensure that the location of the interface between the lower clean zone and the upper polluted zone, a macroscopic approach involving the use of the macroscopic mass balance equations can be used. The location of the interface is approximately taken where the total upward flow rate of the plume and the vertical boundary layer is equal to the supply airflow rate. • however, if the detailed air velocity and turbulence level in the occu- pied region (particularly close to the supply register) is of interest, a microscopic approach involving the use of the differential governing equations of airflow will have to be solved. 4 Yuguo Li
Figure 1.4 Illustration of the air exchange between zones in a building and between a building and outdoor environment.
The airflow phenomena in buildings are often complicated by its inter- action with the outdoor air environment through window openings and leakages. For ventilation and indoor air quality design, it is often required to determine the air-change rate across the building envelope and between the zones (rooms) within the same building (Figure 1.4). When a window is open, it may be termed as natural ventilation flow rate. When the air enters a building through background leakages, it is called air infiltration. Various fluid flow phenomena also occur in heating, ventilating and air- conditioning equipment and components, including ducts, diffusers, fans, air-handling units, airflows, heat exchangers, etc. The fluid flow problems discussed so far may broadly be classified into two categories:
• macroscopic fluid flow problems, in which the objective is to develop overall relationships between some lumped flow parameters. Examples include the required exhaust flow rate (fan power) for achieving a 100 per cent capture efficiency in kitchen range hoods (Li and Delsante, 1996); the dependence of air-change rate on the pressure difference across a building envelope, the heating and cooling load of a house because of air infiltration, the dependence of the clean zone interface in displacement ventilation on supply airflow rates, etc. • microscopic fluid flow problems, in which the objective is to obtain detailed distribution of primitive airflow variables, such as the velocity distribution rather than the volumetric flow rate, the spatial tempera- ture distribution rather than the total heat gain or loss. Airflow, heat and mass transfer in enclosures 5
The macroscopic and microscopic fluid flow problems described above are related in nature. Generally, one could obtain the macroscopic flow quantities by integrating all the necessary microscopic quantities, but not vice versa.
1.2.2 Characteristic scales and non-dimensional parameters One difficulty when attempting to analyse the fluid flow phenomena in buildings is that there are many geometrical and physical parameters that govern or influence the flow. These parameters can be the geometry, type and location of the air supply registers and exhaust openings, the supply air velocity, the heat generation in the room, etc. It is therefore helpful to estimate the order of length, time and velocity scales of airflow system. Such an analysis helps to identify the dominant parameters/factors in the flow and thus provides a reference point for further analysis. The upper limit of the length scale may be the duct diameter, the dimen- sion of the supply register and room or the dimension of furniture causing the flow disturbance. Let this typical length dimension be L. The charac- teristic velocity, U, which can be the supply air velocity or some kind of average velocity. The characteristic overall convection time scale, tc, for a fluid element to be advected along the dimension L by the velocity U is
L t = (1.1) c U A second characteristic time scale because of the viscous diffusion may also be defined as: