Total Heat Recovery: Heat and Moisture Recovery from Ventilation Air

327 Pages·2008·5.56 MB·English

T H R :
OTAL EAT ECOVERY
H M R
EAT AND OISTURE ECOVERY
FROM VENTILATION AIR

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T H R :
OTAL EAT ECOVERY
H M R
EAT AND OISTURE ECOVERY
V A
FROM ENTILATION IR

LI-ZHI ZHANG

Nova Science Publishers, Inc.
New York
Copyright © 2008 by Nova Science Publishers, Inc.

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LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA
Zhang, Li-Zhi.
Total heat recovery : heat & moisture recovery from ventilation air / Li-Zhi Zhang.
p. cm.
Includes bibliographical references and index.
ISBN 978-1-60876-275-0 (E-Book)
1. Heat exchangers. 2. Ventilation. 3. Heat recovery. 4. Condensation. 5. Moisture. 6.
Humidity--Control. 7. Water harvesting. 8. Buildings--Energy conservation. I. Title.
TH7683.H42Z48 2009
621.402--dc22 2008036864

Published by Nova Science Publishers, Inc. (cid:17) New York
CONTENTS

Preface vii
Chapter 1 Total Heat Recovery in Air-conditioning 1
Chapter 2 Energy Recovery Potentials 9
Chapter 3 Estimation of Sorption and Diffusion Properties of Hygroscopic
Materials 15
Chapter 4 Performance of Energy Wheels 55
Chapter 5 Heat Mass Transfer in Bended Sinusoidal Narrow Ducts 75
Chapter 6 Convective Heat Mass Transfer in Plate-fin Channels 99
Chapter 7 Effectiveness Correlations of Total Heat Exchangers 131
Chapter 8 Numerical Simulation of Total Heat Exchangers 155
Chapter 9 Novel Membranes for Total Heat Exchanger 187
Chapter 10 Heat Mass Transfer in Cross-corrugated Triangular Ducts 229
Chapter 11 Applications of Total Heat Recovery 269
Index 307

PREFACE

Energy and environment are two hot topics world wide today. Energy has been described
as “that which makes things go.” It is seen clearly in the transfer of heat and work inside
environmental systems. Heat occurs due to a temperature difference between the system and
its surroundings, while work makes use of that difference to perform a function. The
depleting nature of primary energy resources, negative environmental impact of fossil fuels
and low exergetic efficiencies obtained in conventional space heating and cooling are the
main incentives for developing alternative heating, ventilating and air-conditioning (HVAC)
techniques which can either save energy or employ low-grade thermal energy sources. Today,
air conditioning has accounted for 1/3 of the total energy use by the whole society. The
percentages are still rising in the fast developing economies like China.
The design of HVAC systems for thermal comfort requires increasing attention,
especially in light of recent regulations and standardization on ventilation, so that an optimal
level of indoor humidity may be reached and maintained to ensure a comfortable and healthy
environment and to avoid condensation damage to the building envelope and furnishings.
Fresh air ventilation is necessary, not only for breathing purposes, but also for the prevention
of deadly epidemic diseases like bird flu and SARs. Energy expenses from ventilation are
very heavy. Ventilation air accounts for 20-40% of the cooling load for HVAC industry. The
ratio can be even higher in hot and humid regions where latent load from fresh air is as heavy
as 50% of the cooling load. To reduce this part of energy is very crucial for the reduction of
energy consumption of the whole HVAC system.
Total heat exchangers (enthalpy exchangers, or the so-called energy recovery ventilators)
could save a large fraction of energy for cooling and dehumidifying the fresh air since cool air
and dryness would be recovered from the exhaust stream to the fresh air in summer. Similarly
in winter, the total heat exchangers could also save energy because exhaust moisture and heat
can be recovered to save heating and humidification energy for fresh air. With total heat
exchangers, the efficiency of the existing HVAC systems can be improved too. When they are
combined with independent air dehumidification units, some alternative cooling technologies
like deep well water cooling, chilled-ceiling, phase-change material cooling storage can be
used in practice. They represent a novel trend in the sustainable development of HVAC
industry.
Social resources spent in energy conservation and environmental protection have been
increased substantially. Scientists and engineers worldwide are in active pursuit of novel
energy recovery technologies. The topic has drawn my attention, too. In fact, my research
viii Li-Zhi Zhang
career has been centered around energy recovery and air dehumidification for more than a
dozen years. From these years’ work, I have the deep feeling that total heat recovery, though
very significant and interesting, is not an easy task.
The first obstacle comes from the materials. The novel total heat recovery technology
requires novel adsorbent materials or highly vapor-permeable membrane materials to fulfill
this task. However, there has been only limited information disclosed from public sources
until now. Most of the novel materials are unavailable commercially and even if they are
available, their cost is a problem. The second obstacle comes from the insufficient
accumulation of heat mass transfer data for the total heat exchangers. Novel materials have
led to new heat mass transfer phenomenon. Simultaneous heat mass transfer is the major
phenomenon for total heat exchangers. However, information on mass transfer in heat mass
exchangers is still scarce. The available information in text books and references is for
sensible heat transfer only. Mass transfer in the total heat exchangers, especially with the
novel materials, has been not considered sufficiently. As a result, system design and
performance analysis of total heat exchangers are difficult, which has prevented their market
penetrations.
To overcome these problems, in these years, I have conducted many fundamental
researches on novel total heat recovery systems, from materials fabrication to heat mass
transfer analysis. Numerical modeling has provided me an efficient tool. These results are
helpful to advance this technology from a notion to a real application. However, these data
are still quite sporadic. It’s not easy to have full access to them. On the other hand, a
systematic introduction on this technology is highly desired, because energy saving in the air
conditioning industry has become a hot issue in these days, with oil prices skyrocketing. This
background has prompted me to write this book. I hope this technology can be systematically
introduced to the public through this book.
In this book, the systems and performances used for total heat recovery are introduced.
Energy wheels and membrane based total heat exchangers are specially described. Heat and
mass transfer modeling of the systems are performed. Influences of key material and design
parameters on the system performance are discussed. Novel membranes including
hydrophobic-hydrophilic composite membrane and composite supported liquid membrane are
developed for total heat exchangers and are characterized. Besides material side
intensification, air side intensification measures are taken as well. Plate-fin and cross
corrugated triangular ducts are two important structures that are introduced. The basic
transport data in these structures are provided. Convective heat and mass transfer coefficients
in plate-fin ducts of finite fin conductance with various cross sections are numerical obtained.
Fluid flow and heat transfer in cross-corrugated triangular ducts are estimated by considering
laminar, transitional, and turbulent complex flow regimes.
Based on the fundamental heat mass transfer data, the book illustrates some examples of
the applications of total heat recovery in novel HVAC systems. Chilled-ceiling combined
with desiccant cooling and independent air dehumidification are two pioneering trends in air
conditioning industry. They overcome the shortcomings of conventional all air systems by
decoupling the treatment of latent load from sensible load. Partial or full total heat recovery
are realized in combination with these novel systems, which contribute to reduced energy use
with increased indoor humidity control, even in transit seasons when traditional air
conditioning systems fail to control humidity. The component modeling of various key
equipments like refrigeration cycle, heat pumps, regenerative wheels, heat exchangers,
Preface ix
cooling coils, are conducted to estimate their energy performance and their effects on indoor
thermal and humidity performance.
The book combines theoretical analysis with engineering practices. It covers a wide range
of knowledge from fundamental heat mass transfer to novel systems design and performance
analysis, from materials synthesis, characterization to heat exchanger thermodynamics and
fluid dynamics. I hope the book may give some insight and design guidelines for the total
heat recovery oriented air conditioning energy conservation.
When writing the book, I am keeping in mind that the fundamentals and methodologies
be given the priority. The goal is not only to tell engineers and students what to do, but how
to do. As a basis, first priority is given to the synthesis and characterization of novel materials
for total heat recovery. Another emphasis is on energy system modeling. To evaluate an
energy system, detailed steps from physical model setting up, mathematical model setting up,
solution method, experimental and numerical validation, and parametric studies, are
illustrated. Generally, it is a cross-discipline endeavor which relies heavily on numerical heat
mass transfer for system modeling. I hope peer engineers and research students could benefit
from the methodologies exhibited in this analysis and extend them to the analysis of other
energy systems.
At last, I would like to thank my family for their long-term support of researches on this
topic. I would also like to thank the Natural Science Foundation of China (NSFC) for their
continuing financial support in my researches of total heat recovery. I am indebted to my
colleagues at South China University of Technology, and others all over China and
throughout the worldwide who have provided suggestions and ideas which, in no small way,
have contributed to the fabric of this text. I have always strived to remain conscious of
student learning needs and difficulties, and I am grateful to my many research students, at
South China University of Technology and elsewhere, who have provided positive
reinforcement for my efforts.