International Cryogenics Monograph Series – serie

The purposes of this book are to provide insight and to draw attention to problems peculiar to heat transfer at low temperatures. This does not imply that the theories of classical heat transfer fail at low temperatures, but rather that many of the approximations employed in standard solutions techniques are not valid in this regime. Physical properties, for example, have more pronounced variations at low temperatures and cannot, as is conventionally done, be held constant. Fluids readily become mixtures of two or more phases and their analysis is different from that for a single-phase fluid. These and other problems which occur more frequently at low temperatures than at standard conditions are discussed in this book. Although the title specifies heat transfer, the book also contains a very comprehensive chapter on two-phase fluid flow and a partial chapter on the flow of fluids in the thermodynamically critical state. Emphasis is placed on those flow phenomena that occur at low temperatures. Flow analyses are, of course, a prerequisite to forced-convection heat transfer analyses, and thus these chapters add continuity to the text. The book is primarily written for the design engineer, but does broach many topics which should prove interesting to the researcher. For the student and teacher the book will serve as a useful reference and possibly as a text for a special topics course in heat transfer.

Most descriptions of polymers start at room temperature and end at the melting point. This textbook starts at very low temperatures and ends at room temperature. At low temperatures, may processes and relaxations are frozen which allows singular processes or separate relaxations to be studied. At room temperatures, or at the main glass transitions, many processes overlap and the properties are determined by relaxations. At low temperatures, there are temperature ranges with negligible influences by glass transitions. They can be used for investigating so-called basic properties which arise from principles of solid state physics. The chain structure of polymers, however, requires stringent modifications for establishing solid state physics of polymers. Several processes which are specific of polymers, occur only at low temperatures. There are also technological aspects for considering polymers at low temperatures. More and more applications of polymeric materials in low­ temperature technology appear. Some examples are thermal and electrical insulations, support elements for cryogenic devices, low-loss materials for high­ frequency equipments. It is hoped that, in addition to the scientific part, a data collection in the appendix may help to apply polymers more intensively in low­ temperature technology. The author greatly appreciates the contributions by his coworkers of the Kernforschungszentrum Karlsruhe in measurement and discussion of many data presented in the textbook and its appendix. Fruitful disccussions with the colleagues Prof. H. Baur, Prof. S. Hunklinger, Prof. D. Munz and Prof. R.

The importance of safety in any scientific endeavor is never in question. However, when cryogenic temperatures are involved, safety is especially important. In addition to observing the normal precautions, one must also take into account the variations of physical properties that occur at low temperatures. At these tempera­ tures, some properties not only exhibit large differences from their normal values but also can vary widely over a small temperature range. Before any cryogenic project is started, a thorough knowledge of the possible hazards is necessary. Only in this way can the safest operation be attained. Over the hundred-year history of cryogenic research, this has been shown to be the case. Keeping this requirement in mind is an essential ingredient in the quest for accident-free work. The past four or five decades have seen a great expansion of cryogenic technology. Cryogenic liquids, such as oxygen, nitrogen, hydrogen, and helium, have become commonly used in a number of different applications and are easily available in any part of the United States and, indeed, almost anywhere in the world. Not only are these liquids available, they have become less expensive and also available in ever larger quantities. As quantities increase, so also do the conse­ quences of mishaps. The future seems to hold promise of ever larger and more widespread use of the common cryogens. Thus, the importance of safety also increases as time progresses.

An in-depth survey of regenerative heat exchangers, this book chronicles the development and recent commercialization of regenerative devices for cryogenic applications. Chapters cover historical background, concepts, practical applications, design data, and numerical solutions, providing the latest information for engineers to develop advanced cryogenic machines. The discussions include insights into the operation of a regenerator; descriptions of the cyclic and fluid temperature distributions in a regenerator; data for various matrix geometries and materials, including coarse and fine bronze, stainless steel-woven wire mesh screens, and lead spheres; and unique operating features of cryocoolers that produce deviations from ideal regenerator theory.

The birth of this monograph is partly due to the persistent efforts of the General Editor, Dr. Klaus Timmerhaus, to persuade the authors that they encapsulate their forty or fifty years of struggle with the thermal properties of materials into a book before they either expired or became totally senile. We recognize his wisdom in wanting a monograph which includes the closely linked properties of heat capacity and thermal expansion, to which we have added a little 'cement' in the form of elastic moduli. There seems to be a dearth of practitioners in these areas, particularly among physics postgraduate students, sometimes temporarily alleviated when a new generation of exciting materials are found, be they heavy fermion compounds, high­ temperature superconductors, or fullerenes. And yet the needs of the space industry, telecommunications, energy conservation, astronomy, medical imaging, etc. , place demands for more data and understanding of these properties for all classes of materials - metals, polymers, glasses, ceramics, and mixtures thereof. There have been many useful books, including Specific Heats at Low Tempera­ tures by E. S. Raja Gopal (1966) in this Plenum Cryogenic Monograph Series, but few if any that covered these related topics in one book in a fashion designed to help the cryogenic engineer and cryophysicist. We hope that the introductory chapter will widen the horizons of many without a solid state background but with a general interest in physics and materials.

Cryogenic Engineering: Fifty Years of Progress is a benchmark reference work which chronicles the major developments in the field. Starting with an historical background dating to the 1850s, this book reviews the development of data resources now available for cryogenic fields and properties of materials. The advances in cryogenic fundamentals are covered by reviews of cryogenic principles, cryogenic insulation, low-loss storage systems, modern liquefaction processes, helium cryogenics and low-temperature thermometry. Several well-established applications resulting from cryogenic advances include aerospace cryocoolers and refrigerators, use of LTS and HTS systems in electrical applications, and recent changes in cryopreservation. Extensive references are provided for the readers interested in the details of these cryogenic engineering advances.

Most conventional cryogenic refrigerators and liquefiers operate with pure fluids, the major exception being natural gas liquefiers that use mixed refrigerant processes. The fundamental aspects of mixed refrigerant processes, though very innovative, have not received the due attention in open literature in view of commercial interests. Hundreds of patents exist on different aspects of mixed refrigerant processes. However, it is difficult to piece together the existing information to choose an appropriate process and an optimum composition or a given application. The aim of the book is to teach (a.) the need for refrigerant mixtures, (b.) the type of mixtures that can be used for different refrigeration and liquefaction applications, (c.) the different processes that can be used and (d.) the methods to be adopted for choosing the components of a mixture and their concentration for different applications.

Cryogenic Engineering: Fifty Years of Progress is a benchmark reference work which chronicles the major developments in the field. Starting with an historical background dating to the 1850s, this book reviews the development of data resources now available for cryogenic fields and properties of materials. The advances in cryogenic fundamentals are covered by reviews of cryogenic principles, cryogenic insulation, low-loss storage systems, modern liquefaction processes, helium cryogenics and low-temperature thermometry. Several well-established applications resulting from cryogenic advances include aerospace cryocoolers and refrigerators, use of LTS and HTS systems in electrical applications, and recent changes in cryopreservation. Extensive references are provided for the readers interested in the details of these cryogenic engineering advances.

Most conventional cryogenic refrigerators and liquefiers operate with pure fluids, the major exception being natural gas liquefiers that use mixed refrigerant processes. The fundamental aspects of mixed refrigerant processes, though very innovative, have not received the due attention in open literature in view of commercial interests. Hundreds of patents exist on different aspects of mixed refrigerant processes. However, it is difficult to piece together the existing information to choose an appropriate process and an optimum composition or a given application. The aim of the book is to teach (a.) the need for refrigerant mixtures, (b.) the type of mixtures that can be used for different refrigeration and liquefaction applications, (c.) the different processes that can be used and (d.) the methods to be adopted for choosing the components of a mixture and their concentration for different applications.

Most descriptions of polymers start at room temperature and end at the melting point. This textbook starts at very low temperatures and ends at room temperature. At low temperatures, may processes and relaxations are frozen which allows singular processes or separate relaxations to be studied. At room temperatures, or at the main glass transitions, many processes overlap and the properties are determined by relaxations. At low temperatures, there are temperature ranges with negligible influences by glass transitions. They can be used for investigating so-called basic properties which arise from principles of solid state physics. The chain structure of polymers, however, requires stringent modifications for establishing solid state physics of polymers. Several processes which are specific of polymers, occur only at low temperatures. There are also technological aspects for considering polymers at low temperatures. More and more applications of polymeric materials in low­ temperature technology appear. Some examples are thermal and electrical insulations, support elements for cryogenic devices, low-loss materials for high­ frequency equipments. It is hoped that, in addition to the scientific part, a data collection in the appendix may help to apply polymers more intensively in low­ temperature technology. The author greatly appreciates the contributions by his coworkers of the Kernforschungszentrum Karlsruhe in measurement and discussion of many data presented in the textbook and its appendix. Fruitful disccussions with the colleagues Prof. H. Baur, Prof. S. Hunklinger, Prof. D. Munz and Prof. R.

This 2nd edition volume of Modern Gas-Based Temperature and Pressure Measurements follows the first publication in 1992. It collects a much larger set of information, reference data, and bibliography in temperature and pressure metrology of gaseous substances, including the physical-chemical issues related to gaseous substances. The book provides solutions to practical applications where gases are used in different thermodynamic conditions. Modern Gas-Based Temperature and Pressure Measurements, 2nd edition is the only comprehensive survey of methods for pressure measurement in gaseous media used in the medium-to-low pressure range closely connected with thermometry. It assembles current information on thermometry and manometry that involve the use of gaseous substances which are likely to be valid methods for the future. As such, it is an important resource for the researcher. This edition is updated through the very latest scientific and technical developments of gas-based temperature and pressure measurements using thermometry and manometry, and brings all of the techniques together under one cover. This book fills the gap in international literature, as no other recently published book provides a comprehensive survey for gaseous media closely connected with thermometry.Updates in this new edition include revised appendices and new chapters on Mutual Recognition Agreement of the Comité International des Poids et Mesures and its main applications, and developments in the European Metrology Society.

This book is the first in English being entirely dedicated to Miniature Joule-Thomson Cryocooling. The category of Joule-Thomson (JT) cryocoolers takes us back to the roots of cryogenics, in 1895, with figures like Linde and Hampson. The "cold finger" of these cryocoolers is compact, lacks moving parts, and sustains a large heat flux extraction at a steady temperature. Potentially, they cool down unbeatably fast. For example, cooling to below 100 K (minus 173 Celsius) might be accomplished within only a few seconds by liquefying argon. A level of about 120 K can be reached almost instantly with krypton. Indeed, the species of coolant plays a central role dictating the size, the intensity and the level of cryocooling. It is the JT effect that drives these cryocoolers and reflects the deviation of the "real" gas from the ideal gas properties. The nine chapters of the book are arranged in five parts.•The Common Principle of Cyrocoolers shared across the broad variety of cryocooler types•Theoretical Aspects: the JT effect and its inversion, cooling potential of coolants, the liquefaction process, sizing of heat exchangers, level of pressurization, discharge of pressure vessels• Practical Aspects: modes of operation (fast cooldown, continuous, multi-staging, hybrid cryocoolers), pressure sources, configuration, construction and technologies, flow adjustment, MEMS, open and closed cycle, cooldown process and similarity, transient behavior• Mixed Coolant cryocooling: theory, practice and applications• Special Topics: real gas choked flow rates, gas purity, clog formation, optimal fixed orifice, modeling, cryosurgical devices, warming by the inverse JT effectThe theoretical aspects may be of interest not only to those working with cryocoolers but also for others with a general interest in "real" gas thermodynamics, such as, for example, the inversion of the JTeffect in its differential and integral forms, and the exceptional behavior of the quantum gases.A detailed list of references for each chapter comprises a broad literature survey. It consists of more than 1,200 relevant publications and 450 related patents. The systematically organized content, arranged under a thorough hierarchy of headings, supported by 227 figures and 41 tables, and accompanied by various chronological notes of evolution, enables readers a friendly interaction with the book. Dr. Ben-Zion Maytal is a Senior Researcher at Rafael-Advanced Defense Systems, Ltd., and an Adjunct Senior Teaching Fellow at the Technion-Israel Institute of Technology, Haifa, Israel.Prof. John M. Pfotenhauer holds a joint appointment in the Departments of Mechanical Engineering and Engineering Physics at the University of Wisconsin - Madison.

Twenty five years have elapsed since the original publication of Helium Cryogenics. During this time, a considerable amount of research and development involving helium fluids has been carried out culminating in several large-scale projects. Furthermore, the field has matured through these efforts so that there is now a broad engineering base to assist the development of future projects.Helium Cryogenics, 2nd edition brings these advances in helium cryogenics together in an updated form. As in the original edition, the author's approach is to survey the field of cryogenics with emphasis on helium fluids. This approach is more specialized and fundamental than that contained in other cryogenics books, which treat the associated range of cryogenic fluids. As a result, the level of treatment is more advanced and assumes a certain knowledge of fundamental engineering and physics principles, including some quantum mechanics.The goal throughout the work is to bridge the gap between the physics and engineering aspects of helium fluids to provide a source for engineers and scientists to enhance their usefulness in low-temperature systems.Dr. Van Sciver is a Distinguished Research Professor and John H. Gorrie Professor of Mechanical Engineering at Florida State University. He is also a Program Director at the National High Magnetic Field Laboratory (NHMFL). Dr. Van  Sciver joined the FAMU-FSU College of Engineering and the NHMFL in 1991, initiating and teaching a graduate program in magnet and materials engineering and in cryogenic thermal sciences and heat transfer. He also led the NHMFL development efforts of the cryogenic systems for the NHMFL Hybrid and 900 MHz NMR superconducting magnets. Between 1997 and 2003, he served as Director of Magnet Science and Technology at the NHMFL. Dr. Van Sciver is a Fellow of the ASME and the Cryogenic Society of America and American Editor for the journal Cryogenics. He is the 2010 recipient of the Kurt Mendelssohn Award.Prior to joining Florida State University, Dr. Van Sciver was Research Scientist and then Professor of Nuclear Engineering, Engineering Physics and Mechanical Engineering at the University of Wisconsin-Madison from 1976 to 1991. During that time he also served as the Associate Director of the Applied Superconductivity Center. Dr. Van Sciver received his PhD in Low Temperature Physics from the University of Washington-Seattle in 1976. He received his BS degree in Engineering Physics from Lehigh University in 1970. Dr. Van Sciver is author of over 200 publications and patents in low temperature physics, liquid helium technology, cryogenic engineering and magnet technology. The first edition of Helium Cryogenics was published by Plenum Press (1986). The present work is an update and expansion of that original project.

Scope and Purpose Although conductors based on the Al5 intermetallic compound Nb Sn 3 possess desirable high-field superconducting properties, manufacturing and handling difficulties, coupled with the tendency of their critical current densities to degrade rapidly under stress, have generally restricted their use to fairly straightforward, usually small-scale solenoidal-magnet applica­ tions. Likewise the Al5 compound VGa, which has a wider critical strain 3 window than NbSn but a uniformly lower upper critical field, has not 3 entered widespread service. Strain has been found to have no measurable influence on either the critical fields or the critical current densities of compound superconductors with BI and Cl5 crystal structures, but as yet they are still in the research and development stages. On the other hand, conductors using the binary alloy Ti-Nb or multi component alloys based on it, because of their relative ease of manufacture, excellent mechanical properties, and relatively low strain sensitivities, are now being pressed into service in numerous large-scale devices. Such conductors are being wound into magnets for use in energy storage, energy conversion (i. e. , generators and motors), and high-energy particle detectors and beam-handling magnets. of cold-rolled or drawn Ti-Nb-alloy wire for superconducting The use magnet applications was first proposed in 1961. During the ensuing ten years, while progress was being made in the development of Cu-clad filamentary-Ti-Nb-alloy conductors, Ti-Nb and other Ti-base binary transi­ tion-metal (TM) alloys were being employed as model systems in the fundamental study of type-II superconductivity.

The birth of this monograph is partly due to the persistent efforts of the General Editor, Dr. Klaus Timmerhaus, to persuade the authors that they encapsulate their forty or fifty years of struggle with the thermal properties of materials into a book before they either expired or became totally senile. We recognize his wisdom in wanting a monograph which includes the closely linked properties of heat capacity and thermal expansion, to which we have added a little 'cement' in the form of elastic moduli. There seems to be a dearth of practitioners in these areas, particularly among physics postgraduate students, sometimes temporarily alleviated when a new generation of exciting materials are found, be they heavy fermion compounds, high­ temperature superconductors, or fullerenes. And yet the needs of the space industry, telecommunications, energy conservation, astronomy, medical imaging, etc. , place demands for more data and understanding of these properties for all classes of materials - metals, polymers, glasses, ceramics, and mixtures thereof. There have been many useful books, including Specific Heats at Low Tempera­ tures by E. S. Raja Gopal (1966) in this Plenum Cryogenic Monograph Series, but few if any that covered these related topics in one book in a fashion designed to help the cryogenic engineer and cryophysicist. We hope that the introductory chapter will widen the horizons of many without a solid state background but with a general interest in physics and materials.

With the increased interest in superconductivity applications through­ out the world and the necessity of obtaining a firmer understanding of the basic concepts of superconductivity, the editors of the In­ ternational Cryogenics Monograph series are extremely grateful for the opportunity to add Superconducting Materials to this series. This comprehensive review and summary of superconducting ma­ terials was originally prepared by the Russian authors in 1969 and has been specifically updated for this series. It is the most thorough review of the literature on this subject that has been made to date. Since advances in the development and use of new superconducting materials are largely associated with the general state and level in the development of the physical theory of superconductivity, the physical chemistry of metals, metallography, metal physics, tech­ nical physics, and manufacturing techniques, it is hoped that this monograph will provide the stimulus for further advances in all aspects of this exciting field. The editors express their appreciation to the authors, the translators, and Plenum Publishing Corporation for their assistance and continued interest in making this worthy addition to the series possible.

In writing this monograph, the aim has been to consider the mechanical properties of the wide range of materials now available in such a way as to start with the fundamental nature of these properties and to follow the discussion through to the point at which the reader is able to comprehend the significance or otherwise of the large amounts of data now available in design manuals and other compilations. In short, it is hoped that this volume will be used as a companion to these data compilations and as an aid to their interpretation. In attempting to cover such a wide field, a large degree of selection has been necessary, as complete volumes have been written on topics which here have had to be covered in a few pages or less. It is inevitable that not everyone will agree with the choice made, especially if it is his own subject which has been discussed rather briefly, and the author accepts full res­ ponsibility for the selection made. The book is written at a level which should be easily followed by a university graduate in science or engineer­ ing, although, if his background has not included a course in materials science, some groundwork may be lacking.

* Electron optics involves the influence of electric and magnetic fields on electron beams. In those electron optical instruments utilizing magnetic fields, a replacement of the conventional, i.e .. nonsuperconducting, electron optical parts, is worth considering if the outstanding magnetic properties of superconductors can improve the systems. However, the use of superconductors demands complicated cryogenic techniques and this, of course, dampens enthusiasm. There are fields, however, where there are extreme requirements on the optical systems, namely, electron microscopy and high-energy physics. The great advantage of the combination of electron optics and superconductivity in these domains has been demonstrated in recent experiments. This monograph is mainly concerned with electron micros­ copy. Superconductivity in high-energy electron optics is treated only briefly, in Appendix A, since the author is little acquainted with the details of the projects. Furthermore, the number of experiments as yet carried out is small. In Appendix B, electron microscope studies of basic superconductor phenomena are reviewed. This material is included, even though it is only slightly connected with the main topic of the book, since a breakthrough in this field may be possible by the application of superconducting lenses.

The rapidly expanding use of very low temperatures in research and high technology during the last several decades and the concurrent high degree of activity in cryogenic engineering have mutually supported each other, each improvement in refrigeration technique making possible wider oppor­ tunities for research and each new scientific discovery creating a need for a refrigerator with special features. In this book, Professor Walker has provided us with an excellent exposition of the achievements of this period, the fundamental principles involved, and a critical examination of the many different cryogenic systems which have led to a new era of low-level refrigeration. I feel fortunate to have had a part in the developments discussed in this book. During the early 1930s I constructed several rotary engines using leather vanes. Their performance was not good, but I was able to liquefy air. I had been impressed by the usefulness of leather cups in tire pumps and in Claude-type engines for air liquefaction. I was trying to find a way to avoid that part of the friction generated by a leather cup as a result of the radial force of the working gas on the cylindrical part of the cup. During the 1950s I built two efficient helium liquefiers in which essentially leather pistons were used.

Cryogenics, a term commonly used to refer to very low temperatures, had its beginning in the latter half of the last century when man learned, for the first time, how to cool objects to a temperature lower than had ever existed na tu rally on the face of the earth. The air we breathe was first liquefied in 1883 by a Polish scientist named Olszewski. Ten years later he and a British scientist, Sir James Dewar, liquefied hydrogen. Helium, the last of the so-caBed permanent gases, was finally liquefied by the Dutch physicist Kamerlingh Onnes in 1908. Thus, by the beginning of the twentieth century the door had been opened to astrange new world of experimentation in which aB substances, except liquid helium, are solids and where the absolute temperature is only a few microdegrees away. However, the point on the temperature scale at which refrigeration in the ordinary sense of the term ends and cryogenics begins has ne ver been weB defined. Most workers in the field have chosen to restrict cryogenics to a tem­ perature range below -150°C (123 K). This is a reasonable dividing line since the normal boiling points of the more permanent gases, such as helium, hydrogen, neon, nitrogen, oxygen, and air, lie below this temperature, while the more common refrigerants have boiling points that are above this temperature. Cryogenic engineering is concerned with the design and development of low-temperature systems and components.

This work was begun quite some time ago at the University of Oxford during the tenure of an Overseas Scholarship of the Royal Commission for the Exhibition of 1851 and was completed at Banga­ lore when the author was being supported by a maintenance allowance from the CSIR Pool for unemployed scientists. It is hoped that significant developments taking place as late as the beginning of 1965 have been incorporated. The initial impetus and inspiration for the work came from Dr. K. Mendelssohn. To him and to Drs. R. W. Hill and N. E. Phillips, who went through the whole of the text, the author is obliged in more ways than one. For permission to use figures and other materials, grateful thanks are tendered to the concerned workers and institutions. The author is not so sanguine as to imagine that all technical and literary flaws have been weeded out. If others come across them, they may be charitably brought to the author's notice as proof that physics has become too vast to be comprehended by a single onlooker. E. S. RAJA GoPAL Department of Physics Indian Institute of Science Bangalore 12, India November 1965 v Contents Introduction ................................................................. .

I hope this book will be useful to at least two groups of individuals: the nonspecialist reader with a general knowledge of solid-state science and seeking an introduction to the theory and practice of the Hall effect in metals, and the specialist seeking a contemporary review of the relevant literature. The literature has been surveyed thoroughly up to the middle of 1970, while the more accessible journals have been followed to late 1970. I have been selective in cases where there is a great volume of literature, particu­ larly in the case of old or obscure measurements of low accuracy, but in all cases I have tried to present the reader with sufficient information to judge whether a particular reference matches his interest and is therefore worth tracing. I compiled the book from reading the original publications, but inevitably there will be errors arising in transcription or inadvertent omissions. I hope the reader finding these will be charitable enough to write to me. lt is a pleasure to acknowledge the numerous useful discussions I have had at various times with associates and colleagues, particularly Drs. Mme M. T. Beal-Monod, J. E. A. Alderson, R. D. Barnard, T. Farrell, and P. Monod. Their influence appears at various points in the text-although, of course, they must not be held responsible for anything I have written.

This book is meant for laboratory workers who for one reason or another have a need to cool something down to temperatures below that of liquid nitrogen - notably to 4. 2°K and below. It does not deal with experimental techniques at low temperatures, but I have tried to bring the reader face to face with the brutishrealities of the necessary hardware. As weIl as giving information about sources of supply of equipment, I have gone into so me detail about how some of it can be made in laboratory workshops for the sake of those who are short of money but blessed with competent technical support. So far as highly specialized items such as liquefiers, refrigerators, refrigerant containers, cryostat dewars, etc. , are concerned, I have included aIl sources of supply which I have got to he ar of; in the case of more generaIly available equipment only representative sources of known reliability have been quoted. Any omissions or errors must be put down either to my own ignorance, stupidity, or lack of will toget about the world, or perhaps to the difficulty I have had in extracting information from manufacturers. However, most have gone to great trouble to help, and I hope I have done them justice. Brought up to work indifferently in inches and centimetres and perched between the opposing puIls of the USA and Europe, I have used a mixture of units which may shock the purist.

An in-depth survey of regenerative heat exchangers, this book chronicles the development and recent commercialization of regenerative devices for cryogenic applications. Chapters cover historical background, concepts, practical applications, design data, and numerical solutions, providing the latest information for engineers to develop advanced cryogenic machines. The discussions include insights into the operation of a regenerator; descriptions of the cyclic and fluid temperature distributions in a regenerator; data for various matrix geometries and materials, including coarse and fine bronze, stainless steel-woven wire mesh screens, and lead spheres; and unique operating features of cryocoolers that produce deviations from ideal regenerator theory.

During the past 20 years there have been amazing developments in low temperature physics, engineering, and biology. They form part of the very rapid post-war growth in pure and applied sciences of every kind. During this period several branches of biology including immunology, molecular biology and, of course, cryobiology, have split off from their parent disciplines. One result of this splintering has been the development of separate jargons used by the specialists and sometimes incomprehensible to those working in closely allied fields. The pure physicists, chemists, and the applied scientists, including physicians, surgeons, and pathologists, find the new jargons particularly baffling. We have attempted in this monograph to present to cryogenic engineers a picture of cryobiologists and their problems using as few strange technical words as possible. We hope that this book will help to bridge the gap which has already formed between them in spite of the opportunities for collaboration in many projects. We hope that it may also be useful to scientific research workers and postgraduate students of many kinds united only by curiosity about cryobiology. We are very much indebted to Dr. K. Mendelssohn, who insti­ gated us to produce a monograph and who reassured us that cryogenic engineers are as keen to understand the current trends in biology, and particularly in cryobiology, as we are to enlist their help. We have had much help in preparing this book.

The importance of safety in any scientific endeavor is never in question. However, when cryogenic temperatures are involved, safety is especially important. In addition to observing the normal precautions, one must also take into account the variations of physical properties that occur at low temperatures. At these tempera­ tures, some properties not only exhibit large differences from their normal values but also can vary widely over a small temperature range. Before any cryogenic project is started, a thorough knowledge of the possible hazards is necessary. Only in this way can the safest operation be attained. Over the hundred-year history of cryogenic research, this has been shown to be the case. Keeping this requirement in mind is an essential ingredient in the quest for accident-free work. The past four or five decades have seen a great expansion of cryogenic technology. Cryogenic liquids, such as oxygen, nitrogen, hydrogen, and helium, have become commonly used in a number of different applications and are easily available in any part of the United States and, indeed, almost anywhere in the world. Not only are these liquids available, they have become less expensive and also available in ever larger quantities. As quantities increase, so also do the conse­ quences of mishaps. The future seems to hold promise of ever larger and more widespread use of the common cryogens. Thus, the importance of safety also increases as time progresses.

Practicing engineers and scientist will benefit from this book's presentation of the most accurate information on the subject. The equations for fifteen important cryogenic fluids are presented in a basic format, accompanied by pressure-enthalpy and temperature-entropy charts and tables of thermodynamic properties. The book is supported by ICMPROPRS - an interactive computer program for the calculation of thermodynamic properties of the cryogenic fluids - that can be downloaded from the World Wide Web.

The purposes of this book are to provide insight and to draw attention to problems peculiar to heat transfer at low temperatures. This does not imply that the theories of classical heat transfer fail at low temperatures, but rather that many of the approximations employed in standard solutions techniques are not valid in this regime. Physical properties, for example, have more pronounced variations at low temperatures and cannot, as is conventionally done, be held constant. Fluids readily become mixtures of two or more phases and their analysis is different from that for a single-phase fluid. These and other problems which occur more frequently at low temperatures than at standard conditions are discussed in this book. Although the title specifies heat transfer, the book also contains a very comprehensive chapter on two-phase fluid flow and a partial chapter on the flow of fluids in the thermodynamically critical state. Emphasis is placed on those flow phenomena that occur at low temperatures. Flow analyses are, of course, a prerequisite to forced-convection heat transfer analyses, and thus these chapters add continuity to the text. The book is primarily written for the design engineer, but does broach many topics which should prove interesting to the researcher. For the student and teacher the book will serve as a useful reference and possibly as a text for a special topics course in heat transfer.