Cryocoolers Part 1: Fundamentals THE INTERNATIONAL CRYOGENICS MONOGRAPH SERIES General Editors K. D. Timmerhaus, Engineering Research Center University of Colorado, Boulder, Colorado Alan F. Clark, National Bureau of Standards U.S. Department of Commerce, Boulder, Colorado J. L. Olsen, Laboratorium fiir Festkorperphysik Eidgendssische Technische Hochschule, Zurich, Switzerland Founding Editor K. Mendelssohn, F.R.S. (deceased) H. J. Goldsmid Thermoelectric Refrigeration G. T. Meaden Electrical Resistance of Metals E. S. R. Gopal Specific Heats at Low Temperatures M.G. Zabetakis Safety with Cryogenic Fluids D. H. Parkinson and B. E. Mulhall The Generation of High Magnetic Fields W. E. Keller Helium-3 and Helium-4 A. J. Croft Cryogenic Laboratory Equipment A. U. Smith Current Trends in Cryobiology C. A. Bailey Advanced Cryogenics D. A. Wigley Mechanical Properties of Materials at Low Temperatures C. M. Hurd The Hall Effect in Metals and Alloys E. M. Savitskii, V. V. Baron, Yu. V. Efimov, M. I. Bychkova, and L. F. Myzenkova Superconducting Materials W. Frost Heat Transfer at Low Temperature I. Dietrich Superconducting Electron-Optic Devices V. A. Al'tov, V. B. Zenkevich, M. G. Kremlev, and V. V. Sychev Stabilization of Superconducting Magnetic Systems G. Walker Cryocoolers, Part 1: Fundamentals Cryocoolers, Part 2: Applications Cryocoolers Part 1: Fundamentals Graham Walker The University of Calgary Calgary, Alberta, Canada Springer Science+Business Media, LLC Library of Congress Cataloging in Publication Data Walker, G. (Graham), 1930- Cryocoolers. (The International cryogenics monograph series) Includes bibliographical references and indexes. Contents: pt. 1. Fundamentals-pt. 2. Applications. 1. Low temperature engi­ neering. I. Title. II. Series. TP482.W34 1983 621.5'9 83-2166 ISBN 978-1-4899-5288-2 ISBN 978-1-4899-5288-2 ISBN 978-1-4899-5286-8 (eBook) DOI 10.1007/978-1-4899-5286-8 © 1983 Springer Science+Business Media New York Originally published by Plenum Press, New York in 1983 Softcover reprint of the hardcover 1st edition 1983 All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher High-capacity, three-stage Vuilleumier cycle cryocooler. (Courtesy of Hughes Aircraft Co., Los Angeles.) Foreword 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. A steel core was encased in a stack of leather rings separated by thin steel rings of slightly smaller diameter, the stack of alternate leather and steel rings being compressed to provide a rigid piston which could be machined to fit the cylinder. The wearing quality was excellent. One of these liquefiers provided thousands of liters of liquid helium during about two years of use. Instead of becoming smaller in diameter because of wear, the diameter of the pistons actually increased by absorption of water. The necessity of controlling the water content was a disadvantage. I was also intrigued by Heylandt's crowned piston concept during the 1930s but ruled it out because of the sealing problem at the warm end of the piston. I had seen some of them in action in oxygen plants. A slight vii viii Foreword air leakage was of no importance but a helium leak of such magnitude could not be tolerated. Little did I realize then that by 1960 I would find an adequate solution of the leakage problem by the use of rubber 0-rings. During the late 1930s I experimented with free piston expanders and compressors and with diaphragm engines. When the war began in 1941 I was able to secure substantial funds for the development of oxygen gen­ erators and decided to see if Kapitza's engine could be useful. At least two attempts had been made to reproduce Kapitza's liquefier in the U.S. One was a complete failure and the other almost so. I had tried several combinations of metals for the piston and cylinder. The problem was to produce a fit between piston and cylinder close enough to reduce leakage to a reasonable value but not close enough to encourage seizure. Of the four combinations I tried, Kapitza's choice of bronze and stainless steel was the least desirable. Kapitza found it necessary to employ a relatively wide annular gap and contrived a way for the piston to travel very rapidly during the power stroke so as to minimize leakage. By making both piston and cylinder of nitrided alloy steel (hardness= 90 RC) I was able to reduce the radial clearance by an order of magnitude and thus reduce leakage to a negligibly small value. It became possible, therefore, to use conventional gear for controlling the motion of piston and valves. Several dozen engines with nitrided pistons and cylinders were manufactured for oxygen production during the war. After the war this type of engine was incorporated in helium liquefiers and was manufactured by Arthur D. Little and its successor, CTI-Cryogenics. Between 1947 and 1970, 365 units were marketed. I started using oil-lubricated rubber 0-rings to seal piston rods and valve stems in nitrogen and oxygen plants in 1950 and in helium liquefiers a few years later. Finally, I returned to the Heylandt type of piston, first in nitrogen plants, and in 1960 in a helium liquefier. The piston was made from a rod of laminated phenolic plastic (Micarta) about 2 ft long machined to fit loosely in a stainless steel cylinder sealed at the room-temperature end by a single 0-ring. The advantages over the earlier engine proved to be enormous. The pistons and valves could be removed easily for inspection without disturbing the insulating vacuum or breaking any piping. There was no friction between piston and cylinder except when massive amounts of air or water gained access to the working fluid. The reliability was very high. The loss of efficiency from the flow of gas to and fro in the annular space between the hot and cold ends was very slight and mostly from the mismatch of temperature between cylinder wall and piston surface. The thin (0.002-0.006-in.) annular passageway became an effective regenerator. The inefficiency, as in other reciprocating engines, was mostly the result of irreversible heat transfer between the working gas and the walls Foreword ix of the expansion chamber in response to the relatively great temperature change the gas undergoes. Engines working at lower temperature levels tended to be more efficient because the drop in temperature of adiabatic expansion was smaller. In fact, the efficiency of the two-phase engine was apparently above 90%. Not only was the change in temperature very small, but also the vanishingly small heat capacity of cylinder and piston made the expansion truly adiabatic. When I retired from MIT in 1964, I joined the engineering staff of A. D. Little and soon thereafter put together an experimental model of their standard liquefier equipped with Micarta pistons sealed by 0-rings. A considerable number were produced over the next few years. Concurrently, I developed a slightly larger machine, again with Micarta pistons, which led to the 1400 series, completely replacing the earlier series. There are 200 of these units now in use. A two-phase engine has replaced the Joule-Thomson valve in all of the recent machines I have built and in some that CTI has manufactured. The size of plant on which this change has been made ranges from 5 to 1000 1/hr. The gain in liquid production has been 25% to 33%. One of the Naval Research Laboratory liquefiers has an inverted engine, that is the cold end of the cylinders is at the top. It was astonishing that the two-phase engine behaved normally and gave the expected improvement over the Joule-Thomson. It seemed strange to have liquid helium formed on top of the piston without appreciable loss by flowing down the annular gap toward the hot end of the piston. Apparently convection is very ineffective in thin layers of the order of 0.004 in. when so little time (a period of one stroke) is available. There is a wealth of information in this book. The comprehensive Bibliography and Guide to Cryogenic Engineering Literature (Appendix II of Part 2) is indicative of the prodigious effort which Professor Walker has expended in preparation for writing this book. Professor Walker and those who contributed certain chapters are to be congratulated on this scholarly and timely treatise on a very interesting subject.