Cryogenic helium refrigeration for middle and large powers This book offers a practical introduction to helium refrigeration engineering taking a logical and structured approach to the design building commissioning operation and maintenance of refrigeration systems. It begins with a short refresher of cryogenic principles and a review of the theory of heat exchangers allowing the reader to understand the importance of the heat exchanger role in the various thermodynamic cycle structures. The cycles are considered from the simplest (Joule Thomson) to the most complicated ones for the very large refrigeration plants and finally those operating at temperatures lower than 4.5 K. The focus then turns to the operation ability and limitations of the main components including room temperature cycle screw compressors heat exchangers cryogenic expansion turbines cryogenic centrifugal compressors and circulators. The book also describes the basic principles of process control and studies the operating situations of helium plants with emphasis on high level efficiency. A major issue is helium purity and the book explains why helium is polluted how to purify it and then how to check its purity to ensure that all components are filled with pure helium prior to starting. Although the intention of the book is not to design thermodynamic cycles it is of interest to a designer or operator of a cryogenic system to perform some simplified calculations to get an idea of how components or systems are behaving. Throughout the book such calculations are generally performed using Microsoft® Excel and the Gaspak® or Hepak® software. .