INTRODUCTION

Nicholas P. Theophilos, Ph.D., P.E.

Manager
Standards and Quality Assurance
Praxair, Inc.

Cryogenics (from the Greek kryo-genikos, meaning cold generation) is the science and technology associated with very low temperatures. Depending on one s point of view, any temperature below ?20 F can be set to establish such a demarcation. Here the ?20 F point has been selected because it normally represents the onset of embrittlement for ordinary carbon steels in typical structural applications.

Cryogenics is not a separate branch of physics, since it obeys all laws of ordinary physics. In fact, cryogenics is low-temperature physics. The reasons for its special treatment, therefore, are not because of its uniqueness as a science but rather because of the very special problems it creates as a technology. These problems relate to embrittlement of materials, large displacements (expansion and contraction), rapid change of phase due to large heat fluxes (big delta T), and small latent heats of the fluids involved.

In order to obtain a better appreciation of the special considerations involved in cryogenic piping system applications, it was felt that it would be necessary to review the behavior of materials at cryogenic temperatures and the physical and thermodynamic properties of cryogenic fluids. These considerations are covered in the sections Properties of Cryogenic Fluids and Materials Used in Cryogenic Piping Systems. Additionally, cryogenic piping system design is discussed in the sections Piping Systems Design Fluids and Piping Systems Design Mechanical.

From the strictly heuristic point of view of fundamental applications of scientific principles there are...