ABSTRACT
The nature of low-temperature, or cryogenic, systems makes them highly susceptible to incoming energy in the form of heat due to the large temperature difference between the system and the warm environment. This chapter begins with a review of the fundamental laws of thermodynamics that are applied later in the book. Applications of the principles of energy conservation, that is, the first law of thermodynamics, and the allowable direction of change for a process and losses that are a result of the second law of thermodynamics are fundamental to the heat management of cryogenic systems. Maintenance of low temperatures is achieved by supplying refrigeration either through a mechanical refrigerator or a low-temperature fluid. The energy associated with the production of low-temperature refrigeration through either approach is discussed to provide the reader with an understanding of the need to minimize the energy costs associated with maintaining low temperatures. The relevant modes of heat transfer, including conduction, convection, and thermal radiation, are reviewed, with an emphasis on modes with large temperature differences for heat transfer entering the cold space as well as when temperature differences are small, as required in the heat removal processes that maintain the system at a low temperature. For large temperature differences, the variation of thermal conductivity with temperature is significant in many materials and must be accurately evaluated. At low temperatures, where cooling/refrigeration takes place, these equations are used to guide the design in combination with the correct properties for the temperature range in order to develop an efficient design that provides the required performance. Most cold spaces are maintained at low pressures, and a brief discussion of gas conduction in high vacuum is presented. Some basic correlations for convection heat transfer coefficients are presented for both laminar and turbulent flows. Thermal radiation also plays a significant role, and models for simple enclosures are discussed. In addition, a generalized method for thermal radiation between surfaces of an enclosure is presented. In most situations, a combination of all these heat transfer modes is present, and applications to maintain the cold space are presented by functional components in separate chapters.
