Cryocooler

A Cryocooler is a standalone cooler, usually of table-top size. It is used to cool some particular application to cryogenic temperatures. A review is given by Radebaugh. The present Article deals with various types of cryocoolers and is partly based on a paper by De Waele. A Cryocooler is a standalone cooler, usually of table-top size. It is used to cool some particular application to cryogenic temperatures. A review is given by Radebaugh. The present Article deals with various types of cryocoolers and is partly based on a paper by De Waele. Heat exchangers are important components of all cryocoolers. Ideal heat exchangers have no flow resistance and the exit gas temperature is the same as the (fixed) body temperature TX of the heat exchanger. Note that even a perfect heat exchanger will not affect the entrance temperature Ti of the gas. This leads to losses. An important component of refrigerators, operating with oscillatory flows, is the regenerator. A regenerator consists of a matrix of a solid porous material, such as granular particles or metal sieves, through which gas flows back and forth. Periodically heat is stored and released by the material. The heat contact with the gas must be good and the flow resistance of the matrix must be low. These are conflicting requirements. The thermodynamic and hydrodynamic properties of regenerators are complicated, so one usually makes simplifying models. In its most extreme form an ideal regenerator has the following properties: Progress in the cryocooler field in recent decades is in large part due to development of new materials having high heat capacity below 10K. The basic type of Stirling-type cooler is depicted in Fig.1. From left to right it consists of a piston, a compression space and heat exchanger (all at ambient temperature Ta), a regenerator, a heat exchanger, an expansion space and a piston (all at the low temperature TL). Left and right the thermal contact with the surroundings at the temperatures Ta and TL is supposed to be perfect so that the compression and expansion are isothermal. The work, performed during the expansion, is used to reduce the total input power. Usually helium is the working fluid.