No abstract

A gas distributing system is proposed that curtails the time for attaining the normal operating condition, maintains maximum refrigeration over the whole operation temperature range, and increases reliability and operating life.Analysis of various designs of Gifford-MacMahon (GM) microrefrigerators (MR) shows that the most promising designs of this type of machines are MRs with free motion of the expulsor. The chief merits of the MRs are: adaptability to efficient manufacture, increased operating life and reliability because of a relatively smaller number of seals and moving parts, scope for reducing mass and size characteristics, and relatively low-duty electric drive. Furthermore, in the MRs there is a potential scope for manipulation of the gas distribution processes and the law of expulsor motion, which would allow maximum optimization of the processes of entry and exit of the working medium into and out of the chamber and would ensure stopping of the expulsor at the bottom and top dead centers (BDC and TDC) for increasing the area of the indicator diagram and, consequently, the efficiency of the RM.The GM MRs belong to gas cryogenic machines (GCM), among which can be distinguished two main most popular fundamentally different types, namely, Stirling and GM.In thermodynamic efficiency, Stirling GCMs are undoubtedly superior to other types of cryorefrigerators but have some shortcomings relative to GM GCMs: high vibration level, short overhaul life, and less freedom of mating with the object being cooled.For certain areas of application, refrigerators based on machines operating on Solvay cycle are of maximum interest. Refrigerators of this type are inferior to Stirling GCMs in thermodynamic efficiency but have important advantages: overhaul life more than 10000 h, low vibration level, and scope for locating the refrigerator at a considerable distance from the pressure source and in any spatial position.Because of these advantages, GM machines are used in several branches of the industry with virtually no alternative. This class of MRs ensures efficiency of such devices as cryopumps, IR detectors, thermal shields of cryostats, magnetic resonance imagers, etc.Recent developments of GM microrefrigerators (MRs) are aimed at expanding the sphere of their application, for example, for cooling superconductors having a high critical temperature, helium liquefaction and recondensation, etc. The major problems to be resolved by developers relate to raising reliability and efficiency of the MRs, ensuring ease of mat-

No abstract

No abstract

Studies have been done on a one-stage microcooler in a Gifford-McMahon cryorefrigerator with a gas distribution mechanism including a rotating slide valve.The following relationships have been determined: heat flow in relation to cryostatic temperature with various motor speeds, temperature on the time to reach the steady state, and slide rotation frequency as a function of temperature.Some types of cryogenic vacuum pump use single-stage Gifford-McMahon (GM) cryorefrigerators for cooling the radiation screens as well as cryopanels (in condensation cryopumps). These coolers are also used in air microliquefiers (AML). The performance of a cryovacuum pump or MO AML is dependent on the working parameters of the microcoolers (MC) of the GM, and in that connection it is necessary to raise the cold output.Theoretical studies have been performed on the physics of the processes in gas cryogenic machines that influence the cold production as the cycle frequency varies at several temperature levels. With a small temperature difference, the losses of performance are relatively small, i.e., with an infinitely small temperature difference between the warm and cold sources, the thermodynamic process tends to ideal, and therefore the losses are minimal at the higher working temperature level of the cryorefrigerator.If one assumes that the thermodynamic processes are ideal, one can say that the cold production increases with the cycle frequency. However, actual processes differ considerably from ideal, and the losses associated with increase in hydraulic resistance and the frictional heat increase with the cycle frequency, gas flow speeds, and displacer speed. Also, as the periods of blowing through the regenerator are shortened, there is an increase in the loss of heat recuperation in the exchange of the gas with the packing elements in the regenerative heat exchanger.It has therefore been suggested that the losses dependent on the temperature difference persist at a higher temperature level, and therefore there is scope for increasing the cycle frequency, which would raise the MC cold production. Correspondingly, at the lower temperature, it is necessary to reduce the basic losses as the temperature difference increases, which are due to the hydraulic resistance, frictional heat, and loss of recuperation in the heat exchange.The cycle frequency change is not unambiguous, and there should be a turning point in the dependence of the cold production on the rotation frequency. It is therefore desirable to determine the best cycle frequency at various temperature levels.