Rapid, Low-Loss Liquid Helium Transfers
The transfer of liquid helium from one container to another, can result in evaporation losses as high as 15%, even if the dewar to be filled is already cooled to liquid helium temperatures . However thermodynamic calculations show that heat iniux into transfer tubes, flash losses due to expansion of the liquid by a few psi, etc., cannot account for the high loss rates which have been observed. In most laboratory transfers into research dewars, the real loss rate is usually masked by the fact that the dewar and the research equipment are only precooled, at best, to nitrogen temperature. Transfer tubes are usually small-diameter, vacuum-jacketed tubes. Transfers are usually made slowly, based on the philosophy that in this way not only the refrigeration of the liquid, but also that of the cold vapor, would be optimized. This method of transferring best serves the needs of the experimentalist, but with the advent of large-scale industrial helium liquefaction, transfer problems have gained different dimensions and greater importance. Huge quantities of liquid helium are now being transferred from cryogenic tankers to large-size containers. These shipping containers of 100- to 500-liter capacity must be emptied into smaller storage vessels already at helium temperature in research laboratories for convenient handling at the experimental site. Thus, it was decided to study the causes of transfer losses in some detail.
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