PERFORMANCE OF HEAT EXCHANGER FOR SUBCOOLING LIQUID NITROGEN WITH A GM CRYOCOOLER

Abstract

A heat exchanger to continuously supply subcooled liquid nitrogen at 65 similar to 70 K in thermal contact with a Gifford-McMahon (GM) cryocooler is experimentally investigated. This study is motivated by HTS power applications, where liquid nitrogen is circulated in subcooled state by forced convection and a regenerative cryocooler is used for continuous refrigeration. Since the coldhead of the cooler has a very limited surface area, a cylindrical "cup" made of copper is attached to the coldhead to serve as extended surface. A copper tube for liquid nitrogen flow is spirally wound and silver-brazed on the exterior surface of cylinder. In order to examine the effect of physical dimensions on the cooling performance, different sizes of heat exchangers are fabricated and tested with a commercial GM cooler to subcool liquid nitrogen from 78 K to 65 similar to 70 K. It is clearly shown that there exists an optimal size of heat exchanger cylinder to achieve a maximum cooling of liquid nitrogen. The reason for poor performance with a smaller size is that the cooling surface is not enough, and the reason for poor performance with a larger size is that the effectiveness of extended surface is lower and the heat leak from surroundings is greater.