Heat exchanger for subcooling liquid nitrogen with a regenerative cryocooler

A heat exchanger for continuously subcooling liquid nitrogen in contact with a regenerative cryocooler is analytically investigated as a next step of our recent experimental works. Since the coldhead of regenerative cryocooler has a limited surface area, a cylindrical copper cup is attached as extended surface, and a tube for liquid flow is spirally wound and brazed on the exterior surface of cylinder. Different sizes of heat exchangers are fabricated and tested with a single-stage GM cooler to cool liquid nitrogen from 78 K to 66 K. Analytical model is developed for the heat exchanger effectiveness and thermal resistance, and the results are compared with the experimental data. It is concluded that there exists an optimum for the height and diameter of cylindrical heat exchanger to maximize the cooling rate with a given unit of cryocooler.

Research highlights
► We model a heat exchanger to subcool liquid nitrogen with a regenerative cryocooler.
► The heat exchanger is an extended surface attached to coldhead of cryocooler.
► The analytical model clearly explains our recent experimental results.
► There exists an optimal size of heat exchanger to maximize the subcooling rate.