Design and analysis of concentric helical heat exchanger for dilution fridge

Abstract

The lowest temperature which can be reached in a continuously operating dilution fridge is determined by the performance of heat exchanger. Operation of dilution system requires a continuous tubular heat exchanger (Hex) for heat exchange primarily between the concentrated 3He and dilute 3He solution. The helical Hex is made of two thin walled cupro-nickel capillary tubing with stainless steel ends. The inner tube is spirally wound and inserted inside the outer tube. Based on the proposed flow rate of 15 ymoles/s, lengths and diameters have been optimized as to minimize the viscous and axial conduction heating. Flow impedances of the concentrated and diluted phase flow channels are maintained 10−7cm−3and 10−6 cm−3 respectively. Analytical results show that a base temperature of around 100mK can be achieved with this Hex. Effectiveness of condenser for condensation of helium isotopic gas mixture and temperature dependent quality factor has also been determined. This paper presents the detailed analytical results based on enthalpy and energy balance consideration relevant to the Hex.