Design optimization of the 2 K heat exchanger for the superfluid helium cryogenic systems at KEK

Abstract

At KEK, the superfluid helium cryogenic systems employ laminated-finned type 2 K heat exchangers (2 K HX). Their purpose is to sub-cool normal liquid helium (He I or LHe) from 4.4 K to 2.2 K or above, using the sensible heat carrying capacity of 2.0 K gaseous helium (GHe). This would help in reducing vapor flash losses during the Joule-Thomson (JT) expansion, hence improving the production rate of 2.0 K saturated superfluid helium (He II). Another equally important factor that affects the production of He II is the GHe pressure drop through the 2 K HX. Higher GHe pressure drop compels the GHe pumping system to operate at lower inlet pressures, reducing the flow rates that can be pumped out, hence reducing the He II production. The performance of a 2 K HX is characterized by a factor known as effectiveness and should be > 83.7% to produce ~ 2.2 K LHe, at the outlet of the 2 K HX and before the JT valve. In this paper, a numerical model has been developed to determine the performance parameters (effectiveness and GHe pressure drop) of the current 2 K HX design and is verified experimentally using a heat exchanger test stand. Furthermore, a parametric study is conducted to improve the performance of the current 2 K HX design, which was also validated experimentally. The improved 2 K HXs will be studied in conjunction with the GHe pumping system to determine the optimal 2 K HX design that maximizes the He II production from the cryogenic systems. The optimized 2 K HX design in this study improved the He II production rate by 7%.