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Description
Superfluid helium is another phase of liquid helium when it is cooled below 2.17 K, under saturation condition. The superfluid helium bath cools the niobium superconducting radio frequency (SRF) cavities housed in the helium tanks of the cryomodules. The SRF cavities operate at temperatures of 2.0 K or below, due to its higher operating frequency like 1.3 GHz. At KEK, the superfluid helium cryogenic plants produce superfluid helium continuously by isenthalpic expansion of the normal liquid helium (LHe) (~4.4 K) through a Joule-Thomson (JT) valve, which is connected in series with a 2K heat exchanger (2K HX). The 2K HX recovers the coldness from the 2.0 K gaseous helium (GHe) evaporating from the helium tanks of the SRF cavities. This increases the production rate of the superfluid helium by reducing the incoming LHe temperature from 4.4 K to 2.2 K or above, before the JT valve. As such, reducing the vapor flash losses from 40% to 9.4%, during the JT expansion to produce 2.0 K saturated superfluid helium. At KEK, we have a 2K HX consisting of a helical coil and laminated fins. Its performance is determined and characterized by a factor known as “effectiveness”, which is the ratio of actual heat transfer to the maximum possible heat transfer between the fluids. To produce ~2.2 K LHe at the outlet of the 2K HX and before the JT valve, the required effectiveness is > 84%. A numerical model has been developed to determine the performance parameters (effectiveness and GHe pressure drop) of the current 2K 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 2K HX design, which was also validated experimentally. The improved 2K HXs were studied in conjunction with the GHe pumping system to determine the optimal 2K HX design that maximizes the He II production from the cryogenic systems.
