Hydrogen-Free Liquid-Helium Recovery Plants: The Solution for Low-Temperature Flow Impedance Blocking

Abstract

The blocking of fine-capillary tubes used as flow impedances in 4 He evaporation cryostats to achieve temperatures below 4.2 K is generally attributed to nitrogen or air impurities entering these tubes from the main bath. The failure of even the most rigorous low-temperature laboratory best practices aimed at eliminating the problem by maintaining the cleanliness of the helium bath and preventing impurities from entering the capillary tubes suggests that a different cause is responsible for the inexplicable reduction of impedance flow. Many low-temperature research laboratories around the world have suffered this nuisance at a considerable financial cost due to the fact that the affected systems have to be warmed to room temperature in order to recover their normal low-temperature operation performance. Here, we propose an underlying physical mechanism responsible for the blockages based upon the freezing of molecular H 2 traces present in the liquid-helium bath. Solid H 2 accumulates at the impedance low-pressure side, and, after some time, it produces a total impedance blockage. The presence of H 2 traces is unavoidable due its occurrence in the natural gas wells where helium is harvested, forcing gas suppliers to specify a lower bound for impurity levels at about 100 ppb even in high-grade helium. In this paper, we present a simple apparatus to detect hydrogen traces present in liquid helium and easily check the quality of the liquid. Finally, we propose a solution to eliminate the hydrogen impurities in small-and large-scale helium recovery plants. The solution has been implemented in several laboratories that previously experienced a chronic occurrence of blocking, eliminating similar occurrences for more than one year.

“Clean” Liquid Helium

“…With a boiling point of 4.2 K at 100 KPa, liquid helium is the coldest fluid that exists in nature. Below its critical temperature (Tc = 5.2 K), any unwanted substance present in the liquid phase, that is, any impurity, will be in solid form, resulting in mist, snow, suspensions or particulates [6]. The vapor pressure of these solid impurities will be, in general, negligibly small (<<10 −9 Pa), except for the case of the hydrogen isotopes and their molecular combinations [7] for which this is of the order of 10 −2 Pa and 10 −5 Pa, at 5.2 and 4.2 K, respectively.…”

Reducing the hydrogen content in liquid helium

“…In 2016, Gabal et al [4] and, also in a recent paper, Will et al [9], it was reported that even hydrogen concentrations within parts per billion are sufficient to block the helium flow within a few hours of operation. On the other hand, hydrogen impurities in liquid helium are easy to detect using a dedicated sensor developed at the University of Zaragoza [4]. It consists of a test capillary that is connected to a vacuum pump (see figure 2 in reference [10]) and has been successfully tested elsewhere [9].…”

“…This points to hydrogen as the source of blocking. Many research laboratories around the world have faced this issue at a considerable financial cost because the affected systems have to be warmed up to room temperature to recover their typical low-temperature operation performance [4][5][6][7][8].…”