Purification systems are necessary to support commissioning and operation of medium to large-scale cryogenic refrigeration systems using various cryogenic working fluids. The present research focuses on helium refrigeration systems that operate at 4.5 K (which is just above the normal boiling point of helium), down to 1.8 K (which requires helium with a vapor pressure of 16 mbar). At these very low temperatures, the presence of any substances except helium (contaminants) will result in solidification. Even trace amounts of these impurities in the process fluid can block and/or change the flow distribution in a refrigerator’s heat exchangers and potentially damage rotating equipment operating at high speeds. Therefore, helium purifiers for these refrigerators are typically designed for a low level of impurity (i.e., 1-100 ppmv) removal of moisture and air components, since gross impurities are removed during the initial clean-up and commissioning of the system. Purification of the process gas (helium) is typically achieved by molecular sieve adsorption beds at room temperature for moisture removal and liquid nitrogen (LN) cooled activated carbon adsorption bed for air (nitrogen/oxygen/argon) removal. However, past studies and operational experience show that molecular sieves are unable to remove low level moisture contamination effectively. Freeze-out purification has great potential to reliably remove low-level moisture contamination, but requires careful design. Typical commercially available freeze-out purifiers have a much shorter operating time in between regenerations than should be achievable, are not optimized for low pressure operation, and require large amount of utilities like liquid nitrogen. Furthermore, frost formation in a purifier heat exchanger is not well understood. Developing an understanding of this process and studying the design and process parameters that can improve the process for this critical sub-system is the focus of this research. This work begins with an experimental study of a commercially available helium freeze-out purifier. It is tested under practical operating conditions and controlled operating conditions, under different contamination levels and flow capacity imbalances. Auxiliary equipment was designed, fabricated, tested, and operated to achieve controlled and tunable low-level moisture contamination in the helium stream. The performance and moisture capacity of the purifier heat exchanger was characterized. Following the experimental study, a series of theoretical studies were carried out. First, a heat and mass transfer model on an isothermal surface was developed to establish a base-level understanding of frost formation and relate to the existing literature. This model was used to study the effects of gas pressure, wall temperature difference, reduced temperature differential, absolute humidity, and carrier gas on the frost growth and mass transfer. A simplified estimation to predict frost thickness was developed and found to be accurate within 1%. Second, this model was extended to a heat exchanger surface. This model was validated using test data and used to study the effects of flow imbalance and inlet moisture contamination level. Through this study, it was found that flow mal-distribution within the heat exchanger caused a significant rift between many of the experimental results and the simulation results. Third, in order to eliminate the effects of flow mal-distribution and reduce utility usage, a novel purifier design is studied. It considers a coiled finned-tube design to maximize surface area for heat exchange and mass collection. An initial exergy analysis was done to determine a reasonable reference design geometry. The effects of fin density and heat exchanger mandrel diameter on frost formation and heat exchanger performance were studied. It was found that the novel purifier can hold approximately as much frost as the commercially available purifier, while using significantly less nitrogen for cooling.
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