Purification systems are necessary to support commissioning and operation of helium refrigeration and associated experimental systems. These systems are typically designed for a low level of impurity (i.e., in parts per million), since a 4.5 K or 2 K helium system will solidify, or freeze out every other substance. The trace impurities can block and/or change the flow distribution in heat exchangers and potentially damage turbines or cryogenic compressors operating at high speed. Experimental systems, such as superconducting magnets, require helium purification due to inherent characteristics in their construction. These are also used for the commissioning of sub-systems, like the compressors, and cold boxes. As known from experience, molecular sieves do not remove low-level moisture impurity sufficiently. Typical commercial freeze-out purifiers using molecular sieves have very short operating times between regeneration and are inefficient requiring substantial utilities like liquid nitrogen and high-pressure operation. Based upon proven experience from a freeze-out purifier design for Brookhaven National Lab (BNL) in 1983, a liquid nitrogen assisted freeze-out purifier has been designed. This design includes a multi-pass and multi-stream heat exchanger and an activated carbon bed. The heat exchanger design is expected to minimize the liquid nitrogen usage and extend the capacity and the operating pressure range, thereby the time interval between regeneration. The goal is to provide a simple design procedure to develop and operate an efficient purifier system.

Cryogenic refrigeration and liquefaction systems require ultra-high purity refrigerants (helium, argon, hydrogen, etc.) for proper operation. Common contaminants in the refrigerant gases freeze at the operating temperatures of these systems, causing performance degradation of process equipment. Therefore, ultra-high purity refrigerant gas (1.0 ppmv or less contaminants) is often used. However, removal of low levels of moisture (10 ppmv or less) from the refrigerant gas is particularly challenging. Contaminant freeze-out processes in a specifically designed heat exchanger have the potential to achieve effective and efficient purification. Developing an understanding of the contaminant frost formation process is crucial for the proper design of an effective freeze-out heat exchanger. A transient computational model simulating formation and densification of frost on an isothermal cryogenic surface from a contaminated refrigerant gas stream has been developed. The mass and energy conservation equations are discretized and simultaneously solved to obtain the frost layer thickness and frost surface temperature. The model is validated using available experimental data for frost formation from a humid air stream. Several parameters, namely — contaminated gas stream pressure, surface temperature, flow Reynolds number, and carrier (refrigerant) gas affect the interaction between frost formation and densification. The effect of these parameters on the frost formation characteristics has been systematically studied using the developed numerical model. The developed model can be utilized to predict the freeze-out heat exchanger performance degradation and its moisture collection capacity.

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