Ground Freezing: The Use of Liquid Nitrogen Systems Converted to Brine-Paper

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The paper compares direct-expansion liquid nitrogen (LIN) ground freezing with circulating refrigerated brine and examines projects that begin with LIN before switching to brine.<br /><br />LIN can form frozen ground much faster: in the example analyzed, it achieved the required frozen zone in four days, compared with 17 days for brine. However, LIN’s extremely low temperature extracts substantially more heat than necessary, making it inefficient and often more expensive. Estimates based on a standard heat-extraction value commonly underestimate actual LIN use, while transportation and supply constraints can add cost or make the method impractical. LIN remains useful for small projects, remote applications where a refrigeration plant is impractical, or situations requiring exceptionally cold and strong frozen ground.<br /><br />Circulating brine is generally better suited to larger projects. Although initial freezing takes longer, the system can operate over long distances and at great depths, and its refrigeration capacity can be tailored to project needs.<br /><br />Combining the systems—using LIN for initial freezing and brine for maintenance—may appear to save time or avoid the need for a high-capacity refrigeration plant. In practice, it may not shorten the overall schedule. Installing separate distribution systems, changing over between coolants, repairing pipes damaged during LIN freezing, preventing brine from freezing after conversion, and meeting separate safety requirements can introduce delays and complications. The approach can also consume more LIN than expected and require reheating ground that has been cooled beyond what the design requires.<br /><br />The author concludes that either method, or occasionally a combination, may suit a particular project. But combining them simply because the contractor lacks suitable refrigeration equipment is likely to increase complexity and cost, and may take longer.

Keywords

liquid nitrogen

ground freezing

circulating brine

direct expansion

frozen ground

heat extraction

refrigeration plant

freeze-thaw transition

construction methods

project cost