The vacuum layer inside a liquid nitrogen (LN₂) tank is the most critical component responsible for thermal insulation. Although invisible from the outside, this layer directly determines evaporation rate, static holding time, and overall tank performance. Manufacturing a stable, long-lasting vacuum layer requires precision engineering and strict process control.

1. Double-Wall Structure Preparation

The process begins with the fabrication of the inner vessel and outer shell, usually made from high-strength aluminum alloy or stainless steel. These two components are assembled concentrically, creating a narrow interspace between them. This gap will later become the vacuum insulation layer.

2. Surface Cleaning and Degassing

Before evacuation, both inner and outer surfaces are thoroughly cleaned to remove oil, moisture, and contaminants. Even trace residues can release gas over time and degrade vacuum quality. The assembly may undergo thermal baking, where it is heated under controlled conditions to drive off absorbed gases.

3. Deep Vacuum Pumping

Once sealed, the interspace is connected to high-performance vacuum pumps, such as rotary, diffusion, or molecular pumps. Air molecules are extracted until the pressure reaches ultra-low levels—often between 10⁻³ and 10⁻⁵ Pa. This step dramatically reduces heat transfer via conduction and convection.

4. Multi-Layer Insulation Installation

To further minimize radiant heat transfer, multi-layer insulation (MLI) materials—typically alternating layers of reflective aluminum foil and low-conductivity spacers—are placed within the vacuum gap. These layers reflect thermal radiation away from the inner vessel.

5. Getter Material Placement and Sealing

Finally, getter materials, such as activated charcoal or metal oxides, are added inside the vacuum space. These materials absorb residual gases released during long-term use. After final vacuum verification, the evacuation port is sealed permanently.

Conclusion

Through precision fabrication, deep vacuum pumping, insulation layering, and gas absorption techniques, the vacuum layer becomes a powerful thermal barrier. This carefully engineered system allows liquid nitrogen tanks to maintain ultra-low temperatures reliably over many years.