Heat Transfer Mechanism of Double-Walled Vacuum Powder Insulated Storage Tanks

Double-walled vacuum powder insulated storage tanks are widely used for storing cryogenic liquids such as LNG, LPG, and liquid nitrogen. The design employs a vacuum layer combined with fine powder insulation to minimize heat ingress, reducing boil-off and maintaining low-temperature storage. Understanding the heat transfer mechanism is critical for optimizing insulation performance and ensuring energy efficiency.

1. Structure Overview
The tank typically consists of an inner vessel containing the cryogenic liquid and an outer vessel providing structural support. Between the two walls, a vacuum layer is created and filled with fine insulating powders, such as perlite or silica powder, which significantly reduce heat transfer.

2. Heat Transfer Modes

Conduction:
Heat conduction occurs through solid contact points within the powder insulation and structural supports (such as spacers and support rods). The powder’s low thermal conductivity minimizes this mode, and the use of small particles increases the number of contact points, which further reduces conduction.

Convection:
In the vacuum layer, natural convection is suppressed due to the low density of residual gas molecules. The powder also restricts the movement of gas molecules, effectively minimizing convective heat transfer.

Radiation:
Thermal radiation from the outer vessel to the inner vessel contributes to heat ingress. Reflective foils or coatings on the inner and outer surfaces of the vacuum layer reduce radiative heat transfer. Multilayer insulation (MLI) or aluminized coatings can further enhance performance by reflecting infrared radiation.

Residual Gas Heat Transfer:
Even in high vacuum conditions, residual gas molecules may transfer heat through rarefied gas conduction. Maintaining a high-quality vacuum and using getter materials or cryopumps reduces this effect.

3. Thermal Optimization Factors

Vacuum Quality: High vacuum levels (<10⁻³ Pa) significantly reduce conduction and convection, enhancing insulation efficiency.

Powder Properties: Particle size, density, and thermal conductivity influence the overall heat transfer rate. Fine, low-density powders minimize solid conduction.

Support Design: Minimizing the cross-sectional area of structural supports reduces heat conduction bridges between the inner and outer walls.

Reflective Layers: Incorporating reflective foils in the powder layer reduces radiative heat transfer, improving thermal performance.

4. Practical Implications
Efficient heat transfer management extends storage duration, minimizes liquid boil-off, and reduces operational costs. Proper design and maintenance, including vacuum monitoring and insulation integrity checks, are essential to maintain long-term performance.

Conclusion
The heat transfer in double-walled vacuum powder insulated storage tanks is governed by conduction through supports and powder, radiation between walls, and residual gas effects. Optimizing vacuum quality, powder properties, structural support design, and reflective coatings can effectively minimize heat ingress, ensuring efficient and safe cryogenic storage.

References

ASME Boiler and Pressure Vessel Code, Section VIII – Rules for Construction of Pressure Vessels.

EN 14620 – Design and Manufacture of Cryogenic Vessels.

Barron, R.F. (1999). Cryogenic Systems, 2nd Edition. CRC Press.

Van Sciver, S.W. (2012). Helium Cryogenics, 2nd Edition. Springer.

Bratt, R., & Mort, P. (2015). Cryogenic Engineering: Fifty Years of Progress. Springer.