Cost of Keeping Liquid Nitrogen
The cost of maintaining liquid nitrogen (LN2) depends on the purchase or rental of storage dewars, refilling expenses, natural evaporation loss, and equipment maintenance.
Purchasing and Renting Dewars
Large dewars, such as 160-liter containers, cost less than $100 to fill initially. Renting dewars generally costs under $100 monthly. This initial or monthly outlay forms the foundation of LN2 storage expenses.
Refilling Costs and Frequency
- Refill prices vary by supplier and location. Some suppliers, especially when delivering other gases, refill dewars for free. When they visit only for LN2, charges may run about $30.
- Welding supply stores typically charge around $100 per refill.
- A 50-liter dewar may last roughly six weeks before being nearly empty, requiring regular refills.
- Smaller tanks with higher insulation, such as a 36.5-liter MVE cryo-tank, can hold LN2 for several months before needing a $40–50 refill.
Boil-Off and Storage Duration
LN2 stored in dewars slowly evaporates due to natural boil-off; refrigeration is not used to keep it liquid. For example, a 6-liter dewar typically loses its content over two weeks under standard conditions. Efficient insulation reduces losses and extends storage duration.
Maintenance for Efficiency
Vacuum quality inside the dewar affects boil-off rates significantly. If a dewar loses several liters daily, pumping the vacuum to below 100 microns can enhance efficiency. This maintenance can take 2-5 days to complete.
Capital Investment: LN2 Generators
LN2 generators have a payback period of approximately 18 months when used with second-hand components and low electricity costs. However, reliability issues and initial capital may influence this decision.
Additional Costs and Considerations
- Individuals often face difficulty purchasing LN2 directly due to supplier liability concerns.
- Transport safety is critical. LN2 tanks emit nitrogen gas that displaces oxygen, posing asphyxiation risks if spilled or tipped, especially in enclosed spaces.
- Transporting tanks safely typically requires open-air vehicles or non-occupied enclosed spaces.
Key Takeaways
- Initial or rental cost of dewars is under $100 for common sizes.
- Refilling costs range from $30 to $100 depending on provider and location.
- Natural boil-off causes gradual LN2 loss; efficient dewars reduce this.
- Vacuum maintenance enhances dewar efficiency and reduces LN2 loss.
- LN2 generators can be cost-effective with an 18-month payback.
- Safety and liability affect purchase, transport, and handling costs.
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