A nitrogen dewar flask, also known as liquid nitrogen dewar, simply called dewar or LN2 tank, is a specialized cryogenic storage container used for for storing and transportation of cryogenic liquids, such as liquid nitrogen (LN2), with minimal evaporation loss.

Structure

1. Double-layer Stainless Steel Cavity

The core of the dewar is a double-walled vessel. It consists of an inner tank and an outer shell. The inner tank is usually made of 304 stainless steel and is polished to reduce heat radiation. The outer shell is a corrosion-resistant metal layer.  With the space between the walls evacuated to create a high vacuum. This eliminates conduction and convection, as there is no air or other medium to carry heat.

2. Thermal Insulation Protection System

In modern dewars, the vacuum space often contains a reflective material (like aluminized Mylar) called "multi-layer insulation" (MLI). It includes multiple layers of aluminum foil reflective screens, usually 20-50 layers stacked alternately, a glass fiber or aerogel composite material filling layer, and an activated carbon or molecular sieve adsorbent to maintain the vacuum degree. This reflects radiant heat, drastically reducing heat transfer via radiation.

3. Neck Tube Assembly

The point where heat enters most easily is the neck, where the inner chamber connects to the outside. Dewars are designed with a long, narrow neck made of materials with low thermal conductivity to minimize this heat leak. It is equipped with a vacuum-sealed flange, a bellows shock-absorbing structure, and a silica gel or polyurethane sealing ring.

4. Safety Protection Devices

Pressure Builder Coil: A tube that runs through the inner chamber. When you open the pressure-building valve, some liquid is gasified here, increasing the internal pressure to push liquid out during dispensing.

Pressure Release Valves: Crucial for safety. These valves automatically vent off excess pressure that builds up from normal evaporation, preventing a dangerous pressure buildup.

Vent Valve: A manual valve that can be opened to release pressure, typically used during filling or when no pressure is desired. Liquid Dispensing Valve: The valve used to withdraw liquid nitrogen.

Gas Dispensing Valve: Used to withdraw gaseous nitrogen (which is created naturally from evaporation). Level Gauge: A visual dipstick or an electronic gauge to measure how much LN2 remains.

Common Uses and Applications

Nitrogen dewars are well used in settings that require ultra-cold temperatures:

Biological & Medical: Storage of biological samples (cells, tissues, semen, embryos) in cryogenic freezers or directly in the dewar. Preservation of blood, stem cells, and other medical materials. Cryosurgery for removing warts or cancerous cells.

Medical Cold Chain Transportation: It is used for the transportation of medical products that require low-temperature storage, such as vaccines and biological reagents.

Industrial: Cold trapping in vacuum systems. Shrinking-fit metal assembly (contracting metal parts for insertion). Materials testing at low temperatures.

Food Industry: Flash freezing food for preservation and transport.

Research & Science: Cooling detectors (like in electron microscopes or X-ray detectors) and superconducting magnets (like in MRI machines).

Other: Used by chefs for creating instant frozen desserts and cocktails.