The purpose of replacing the Dewar cylinder with nitrogen during vacuuming

Welding Dewar cylinders are replaced with nitrogen during vacuuming, which is determined by the characteristics of nitrogen. Compared with general air, high-purity nitrogen gas is oxygen-free, almost water-free and oil-free, its thermal expansion coefficient is low, thermal conductivity is low, temperature rise is slow, and the rate of heat accumulation is reduced, so it can greatly ensure the thermal insulation performance of the Dewar .
 
1. The inner tank is heated to 250 degrees to evacuate, mainly to allow the air inside to be fully heated to expand, so that the air can be extracted to the maximum extent, and a higher vacuum is formed after cooling.
 
2. If the level of evacuation process and welding process is very high, it is possible to ensure that the vacuum is not destroyed and not filled with nitrogen, because under high vacuum conditions, because there are few heat transmission media, the heat transfer is fast It is also very slow and can basically meet the needs of relative insulation. 3. Nitrogen filling is mainly used for compensation when the above 2 processes are immature. As long as 1 process appears in 10 bottles, it should be filled with nitrogen. After filling with nitrogen, the pressure in the bottle is close to normal pressure, so that in case of poor welding quality, air will not easily leak in, and the nitrogen filled in the bottle can be maintained at the initial stage of use due to its low thermal expansion coefficient and low thermal conductivity. Relatively insulated.
 
4. The welding process is the key issue. If the welding quality is not enough, during the use of the Dewar tank, due to the expansion and contraction of the heat, nitrogen will overflow the cylinder when heated, and the air will leak during cooling. After a long time of use, the nitrogen will be replaced by air, so that the cylinder Loss of thermal insulation performance can easily induce abnormalities such as bottle explosion.