The general rule for liquid nitrogen tanks – the larger the diameter, the faster it evaporates! The liquid nitrogen with the same capacity and larger caliber has a shorter storage time and requires a higher frequency of rehydration. According to this rule, wouldn’t the wide-mouth liquid nitrogen low-temperature freezer need to add liquid nitrogen every day? After all, its diameter is above 325mm, and it uses 304 stainless steel material whose thermal insulation effect is slightly inferior to aviation aluminum!
Is that so?
Of course not!
Although liquid nitrogen cryogenic freezers have two storage modes: vapor phase and liquid phase due to capacity reasons, considering resource investment and maintenance costs, the vapor phase is actually used. Vapor phase preservation uses evaporated liquid nitrogen, which means nitrogen vapor is used to cool down. When in use, the evaporator in the liquid nitrogen storage area works, and the vaporized liquid nitrogen expands and then rises to maintain low temperature.
In this storage mode, unlike the liquid phase immersed in liquid nitrogen, the total amount of liquid nitrogen is reduced by 1L for 1L of liquid nitrogen volatilization. The volatilization of 1L of liquid nitrogen is gaseous nitrogen that expands a hundred times, and then fills the entire tank. The temperature will drop only when the gaseous state slowly dissipates. This dissipation is very slow in a vacuum adiabatic vapor-phase refrigerator. Therefore, there is no need to add liquid nitrogen every day.
Moreover, the corresponding test data also shows that the liquid nitrogen cryogenic freezer for vapor phase preservation can maintain a low temperature of -185°C for more than 7 days and -135°C for about 20 days as long as the lid is not opened when the supply of liquid nitrogen is cut off. ℃. When the lid is forgotten, the temperature is still around -150°C for 48 hours.
It can also be seen from here that the liquid nitrogen cryogenic freezer does not need to add liquid nitrogen every day!