No Coffee Carafes in Cryogenic Applications
On the other hand, a coff ee carafe can also be produced of stainless steel. When buying a stainless steel coff ee carafe, the quality of the stainless steel is not apparent to the customer. T erefore, the user has no knowledge about whether or how long the material of the carafe made of stainless steel will withstand the thermal stresses that occur during fi lling with liquid nitrogen. Inappropriate types of steel can become brittle at low temperatures, which can lead to the destruction of the vessel.
Surface Damage and Its Consequences Damage, such as surface scratches on a Dewar fl ask or a
coff ee carafe with vacuum insulation, can lead to the implosion of the fl ask when exposed to the extreme thermal stress experi- enced when fi lling the vessel with LN 2 . If an implosion occurs, the liquefi ed gas and the broken pieces remain within the protective casing evaporating the liquid gas. It is quite important when tightening the lid of the LN 2 container that it not be fi xed or have ice on top, otherwise any overpressure might not be released fast enough. T is could lead to an explosion of the fl ask or coff eepot, which could cause serious injuries or damage. In the article [ 1 ] you can fi nd that there is a fi xed connection between the carafe and the lid through icing. If implosion, takes place, pressure will not be released effi ciently, which could lead to an explosion.
If icing cannot be prevented through the testing procedure, we recommend installation of a small fan, like the type used in computer housings. T e fan would blow away the humidity and prevent the lid from icing ( Figure 2a ). A glass Dewar fl ask manufactured according to DIN 12492 or ASTM E1379-90 has an expected lifetime of at least ten years when handled properly. T is statement can be made
because of the high standards used for the material and its manufacture when it is to be used in vacuum and cryogenic applications. Dewar fl asks made of stainless steel have a higher mechanical strength compared to the glass Dewar fl asks. T e disadvantage compared to a glass Dewar is the higher thermal conductivity of the stainless steel fl asks. However, both materials meet necessary standards for safe use when handled properly.
Surface damage may occur in a metal Dewar or coffee carafe. This can be caused by mechanical stress or corrosion, allowing a crack or a leak to develop in the vacuum space. The liquid nitrogen would be then sucked into the vacuum and come into contact with the warm outer wall of the flask. This would lead to a rapid evaporation during the phase change from liquid to gas, and the volume would increase by about 700 times. This leads to a rapidly increasing pressure inside the vacuum space. The increased pressure cannot be compensated by exhausting gas through the leak, and the valve may become damaged by the extreme overpressure. In order to reduce this overpressure, only Dewar flasks made of stainless steel in accordance to the “Directive on Pressure Equipment - 97/23/EC” have a combined vacuum/ overpressure valve placed at the vacuum chamber ( Figure 2b ). A coffee carafe, for example, does not have this combined vacuum and pressure relief valve. Penetration of LN 2 into the vacuum chamber of a carafe may lead to an explosion in extreme cases.
Such destructive explosions cannot occur when using a glass Dewar correctly. Only implosions happen; the glass splinters collapse and are not ejected with high speed. Because there is a protective casing out of metal or aluminium, the shards of glass as well as the liquid nitrogen will come to rest inside the casing.
Only a tightly closed Dewar fl ask can lead to an explosion. T erefore, it is standard for Dewar fl asks to be operated only with loose plugs when they are fi lled with cryogenic liquids, such as LN 2 or cold CO 2 . When the pressure rises from gassing liquids in a tightly closed Dewar, they cannot exhaust. T e increase in pressure would inevitably lead to an explosion. If a lid is required for a specifi c application of a Dewar vessel, the lid must be provided with an exhaust hole or some other suitable opening so the resulting gas can escape.
Reducing Risks
Figure 2 : (a) Schematic of a Dewar with a cold trap and fan to prevent icing of the lid. (b) A gas outlet tube (projecting toward the back of the photo).
32
Safety is of prime concern for all laboratory workers. To avoid workplace risks, users of LN 2 containers should strictly adhere to the following: (a) respect all country-specifi c requirements for handling liquefi ed gases, (b) use
www.microscopy-today.com • 2016 January
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