Creating The Vacuum

The main "substance" used in cryogenic storage is Liquid Nitrogen, abbreviated LN2. LN2 is usually stored in a Bulk Tank or pressurized liquid cylinders, which is then piped to the freezers for sample storage. The temperature of LN2 is roughly -196° C, so it is obvious that bringing a substance in contact with LN2, or even pressurized N2, will cool a substance to below the accepted Tg of -132° C.

One of the fundamental rules of all science is the conservation of energy; that is to say that energy can be neither created nor destroyed. Thermodynamics is the study of the particular form of energy known as heat. Applying the principal of Conservation of Energy to thermodynamics means that heat always exists, so the fundamental problem of the cryogenic engineer is how to keep things cold; or another way of asking the same question is how do we limit heat transfer?

Energy in the form of heat, as opposed to others such as electromagnetic waves, i.e. light, needs a substance to transfer through. There are three methods heat may be transferred: (1) Conduction, the direct contact of one substance to another such as putting your hand to a hot oven element; (2) Radiation, a paradoxicaly simple yet complex form of energy transfer through multiple substances; and (3) Convection, the transfer of energy by means of atoms. The cryogenic engineer must account for all three of these energy transfers when designing a cryogenic system. One fact remains constant in thermodynamics and that is this: If there is no substance to transfer through then heat cannot be transferred. By definition that lack of any substance is a vacuum. The fundamental problem faced by the cryogenic engineer is resolved by the creation of a vacuum. The vacuum is the solution but how do we maintain such a thing, how do we minimize the direct path of energy, how can we reflect energy away from LN2 and how do were minimize the pressure when transfering LN2? These are all questions the cryogenic engineer faces on a day to day basis.

Essentially, how do we keep the LN2 cold? By creating a vacuum. Cryogenic engineers use vacuum jacketed piping to transport LN2. There are several different forms of LN2 piping. The main three are a simple stainless steel or aluminum insulated pipe, then there is a dynamic vacuum jacketed pipe, and finally there is a static vacuum jacketed pipe. Of these three, static vacuum insulated pipe is the superior method of LN2 transportation. For more information on pipe designs visit the piping design page.

Holding a vacuum level below 35 microns and supplemented by a chemical gettering system is imperative for the success of any VJ piping systems, for this reason PRINCETON CryoTech INC has developed a Preventative Maintenance program to ensure a long life for your system.

For more information on Thermodynamics please visit Hyperphysics.