Liquid nitrogen refrigerant traps offer an extremely effective method for trapping condensable vapors such as water. Liquid nitrogen consumption is largely dependent on the level of vacuum maintained in the roughing line (the deeper the vacuum level the longer the LN2 will last). Other factors such as frequency of vacuum cycling, ambient relative humidity and gas load. In general, reported LN2 hold times are only valid at vacuum levels of 10^-2 torr (10 millitorr or less).
Trapping with LN2
All traps that have a contained volume of LN2 require rigorous, yet simple maintenance—the LN2 level must be kept constant. Allowing long periods between fills causes large level changes and runs the risk of releasing vapors previously frozen on the trap’s surfaces. With the re-evaporation of vapor comes two risks: vapor entering the pump or pressure buildup (if the system is shut off but not vented). Obviously, any pressure buildup may cause an explosion at a structural weak point such as a glass envelope ion gauge or viewport. Less obvious but perhaps more dangerous is the potential chemical, toxic, or fire hazard created as the operator opens the chamber connected to a trap that is now at ambient temperature. As a safety precaution, any application in which questionable vapors are frozen should be thoroughly examined by the facility’s safety group.
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