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Some Cautions on Liquid Nitrogen Trap Selection


We have found some things out recently on these LN2 traps that we do not particularly like, so tell me about your application and I will try and be as candid as I can about drawbacks of LN2.  The big thing is length of time they last.  Our people tested them for time and the stated numbers are based on pressures of 10-6 Torr.  Now at these pressures there is not a whole lot of gas flow so therefore there is not a lot of heat transfer between the cold trap (LN2) and the gas being pumped and the LN2 lasts.  The traps last a long time at these pressures and are not exactly "real world" applications.  At higher pressures they do not "last" anywhere close to the stated time frames.  Ours or anyone else's.  We are working on some newer models, but are just at the end of the test phase for our new LN2 traps that do not even incorporate this new idea.  It was determined during testing of these traps.

Now the thing is many users are using these to stop say water, some aliphatic alcohol or other low boiling chemical out of the pump or to reduce it in their sample.  In this example the pressure stays, for talking sake with water, at like roughly 10 Torr until all the water is turned into a gas and pumped by the pump.  The pressure will stay at this elevated “high pressure” and as a result, the trap, sees this elevated gas flow and the liquid nitrogen is consumed at much higher rates.  Understand the application before getting a trap and ending up with a false sense of security.  We can help.  Ask us.

It would be another 2 months if our hypothesis is correct and we indeed have a higher pressure, longer term LN2 based trap.  With current traps in my opinion, in higher pressure application there is also a need for an LN2 keep full pumping station that typically adds another healthy kick to the price of the traps in order to maintain LN2 levels in the trap and maintain affectivity of the trapping function.  In this scenario, the trap has a larger LN2 storage container placed nearby.  A special LN2 transfer pump is incorporated into the system design to keep the trap maintained from the storage container so that trapping and the evacuation process can proceed without constant monitoring by the end user.  It is also bad to have the trap lose temperature during evacuation as all the condensable vapors trapped so far then re-evolve and end up in the pump anyhow.  I will be writing this all up soon and posting it soon as we get all our ducks in a row for our 2nd "new" design and have a better mousetrap which I think we will have in the relatively near future.

Dry Ice Traps typically may be better solutions for high pressure applications but have some drawbacks namely availability of supply that may impact user choice.

11/26/2005 ©Copyright HyVac Products, Inc. All Rights Reserved
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