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Thermo Fisher Scientific

CO2 Versus LN2 Back-up Systems

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Issue

Which is better, a CO2 back-up system or an LN2 back-up system?

Environment

All ULT Freezers

Resolution

The following information may be helpful regarding CO2 and LN2 Back-Up Systems.

The back-up systems are factory set for -60C, but are adjustable. The choice between CO2 verses LN2 is strictly up to the customer. There are pros and cons to both units. All you can really do is explain to the customer how each system operates and let them make the choice.

The CO2 back-up system uses liquid CO2 , which is -78 C. The CO2 tank does require a siphon tube in order to draw the liquid from the bottom of the cylinder. Liquid CO2 can remain pressurized until it is needed. Therefore a tank can be connected to the back-up system and still be full (if the back-up system has not been activated) in 6 months, a year, or more, so it will be ready when the back-up system is activated. One standard 50 lb. cylinder will keep a freezer that is 3/4 full to full at -60 C for  approximately 8 hours depending on the size of the freezer, ambient temperatures, and door seal. Tanks would need to be changed every 8 hours or just manifold several tanks together to gain additional time.

The LN2 back-up system of course uses liquid nitrogen, which is -196 C. LN2 cannot remain pressurized and therefore constantly boils off whether they use it or not. The customer cannot connect a tank of LN2 to a back-up system and expect it to have any LN2 within 6-8 weeks. However a fresh 160-liter cylinder of LN2 will keep a freezer that is 3/4 full to full at -60 C for 45
to 50 hours depending on the size of the freezer, ambient temperatures, and the door seal.

If they choose an LN2 back-up system they have to understand that they will have to change out the LN2 tank approximately every 3 - 4 weeks if they intend to keep one connected all the time. The best solution for an LN2 back-up system is to connect the freezer to a telephone dialing system and have a procedure that a designated person is to connect a fresh LN2 tank and open the valve on the tank if they receive a call the freezer is in an over-temp alarm.

If the customer wants a colder temperature than -60 C they would need to have an LN2 back-up system.

No matter which they choose, they should consider some form of back-up in case of a power failure. Whether it is a CO2/LN2 back-up system or a Sensaphone telephone dialing system, extra security or extra time can provide some peace at mind.