Cryogenic Cooling Safety

Liquid Nitrogen (L-N2), Liquid Carbon Dioxde L- CO2) and other cryogenic coolant coolants have potential risks but can easily be safely managed like many things in the lab. The effecitve and rapid cooling these fluids produce is a great convenience for thermal testing.

 

Similar to other potential hazards such as flamable liquids, electricity, hot surfaces, chemicals, etc. Cryogenic fluids can be safely used when aware of a few basic precautions.  The main precautions are as follows:

 

  • The extreme cold temperatures of cryogenic liquids can cause burns
  • Pressurized liquids and gasses require proper pressure regulation / venting
  • Tanks are often very heavy and pose risks should they fall over
  • In extreme cases, excessive use can displace breathable Oxygen in the air

Taking Proper Precautions:

When making or undoing connections to cryogenic fluids, always wear gloves and eye protection.  Preferably heavy leather gloves full face shield.  In General, the rule is hands off of plumbing and devices at cryogenic temperatures.

 

Be aware of the controls / safety features of any cryogenic liquid delivery system. Know how to use these controls. Locate safety vents or rupture disks that might unexpectedly produce a stream of cold vapor.  Here is a supplier’s link with more detailed LN2 safety infomation and explanation of the controls on a cryogenic liquid vessel. Tanks have many built in safety devices to prevent over pressurization and general safety.

 

Never tamper with any of the safety devices on a tank and do not adapt connections to other than standard LN2 designated connections. Not just your eyes and flesh but many items in the lab become damaged or behave differently at these extreme cold temperatures. For example typical foam pipe insulation from materials such as polyethelene becomes extremely brittle and shatters at temperatures of Liquid Nitrogen which is typically -185 C.

 

Be aware that cryogenic liquids expand dramatically and forcefully in the process of use and also during unintended vaporization in the hose. When connecting equipment to a source of cryogeic liquid, there is often an automatic valve at the downstream connection that is normally closed.  After the system is used and the operator believes that he is safely shutting off the coolant source at the tank, an usafe condition can be created. With a closed valve at both ends of a liquid filled hose, the increasing pressure as the liquid warms can rupture the hose. The hand valve at the tank should only be closed when you are sure there is no cold liquid in the hose.  For this reason, our LN2 hoses employ a pressure relief valve that will limit the maximum pressure to a safe level. Likewise any device using coolant must have a functioning exhaust to vent the spent coolant.

 

The contents of a high pressure (~900psi) CO2 tank are stored at room temperature so although the pressure is much higher in the hose to begin with, it does not increase as it sits in the hose.  You can close the hand valve on a CO2 tank at any time without risk.

 

LN2 tanks or dewars as they are known (vacuum insulated thermos-like bottles) rarely tip over due to the wider base but must be placed on a stable level footing. Slimmer High pressure CO2 tanks are more prone to tip over and must alway be restrained when stored in the lab for even short periods of time. The valve on the top of the tank can break if the tank falls over and cause very dangerous situations. LN2 tanks typically have a steel “halo” around the controls and ports at the tank. This increases the safety of an many tank accidents.

 

CO2 and LN2 are both non-toxic and largely part of what we normally breath.  Use of these cryogenic fluids in a normallyventilated area is not a problem however if the ventilation is abnormally low or there is a very large amount of coolant being used, an Oxygen monitoring device should be used to monitor the Oxygen level in the room. Exit the room without delay if Oxygen levels go below 20%.

 


CONCLUSION:

 

Cryogenic coolants like many things in the lab are easily handled in a safe and efficient manner by paying attention in general and being aware of a few basic concerns.

 

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