All cryogens are used near their boiling points. This means they are rapidly evolving gases at all times. The following shows both the extremely low boiling points of cryogens and the enormous amounts of gas these liquids could produce if they were to boil.
- Liquid Helium
- boiling point of 4.2 K, -269 C, -452 F
- volume ratio of liquid to warm gas is 1 : 757
- Liquid Nitrogen
- boiling point of 77.3 K, -196 C, -321 F
- volume ratio of liquid to warm gas is 1 : 696
are used to store and transport cryogenic liquids; gas boiloff is limited by keeping the liquid cold with thermal insulation and by allowing the volume to pressurize slightly. Dewars are protected against explosion by relief valves and burst disks. For a sobering account of what happens if such pressure relief is disabled, read this
. For a harrowing account of what happens if a dewar is damaged, read this
Liquid Helium (LHe)
LHe is used for equipment and experiments at very low temperatures (around 4-5 K). LHe cools the CTA magnet in the L0 blue room, CESR wiggler magnets, and superconducting RF cavities in CESR, ERL, and the HTC, as well as those at Newman Lab.
Like other cryogens, LHe is transported and stored in thermally insulated containers called dewars
. Never tip LHe dewars when transporting them. Always move slowly and pull them only by their handles, not by the protective ring around the top. If you must crane-lift a dewar, attach the cables to the lifting lugs only.
Transferring LHe requires detailed understanding of the procedure, including the use of vacuum-jacketed transfer lines, which can be quite cumbersome and awkward. You must receive on the job training by someone familiar with the transfer process before attempting a transfer.
Both Wilson and Newman Laboratories have helium recovery/liquifaction systems which act to avoid helium loss by recooling evaporated helium into a liquid state. (In comparison, liquid nitrogen is relatively cheap, and no recovery system for nitrogen exists.) These systems contain refrigerators, large liquid helium reservoirs, liquid transfer lines, pumps, and large gaseous helium storage tanks which also act as a location for emergency pressure relief. These systems contain numerous safety features should be operated only by trained experts.
Liquid Nitrogen (LN2)
LN2 is sometimes used in the insulating jackets around LHe transfer lines and dewars
to prevent rapid evaporation of LHe by radiant heating. LHe boils at a temperature of about 4 K and has a latent heat of about 20 joules per gram, whereas LN2 boils at about 80 K and has a latent heat of about 200 joules per gram. Room temperature is about 300 K. LN2 cools the thermal barrier necessary between LHe and room temperature.
Full LN2 portable dewars weigh about 600 pounds. Use caution when using and transporting them. Transport and tip them only with approved handcarts that can support their full weight. You should receive instruction from someone familiar with the process before filling or transporting a dewar.
Whereas the cost of helium makes the CLASSE helium liquification and recovery economical, liquid nitrogen is purchased commercially and delivered to large storage tanks (emblazoned with the "PRAXAIR" logo) outside Newman and Wilson. Liquid from these tanks is then piped via insulated transfer lines
to the locations where large quantities are frequently or continuously needed. Small quantities of liquid N2 are sometimes useful, and small dewars can be filled by hand. Such filling requires the use of appropriate protective attire (see below).
Personal Protective Clothing
Avoid contact with cryogens or surfaces cooled to cryogenic temperatures to avoid cryogenic burns. Wear dry clothing when transporting or working near cryogens because the risk of cryogenic burns increases with wet skin or clothing. Wear long pants and long-sleeve shirts to avoid trapping any cryogens next to your body. You should also wear high-topped shoes, safety glasses or a face shield, and leather gloves. The gloves should be slightly large so you can remove them quickly in the event of a spill. Remove rings, watches, bracelets, and any other jewelry. Wear a waterproof tyvek suit, available in the stockroom, when there is a danger of being sprayed by cryogenic liquids.
If you are burned by a cryogen or a surface cooled to cryogenic temperatures, seek medical attention immediately (for anything other than minor burns, call 911). While waiting for help to arrive, bathe the burned skin with warm, not hot, water. Remember that the skin may be both frozen and burned.
Cryogens and Oxygen Depletion
Modest volumes of liquefied gases can displace much larger volumes of air when heated suddenly, leaving the air with a reduced oxygen supply. For this reason it is important to review the hazard in a given room or lab where cryogens are used and engineer appropriate safeguards. Examples where engineered controls were required include the liquid helium and liquid nitrogen used in the CESR tunnels and in the Newman SRF Processing Pits. Whenever cryogens are used in labs, rooms or other restricted volume areas, the volume of the cryogen used and the maximum delivery rate need to be compared to the volume of the use area and its ventilation rate. A simple summary should be discussed with the Safety Director.
Although the details of the use of cryogens are usually evaluated when there is a system design, there are guidelines that should be observed from the beginning of the design and included in the Safety Plan
. Some of these include
- All normal exhausts of evaporated cryogens are vented outside the building or into a large, well-ventillated volume such as L0 at Wilson Lab
- Stored volumes and supply rates are limited to only what is needed
- All reasonably anticipated events that result in a venting of cryogen are engineered to vent outside the limited volume area. These would include quenches, simple vacuum failures, simple operational errors, and warm ups. This is usually accomplished by having a set of pressure relief systems that discharge into an appropriately sized and engineered vent line
- There is adequate ventilation
- Cryogenic storage vessels and delivery lines are protected against casual damage
- No trapped cryogenic volumes
Other Cryogenic Hazards
Cryostats are insulated containers that help maintain equipment at cryogenic temperatures. Each must be equipped with an adequate pressure relief system on any closed volume such as the insulating vacuum, any LN2 jacket, and the LHe vessel. Maximum working pressure of the cryostat must not be exceeded. Some dewars and cryostats use flexible "pigtails" which must be secured to prevent them from whipping around while cryogens are first being transferred through them.
Surprisingly, LHe and LN2, which are not flammable in their natural states, can pose fire hazards as cryogens. Because cryogens are so cold, they can cause oxygen from the surrounding air to condense on transfer lines as they flow through them. Concentrated oxygen is a fire hazard, so use insulated transfer lines to prevent condensation.