Containers for cryogenics

In containers for cryogenic liquids, we must be able to determine the level of the liquid. Since most dewars are metallic, we cannot detect the level optically. 

Beakers and Flasks. Beakers and flasks are the least effective containers for cryogenic materials because there is no insulation whatsoever. However, if the coolant is only water, ice, or a salt/ice mixture, not much insulation is required. There is little concern for rapid material loss with these coolant solutions because they are easy and inexpensive to replace. In addition, it is (usually) safe to pick up these containers with your bare hands. If any ice forms on the sides of a container, it is simple to use gloves or tongs to pick up the beaker or flask and prevent possible skin damage. 

Most storage containers for cryogens are designed for a fO percent ullage volume. The latter permits reasonable vaporization of the contents due to heat leak without incurring too rapid a buildup of the pressure in the container. This, in turn, permits closure of the container for short periods of time to either avoid partial loss of the contents or to transport flammable or hazardous cryogens safely from one location to another. 

The main tank is filled with liquid hydrogen from a trailer. Despite the sophisticated heat insulation in any container for cryogenic liquids, the small amount of remaining heat input vfill trigger off a warming process in the tank which causes the liquid in the container to evaporate and the pressure to rise. After a certain pressure build-up time the maximum operating pressure of the tank is reached. The pressure relief valve has to be opened. From this point onwards, gas must be released (boil-off). The container now acts as an open system with gas usually being lost to the environment. 

The most common container for cryogenic fluids is a double-walled, evacuated container known as a Dewar flask, made of either metal or glass. The glass container is similar in construction and appearance to the ordinary Thermos bottle. Generally the lower portion will have a metal base which serves as a stand. Exposed glass portions of the container should be taped to minimize the flying glass hazard if the container should break or implode. 

Typical containers for cryogenic liquids are pressure vessels, spheres or tanks. In any case the normal storage device consists of a double wall, much like a thermos. Between the two walls is an insulation barrier to keep the cold within the inner vessel. Where the service requirements become critical in terms of minimizing heat loss, evacuation of the inner space becomes economically justifiable. The maintenance of this vacuum becomes one of the long term service problems. Vacuum requirements frequently begin for temperatures below (-) 200°F. 

Containers for cryogenic liquids must be kept clean and restricted to this service. These containers must be made of Type 300 series stainless steel, copper, brass, aluminum, etc., except for special laboratory equipment made of glass. Several materials such as carbon steel become brittle at cryogenic temperatures and cannot be used for handling these liquids. 

Dewan A double-walled container for cryogenic liquids, witti evacuated annulus and heat-reflective surfaces facing the vacuum. 

By Rail In cylinders as a compressed gas, and as a liquid in special containers for cryogenic gas. 

Up to this point we have considered vacuum-insulated containers for cryogens after the UUT is mounted, the vacuum space is evacuated and the cryogen tank is filled, and we wait for the UUT to reach thermal equilibrium before beginning the test. That process takes at least an hour. 

This simple technique can be applied to all cryogenic systems, including small-scale refrigerated systems, all storage containers for cryogenic liquids, and the large scale storage of LNG, LPG and other hydrocarbon liquids. 

Because of Hquid helium s uniquely low temperature and small heat of vaporization, containers for its storage and transportation must be exceedingly weU insulated. Some containers are insulated with only a fairly thick layer of very efficient insulation, but containers with the least heat leak use an inexpensive sacrificial cryogenic Hquid, usually Hquid nitrogen, to shield thermally the Hquid helium contents. 

Use only labelled, insulated containers designed for cryogens, i.e. capable of withstanding rapid changes and extreme differences in temperature, and fill them slowly to minimize thermal shock. 

Equation (5.2) indicates that the time vapor is advected downwind will increase as zt (height of vapor containment box) increases, as this increases the time to fill the vapor box. This additional time allows for the effects of decreasing rates of conductive heat transfer from the dike floor for cryogenic materials, or decreasing convective mass transfer for materials with boiling points that are higher than ambient temperatures, to take effect. 

Foam-insulated containers are adequate for some slush baths, but will require more effort to maintain the coolant. Foam-insulated containers should never be used for long- or short-term storage of cryogenic liquids. They may be used for cryogenic liquid transport, but there will be significantly more loss of the liquid coolant (even in limited transport) than from a regular Dewar. 

We have successfully used a Dry Shipper (such as the Arctic Express Thermolyne CY 50915/50905 available from VWR Scientific, San Francisco, CA) and prefer it for cryogenic collections because of the ease with which samples can be transported by air. The absorbent walls of the shipper are precharged with liquid nitrogen and the flask then maintains ultracold conditions for upward of 3 weeks if left upright. No special restrictions apply to transport of samples in this container, because there is no liquid nitrogen left to spill or pressurize. 

The containers used for transporting and storing the majority of cryogenic liquids are insulated cryogenic tankers, liquid cylinders, and stationary storage tanks. These containers are similar in design to a Thermos bottle. There is an inner vessel, which contains the cryogenic product, and an outer vessel. The space between the two vessels is under a vacuum and usually contains a wrapped insulation material such as aluminized Mylar, or it may contain a powdered insulation such as expanded perlite. The liquid cylinders, usually 160-250 L in capacity, are constructed to a... 

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