What is Cryogenic Storage?
Cryogenic Storage refers to the storage of Cryogenic liquids, which are used for their low temperature properties. Cryogenic liquids are extremely cold, with boiling points below -150°C. Such liquids include, liquid helium (-269°C), liquid hydrogen (-253°C) and liquid nitrogen (-196°C), which is the most widely used cryogenic liquid. There are two types of storage equipment that use liquid nitrogen (LN2) as the cryogenic liquid:
- Mechanical freezing – where a refrigeration system is used to cool samples.
- Cryogenic Storage tank – where the temperature is maintained using a vacuum insulated tank and liquid nitrogen.
Cryogenic Storage Tanks, which are also known as Cryogenic Storage Tanks, are the predominant type of cryogenic storage product within the marketplace and the only type of storage with the Cryovita range from Froilabo.
Cryogenic storage tanks can be categorised by:
- Fill type- auto-fill or manual-fill
- Use- in the liquid phase, vapour phase or ‘dry system’
- Sample type- for storing vials, straws or bags
- Sample quantity
- Storage term- long or short term storage
- Software/alarm systems
Cryogenic storage Tanks – Manual Fill
Typically, manual-fill cryogenic storage tanks/dewars are used for sample quantities in the range 100 and 6,000 vials. One disadvantage is the need for manual filling, however, these cryogenic storage tanks/dewars tend to have narrow neck openings which reduces the static evaporation rate and increases the holding time.
Cryogenic storage Tanks – Auto Fill
The auto-fill cryogenic storage tanks typically contain between 6,000 – 50,000 samples, sometimes more. The large capacity enables the cryogenic storage of many samples, but the capacity and the wide neck opening requires a large amount of liquid nitrogen.
Cryogenic storage Tanks – Liquid Phase
The complete submersion of samples in LN2 is known as liquid phase storage. This is the traditional form of cryogenic storage and maintains samples at -196°C. This cryogenic storage method is simple to operate, requires minimal electronic control and LN2 level can be checked visually. However, there is an increased risk of cross contamination since all samples are contained within a shared liquid. If a sample bag or vial were to split or leak, then the contamination could easily spread to the other samples. There is also a consideration with safely handling the submerged samples or if manual LN2 filling of the cryogenic storage tank is required. Customers may choose this method of cryogenic storage due to the associated safety margin. In the event of a power failure or a natural disaster where the LN2 supplier could not deliver then there are several days of safe storage due to the low static evaporation. Knowing the level of LN2 and the static evaporation rate the user can estimate how many days of safe cryogenic storage is available if the vessel remains unopened, thereby allowing time to make alternative arrangements to preserve samples.
Cryogenic storage Tanks – Gas (Vapour) Phase
Conventional gas (vapour) phase storage is where samples are stored on a platform or within a rack above a reservoir of LN2. The risk of cross contamination is reduced with this method since the samples are not stored in a shared liquid and there is safer manual handling since samples are not directly submerged. This type of cryogenic storage is more efficient than liquid phase storage, but this has a cost implication since more complex electronic control is generally required (on larger units). The design of the cryogenic storage method means that a visual check on the LN2 level is not easy and it is often difficult to maintain temperatures at the top of the tank, generally the achievable temperature within the tank is approximately -150°C. Additionally, there is a much smaller safety margin in the event that LN2 refill could not be performed.
Cryogenic Storage Tanks – Dry Systems: Liquid Nitrogen (Ln2) Storage Tank
Dry cryogenic storage is where there is complete separation of samples and the LN2. Typically, the LN2 is contained within a ‘jacket’ which surrounds a sealed inner chamber that contain the samples, which enables a more uniform temperature profile throughout the tank, the achievable sample storage temperature is approximately -190°C. Dry cryogenic storage systems have a low risk of cross contamination and have the added benefit of greater safety for the user. Since the LN2 is stored separately to the main sample storage chamber there is no risk of the user coming in to contact with the liquid. The full volume of the chamber can be utilised (no loss of storage due to the sample platform in base, like in vapour phase systems).
Cryogenic Storage Considerations
Historically, stainless steel was the preferred material in the manufacture of vacuum vessels for cryogenic storage since it was easy to produce leak-tight welds, easy to clean and the wide availability of standard sealing systems. As an alternative to stainless steel is aluminium. Aluminium has other advantages in that it is lightweight, easier to machine, and the price is generally much lower than stainless steel.
Inventory control systems can be utilised within vessels and are either constructed from aluminium or stainless-steel. Depending on the cryogenic storage method and the sample type there is a choice of standard square racks, pie shaped racks, ladder racks, canisters and blood boxes to maximise space. Aluminium racking has good cold transfer compared with stainless steel, and the boxes may be constructed from fibreboard, plastic, stainless-steel or aluminium.
Many of the cryogenic storage tanks come with handles that facilitate easy handling and optional roller bases or cradles are also available. It important to recognise that narrow neck diameters will limit liquid nitrogen evaporation and that static evaporation rates are provided as an indication since these may be affected by working environment (temperature, atmospheric conditions etc).
Get in Touch for More Information about Froilabo’s Cryogenic Storage Systems
Discover Froilabo’s comprehensive range of Cryogenic Storage Tanks/Dewars. For further information on our range or to talk to a member of our sales team, please don’t hesitate to get in touch. We look forward to helping you.
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