How Do Ultralow Temperature Freezers Work?

How Do Ultralow Temperature Freezers Work?

Ultralow temperature freezers are critical tools for biological sciences

Andy Tay, PhD

Ultralow temperature (ULT) freezers—more commonly known as -80 freezers—are pieces of equipment used extensively in life sciences for long-term storage of biomolecules. They typically provide a temperature between -40 to -80°C or -40 to -123°F to protect samples containing biomolecules like oligonucleotides and proteins from heat-associated denaturation. ULT freezers are also crucial for the shipment of SARS-CoV-2 vaccines composed either of proteins or messenger ribonucleotide (mRNA) to protect these heat-sensitive biomolecules from being damaged. This article will discuss the various components of the ULT freezer and its operating mechanism.

Key components of ULT freezers

ULT freezers come in two main shapes—upright or chest. Upright freezers are the preferred model as they have a better physical footprint and use space more efficiently. However, the chest-shaped freezer, which is shorter, is likely to be more inclusive to the needs of researchers with physical disabilities.

All ULT freezers come with two doors, with the inner door serving to reduce unwanted heat input into the freezer interior for maintaining a uniform temperature. The doors can also be shut very tightly through the use of an external knob and silicone, gel-like gasket seals that line up along the perimeters of the doors. Furthermore, the walls of ULT freezers are fitted with polyurethane material for maximum heat insulation from the external environment.

Many modern ULT freezers also come with a display panel that shows the interior temperature and other key parameters that can be remotely accessed. Most ULT freezer designs forbid users from opening the freezers too frequently and are equipped with an alarm system if the interior temperature is too high.

Achieving ultralow temperature

ULT freezers usually make use of cascade refrigeration, which consists of two or more vapor compressors using different refrigerants and an inter-stage heat exchanger to cool the freezer over its desired temperature range. 

The typical refrigerants used in ULT freezers are gaseous compounds like propane and ethane that can be easily liquefied, with well-characterized properties like evaporation temperature; these compounds also have limited toxicity.

Imagine a cascade refrigeration system with two compressors, A and B, with A being the one with the lower temperature system. The evaporation of refrigerant A removes heat from the space to be cooled (this concept is similar to the cold feeling you have probably experienced after emerging from the pool as water evaporates and removes heat from your skin) and transfers it to a heat exchanger, which is then cooled by the evaporation of refrigerant B in the higher temperature system. The higher temperature system B then transfers heat to a conventional condenser where the entire heat output of the ULT freezer can be fan or water-cooled. The ULT freezers used in labs are usually air-cooled, which explains their continual loud noise. As a result of continual cooling, ULT freezers generate a substantial energy footprint, which has led to a push to maintain ULT freezers at -70 degrees Celsius (instead of the -80 degrees Celsius they are designed for) and innovate in design features, such as quick restoration of a freezer’s interior temperature after door opening.

Schematic of a cascade refrigeration system.
Adapted from Wikipedia

How much do ULT freezers cost?

A typical ULT freezer from VWR is expected to cost between $12,000 and $40,000 USD, depending on the dimensions. Generally, the larger the volume of items the freezer can store, the more expensive the freezer will cost. Information from Thermo Fisher Scientific shows that the daily energy consumption of their ULT freezer ranges between eight and 12 kilowatt hours (kWh) per day. Considering an average pricing of 11 cents per kWh in the United States, the costs of running a typical ULT freezer would be around $480 per year. However, this does not take into consideration lab-specific factors such as frequency of opening the freezer, volume of biological samples stored in the freezer, and build-up of frost which can affect uniform temperature distribution and quickly increase the operating costs.

ULT freezers are crucial to biological sciences and they will be critical to enabling safe and efficient transport of protein and mRNA-based vaccines against SARS-CoV-2.