Low-temperature laboratory freezers can achieve temperatures of about -40° C or lower. Kitchen freezers, by contrast, operate to about -20° C. The temperature “sweet spot” for lab freezers is about -86° C. “Mechanical” (compressor-based) freezers reach the -40° to -50° C range with a single compressor. A second compressor is required to cool below that point. To achieve energy efficiency and extend the life of the compressors, the low-stage compressor turns off automatically when the set-point temperature is reached, while the high-stage compressor remains on continuously.
What is a ULT Freezer?
Ultra-low temperature freezers, or ULT freezers, range between -30 and -40 degrees Fahrenheit. They’re used for the storage of drugs, tissue samples, enzymes, and other biological materials.
ULT freezers have become essential for clinical environments. Lab technicians in many fields consider them to be a vital part of their work and research.
Let us resume them below, thanks to our 50 years experience in the field and our collaboration with some of the most important laboratory researchers throughout the world.
Two versions are generally available: upright freezer or chest freezer with access from the upper part. The most common one is the upright version, as laboratories are increasingly looking to save space to make it easier to move around.
The internal storage volume of the low temperature freezer can start from an internal capacity of 200 litres up to a maximum capacity of 800 litres. Some manufacturers, including FDM, can provide customized volumes on requests.
The Ultra freezer has shelves on the inside where research samples are placed. Each shelf is closed by an internal door so to maintain the temperature as uniform as possible on the shelves where no sample is removed or placed.
The insulation is an essential part of the Ultra freezer. This polyurethane insulation is free of polluting substances and it is designed to keep the cold stored inside the cabinet, between the external and internal walls. The high insulation coefficient keeps the temperature stable inside the cabinet for hours.
The gasket seal is positioned all along the perimeter of the freezer cabinet. It seals the freezer cabinet when the access door is closed. The most efficient ones are silicone quadruple-step seals, which guarantee even greater temperature stability.
The main access door also insulated with polyurethane, guarantees to keep the cold, being blocked by an external closure generally fitted with a safety key.
More practical freezers are equipped with swivel castors at the bottom, to facilitate movement inside the laboratory.
The best freezers are equipped with an electronic PLC controller, and a display that indicates freezing parameters. The entire control system is generally equipped with a back-up battery, which ensures that the digital temperature display works even in case of power outages.
How does the refrigerator circuit of an ultra freezer work?
Refrigeration systems can be a single high-power compressor hermetically sealed or two cascade compressors. The two cascade solution, is two refrigeration circuits connected so that the evaporator of one cools the condenser of the other, facilitating the condensation of the compressed gas in the first circuit.
Air-cooled condensers are generally used in laboratory refrigeration systems. They consist of tubular batteries (copper or copper-aluminium) arranged to provide as much surface heat transfer as possible. In Ultra freezers, the circulation of the cooling air is forced by an engine driven fan and the expansion of the refrigerant fluids is obtained by capillary tubes.
Evaporation takes place through steel plate heat exchangers, located inside the chamber, or by means of a coil. In chest freezers, the copper or copper-aluminium coil evaporator is placed along the walls and bottom, while in the upright freezers it runs along the walls and at the top. The coil in the cabinet eliminates the efficiency issue in the heat exchange of freezers with the coil in the insulation cavity.
Refrigerant fluids used in the ULT freezers
Refrigerant fluids are selected from gaseous chemical compounds that can easily be liquefied and characterized by certain properties such as high critical temperature and low critical pressure, limited hazard and toxicity.
They are indicated by the letter R followed by a number that refer to the molecule’s atoms.
Each refrigerant system must be sealed and contain eco-friendly refrigerant HFC gas, free from CFC and HCFC as well as being non-explosive, non-flammable and non-toxic.
The most efficient, environment-friendly refrigerant fluids used in ultra freezers are typically R170, R23 and R290.
With the European Regulation UE 517/2014, the Ultra freezer manufacturers must follow the guideline to allow its use even after 2030. So they must use green refrigerants:
R744 (Carbon dioxide)
R290 (CARE 40) Propane
R600a (CARE 10) Isobutane
R1270 (CARE 45) Propylene
Typical Applications for Ultra-Low Temperature Freezers
Ultra-low temperature freezers find wide application for biological and biotech storage applications in
- research universities
- medical centers and hospitals
- blood banks
- forensic labs for long-term evidence storage
- establishing performance specs for parts used in extreme environments
As to more specific examples of the above, ultra-low temperature freezers are used to store biological samples such as DNA/RNA, plant samples and insect artifacts, autopsy materials, blood, plasma and tissues, chemicals, drugs and antibiotics.
Manufacturing firms and performance testing labs use ultra-low temperature freezers to determine the ability of products and machinery to perform reliably under severe low temperature conditions such as found in Arctic and Antarctic regions.
These myriad applications suggest that ultra-low temperature freezers may house multiple and different experiments, some of which may require frequent checking while others remain undisturbed for lengthy periods of time. And in fact if they do, this becomes a feature that can be applied to your selection process.