A ULT freezer is simply a refrigerator whose contents are stored at between -40 and -86°C. ULT is the abbreviation for ultra low temperature. There are upright and chest freezers. The inner volume is in general between 300 and 800 L.
ULT freezers that employ the cascade refrigeration (CR) system use up to 20 times the energy footprint of household fridges, and used to refrigerate with greenhouse gas fluids (typically hydrofluorocarbon R-508B).Modern ULT freezers employ HC gas mixtures. Typically Ethane and Propane. This has also improved their efficiency by up 30% over the conventional CFC or HFC gassed freezers.
In contrast to short term sample storage at +4 °C or -20 °C by using standard refrigerators or freezers, many molecular biology or life science laboratories need long term storage for biological samples like DNA, RNA, proteins, cell extracts, or reagents. To reduce the risk of sample damage, these types of samples need extremely low temperatures of -80 to -86 °C. Cells are stored in tanks of liquid nitrogen at -196 °C.
The samples in ULT freezers are typically stored in polymer tubes or microtubes. Typically, a rectangular grid of 64, 81 or 100 of these microtubes are located in one box made of cardboard or polymer. Standard ULT freezers can store ca. 350 to 450 of those standard boxes for microtubes.
When using an ULT freezer at full capacity, 40,000 or 50,000 microtubes are stored in one instrument. Clear sample labeling (type of sample, concentration, volume, owner, date, etc.) and exact documentation of the sample location within the freezer is mandatory to find the sample tubes.
The pull down time is defined as the necessary time to cool down the ULT freezer from ambient temperatures to the selected temperature of -80 to -86 °C. The time strongly depends on the type of insulation, the efficiency of the compressor system as well as the installed metal shelves within the freezer. At the start of the 21st century, ULT freezers were able to cool down within 3 to 5 hours. Warm up time is typically 1/8 degC per minute.
Due to the low temperature, ULT freezer consume high amounts of electric energy per day. Depending on the volume of the freezer, the opening frequency of the users as well as the number of samples, the energy consumption starts from ca. 11 kWh/ day and higher. To reduce the energy consumption, the insulation should be as efficient as possible. Additional inner doors reduce the loss of temperature when opening the main door. Icing within the ULT freezer should be reduced to a minimum. Modern ULT freezers employ variable speed drives for both the compressors and fans. This has reduced energy consumption a further 30% to typically 8.5 kWh/day.