What is a ULT Freezer

Ultra Low Temperature Freezer

A Ultra low temperature (ULT) freezer, often referred to as a minus eighty freezer, is much the same as your fridge or freezer at home or in your lab. With of course, a notable exception. The ULT description generally applies to a freezer which operates in the range from -60 to -80 deg C.
Because they are so cold inside they have more and better insulation than your home freezer. The walls and outer door sometimes  feature a partial vacuum with the foam packing. There is usually more than one inner door separating compartments so that  only a fraction of the contents are exposed to ambient conditions at a time.

There are different ways of achieving such a low temperature. Most common is to have two compressors. Each a little different and containing coolant which is liquid at different temperatures. One compressor cools the other (a cascade) which cools the contents of the freezer. Now days CFC's (which deplete the ozone when they escape from the freezer) are not allowed and even their substitutes, HFC's are now frowned upon  because they have a high global warming index. So now the best gases are common hydrocarbons such as methane, ethane and propane in various mixtures. Using these gases also increases the efficiency of the cooling system.

Another modern step to further increase efficiency and reduce operating costs is to employ variable speed drives for both the compressors and the cooling fans. This has the added benefit of reducing noise as well. A modern ULT with all of these improvements may typically consume 8.5 kWh/day. That's about $2.
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