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.
