Another ULT freezer option is to have
two compressors. In most freezers, twin
compressors don’t function as backups. They
are independent and one functions as a high
stage compressor that cools down the low
stage condenser. This isn’t a standard option in
the industry yet because it is price prohibitive,
but some manufacturers justify the price with
performance improvements.
After CFC’s were banned, ULT freezers
switched to HCFC refrigerants. Currently,
freezers which are the most energy ecient
use hydrocarbon refrigerants (HFCs).
The ULT freezer industry is trending towards
using highly ecient, but ammable,
refrigerants such as R170 Ethane, and R270
Propane. The United States and France do not
yet allow ammable refrigerants despite the
many years both have been in use elsewhere.
ULT freezers using ammable refrigerants are
popular in the European pharmacy industry,
but not yet available in the United States due
to regulatory restrictions. ULT freezers using
ammable refrigerants are 50% more energy
ecient than models using conventional
refrigerants after pull down, and have
decreased potential for oil logging.
How long does it take for ULT
freezers to cool?
The pull down time is the time it takes to get
to -80°C after set-up or installation. After a
power failure, the time it takes to reach -60°C
is called the holdover time.
After a door opens ULT freezers are designed
to recover the temperature as soon as
possible. Every time an upright ULT freezer
door is opened it takes 10 minutes to recover
its temperature back to its setpoint.
Flammable refrigerant gas helps pull down
faster, and saves around 15% electricity.
Warm up can be slowed down by having
eective insulation, door gaskets, and a claw
door. All three are important.
Quality manufacturers supply graphs
to show how their systems compare to
others with data on pull down, heating
up, power consumption. Pull down and
warmup numbers vary widely, with some
systems taking 3 times as long as others.
Be aware of the location inside the freezer
where temperature was measured in the
manufacturers’ data, as pull down times
measured in the middle of a chamber can
vary considerably versus measuring at the
top or bottom.
How much temperature variation
inside can be expected?
It’s important to select a ULT freezer with
the best temperature uniformity. Typically,
temperature variation is only a few degrees
away from set point, but it is best to review
the manufacturer’s test data. The data should
specify the location of the testing probes,
typically top, middle, and bottom of inner
chambers, and explain how loaded the freezer
was during testing. It is typical that uniformity
is better when a freezer is fully loaded.
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