Environmental testing
ULTRA-COLD
CONSIDERATIONS
How to choose your cooling method for
thermal testing at ultra cold temperatures
// JOHN BOOHER
Aerospace electronics and many other
products require testing at a wide
range of temperatures. Generally,
within the prescribed range of +185 to
-75°C. Hot testing is often very much
cut and dry with resistance heating.
Cooling, on the other hand is a little
more complicated.
There are three main methods
frequently used to reach these colder
temperatures and there are some trade-offs
for each. Liquid nitrogen (L-N2), liquid
carbon dioxide (L-CO2) and two-stage
cascade refrigeration, including
autocascade systems.
LIQUID NITROGEN AND
CARBON DIOXIDE
With a boiling point of -196°C liquid
nitrogen has a seemingly unlimited range
and heat removal capability for most uses.
It is readily available, reasonably priced
and a good choice when fast transitions are
important and if repeated temperature pull
downs are needed. It is also desirable when
massive or active loads are to be cooled.
Other than the fact that L-N2 by itself is
extremely cold there are very few problems
cooling and controlling temperature with
L-N2. Simplicity often wins. A temperature
controller and a control solenoid is really
all it takes to accomplish the task.
Typical thermal test systems using L-N2
either have a portable tank located next to
the test system or a vacuum insulated
distribution system connected to an
outdoor tank. These systems used for
thermal test are often limited to
temperatures above -100°C because of
limitations from other system components
and the risk of liquifying Oxygen out of
the air on enclosed surfaces exposed to
temperatures below -150°C. Most test
requirements luckily call for temperatures
-100°C or above but in general, more
complications can occur at the more
extreme
temperatures.
In a low-pressure
distribution system liquid
carbon dioxide can easily be
cheaper than other methods.
However, it is limited to about -60°C and
has some additional issues such as the
possibility of dry ice freezing solid in the
delivery line if careful attention is not paid
to pressure drops and restrictions in the
line. Water-ice accumulation can be an
additional problem in some cases,
particularly if a delivery drop line in a
distribution system is not used for a long
period of time. Heat removal capacity per
pound is very good with L-CO2 but not as
good as liquid nitrogen, but L-CO2 is
cheaper than liquid nitrogen.
REFRIGERATION SYSTEMS
When availability of L-N2 or L-CO2 is not
practical, temperatures down to -35 or
-40°C can be easily achieved with basic
refrigeration systems such as are
commonly used in homes and industry.
When colder temperatures are required, a
system with multiple refrigerants must be
used to achieve temperatures below
-40˚C. For systems in the range of
ambient to -75°C, a dual refrigerant
cascade refrigeration system is the typical
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1 // The cover
allows temperature
conditioning by convection
and conduction
2 // LN2 tank for a
large facility
1
choice for achieving
these temperatures
without expendable
coolant, only
using electricity.
Autocascade compressor
systems use phase separators to let
each of multiple refrigerants to operate
in the pressure and temperature range
where they can effectively do their work.
The capacity is greatly reduced over a
single stage with the same size compressor
using a single refrigerant but good heat
removal capacity is available at
temperatures in the range of -75°C. The
autocascade systems are often less
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