The range of temperatures that may be produced
inside a chamber is one of the most important characteristics that distinguishes
one type of chamber from another.
Common upper-range (high) temperatures offered by most chamber manufacturers
are 177 degrees C to 190 degrees C; this range is especially common in
reach-in and walk-in types. Using special construction and insulation,
temperatures up to 593-degrees C may be attained.
Cooling environmental temperature chambers can be done in a number of
different ways. The two most commonly used ways are:
expendable refrigerants -- liquids/gasses that are injected
into the space being cooled. As refrigerant liquid enters a chamber
(directly or via a fin coil) it absorbs heat and flashes to gas. The
gas is then vented out side the chamber (and out of the building). Liquid
nitrogen and liquid carbon dioxide are the two most popular refrigerants.
Cryogenic temperatures down to -184 degrees C can only be attained using
liquid nitrogen. Liquid carbon dioxide (CO2) can't produce temperatures
below -68 degrees C. Both liquid nitrogen and liquid CO2 are environmentally
safe and may be vented to the atmosphere. However, it's very important
that these gasses be vented outdoors -- they displace oxygen, which
can lead to asphyxia if the chambers is not properly vented.
mechanically-cooled refrigeration -- these type of chamber
cooling systems are similar to those found in common refrigerators.
A compressor is used to circulate refrigerant in a closed-loop system.
The lowest temperature required by one's testing goals determines the
type of refrigeration system needed. Single-stage systems can drop a
chamber's temperature down to -34 to -40 degrees C. For continuous operation
at and below -40 degrees C, most manufacturers recommend a cascade refrigeration
system. Cascade systems are comprised of two separate cooling systems
to cool the chamber to a maximum low of around -85 degrees C.