Product Intelligence Ultralow-Temperature Freezers: Antarctica in a Box by Josh P. Roberts I
n most common parlance, ultralow-temperature freezers (ULT freez- ers) are containers capable of maintaining an interior temperature of about –80 °C, typically by mechanical means. While they come in different orientations and sizes, they are most often seen as upright floor-standing models similar to a single-door home refrigerator.
A large university with a medical, veterinary or agricultural school, or a biotechnology or pharmaceutical company, may have several thousand ULT freezers spread throughout the campus in individual laboratories as well as in centralized facilities. These are used to store drugs, enzymes, chemicals, DNA and protein extracts, viruses, bacteria, tissue and cell preparations and agricultural and environmental samples, in tubes, plates, bags and other containers designed to withstand such cold conditions.
How is it so cold? In its essence, a ULT freezer is a highly insulated cabinet fitted with a
means to bring and keep the temperature inside the box down. The most common way to accomplish this, called a cascade refrigeration system, is to use a pair of vapor-compression refrigeration systems interacting in series. The first absorbs heat from the interior of the cabinet and transfers it to the second system, which then transfers the heat to the exterior.1 The physical properties of cascade freezer coolants have historically enabled ULT freezers to achieve –86 °C, says Neill Lane, president and CEO of Global Cooling (
www.stirlingultracold.com), and “so freezers sold as ultralow freezers can typically operate at–70 or –80—and there’s no differentiators” in terms of the coldest temperatures they can achieve.
There are, of course, variations on the theme of the cascade freezer. The compressors in Twin Guard freezers, for example, work in parallel rather than as a cascade—that is, both are used to directly cool the cabinet, so “if one shuts down the other can keep functioning by itself to keep the freezer at a temperature of –65 °C,” points out Carl Radosevich, product specialist at manufacturer Panasonic Healthcare (
us.panasonic-healthcare.com). So-Low (
www.so-low.com) offers an extra-wide, 30-cubic-foot-capacity ULT freezer with two redundant cascade systems, each with its own independent electrical system and power cords.
A new player on the scene is Global Cooling’s highly efficient Stirling Ultracold freezer. Here, an electrically driven piston is used to expand (and cool) helium gas in what is termed a Stirling engine. Ethane,
With real estate at a premium, it’s not uncommon to fi nd manufacturers citing their unit’s storage capacities (in cubic feet, or boxes, or vials) per square foot of fl oor space.
contained in tubing wrapped around the cabinet, absorbs heat from the cabinet and transfers it to the helium.
Several vendors also offer freezers that use nitrogen (either in vapor or liquid form) to cool the cabinet and are capable of achieving and holding significantly lower temperatures. While these can serve many of the same needs as a traditional mechanical ULT freezer, they require infrastructure commitments beyond what are found in a typical research laboratory and thus will not be further considered in this article.
Form factor Because cold air falls and warm air rises, “chest freezers have the great
benefit that if you open the door you don’t have all the cold air falling out, so they’re much better in terms of recovery,” meaning the amount of time it takes to return to their setpoint, notes Lane. “But they’re not very common.” This is due in part to the way they’re used: samples in chest freezers are not as readily accessible as those sitting in racks on the shelves of an upright—whether floor-standing, benchtop or under- cabinet. In addition (especially in high-rent areas like the East and West Coasts), scarcity of floor space makes uprights more attractive.
With real estate at a premium, it’s not uncommon to fi nd manufacturers citing their unit’s storage capacities (in cubic feet, or boxes, or vials) per square foot of fl oor space. The Thermo Scientifi c higher-end units, with more effi cient insulation, for example, “can hold about 20% more samples in the same footprint… it’s not taller; it just has thinner walls and keeps the
AMERICAN LABORATORY • 38 • MARCH 2015
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