Please see our Ultra Low Temperature Freezer (ULT Freezers) section to find manufacturers that sell these products
In most common parlance, ultralow-temperature freezers (ULT freezers) 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 (www.panasonic-healthcare.com/us/). 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, 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 find manufacturers citing their unit’s storage capacities (in cubic feet, or boxes, or vials) per square foot of floor space. The Thermo Scientific higher-end units, with more efficient 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 energy just as well,” says Chris Wilkes, product director for Thermo Fisher Scientific’s (www.thermoscientific.com) cold storage business.
Similarly because Stirling Ultracold’s cooling system is much more compact, “for a given height, we use less of that height for the cooling system, so our storage capacity per square foot of floor space is substantially higher,” notes Lane.
A variety of racking and shelving solutions are available for efficient storage of tubes, vials and microplates, for example, to fit almost any freezer. “There are only a few different types of configurations of racks for uprights—usually they’re four or five boxes deep,” says Wilkes.
Most upright ULT freezers feature a single outer door and several inner doors—which may or may not be insulated—to help keep air from falling out of one level of the cabinet when the outer doors are opened to access another level. Thermo Fisher Scientific offers an optional double outer door version of the Forma line: “the bottom is used more like a chest freezer— archival samples are stored in the bottom, and then working samples are stored on the top,” explains Wilkes.
Making sure that the temperature stays within range is important for maintaining the integrity of the stored samples in the event of a power outage or other failure. “Alarm systems are absolutely standard, and a backup battery to control alarm systems and control set[point] is also standard,” Wilkes points out, urging users to make sure the alarm is properly connected to a building management or other monitoring system in order for it to do its job. Many ULT freezers can also be equipped with an optional CO2 or liquid nitrogen backup, allowing cold gas to be injected directly into the cabinet if the temperature gets too high.
Choosing a ULT freezer
When the Biospecimen and Processing Core Laboratory at Mayo Clinic (www.mayoclinic.org) moved and expanded last year, they took the opportunity to evaluate several different freezer solutions in terms of such metrics as capacity, temperature profile under light-to-moderate and moderate-to-heavy usage, temperature stability both with and without power, initial cost and lifetime cost and ease of use. Director Mine Cicek explains that for her biobank maintaining sample integrity ultimately trumps all else: “If I have an upright freezer that I use daily for putting samples in and pulling samples out, then I want an upright freezer that can cool back down quickly when I close the door.” After that comes cost and ease of use.
Most purchasers don’t have the luxury of simultaneously putting multiple ULT freezers to the test under controlled conditions over an extended period, and there is no industry standard as to how vendors should do so, Wilkes explains. In addition, “you can’t just look at the display and determine what’s going on because temperatures displayed on the front of the freezer are kind of a rolling average, or controlled by some other algorithm.” Thus it’s not uncommon to find competing vendors making conflicting claims about which units offer the quickest time to reach setpoint, the most stable performance, temperature uniformity within the cabinet or the best recovery from a door opening, for example.
Wilkes notes that Thermo Fisher Scientific and most other leading vendors have “a better–best platform,” in which top-of-the-line models sport superinsulated panels, upgraded control sets with more reliability indicators and other bells and whistles compared to more basic offerings.
Figure 1 – Revco upright ULT freezer from Thermo Fisher Scientific.Shoppers should expect to pay in the neighborhood of $8–13,000 for an upright ULT freezer,with some specialty or premium models costing more.
If your lab pays the electric bill, or is otherwise motivated toward energy efficiency, keep in mind that a single, average ULT freezer uses about as much electricity as a small house. A 2014 report prepared for the U.S. Department of Energy found that high-efficiency ULT freezers saved between 20 and 66% compared to similarly sized and similarly aged standard ULT freezers— with the Stirling Ultracold saving an estimated $570 per year, recouping its higher initial cost in less than three years.1
Table 1 – ULT freezer purchasing considerations
Cicek recommends that researchers find a freezer that holds a stable temperature and cools back down to –80° as quickly as possible, since they’re likely to be opening the door multiple times in a day. In this regard, the Thermo Scientific Revco (see Figure 1) fared best of the upright ULT freezers tested by Mayo Clinic (which doesn’t surprise Wilkes, who points out that there is often a tradeoff between performance and energy usage). And, to get more bang for the buck, she recommends getting the highest capacity available for the footprint.
Table 1 lists things to consider when shopping for a ULT freezer.
This article has provided a sampling of ULT freezers and purchasing considerations. Other innovative systems include the HEF High-Efficiency, -86 Ultra-Low Temperature Chest Lab Freezer from Eppendorf, Glacier NU-9483 Ultra Low Temperature Laboratory Freezer from NuAire and UF V series ULTRA.GUARD™ Ultra Low Temperature Freezers from BINDER.
Reference
- Legett, R. Field Demonstration of High Efficiency Ultra-Low-Temperature Laboratory Freezers; http://energy.gov/sites/prod/files/2014/11/f19/ult_demo_report.pdf
Josh P. Roberts has been a full-time biomedical science writer for more than a decade. After earning an M.A. in the history and philosophy of science, he went through the Ph.D. program in molecular, cellular, developmental biology and genetics at the University of Minnesota, with dissertation research in ocular immunology; e-mail: [email protected]
Please see our Ultra Low Temperature Freezer (ULT Freezers) section to find manufacturers that sell these products