How Low Can You Go?
Low-temperature laboratory freezers are cooling boxes that achieve temperatures of about -40° C or lower. Kitchen freezers, by contrast, operate to about -20° C.
The temperature “sweet spot” for lab freezers is about -85° C. “Mechanical” (compressor-based) freezers reach the -40° to -50° C range with a single compressor. A second compressor is required to cool below that point. To achieve energy efficiency and extend the life of the compressors, the low-stage compressor turns off automatically when the set-point temperature is reached, while the high-stage compressor remains on continuously.
“Some systems using exotic refrigerants can get much lower than -85° C,” explains Buckner Richerson, VP of international sales at NuAire (Plymouth, MN). For example, ultra-low-temperature freezers reach temperatures of -150° C, which is useful for storing bone marrow for medical procedures. “But these are quite expensive, and not too many are sold,” Richerson tells Lab Manager.
Blood storage is a growing application for low-temperature freezers. Whole blood and blood products must be stored at -33 C or cooler. One special type of device, a blast freezer, chills blood quickly and maintains it at -40° C or lower. Other applications include storing viruses, bacteria, and human or animal tissue.
Features and choices
NuAire provides these high-level differentiators for low-temperature laboratory freezers:
- Sensitivity and accuracy of temperature controls
- Precision of electronic microprocessor controls
- Long-term reliability and protection of automated backup systems
- Noise level
- Available options and accessories
Specific design attributes include communications and data storage, alarms, quality/configuration of the inner and outer door closures, vacuum release that permits reopening a freezer immediately after door closure, temperature uniformity, and temperature recovery after opening.
Where floor space is at a premium, users can opt for vacuum-insulated panels, which take up less room than polyurethane insulation. Vacuum insulation used to be a rather expensive option, but it has come down in price, Richerson says. Another cold-conserving feature is double outer doors or double inner doors to minimize the loss of cold air.
Alarms are critical for regulated industries like pharmaceuticals and where stored materials are used in legal cases. In both instances, users must demonstrate the ability to validate cooling and convince “higher authorities” that samples have been properly stored. Higher-end freezers can also provide, through a USB data hookup, a history of temperature and usage. Entry codes and keys have become more popular for freezers that store biohazards like germ warfare agents.
Users pay more for advanced data and recording features, but not all will be in a position to use them. Chris Wilkes, director of product management for cold storage at Thermo Fisher Scientific (Asheville, NC), notes that regulated industries have validated standard operating procedures around chart recorders and are therefore unlikely to change.
High-end freezers should maintain uniform temperature throughout the unit and recover quickly from openings. Opening the door of an ultra-low-temperature freezer introduces warm, moist air that causes the device to work harder to retain its set-point temperature and causes condensation and freezing of water vapor inside the unit and on samples. The speed at which a freezer recovers from temperature excursions is a function of BTU reserve capacity, which, according to Thermo Fisher Scientific, is defined as “a measure of a freezer’s ability to maintain a cold temperature across the entire cabinet in the presence of a heat load.” Higher BTU reserve is better.
Low-temperature freezers should maintain their coldest temperature even under balmy ambient conditions. Temperature uniformity is critical as ambient temperature rises, for example, in laboratories or medical centers located in the tropics.
Environmental issues are driving changes that include greater energy efficiency and the move away from halogenated hydrocarbon refrigerants. Manufacturers are investigating novel alternative refrigerants, as well as older alternatives such as low-boiling hydrocarbons. But newer refrigerants are expensive and may pose health or environmental hazards of their own, while hydrocarbons present a serious risk of fire or explosion.
Concerns with carbon footprint are spurring innovations in low-temperature refrigeration. Among the trends are alternative cooling mechanisms such as liquid nitrogen, which boils at -196° C. Cryogenic nitrogen is readily available and relatively inexpensive and remains liquid for extended periods, provided its container is insulated. A liquid nitrogen freezer does not use a compressor, and electrical consumption is less than for conventional freezers. But while the acquisition costs are comparable, ongoing costs for liquid nitrogen are higher than for the electricity it takes to run a mechanical freezer.
Purchasers of low-temperature laboratory freezers can select from numerous options and temperature ranges, but as Thermo’s Wilkes notes, “Once you get below -135° C, there is not a huge benefit to getting colder.”
Richerson of NuAire concurs: “Ninety percent of our customers just want a reliable, inexpensive freezer that will last.”
Angelo DePalma holds a Ph.D. in organic chemistry and has worked in the pharmaceutical industry. You can reach him at firstname.lastname@example.org.
Low-temperature Lab Freezers: Are you using a low-temperature freezer in your lab? Are you considering purchasing a lowtemperature freezer soon? Lab Manager Magazine’s online surveys help improve the purchasing process and provide you with greater confidence in your final purchasing decision. To take the survey, please visit www.labmanager.com/surveys/freezers.