Low-temperature laboratory freezers can 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 said.
Factors to consider when purchasing 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 vacuuminsulated 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.
High-end freezers should maintain uniform temperature throughout the unit and recover quickly from openings. Opening the door of an ultralow- 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. 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.
Concerns with carbon footprint are spurring innovations in lowtemperature 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 Chris Wilkes, director of product management for cold storage at Thermo Fisher Scientific (Asheville, NC) notes, “Once you get below -135° C, there is not a huge benefit to getting colder.”
• Uses HFC CFC-free refrigerants
• Cascade system uses two air-cooled compressors
• Features a temperature range of -50°C to -86°C
• Features two access ports: one on back for CO2 backup; one on bottom for recording
• Upright models feature inner doors with anti-shock panels in polystyrene on a polyurethane core
• Cooling system is based on a cascade cycle using two hermetic compressors
• Plus models feature BIOGUARD®, a proprietary door locking system with a personal identification tag for each user
Revco® PLUS Series
• Available in 17, 21 and 25 cubic foot capacities
• Feature up to 20% more heat removal capacity compared to ordinary freezers
• Introduce up to 15% less heat into the lab, to minimize air conditioning costs
• Feature an automatic voltage buck/boost system
Thermo Fisher Scientific
UF 755G and UF 455G
• Feature temperature set points from -20°C to -86°C
• Vacuum Insulation Panel (VIP) offers improved insulation at a lower wall thickness
• Features stainless steel “Rollbond” evaporator technology for uniform and stable temperatures
• Inner chamber temperature variation: ±3°C at -82°C (set point, preset)
Baker BioScience Solutions