Problem: Frozen storage of biological materials has become an essential activity in many laboratories. Practical technologies for achieving long-term frozen storage have historically been constrained to two modes—(1) upright-style mechanical, compressor, or engine-based freezers typically operated near -80°C and (2) tank-style cryogenic, liquid nitrogen-based freezers typically operated with samples held in cold nitrogen vapor near -190°C. The upright design of today’s mechanical freezers offers superior sample access and ergonomics. Users load or retrieve samples by opening one or more doors on the front of the freezer and reaching straight ahead into the unit. Vapor-phase liquid nitrogen freezers require a user to lift a lid off the top of the freezer and reach down through a small aperture to pull up a vertically oriented rack containing the samples. The ergonomic advantages of upright mechanical freezers make them the preferred choice for laboratories where working samples are accessed frequently.
The main drawbacks of mechanical versus cryogenic freezers center on reliability and temperature performance. Compressor or engine-based refrigeration systems can fail due to power disruption or mechanical breakdown, putting precious specimens at risk. When the door is opened to load or retrieve samples, heat flows from the warmer surroundings into the cold freezer interior, and mechanical refrigeration systems can take one or more hours to return the interior to the necessary set point temperature. Cryogenic freezers provide superior frozen sample security with their reliance on simple liquid nitrogen for cooling rather than a complex mechanical system that might fail. Modern vapor-phase liquid nitrogen freezers are so well insulated that the sample space temperature is maintained at or below -190°C even if the lid is kept off.
In applications where storage temperatures above -150°C are acceptable, laboratory personnel have had to accept these tradeoffs between usability and reliability in selecting among conventional mechanical and cryogenic refrigeration technologies.
Solution: The Cryometrix freezer is an upright-style freezer that employs a novel cryogenic refrigeration strategy to deliver user-defined storage temperatures down to -90°C (T-90 model) or -150°C (T 150 model). With its upright design and liquid nitrogen-based refrigeration system, it combines the convenient ergonomics common to mechanical freezers with the reliability and temperature performance of a cryogenic freezer.
The patented contained-liquid nitrogen delivery system circulates the cold nitrogen within the walls of the freezer, keeping the contents frozen while preventing direct liquid nitrogen exposure to users. A controller opens or closes the liquid nitrogen flow valves to regulate the freezer operating temperature within ± 3°C of the user set point based on a thermocouple placed in the interior sample space. A second, redundant thermocouple is located within the freezer in case the primary thermocouple should fail. The cryogenic refrigeration system rapidly removes heat, allowing the freezer to recover to a -80°C set point temperature in less than 15 min after a two-minute door opening versus a more than 90-minute recovery for a typical mechanical freezer under the same conditions. The Cryometrix freezer uses a small portion of the exhaust nitrogen gas to purge the interior sample space during door open-close events and thereby dramatically reduce the moisture ingress and frost build-up commonly experienced by mechanical freezers. An onboard battery back-up powers the controller and valves for up to 72 hours in case of power outage, and a manual by-pass valve allows liquid nitrogen to flow through the freezer from an onsite tank should power failure persist.
Similar to cryogenic tank-style freezers, the Cryometrix upright freezer has no mechanical refrigeration system to maintain or repair, utilizes very little electrical power, exhausts essentially no heat to the surrounding room, and operates quietly. By combining the virtues of a cryogenic refrigeration system with a preferred upright design, this freezer gives laboratories a powerful new option for storing their frozen samples.
For more information, contact Robert Sever, PhD, market manager, laboratories and life sciences at Praxair Distribution, at Robert_Sever@praxair.com or 630-320-4212