6 min read
Laboratory Freezers: What they are and the various types
Dec 12, 2020 3:39:52 PM
Dr. Overend is a virologist by training. He received in a PhD from the University of Connecticut for work on the PRRSV virus and postdoc at Virginia Tech with Dr. Xiang-Jin Meng, he now serves as Biosafety Coordinator with the University of Florida. Dec 12, 2020 3:39:52 PM
When purchasing a freezer for the lab, it might be tempting to save money and purchase from a home appliance store. This can be a costly mistake.
What is a Laboratory Freezer?
To better understand what a laboratory freezer is, it would be helpful to first explain what a laboratory freezer is not: a residential grade combination refrigerator/freezer. A residential refrigerator is designed with 3 primary factors in mind. First, they are set up for organizations. These units come standard with glass shelving throughout the chamber and on the doors. They also contain several drawers, many with the capacity to maintain different temperature and humidity levels that the main storage compartment. The organization and glass make it visually easier for the user to locate specific items.
Second, a residential unit is designed with efficiency in mind. These units sacrifice precision in that a greater degree of temperature variability is tolerated in order to reduce the amount of time and frequency that a compressor must run to cool the storage chamber. Finally, they are designed for simplicity, both with respect to manufacturing as well as daily use. A combination refrigerator/freezer unit will typically have a single compressor to cool the freezer compartment. Any excess cool air is then shunted into the refrigerator compartment to maintain a higher but still cool temperature. As for user convenience, residential units tend to have simpler temperature adjustments, and automatic defrost capabilities.
In contrast, laboratory freezers are primarily designed with precise temperature control, reliability, and storage capacity in mind. While it is possible to find units that share features with residential models, they are not standard features. For instance, Thermo-Fisher does sell combination units such as this one, they are typically either dedicated refrigerators or freezers. The model linked above also features auto-defrost and is flammable rated. Typically, lab freezers lack the auto-defrost feature. This is due to the fact that the defrosting mechanism briefly heats up the interior of the unit to evaporate any condensation that may be present. While the change in temperature may be inconsequential for food storage, this fluctuation in temperature can have disastrous effects on the potency of many biological reagents such as enzymes.
Like residential freezers, the cooling elements may consist of coils carrying a refrigerant embedded within the walls of the chamber, or in the shelves themselves. Other designs will be cooled using an external compressor and coil and will regulate chamber temperature using forced-air circulation. This mechanism is often found in GLP/GMP facilities as well as clinical settings subject to CDC/FDA oversight, due to the fact that the temperature is maintained within a very narrow margin and is able to maintain unmatched temperature homogeneity throughout the storage chamber.
Some additional features that are also found in laboratory freezers may include locks (for secure storage of infectious, toxic, or regulated agents), status alarms, and temperature monitors. However, it is far less common to find chamber illumination in laboratory freezers than in residential units.
Type of Laboratory Freezers
While freezers intended for laboratory use are often considered a category in and of themselves, there are actually many different types, all with specialized roles within research or clinical settings. Here, some of the most common types will be discussed.
First, there is the “minus 20” freezer, which is probably the most ubiquitous type. While this type of freezer most often resembles that of a residential unit, they are very specialized in their operation. The -20 designation refers to the typical operating temperature of -20o Celsius (C). The workhorses of the laboratory environment, these freezers are used for materials that need to remain frozen, while keeping the materials readily accessible for routine use.
One specialized type of -20 freezer would be the enzyme freezer. Typically, enzymes used in life science research are very delicate. They can lose the majority of their functional capabilities if not stored within a very small temperature range. Therefore, these are not going to be auto-defrost capable, as the brief warming cycles will damage the enzymatic activity, rendering costly experiments invalid. While enzyme freezers can be found in larger formats, most academic labs have a smaller format under-counter model. However, these should not be confused with dormitory-style freezers that share a similar form as they lack the precise temperature control necessary for enzyme storage.
The next common lab freezer is the ultra-low temperature or ULT such as the models found here. These freezers can be found in either vertical orientation or horizontal (chest style) configurations. Sometimes referred to as -80 freezers for the storage compartment operating temperature of -80oC (similar to the temperature of dry ice), these freezers are intended for medium to long-term storage of items that are accessed at a lower frequency than those stored in the -20 freezers described above. These freezers tend to have stainless steel interiors with multiple shelves compatible with accessories, such as racks for holding many boxes of small cryogenic storage vials.
These storage racks greatly improve the organization and accessibility of samples, particularly when combined with a LIMS program. The ULT freezers typically have very thick walls containing ample insulation in addition to both interior and exterior doors to reduce energy requirements. The exterior doors typically contain a lever style latch to ensure that a tight seal with the gasket is formed, and also contains a lock for secure storage of regulated and high hazard samples.
The final general type of freezer is designed for the storage of flammable materials and must comply with NFPA 45 as well as OSHA 29 CFR 1910.307. Many freezers may have lights, thermostats, or other electrical components within the storage chamber, or they do not have an airtight storage chamber. If flammable items were stored in such a freezer, a spill or vapor leaks from ill-fitting caps can result in fires or explosions when the electrical components are activated. Flammable storage freezers have sealed storage chambers that isolate materials from any electrical components. This greatly reduces the risk of laboratory fires and explosions.
Considerations for selecting a lab freezer
When the time comes to purchase a new lab freezer, there are numerous factors that should be considered before making this investment.
First, the nature of the contents to be stored. What storage temperature is going to be required? How precise must the temperature control be for the samples? While many freezers, even residential models can reach -20oC, it is not recommended that the freezer be set to either temperature extreme of its capabilities as this can place excessive strain on the compressor, and it will have trouble maintaining the desired temperature. Instead, your desired temperature setting should be close to the middle of the freezer’s operating range. Furthermore, if precise temperature control is required, a forced air unit would be a good choice, as they provide unmatched temperature uniformity throughout. Similarly, if temperature stability is necessary, then an auto-defrost freezer should be avoided. However, this will likely require that a backup freezer be available when the inevitable manual defrosting is required.
Next, what are the space constraints for the desired freezer? While a chest-style ULT freezer possesses a relatively high degree of energy efficiency, their footprints are quite large. For this reason, they are far less common. Perhaps a standard or large sized freezer would be required. However, for small spaces with limited numbers of samples, or those with high turnover and short expiration dates, an under-counter style freezer might be a better option.
Finally, does the intended use of the freezer require any special accessories? If the samples to be archived are irreplaceable, it would be wise to consider a ULT with a liquid nitrogen backup cooling system. In the event of a power outage, these units can maintain the critical storage temperatures required.
If a highly organized storage system is desired, consider a freezer with numerous shelving arrangements with optional racks or bins. This can be helpful in clinical settings where vaccines should be rotated so that the soonest expiration dates get accessed and used first. Similarly, clinical facilities that are storing vaccines or other supplies for clinical trials have additional requirements. For example, a temperature monitor may be required, from which personnel can check and record temperature multiple times a day. This helps to ensure that the potency of the stored material is maintained. As an additional precaution, a unit with a status alarm can be helpful to rescue precious samples in the event of an equipment or power failure. Some newer models even have the capability of sending a text or email alerts if the chamber temperatures deviate from pre-set ranges.
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