Points to Consider When Purchasing Ultra-Low Temperature Freezers

Although ultra-low temperature (ULT) freezers are specifically designed to work between -86°C and -45°C, they often work at a set temperature of -70°C or -80°C. These freezers can last for about 12 to 15 years.

If a ULT freezer of 10 years old, or older, has to be replaced, many variables have to be taken into account. Some of these problems present in older freezers can be solved by novel technology, which reflects the latest trends in the design of ultra-low range (ULR) freezer.

Image credit: NuAire

Types of ULT freezers and popular options

• There are two basic types of ULT freezers —upright freezers and chest freezers. The latter is more efficient than the former, but is generally not preferred because of usual space limitations. In fact, chest style freezers account for less than 5% of ULT freezer sales. Yet, these freezers have a longer service because they have an efficient design which requires only minimum usage of compressor. The same efficiency makes it possible to reach temperature set points more rapidly and this is maintained continuously.
• The twin ULT freezer equipped with separate compressors is a popular option. Dual compressor systems cycle high stage and low stage refrigerants to maintain a set point temperature. If a compressor fails a ULT freezer will have a hard time maintaining set point temperature. Back-up systems are available to maintain set point until a repair is done. Dual compressors help separate the work load needed to maintain the system and prevent oil logging..ULT freezers equipped with vacuum insulated panel lining provides a good option for space-constrained environments. These freezers come with more capacity and have a small footprint. Owing to the lower thermal conductivity of a partial vacuum, a VIP panel is generally thinner compared to the traditional foam insulation required to obtain the same level of thermal insulating properties. The integrity of the VIP panels should be protected. Temperature fluctuations can cause cracks, which can affect the partial vacuum conditions and thus reduce the insulating properties. Cracks can also occur in the insulation and lead to failures if there is frequent opening and closing of door of a VIP insulated freezer.
• Two compressors is yet another ULT freezer option. In majority of freezers, twin compressors do not work as backups. These are independent compressors, with one functioning as a high stage compressor device that cools down the temperature of low stage condenser. However, this is an expensive option and is yet to become a standard option in the industry. Despite this fact, some manufacturers justify the high price in view of improved performance.

Vacuum Insulated Panel inside a ULT freezer wall. Image credit: NuAire

Refrigerants

• After the ban of chlorofluorocarbons (CFCs), ULT freezers moved to HCFC refrigerants. At present, hydrocarbon refrigerants (HFCs) are used in the most energy efficient freezers.
• Flammable but highly efficient refrigerants such as R270 Propane and R170 Ethane have become the trend in the ULT freezer industry. However, flammable refrigerants are still banned in France and the United States, although both refrigerants have been used elsewhere for many years. In the European pharmacy industry, ULT freezers that use flammable refrigerants are very popular but this is yet to be available in the US because of regulatory restrictions. When compared to freezers using traditional refrigerants, ULT freezers that use flammable refrigerants are found to be 50% more energy efficient after pull down and also have reduced potential for oil logging.

How long does it take for ULT freezers to cool?

• Pull down time refers to the time taken to reach -80°C following installation or set-up. Holdover time is referred to the time taken to reach -60°C following a power failure.
• When a door is opened, ULT freezers should be able to recover the temperature quickly. Whenever the door of an upright ULT freezer is opened, it takes as much as 10 minutes to bring back its temperature to its setpoint.
• With flammable refrigerant gas, pull down is faster and about 15% electricity is saved
• A claw door, door gaskets, and effective insulation can slow down warm up. These three features are very crucial.
• Graphs are supplied by quality manufacturers to demonstrate how their systems compare to others systems in terms of power consumption, heating up, and data on pull downs
• Warm up and pull down differ considerably, with certain systems taking three times as long as other systems. Customers should know the location within the freezer, where temperature was calculated in the data provided by manufacturers, as pull down times determined in the center of a chamber can differ widely against measuring at the bottom or top.

How much temperature variation inside can be expected?

A ULT freezer with the most optimum temperature uniformity should be selected. While temperature fluctuation is just a few degrees away from the set point, reviewing the manufacturer’s test data is the best solution.

The exact location of the testing probes, usually top, bottom, and middle of the inner chambers should be specified by the data. The data should also explain the amount of load in the freezer during the time of testing. A fully loaded freezer ensures a better uniformity.

Temperature Sensor Location. Image credit: NuAire

Size matters – capacities and space

• In order to establish the required freezer size, users typically check the capacity of the freezer as well as the number of sample boxes and racks it can store.
• Freezers with pull-out drawers are the latest trend, particularly in freezers of large capacity. However, some technicians can find it difficult to reach and access the freezer’s top compartments because of heavy inventory racks and drawers. To resolve this issue, some manufacturers provide options where drawers can be accessed from the side of the freezer. This makes it much easier to open, identify, and return samples.
• When efficiency and capacity are being considered, the outer size of a ULT freezer is very important. When compared to large freezers, consumption of energy for each cubic foot is higher in the case of small freezers but this may not necessarily be the main factor when choosing a freezer.
• In laboratories where space is limited, smaller freezers based on size constraints are often used. In research or university settings, energy expenses become very crucial for users and many prefer to buy large freezers and split the space between groups to reduce their energy consumption. Also, in order to find out the most efficient way to create frozen vials, users often look for the largest capacity freezers at the lowest price possible for each vial.
• A combination of strategies and total efficiency in addition to local requirements should be considered by users as this may prove suitable in the long run
• Another critical factor is the time required for the samples to be stored in the freezer. Majority of ULT freezers are made such that a consistent set point is reached by them which will last that way for over a year.

Where the freezer will be located matters

• Location where a freezer has to be placed decides the type of model as well as the size of the freezer to be used
• Another important thing to be considered is whether the freezer will be located within a group of freezers, or alone
• Average air temperature and other regional considerations should be factored in if a freezer has to be placed in a space that lacks climate control
• ULT freezers are neither blast freezers nor flash freezers. They are mainly intended to store and preserve the already frozen product. Conventional ULT freezers will break down over time if loaded fully with warm products, while high-quality freezers may withstand that type of use. Loading the chamber nearly a quarter full with free-frozen products is the best solution, and warm product should not be placed into it. A blood bank facility has a high volume of warm product that should be rapidly cooled. For such facilities, blast freezer would be more suitable instead of a ULT freezer. If a ULT freezer is continuously used to cool a warm product rapidly, the freezer can experience mechanical failure much more quickly.
• Proper ventilation should be there for freezers, allowing the warm air to reach the return registers. There should be a minimum clearance of 5” (127 mm) on the back and sides and a space of 8” (203 mm) on top of the freezer. Unless a ULT freezer is placed in a specially designed freezer farm, side by side spacing should be avoided at all cost. A ULT freezer should not have storage on top.

Image credit: NuAire

• The freezer must pass through a door opening of standard size which is another key consideration. Majority of freezers are built according to international standards of 2 m (78 3/4”) high and 90 cm (35 1/2”) wide — the size of a standard European door.
• A thumb rule proposed by manufacturers is to buy as big a freezer as possible at the best price per ft³ capacity that can still pass via the door. Parts should be there in larger freezers which can be taken off to accommodate it through a door.
• If a ULT freezer has to be placed next to a quiet work space such as an office, its decibel rating becomes imperative. Noisiness is not a parameter of good quality freezers; ULT freezers manufactured by NuAire are rated at just 47dba, against competitive freezers that are rated as high as 73dba.

Avoiding frost

• Whenever water vapor gets into contact with the cold interior surfaces, frost can build up on ULT freezers. The accumulation of frost can obstruct with closing the inner doors, or if extreme frost turns into ice, even the outer door. Frost will accumulate in each ULT freezer, which is merely the nature of extremely low temperature surfaces in the presence of humidity.
• Features are included in the best freezers to reduce the buildup of frost. For instance, claw latches to constrict the hinge against the gasket, or secure the inner freezer doors. Gaskets around the outer door are kept clean by low-wattage halo bypass heaters. Temperature is maintained as much as possible through insulation on the inner and outer doors.
• Majority of ULT freezers developed for environments have a maximum ambient air temperature of 32°C and maximum humidity of 80%. Air-cooled condensers are traditionally used by ULT freezers, and therefore cooling efficiency is reduced if there is very high ambient temperature.
• A ULT freezer should never be placed in an enclosed area without sufficient ventilation, or an area that is exposed to direct sunlight. Water cooled condensers may be required if the freezers have to be placed closer than 10 cm within an enclosed area.
• A major issue in ULT freezers is the accumulation of ice or oil logging in the refrigerant. In the mid-1980s, this problem was there in nearly all ULT freezers because of paraffin wax-based oils, which tend to solidify in capillary tubes. However, this issue was eliminated by wax-free synthetic oils, but other reasons can still promote the formation of ice in capillary tubes.
• To address the issue of oil logging, heaters are used within the capillary tubes so that oil is prevented from becoming very cold. In order to remove ice and thus cut down maintenance down time caused by oil logging, oil can be allowed to run through separators located outside of the system.

What facilities concerns are there?

• Reliable power and chilled process water are required by ULT freezers.
• When vapor lock occurs, operators can manually use high-quality freezers to release the door with a vacuum release valve.
• Data ports are required by higher end freezers for remote temperature sensing or to communicate logged data, and some freezers require battery backup power.
• Brackets are provided to freezers that are located in seismic-sensitive areas. Through these brackets, the unit is secured to the walls.
• Correct receptacles in the area and proper power source are vital where the ULT freezer is placed. Most power source range from 208V to 2230V at 60 Hz, with 15Amp single-phase circuits, but 115V with 20Amp single-phase circuits are required by some customers. There are very few freezers that can maintain even -57^◦C at 15Amp and 115V, except for the small and low interior volume freezers.
• Voltage boosters are provided by certain ULT freezers for buildings that offer less than the rated 230V in Europe or 220V in the USA. In the United States, where 208V is very common rather than 220V, there is a large demand for voltage boosters.
• If a freezer compressor operates at a lower than correct voltage, it will fail much more rapidly. Here, the life of a compressor can be extended by the voltage booster.
• If a freezer is placed in areas that experience frequent power outages, backup systems become a consideration. In the event of a power problem, CO₂ backup kits alert the personnel and at the same time maintain a set temperature for a short period of time to enable the staff to fix the issue. Backup options should be considered if a ULT freezer is located in an area where frequent power failures are common (which does not have backup generators) or sensitive to natural disasters, including hurricanes.

Energy concerns

• A ULT freezer will cause additional heat load and this fact should be known to the facility manager. As part of the energy management strategies, this additional heat load can be mitigated by consolidating all the freezers into a single room with sufficient cooling, or dispersing the additional heat load by separating the ULT freezers to various locations.
• With latest developments in the compressor design of ULT freezers, cabinet design and insulation have led to better efficiencies for storing samples. Yet, despite this fact, the efficiency of ULT freezers reduces with time because of refrigerant loss, loosening seals, poor maintenance, fatigue in mechanical systems, or degraded lubricants. Every year of a ULT freezer’s age translates to about 3% increase in terms of energy usage.
• There are certain ULT freezers that age well and hence it is strongly recommended to test all the freezers to validate the requirement for replacement.
• Certain customers have a committee who manage cold storage and keeps track of performance and energy consumption over time.
• Old freezers should be routinely tested over time to identify which ones deliver poor performance. The voltage, amperage, and wattage drawn for each freezer can be compared through a four to six hour test using a plug-in energy meter.

Maintenance often depends on lab cleanliness

• EERE states that less energy is used by dust-free freezers when compared to dusty freezers operated at the same temperature. Plenty of ventilation is required by ULT freezers. The presence of grime and dust on the filters will block air flow and reduce the potential to dissipate heat. The greater the dirt on the condenser, the higher the amount of energy required by the compressor. The freezer’s life can be extended by reducing the dust as much as possible and by making sure that it does not enter into the freezer.
• Condenser filters should be regularly checked, washed, cleaned, or replaced, and coils should be cleaned with a wet towel, brush, or vacuum. Condenser filters can be reused and hence do not have to be replaced. These filters that have front access and therefore can be maintained much more easily and quickly because tools are not required. However, if there is no front access, almost all freezers are provided with lockable wheels so that they can be easily moved for cleaning.
• There would be minimal maintenance if a freezer is placed in a temperature-regulated clean room. However, many freezers are located in industry locations close to open doorways, windows, or high traffic areas. Moreover, the proposed maintenance is not frequently carried out by certain laboratories. This includes cleaning the condenser fan and filters and removing the frost buildup every three months.

Price is always a consideration

• Manufacturers do away with certain components to provide low cost freezers, but such low cost freezers places users at risk who require reliable and stable performance and other similar options that specify product quality
• Freezers that are moderately priced will provide a reliable temperature of -85^°C and also ensure protection against oil logging. Sometimes, USB ports may be provided in these freezers to plug in and download the operation data manually. This category also includes VIP options, and the performance of the same freezer can be obtained by paying double the price.
• Higher end freezers are employed by users who require quality control to corroborate their product or keep histories such as in the pharmaceutical sector. Automated data logging and better refrigeration options are provided by higher end freezers that that track the events, operations, alarms, and temperatures.

NuAire laboratory equipment supply

NuAire develops ergonomic scientific laboratory instruments and offers personnel, product and even environmental protection in key research environments. The following range of laboratory equipment is provided by NuAire:

Biological safety cabinets. Animal research products. Laminar airflow products. Image credit: NuAire

CO₂ incubators. Centrifuges. Ultra-low temperature freezers. Image credit: NuAire

Polypropylene fume hoods and casework. Image credit: NuAire

References

• Doyle, Allen, and Leo Angelo Gumapas,. “Everything You Wanted to Know about Running an Ultra Low Temperature (ULT) Freezer Efficiently but Were Afraid to Ask.” Store Smart (2013): n. pag. ULT Freezer Management Guide. Store Smart, 8 Mar. 2013. Web.
• Buie, John. “Independent Guide to Purchasing a Lab Freezer.” Lab Manager Magazine. Lab Manager Magazine, 7 Dec. 2009. Web.
• Roberts, Josh P. “Ultralow-Temperature Freezers: Antarctica in a Box.” Labcompare, 25 Feb. 2015. Web.

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