How Does a ULT Freezer Work?

Ultra-low temperature freezers, or ULT freezers, are key in worldwide medical research and healthcare processes. They use two-stage cascade refrigeration to reach temperatures that standard kitchen and lab freezers cannot. This lower critical operating range allows them to store and preserve pharmaceuticals and sensitive biological materials, such as vaccines, DNA and tissue samples.

ULT freezers have complex mechanisms that can vary from freezer to freezer. Teams in laboratories, hospitals and other important facilities all benefit from understanding basic operating principles and proper ULT freezer maintenance. Explore how a ULT freezer works below.

How Do Ultra-Low Temperature Freezers Work?

ULT freezers leverage basic refrigeration technology — they remove heat from a space rather than adding cold. They use double-stage cascade refrigeration, which allows them to achieve lower temperatures than traditional freezer systems using single-stage refrigeration.

ULT freezers can reach and maintain stable temperatures within the low-temperature range of minus 40 degrees Celsius to minus 80 degrees Celsius. 

Some can reach temperatures as low as minus 86 degrees Celsius. Kitchen and lab freezers can only reach minus 40 degrees Celsius at most, which is not cold enough to preserve the vaccines or pharmaceuticals.

How Does Two-Stage Cascade Refrigeration Work?

ULT freezers utilize two compressors, each handling its own refrigerant, to reach lower temperatures than single-stage refrigeration systems can. As a result, two-stage cascade refrigeration uses the following components:

1. Two compressors: Unlike traditional freezers, which only have one compressor, ULT freezers have two different compressors. They are the primary mechanisms that allow the freezer to reach minus 40 degrees Celsius and below. The first compressor is responsible for pressurizing the refrigerant gas. The second is responsible for pressurizing the refrigerant gas a second time once it completes the first stage of two-stage cascade refrigeration.
2. Condenser: Once the first-stage refrigerant is pressurized, it is fed into the condenser. This mechanism condenses the refrigerant by rapidly cooling it. This sudden change in temperature turns the refrigerant from a gas into a liquid.
3. Oil separator: The second-stage refrigerant is fed into this mechanism once it has been pressurized. The separator strips oil from the refrigerant gas, increasing the amount of refrigerant that can flow through the rest of the freezer and protecting the system from clogs.
4. Filter drier: This mechanism is responsible for trapping microscopic moisture, acid and debris that may be contained within the now liquid refrigerant. By trapping these contaminants, the filter drier protects the rest of the freezer mechanisms from blockages and corrosion.
5. Capillary tube: The liquid refrigerant can't flow into the interstage heat exchanger all at once, so it is first fed through this mechanism. The capillary tube, or expansion valve, regulates this flow rate while allowing the refrigerant to expand into a low-pressure mist form. 
6. Interstage heat exchanger: This component is where the first and second stages of the refrigeration process meet. The mist refrigerant we've been following is fed into the interstage heat exchanger to absorb heat from the refrigerant that passed through the second compressor. This chills the second-stage refrigerant, lowering its temperature to new extremes.
7. Evaporator: The ultra-cold second-stage refrigerant flows through the evaporator tubing coils around the freezer, absorbing the heat from the air and cooling the freezer.
8. Accumulator: The accumulator is responsible for recycling both refrigerants back into the compressors. Both the misty first-stage refrigerant and the liquid second-stage refrigerant are collected within it before being funneled back out to the compressors again so that they can continue the cycle.

How Does Single-Stage Refrigeration Work?

Single-stage refrigeration is a bit simpler than double-stage refrigeration. It eschews the double compressor model and the interstage heat exchanger entirely, which prevents it from reaching the same depths as freezers that leverage double-stage cascade refrigeration. This is enough for most kitchen and laboratory freezers.

As with double-stage refrigeration, the process begins with the compressor. Refrigerant gas is pressurized within it and sent through the condenser's coils, where the refrigerant's heat is absorbed and it is transformed into a liquid. 

The filter drier strips the liquid of contaminants before passing it along to the capillary tube. This mechanism manages the flow of the refrigerant as it moves into the evaporator piping, where it absorbs heat from the freezer itself. Then, it is collected in the accumulator, and all remaining liquid refrigerant is separated from the refrigerant that has become gas due to the heat. This gas refrigerant is finally cycled back into the compressor. The cycle starts again.

This process can produce temperatures as low as minus 40 degrees Celsius. As a result, single-stage refrigeration can preserve foodstuffs, cool commercial buildings and more. They are extremely flexible and extremely common. ULT freezers are designed to fill the gaps that single-stage refrigeration technology cannot.

Practical Tips for Maintaining a Healthy ULT Freezer

Because ULT freezers are heavily used in the medical sector, taking care of them is vital. Failure to do so can lead to losing vaccines, medicines and biological samples. It can also lead to ULT freezer failure outright.

Some of the best ULT freezer maintenance practices include:

• Cleaning the condenser filter and coils: When the refrigerant passes through the condenser, it changes from a gas to a liquid. The liquid refrigerant can stick and clog the condenser as a result. This can cause the compressor to waste energy and wear down faster. Cleaning the condenser filter monthly and the condenser coils quarterly is important.
• Wiping down and replacing door gaskets: The door of your ULT freezer prevents warm, humid air from entering the freezer. Over time, the door gaskets can crack and grow brittle, allowing external air to enter the freezer. Gaskets should be regularly cleaned and lubricated to prevent this outcome. If the gaskets have cracked and compromise freezer function, they should be replaced.
• Clearing away ice and frost: Over time, ice and frost will build up on the inner doors and walls of the freezer. They act as an insulator, forcing the freezer to expend more energy to reach and maintain the same low temperatures. Ice and frost should be periodically cleared from the freezer's interior.
• Testing the backup battery: Many ULT freezers have backup batteries that kick in should a power failure occur. These backups should be periodically tested to ensure they operate properly when needed.

Partner With New Life Scientific for Used Lab Equipment

Whether you're looking for a ULT freezer or other used lab equipment, New Life Scientific has you covered. We offer everything from centrifuges to cytometers, and every piece of equipment is quality-assured in-house before making it available. Institutions like Harvard University and Syngenta Global AG have trusted us to provide high-quality equipment. We pride ourselves on offering lab equipment at a price you can afford.

To learn more about our ULT freezers and other used equipment, fill out our contact form or give us a call at 567-212-2460.