1. Operating Principle of Ultra-Low Temperature Freezers

Ultra-low temperature freezers typically use a cascade refrigeration method. In an ultra-low temperature freezer, there are generally two hermetic compressors used as high temperature and low temperature compressors. When the power is turned on, if the set temperature is lower than the temperature displayed on the freezer panel, the first-stage compressor starts, and the first-stage refrigeration system begins to operate. When the temperature in the heat exchanger drops to about 1/2 to 1/3 of the overall refrigeration temperature, the second-stage compressor starts, and the second-stage refrigeration system begins to work, lowering the internal temperature of the ultra-low temperature freezer. The heat released by the condenser of the second-stage refrigeration system is completely absorbed by the evaporator of the first-stage refrigeration system, while the heat discharged by the first-stage condenser dissipates into the air. When the internal temperature of the freezer drops to the set temperature, the temperature sensor transmits this information, causing the control relay to cut off power, and both stages of the refrigeration system stop working. When the temperature rises, the above process repeats, keeping the internal temperature of the ultra-low temperature freezer close to the set temperature.

2. Typical Faults of Ultra-Low Temperature Freezers

A statistical analysis of faults in ultra-low temperature freezers reveals that the refrigeration system is the most prone to failure. During actual maintenance and under the guidance of predecessors, a set of “observe, touch, test” maintenance methods has been summarized. The specific methods and principles are as follows: To ensure the normal operation of the ultra-low temperature freezer, refrigerants and low-temperature compressor lubricating oil are charged into its refrigeration system. Therefore, if there is a leak, both substances will leak simultaneously. Refrigerants are volatile and evaporate easily, while lubricating oil leaves traces at the leak points. We can locate the fault point based on the traces of the lubricating oil, which is the so-called “observation.” When the ultra-low temperature freezer is working normally, the compressor continuously operates. Therefore, any normally functioning compressor will have a certain temperature; under normal conditions, the temperature of the compressor shell is about 70°C, which can be felt when directly touched. This temperature is neither too hot nor too low, which is referred to as “touch.” “Testing” involves using a clamp ammeter to non-destructively measure the current passing through the compressor during operation, and comparing the measured result with the rated current to determine the operational state of the compressor.

2.1 The Ultra-Low Temperature Freezer Works Normally, but the Noise is Particularly Loud

This situation may be caused by the unstable placement of the freezer, objects touching the back of the freezer, or poor vibration damping of the compressor. Since the ultra-low temperature freezer produces vibrations during operation, any instability or poor damping can lead to excessive noise. Additionally, if objects are touching the back of the freezer, this can also cause excessive noise. If the noise is due to the unstable placement of the freezer, a leveling tool can be used to correct the balance by adjusting the height of the feet to stabilize the unit. If objects are touching the back, simply removing them will resolve the issue. Inspection revealed that due to age and prolonged use, the vibration damping pads of the compressor had aged. Replacing the compressor’s vibration damping pads resolved the fault.

2.2 The Ultra-Low Temperature Freezer’s Temperature Stops Dropping Around -45°C

First, observation showed no obvious oil traces, ruling out the possibility of refrigerant leakage. Next, touching both the first-stage and second-stage compressors revealed that the first-stage compressor felt normal, while the second-stage compressor was hot to the touch. Additionally, using a clamp ammeter to measure the current of the second-stage compressor showed that the current far exceeded the rated current. Further, measuring the high and low-pressure sides of the second-stage refrigeration system with a pressure gauge revealed that the high-pressure side was far above the rated value, while the low-pressure side was close to a vacuum, indicating a blockage in the refrigeration system’s piping. To prevent further faults, the unit should be turned off immediately. After removing the oil separator, it was found to have insufficient refrigerating oil, indicating that the oil separator was damaged and could not completely separate the oil from the blocked system. The piping was thoroughly flushed with nitrogen at 0.8 to 1.5 MPa to clean out the refrigerating oil, and the drying filter and oil separator were replaced. The piping was then welded back together, and high-pressure nitrogen at 1.3 to 1.5 MPa was used for leak testing. The low-pressure process pipe of the compressor was connected to a vacuum pump for secondary vacuuming, and refrigerant was added to the second-stage compressor according to the type and filling amount specified on the nameplate. After all these steps, the unit was powered on for trial operation, and it functioned normally, resolving the fault.

2.3 The Ultra-Low Temperature Freezer’s Temperature Stops Dropping Around -78°C

Generally, ultra-low temperature freezers can achieve refrigeration temperatures of around -85°C, while this particular unit only reaches around -78°C. Here, we again used the “observe, touch, test” method to troubleshoot. First, observation revealed no signs of oil leakage, eliminating the possibility of refrigerant leakage. Touching both the first and second-stage compressors showed normal temperature increases. Using a clamp ammeter for measurement revealed that the current of the second-stage compressor was slightly below the rated current, indicating slightly poorer performance. Touching the inlet and outlet copper pipes of the condenser showed no significant temperature difference, suggesting that the second-stage condenser’s heat dissipation efficiency might be low. A thorough inspection revealed that the condenser was covered in dust. Since the ultra-low temperature freezer operates 24 hours a day, even though there is a filter in front of the condenser, the heat dissipation fan continuously draws in air, allowing dust to accumulate on the cooling fins, resulting in low heat dissipation efficiency. After thoroughly cleaning the condenser, the freezer operated normally, resolving the fault.

2.4 The Ultra-Low Temperature Freezer’s Temperature Stops Dropping Around -40°C

First, observation showed no signs of oil leakage, indicating no refrigerant leakage. Touching both the first and second-stage compressors revealed that the first-stage compressor’s temperature increase was normal, while the second-stage compressor was too hot to touch and had entered a thermal protection state. Further inspection revealed that the condenser’s temperature was high, and the cooling fan was burned out, unable to dissipate heat. The first-stage compressor cools by expelling heat from the second-stage compressor, and the condenser at the bottom of the ultra-low temperature freezer is typically air-cooled by a fan. When the fan is burned out, the condenser cannot cool down and thus cannot complete the hot and cold cycle. Replacing the cooling fan restored normal operation of the freezer, resolving the fault.

2.5 The Ultra-Low Temperature Freezer’s Temperature Stops Dropping Around -30°C

First, observation showed no signs of oil leakage, indicating no refrigerant leakage. Touching both the first and second-stage compressors revealed that the first-stage compressor’s temperature increase was normal, while the second-stage compressor was unresponsive to touch, showing almost no temperature increase, and the condenser fan was functioning normally. Measuring the current of the second-stage compressor with a clamp ammeter revealed no current flow. Further inspection found that the power relay for the second-stage compressor was not functioning. This ultra-low temperature freezer has an electronic temperature control system for both stages of the refrigeration system, and the second-stage compressor only starts when the temperature of the first-stage compressor drops to around -30°C, indicating that the electronic sensor for the second-stage compressor had failed. Replacing the electronic sensor for the second-stage compressor restored normal operation of the ultra-low temperature freezer, resolving the fault.

3. Preventive Maintenance

To keep the ultra-low temperature freezer in good working condition, reduce the occurrence of faults, and extend its lifespan, regular maintenance and care are essential. To ensure the normal operation of the ultra-low temperature freezer and prevent equipment damage, the following points should be noted:

１．Ensure a Good Ambient Temperature: The operating environment temperature for the ultra-low temperature freezer must be below 30°C and well-ventilated. Poor ventilation or high temperatures can easily cause the equipment to overload and become damaged. If necessary, a more powerful air conditioner should be installed in the room where the freezer is located to ensure a good temperature environment.

２．Regularly Check the Freezer Door Seals: If the door seals are aged and not tight, the refrigeration effect will be severely reduced, causing the compressor to operate under overload for extended periods, leading to damage. Door seals that are aged should be replaced promptly to ensure the freezer door closes tightly. Additionally, the door panels, seals, and inner doors should be defrosted in a timely manner.

３．Regularly Clean the Heat Dissipation Filter: Generally, this should be done once every two months. If the filter is not cleaned for a long time, dust can clog the mesh, leading to poor heat dissipation, damaging the cooling motor, and subsequently damaging the compressor. The cleaning method should strictly follow the operating instructions.

４．Monthly Cleaning of the Ultra-Low Temperature Freezer: Ensure cleanliness by using a dry cloth to wipe off dust from the inside and outside of the freezer and its components. If the freezer is very dirty, it should first be cleaned with a neutral detergent and then thoroughly rinsed with purified water. However, care must be taken not to spray water directly inside or on top of the freezer, as this can damage insulation materials and lead to faults. There is no need to use lubricating oil on the compressor and other mechanical parts. Cleaning the compressor’s fan should be done with caution.