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Reefers require a precisely calibrated cooling system to maintain a constant temperature for their perishable cargo. At the heart of this system is the cooling fluid (also called refrigerant), which circulates continuously through the refrigerated container. Its function is to extract heat from the container's interior and release it to the surrounding environment. Without this constant heat exchange, the system could not guarantee the necessary controlled conditions.
During its circulation through the system, the cooling fluid repeatedly changes pressure and phase. This allows it to absorb large amounts of heat in one area and release it again in another. In the evaporator, it absorbs heat from the air circulating through the container. It then flows through the compressor and condenser, where the absorbed heat is released to the ambient air.
This cycle enables refrigerated containers to maintain a constant temperature throughout long transport chains that can span thousands of kilometres and multiple climate zones. Efficient heat dissipation from the container is crucial for keeping the cargo within the required temperature range.
Refrigerants are therefore the mechanism that enables reliable temperature control. If it cannot circulate properly, the entire cooling process is compromised. This is why monitoring cooling performance and detecting potential problems early is so important for protecting sensitive goods throughout the entire cold chain (see also: Reefer management).
The main task of a reefer is to keep perishable freight at a temperature below the surrounding temperature - to “cool” it. This is achieved with the help of a compressor, which supplies mechanical energy. By compressing the refrigerant gas, the system increases its pressure and temperature, thereby releasing heat to the outside. The cycle repeats itself, effectively transferring heat from the inside of the reefer to the surrounding air, even if the air is already warm.
Although cooling is the primary function associated with reefers, they can also heat. Many products must be protected not only from overheating but also from excessively low temperatures; otherwise, textural alterations, discolouration, flavour loss, uneven ripening, dehydration, and separation can occur. Therefore, reefers function not only as cooling systems but also as temperature management systems.
As described above, the refrigerant extracts heat from the air circulating in the container and releases it to the outside via the condenser. So how does heating work?
A typical cargo example that often requires controlled temperatures above the ambient temperature is bananas. They are typically transported at around 13–14 °C to prevent premature ripening or chilling injuries (under-peel discolouration, poor texture and taste, highly susceptible to bruising). In colder climates or during winter transport, the outside temperature can drop significantly below this range. In such cases, the cooling unit activates its heating function to ensure the correct cargo temperature.
Typically, electric heaters integrated into the air circulation system work together with the cooling components and the control system to maintain a constant, predefined temperature. The refrigerated container's sensors continuously monitor the internal climate and adjust cooling or heating as needed. The cooling components must be optimally coordinated at all times to regulate airflow, temperature stability, and humidity.
The ability to switch between cooling and heating modes ensures that refrigerated containers can protect cargo in a wide variety of climates and on diverse transport routes (see also: Reefer container temperature monitoring).
The choice of cooling fluid is not arbitrary. Cooling fluids used in reefers must meet several technical requirements. They need a suitable boiling point to enable efficient heat absorption in the evaporator, stable thermodynamic behaviour across a wide temperature range, and chemical compatibility with compressors, seals, and other refrigeration components. Safety considerations also play a role, including low toxicity and controlled flammability characteristics.
Originally, starting in the 1950s, the refrigerant R12, known under its brand name Freon, was primarily used. As increasingly lower temperatures became necessary, R22 was also introduced in the 1970s. Both were eventually replaced in the 1990s, primarily by R134a (which, for a time, was used in over 96% of reefers). (1) Although R134a is - unlike the two that came before - no longer a chlorofluorocarbon that threatens the ozone layer, it is a potent greenhouse gas with a global warming potential (GWP) of 1430.
As a drop-in replacement, meaning it can often be used in existing plants without technical modifications, new cooling fluids such as R513A, which has less than half the GWP (630), have been available since the late 2000s/early 2010s. And there is another advantage: while R134a is difficult to ignite (can burn, but ignites only with difficulty and usually extinguishes itself as soon as the external ignition source is removed, flames spread slowly), R513A is not flammable (cannot burn at all under normal conditions, flame formation is not possible).
New regulations are intended to accelerate the transition. For example, cooling fluids used in intermodal containers for domestic use in the USA must henceforth have a GWP below 700. If we look at the Star Cool fleet as an example, all cooling units have been optimised for R513A since mid-2017; this corresponds to one-third of all Star Cool units ever produced. Approximately two-thirds more can operate with it, but will experience a performance reduction of 8-10%. Only about 6% cannot be operated with the new refrigerant. (2)
More detailed regulations will be needed. Without them, solutions that are better, but still not truly climate-friendly, would continue to be used. R513A, for example, is really only a transitional solution. Another possible substitute is R1234yf, with a GWP of less than 1-4. However, it can degrade into persistent by-products such as trifluoroacetic acid (TFA), which raises environmental concerns. (3)
The most sustainable (gold standard) solution would be R744 (CO₂) with a GWP of approximately 1. It is ideal for deep freezing down to -40 °C, but can only be used in purpose-built new systems. It has been field-tested since 2011, but has remained a niche product ever since. One manufacturer that uses R744 is Carrier Transicold with its NaturaLine system. (4)
Learn also about: the reefer genset
The amount of cooling fluid required to operate a specific refrigeration unit is precisely defined. If the system loses this fluid—for example, through leaks or component wear—its performance will change. While the unit may continue to run for a while, various warning signs indicate that the refrigerant level is no longer optimal.
A very common sign is reduced cooling capacity. The system takes longer to reach the target temperature or struggles to maintain it at higher ambient temperatures. When less refrigerant is present, the evaporator cannot absorb heat as efficiently, and cooling capacity decreases.
Longer compressor run times can also be a symptom. The system has to work harder to dissipate the same amount of heat, which means the compressor runs longer or more frequently.
Temperature fluctuations inside the container—especially when switching between cooling phases—can also result from a lack of cooing fluid. This is another reason why continuous monitoring of each individual reefer is recommended.
Maintenance technicians can identify further signs during inspections, such as unusual pressure readings or alarms from the control system. However, many refrigerant losses develop gradually and may not be immediately apparent during routine checks.
Performance and temperature monitoring, therefore, play a vital role; they can provide early warnings that the refrigeration system is no longer operating at maximum efficiency.
Low cooling fluid levels can have significant consequences for both the cargo and the refrigeration system. Since it is responsible for heat transfer from the container, any reduction in its effectiveness directly affects the system's ability to maintain stable temperatures.
If the system cannot dissipate heat efficiently, temperature deviations from the target value can occur inside the container. Even slight fluctuations can affect sensitive goods such as fresh produce, seafood, or pharmaceuticals. Especially with products that tolerate only a very small range of fluctuations—like the bananas we discussed earlier—it doesn't take long for threshold values to be exceeded and the damage done.
A refrigeration unit is designed to operate with a precise cooling fluid level, and even minor deviations can disrupt the balance of its core components.
One of the first effects impacts the evaporator. With a lower cooling fluid level, the evaporator cannot absorb heat evenly, resulting in reduced heat exchange and inefficient operation. This often forces the system to compensate with longer cycles, increasing the load on the compressor. It can overheat, and lubrication can also be affected, as refrigerants assist in oil transport within the system. Over time, this increases mechanical wear and the risk of premature component failure.
Ultimately, insufficient filling levels put a strain on the entire refrigeration system. What begins with a small loss can escalate into reduced efficiency, increased energy consumption, and, in the worst case, mechanical failure. Maintaining the correct level is therefore crucial not only for cooling performance but also for the long-term functionality of the refrigeration system itself.
Cooling systems are closed circuits, meaning that in principle, the refrigerant remains in the system for a long time without being consumed. In practice, however, losses can occur over time – these happen due to mechanical wear, aging components, and damage.
Leaks are most common, occurring at joints, valves, seals, and junctions. They are caused by vibrations, temperature fluctuations, and mechanical stress. The wear and tear of seals and sealing rings, which harden or crack, as well as corrosion and damage, can be a reason. Losses can also occur during maintenance and repair work: The system is opened and not properly refilled and closed.
It is important to note that: Even small leaks can lead to significant losses if left undetected for an extended period. The refrigeration system can continue to run for quite some time without major deterioration occurring; therefore, early detection is particularly important to prevent larger problems.
Measuring refrigerant levels is not always a function of monitoring systems. However, they can play an important role in the early detection of warning signs.
When the level begins to drop, subtle changes in performance often become apparent. It may take longer to reach the set temperature, compressor cycles may become more frequent or longer, or the temperature stability within the container may begin to fluctuate.
Therefore, by continuously recording operational data such as temperature, alarms and other system behaviour, operators can learn to identify patterns over time that indicate that normal coolant conditions are disrupted.
Although the signals don't always indicate a loss, they can be a valuable indicator to check the filling level. This ability to interpret performance anomalies as a potential cooling fluid problem is crucial for preventing equipment failures and cargo risks through early troubleshooting.
Modern refrigerated container monitoring, supported by continuous data, is increasingly replacing the traditional method of manual, scheduled checks. This allows the reefer's performance to be tracked in real time and irregularities to be detected.
This reveals deviations that would otherwise go unnoticed for hours. Patterns such as longer cooling cycles or recurring alarms indicate that the refrigeration system is gradually losing efficiency. In some cases, refrigerant leaks may be the cause.
This approach enables a proactive maintenance strategy. If a relevant problem occurs, examining the cooling fluid level is added to the checklist, along with other possible causes such as faulty sensors, power problems, or condenser blockage.
Especially on large terminals with thousands of refrigerated containers, this helps to identify and resolve the true causes of problems more quickly.
Data-driven monitoring thus complements traditional maintenance methods and provides the necessary insights for effective refrigeration system management.
Heat naturally flows from warmer to colder objects (second law of thermodynamics) until equilibrium is reached. In a warmer object, the molecules vibrate or move faster. When these molecules collide with slower molecules in a colder object, energy is transferred, increasing the motion of the colder molecules. This energy exchange continues until both objects reach thermal equilibrium—that is, until they possess the same average kinetic energy (energy of motion).
When a cooling fluid passes through the expansion valve, its pressure drops suddenly. Pressure and temperature in gases are closely related: when pressure decreases rapidly, the temperature of the refrigerant falls as well. This is because it expands and its molecules spread out, reducing the energy available as heat. As a result, it enters the evaporator as a cold, low-pressure liquid–vapour mixture. In this state, it can absorb heat efficiently from the air inside the reefer container. The absorbed heat causes the cooling fluid to evaporate, which is the key step that removes heat from the cargo space and maintains the required temperature.
Cooling fluids are essential for reliable temperature control. Their ability to reliably transfer heat ensures that sensitive goods are protected in diverse climates during long transport routes.
If the filling level is correct, cooling performance remains stable and predictable. However, even minor deviations—caused by leaks or damage—can disrupt this balance. This results in reduced efficiency, increased stress on the system, and a growing risk to the cargo.
This is where modern monitoring can play a crucial role. By identifying performance patterns such as cooling cycles, temperature stability, and alarm behaviour as potential early indicators of system imbalances, failures and damage can be prevented.
Delve deeper into one of our core topics: Reefer Monitoring
Calibration is the process of comparing the readings of a measuring instrument (thermometer, scale, sensor, data logger, etc.) with a known reference standard and then adjusting the instrument so its output matches that standard within defined tolerances. It typically involves taking measurements at one or more test points, determining the error, and, if necessary, applying corrections or internal adjustments. Regular calibration ensures accuracy, repeatability, and traceability of measurements, which is essential for quality control, safety, and regulatory compliance in technical and scientific applications. (5)
A phase (physical state) is a region of matter that is chemically uniform and has consistent physical properties (such as density and structure) throughout. A single system can contain multiple phases: for example, ice, liquid water, and water vapour in one container are three distinct phases. Common phases include solid, liquid, gas, and plasma, but materials can also form specialised phases like glassy, crystalline, or liquid‑crystal states. Phase changes occur when conditions such as temperature or pressure cross specific thresholds, altering these properties. (6)
References:
(3) https://www.green-cooling-initiative.org/fileadmin/Greener_Reefers.pdf
(4) https://www.coolingpost.com/features/carrier-co2-reefers-used-as-grocery-collection-units/
(5) Holman, J.P. (2011). Experimental Methods for Engineers. McGraw‑Hill, 8th ed.
(6) Atkins, Peter; de Paula, Julio. 2014. Physical Chemistry. 10th ed., Oxford University Press.
Note: This article was partly created with the assistance of artificial intelligence to support drafting.