How Reefer Containers Work: Maintaining Temperature, Not Reducing It

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Chilled and frozen cargo airflow patterns shown with arrows inside storage containers.

In cold chain logistics, reefer containers play an indispensable role. Whether it's fruits, meat, seafood crossing oceans, or temperature-sensitive pharmaceuticals and flowers, these "mobile fridges" ensure that cargo reaches its destination safely. However, there is a common misconception about how reefer containers work: their core function is not to lower temperature, but to maintain it.

Reefer vs. Household Fridge: Different Principles

Reefer containers are often described as "big fridges," but their working principle is not exactly the same as household appliances. A reefer container is a bottom-air-delivery system. It uses specially designed T-shaped decking to distribute chilled air evenly from the floor upward. The cold air passes through gaps between cargo, removing heat and gases, and ensuring a consistent temperature throughout the container. This design is particularly suitable for chilled cargo that requires continuous ventilation, such as fresh fruit and vegetables.

For frozen cargo, the principle is different. Frozen cargo requires cold air to circulate around the cargo, not through it. Therefore, the cargo must be block-stowed with no gaps between the cargo and the container walls, ensuring the air follows the correct path. The "red load line" on frozen cargo labels serves this purpose — cargo must not be loaded above this line to ensure sufficient air circulation space above.

Key Point: Maintaining Temperature, Not Reducing It

Reefer containers are designed to maintain the cargo at its pre-cooled temperature, not to significantly reduce the cargo temperature during transit. This means cargo must be adequately pre-cooled before loading. If cargo is loaded at too high a temperature, the reefer unit will struggle to bring it down to the desired level during the voyage.

For shipments that require warming, reefer containers are also equipped with heating capabilities, able to maintain temperatures up to 30°C even in freezing outside conditions.

Cold Treatment: Special Requirements for Certain Destinations

Some fruit types may carry potential pathogens. As a result, importing countries such as China, Japan, and Nigeria require cold treatment (commonly known as "steri shipments"). These shipments require pre-cooling to a lower temperature than normal commercial shipments, and typically three probes are inserted into the fruit pulp to monitor the core temperature. There is a minimal tolerance allowed for temperature variance. If exceeded, the cargo may require additional cooling hours or days — or even be rejected at the destination country.

Loading Guidelines

Proper loading is critical to ensuring reefer cargo arrives safely. Key points include:

Cargo must not be stuffed beyond the end of the T-floor
Cargo must not be loaded above the red load line
Cargo must be stable on the floor and tightly wedged to prevent shifting during transit
The unit must be set to the proper carrying temperature according to the cargo type
Dehumidification controls must be checked
Pre-cooling must be done before loading (unless loading occurs in an airlocked cold tunnel)
Ventilation settings are essential and must be set correctly
Any gaps between pallets and the doors must be closed using cardboard or wood to prevent warm air from accumulating

Refrigeration Unit Brands

The most common reefer unit brands in the market today are Carrier, Thermo King, and Daikin. Each brand has its own strengths in terms of energy efficiency, temperature accuracy, and application suitability. Customers can choose the right configuration based on cargo type and shipping route.

The technical details of reefer containers are more complex than many people realize, but understanding how they work is the first step to ensuring safe cold chain transport.

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