Knowledge / Maritime cargo

Reefer transport: air temperature, product temperature and cold-chain evidence

Examine precooling, airflow paths, sensor location, power interruptions and temperature-record interpretation in refrigerated containers.

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The set point on a refrigerated-container display is not the actual temperature at every location in the cargo. Initial product condition, packaging, stowage, airflow paths and power availability throughout the journey act together. A reliable cold-chain assessment distinguishes evidence that the equipment operated from evidence that the product remained within its required conditions.

Begin with the cargo's carriage requirements

Temperature, humidity, fresh-air and controlled-atmosphere requirements can differ between commodities. Chilled and frozen products also have different thermal behaviour. The same instruction, tied to cargo identity, must reach the booking, packing team and monitoring party with the same meaning. Celsius and Fahrenheit, or volumetric fresh-air flow and percentage vent opening, must not be substituted for each other.

A completed pre-trip inspection is evidence about container equipment. It does not establish that cargo arrived at the correct temperature or in suitable packaging. At handover, the measurement location, whether it represents surface or core, and instrument condition matter. Measurements from a few accessible cartons do not automatically represent the centre of every pallet.

Precooling product differs from cooling an empty container

Maersk's loading explanation emphasises bringing product to carriage temperature beforehand and avoiding treatment of the container as a rapid product-cooling facility. It also explains that opening a precooled empty container in warm, humid surroundings can introduce condensation. A local carrier instruction should not be generalised into one procedure for every packing facility.

A closed cold-loading connection and loading on an open humid quay have different heat and moisture ingress. Product precooling can therefore be required while empty-container precooling depends on different conditions. Describing both activities with the same word and treating them as the same decision can produce well-intentioned but unsuitable preparation.

Does circulating air reach the actual cargo?

Cold air taking a short path directly back to the unit can leave some regions warm even while sensors look satisfactory. Floor channels, loading height, the return path near the doors and carton ventilation openings belong to the same airflow circuit. The location of a void can matter as much as its total volume.

MSC's reefer loading guide distinguishes circulation around frozen cargo from flow through fresh products that generate respiratory heat. This general distinction does not specify one packaging or gap arrangement for every commodity. Fresh-air exchange is also different from internal circulation: admitting outside air can change gas composition while adding heat and moisture load.

The energy carried by warm cargo

Assume a hypothetical cargo with no phase change, mass 20,000 kg, specific heat 3.5 kJ/(kg·K) and temperature 5 K above its target. Sensible energy to remove is Q = m c ΔT = 350,000 kJ ≈ 97.2 kWh. Under the ideal assumption that a continuous net 10 kW reaches the product, the time would be 97.2 / 10 ≈ 9.7 h.

That duration is neither a carriage recommendation nor a safe exposure time. It excludes wall heat gain, respiration, packaging, defrost and temperature differences within the product. In practice, core temperature may change more slowly than surface temperature, and the refrigeration unit's catalogue capacity is not entirely available to the cargo. The calculation illustrates why hot loading is not a problem solved by a small set-point adjustment.

Read supply, return and product measurements separately

A supply sensor measures air leaving the unit; a return sensor measures air returning from the cargo space. A product sensor represents its own location. The MSC iReefer technical FAQ distinguishes these quantities and explains brief return-sensor rises during defrost. A model-specific control threshold must not be applied to all reefer units.

Assuming normal steady circulation and valid, comparable sensor readings, if supply reads 0.5°C and return 2.0°C, for example, the 1.5 K difference indicates heat uptake by the circulating air. It does not demonstrate that every product location stayed between 0.5°C and 2.0°C. Sensor position, short-circuit airflow and initially warm or cold regions can produce different product temperatures. Treating one return-air spike directly as product warming is another representation error.

Power interruption, data gaps and delivery assessment

Missing remote data does not prove loss of power. Communication coverage can disappear while the unit continues operating and recording locally. MSC's same technical explanation describes circumstances in which buffered history is transmitted after connectivity returns. Conversely, a temperature close to the earlier reading after a gap does not establish that all conditions were acceptable throughout that gap.

Assessment should align timestamps and time zones, power events, equipment alarms, set-point changes, door openings and product measurements on one timeline. This helps distinguish an actual excursion from a sensor or communication problem. Cargo acceptance depends on the product specification and authorised quality assessment. This article sets no temperature threshold for food safety, remaining shelf life or an insurance claim.

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