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Refrigerated (reefer) cargo — perishable food, pharmaceuticals, and other temperature-sensitive goods — depends on maintaining an unbroken cold chain from packing to final delivery. A single gap in temperature control, even briefly, can spoil the cargo or shorten its remaining shelf life beyond commercial use, so reefer operations are built around continuous monitoring, pre-trip verification of equipment, and close attention to the difference between the unit’s set-point and the actual supply/return air temperature it is achieving.
Self-contained ISO containers with a built-in refrigeration unit at one end, plugged into the ship’s (or terminal’s) electrical supply. Dominant mode of reefer carriage today, since it lets reefer boxes move on ordinary container ships alongside dry cargo.
Dedicated reefer vessels with insulated, mechanically-refrigerated cargo holds — historically the standard for bulk perishable trades (bananas, other fruit) before containerisation. A shrinking but still-active specialist fleet segment.
Non-integral (porthole) containers without their own refrigeration unit, cooled instead by conditioned air ducted through the ship’s central refrigeration plant via portholes in the container end wall — the Conair system. Requires a ship fitted with the matching central air-delivery infrastructure, and is now a minority carriage mode compared with integral containers.
The set-point is the target temperature specified for the cargo by the shipper. The reefer unit does not measure the cargo itself in normal operation; it measures and controls the supply air (air leaving the unit into the cargo space) and return air (air coming back from the cargo space to the unit), and the gap between them indicates how effectively the refrigeration is penetrating a densely packed load. A widening supply/return gap, or a return-air temperature drifting away from set-point, is an early warning sign that airflow through the cargo is restricted — often because the load was packed too tightly or the floor T-bar channels were blocked.
For certain fruit cargoes (notably bananas and other climacteric fruit that continue to ripen after harvest), temperature control alone is not enough — the atmosphere inside the container or hold is actively managed to slow ripening. Controlled atmosphere (CA) systems actively regulate oxygen, carbon dioxide, and sometimes ethylene levels inside the sealed space; modified atmosphere (MA) approaches passively rely on the fruit’s own respiration and a sealed environment to shift the gas balance without active injection/scrubbing. Both aim to slow the ripening process and extend transit shelf life.
Fresh-air ventilation settings on a reefer unit exchange a controlled amount of outside air with the cargo space to manage the buildup of carbon dioxide (from cargo respiration) and, for ethylene-sensitive produce, to vent the ripening-hormone ethylene given off by ripening fruit before it accelerates ripening in surrounding boxes. Ventilation rate is cargo-specific and is normally set at the shipper’s instruction, since too much or too little airflow can each accelerate spoilage for a given commodity.
A pre-trip inspection (PTI) tests a reefer unit’s refrigeration cycle, controller, sensors, defrost cycle, and door seals before it is released for booking, catching mechanical or electronic faults before cargo is loaded rather than after. PTI is typically run at the container depot on a defined schedule (for example, before every booking or on a periodic cycle) and is a precondition many carriers require before accepting a unit for a temperature-sensitive booking.
Where practical, the container itself is pre-cooled to close to the intended set-point before the cargo is loaded, so the refrigeration unit is not required to pull the whole cargo mass down from ambient temperature after stuffing — a process that stresses the cold chain during the period the cargo is most vulnerable and that the unit is generally not sized to perform quickly for a full load.
Some importing countries require a verified, sustained low-temperature exposure during transit as a phytosanitary treatment — holding the cargo at a specified temperature for a specified duration to control pests such as fruit flies — before the cargo may be admitted. Where such a protocol applies, the vessel or container’s temperature recorders must demonstrate continuous compliance for the required period, and national plant-health authorities (for example, the USDA in the United States) set and audit the specific temperature/duration combinations for each commodity and origin.
Modern integral reefer containers log supply/return air temperature, humidity, and (increasingly) location data continuously, either downloaded at destination or transmitted in real time by remote-monitoring telematics. A broken cold chain — a power interruption, a defrost fault, a mis-set point, or a door left open during handling — may not be visually detectable on arrival even though the cargo’s remaining shelf life or safety has been compromised, which is why continuous data logging rather than spot-checking is the standard of care for temperature-sensitive cargo.
Integral reefer containers require continuous electrical power throughout the voyage, supplied from the ship’s reefer plug points where slotted in dedicated reefer bays, or from standalone diesel generator sets (gensets) clipped onto the container itself where deck reefer-plug capacity is exceeded or where the stow position has no plug access. Reefer plug capacity is a real constraint on how many reefer boxes a given vessel can carry, and is planned for explicitly in container stowage planning.
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