Refrigerated Container

This is the point on which reefer shipments most often fail, and it is worth stating without qualification. The refrigeration plant on a container is dimensioned for a steady-state duty: replacing the heat that leaks through the insulation from the outside air, and removing the heat that living produce gives off as it respires. Both are small compared with the field heat in a pallet of fruit picked in the sun. Pulling that heat out is the job of a precooling operation — forced-air, hydro-cooling, or vacuum cooling — performed at the packhouse, using equipment with many times the capacity, before the container is ever stuffed.

The problem compounds itself. A tightly packed stow of warm cartons resists airflow, so the limited cold air the unit produces cannot reach the middle of the load anyway. The unit runs continuously, the return air reads warm, and an inexperienced operator may conclude the machine is faulty. It is not: it is doing what it was built to do, against a load it was never intended to handle. By the time the container is opened at destination, the produce has spent the voyage at a temperature at which it was ripening, softening, and decaying. Carrying temperatures for each commodity are set by the applicable national and trade guidance; the requirement to reach that temperature before stuffing is universal.

A reefer uses bottom-air delivery. Conditioned air leaves the unit at floor level, travels the length of the container along the channels of a T-bar decking, rises through the cargo, and returns along the ceiling to the unit’s intake. That geometry is deliberate: air is forced to pass through the load rather than around it, so the cargo, not the box, is what gets cooled. But it also means the stow determines whether the system works at all. Air takes the easiest path, and any gap it can flow through instead of through the cargo is a path it will take.

Hence the conventions. Cartons are stowed to leave the T-bar channels clear, because a pallet base sitting flat on the floor blocks the delivery path for everything behind it. The load is kept below the red line marked inside the container, because stowing to the ceiling blocks the return path and the air short-circuits straight back to the unit without traversing the cargo. Gaps at the sides and between blocks are managed so that air is not offered a free route past the load. A badly stowed reefer produces a distinctive claim pattern — sound produce near the unit, damaged produce at the door end — that reflects air never having got that far. Stow patterns and load limits are set by the carrier’s and the shipper’s own guidance for the commodity and packaging.

A container of fruit is not inert freight. It respires — consuming oxygen and producing carbon dioxide, heat, and in many cases ethylene — for the entire voyage, in a sealed insulated box. Left entirely closed, the atmosphere inside changes: carbon dioxide accumulates and oxygen falls, which for some commodities is injurious and for others is beneficial. This is why reefers have a fresh-air vent, an adjustable opening that exchanges container air with outside air, and why its setting is a commodity-specific decision rather than a default.

Ethylene is the other consequence of living cargo. Ripening fruit produces it, and it acts as a ripening signal on other produce in the same box, which is why commodities that produce it and commodities sensitive to it should not be mixed and why the vent contributes to removing it. Some reefers go further and actively regulate the gas composition throughout the voyage, which converts the box into a controlled-atmosphere store on wheels — a genuinely different system from a plain reefer, requiring gas control, monitoring, and the safety regime that any deliberately oxygen-deficient space demands.

Versus the cold chain

The cold chain is the whole unbroken sequence from field to consumer. A reefer is one link in it — the long-haul link — and the links around it, at ports and on inland legs, are where it most often breaks.

Versus precooling

Precooling removes field heat with high-capacity equipment before shipping. A reefer maintains the result. They are complementary, and a reefer cannot substitute for precooling.

Versus a refrigerated warehouse

A warehouse cools a room and its air; a reefer forces air through a fixed stow in a sealed box, and is moving. The warehouse can be re-stacked and inspected; the reefer cannot be touched for weeks.

Versus controlled-atmosphere storage

A plain reefer only vents; a controlled-atmosphere reefer actively regulates gases in transit. That is CA applied to a moving container, with the same physiological risks and the same confined-space hazards.