Reefer containers: temperature control across a sea leg

The unit maintains temperature; it does not create it

A reefer circulates chilled air through a T-shaped floor, up through and around the cargo, and back to the machinery. It is sized to hold a temperature against heat entering through the walls, not to pull down a warm load. Cargo therefore has to be pre-cooled to its carriage temperature before it is packed. Loading warm produce into a reefer and expecting the unit to fix it is the most common and most expensive mistake in the trade. Airflow discipline follows from the same design. Cargo is stowed so air can travel up the floor channels and return above the load, packaging is vented where the commodity needs it, and the red load line on the walls is respected so the return path is not blocked.

Power at every step, and the gaps between

The unit needs electricity continuously. Aboard ship it plugs into vessel sockets monitored by the crew; in a terminal yard it plugs into a reefer rack; at a depot it plugs into a plug point; on the road it draws from a diesel generator clipped to the container or mounted on the chassis. Every transfer between those sources is an unplugged interval. Short gaps are absorbed by the insulation and the mass of the cargo, but repeated or extended gaps accumulate into a genuine excursion.

Setpoint, ventilation and modified atmosphere

Chilled cargo and frozen cargo behave differently: frozen goods want a stable low temperature and no air exchange, while respiring produce gives off heat, carbon dioxide and ethylene and needs a fresh-air vent opened by a defined amount. Controlled-atmosphere units go further, actively managing oxygen and carbon dioxide to slow ripening on long voyages. Setting a vent as though fruit were frozen cargo will spoil a load as surely as a power failure.

Proving what happened inside the box

The unit records supply and return air temperatures throughout the voyage, and independent loggers are often placed with the cargo as well. Modern boxes report remotely so an alarm can be acted on while the vessel is still at sea. When cargo arrives out of specification, that recorded trace is what decides whether the fault lay with pre-cooling, with stowage, with a power gap or with the machinery itself.

The predictable failure points

Packing cargo that was never pre-cooled, blocking the return air path with over-height stowage, leaving the box unplugged during a terminal move, doors held open while a container is stripped in warm ambient conditions, and setpoints entered incorrectly at booking account for the great majority of claims. None of them are refrigeration faults.

Frequently asked questions

Can a reefer cool cargo that was packed warm?

Not effectively. The unit is designed to hold a temperature, not to remove the heat already in a load, so a warm consignment may never reach specification and will arrive with a recorded excursion. Pre-cooling before packing is the control.

What is the load line inside a refrigerated container?

A marking showing the maximum stowage height that still leaves a clear return air path above the cargo. Stacking above it starves the circuit, so air short-circuits back to the unit and parts of the load never reach temperature.