Every part of a cold chain shipment can be specified correctly and still fail during loading. The doors are open, the cargo is on an ambient dock, and the machinery is either off or fighting a losing battle against the outside air. This is where most temperature histories go wrong.
Pre-cool the cargo, always
A reefer holds temperature, it does not create it. Cargo must arrive at the loading dock already at its carriage temperature, brought there by a proper cooling process: forced air cooling, hydro-cooling, vacuum cooling or a blast freezer, depending on the commodity.
Check it with a pulp temperature probe in the product itself, on multiple pallets, and reject product that is out of range before it goes in. A surface reading tells you nothing about the middle of a pallet.
Do not pre-cool the empty container at an ambient dock
This is the counter-intuitive one, and it is the opposite of what most people assume.
If you chill an empty container and then open it on a warm humid loading bay, two things happen immediately. Warm moist air floods in and condenses on every cold interior surface, wetting the walls, the floor and then the cartons. And that moisture goes on to ice up the evaporator coil, which degrades airflow for the rest of the voyage and forces repeated defrost cycles.
So the rule depends on the dock:
- Loading at ambient: do not pre-cool the container. Deliver it at ambient with the unit off, load fast, close the doors, then start the machinery and let it pull the box down around already-cold cargo.
- Loading through a sealed refrigerated dock, where the container is docked against a seal and the loading area is itself at the carriage temperature: pre-cool the container. There is no warm humid air to admit. This is the correct method for fresh produce and for cold treatment shipments.
Where a cold treatment protocol or a shipper's own procedure specifies pre-cooling, follow it, and load through a dock seal.
The T-floor
A reefer floor is corrugated, forming channels along the length of the container. This is not drainage, it is the air delivery duct: the unit pushes cold air along the floor to the far end, up through and around the cargo, and back along the ceiling to the return intake.
Two rules follow, and both are broken routinely:
- Never block the T-floor. Do not lay flat sheets, cardboard, plastic film or pallet liners on the floor. Cargo must sit on the corrugations so air can pass beneath it. A single sheet of cardboard laid down for cleanliness can starve the far end of the container.
- Do not load past the red load line. The line marked near the ceiling is the return air path. Stack above it and the cold air short-circuits straight back to the intake without going through the cargo, so the machine reads a satisfied return temperature while the load stays warm.
The stow pattern depends on the cargo
This distinction is genuinely important and often missed:
- Chilled cargo, especially fresh produce, needs air to flow THROUGH the load. The product is generating heat by respiration and it has to be carried away continuously. Stow with spaces, use vented cartons, and align the vents so the air path is continuous from pallet to pallet.
- Frozen cargo needs air to flow AROUND the load. The product generates no heat and the job is only to hold it. A tight block stow with minimal voids is correct, because it reduces the air volume to be cooled and slows warming if the unit stops.
Applying the frozen method to fresh produce is a common and expensive error. See shipping fresh produce.
The loading sequence
- Inspect the container. Clean, dry, odour free, door seals intact, drains clear, insulation undamaged. Run the unit and confirm it holds setpoint before you commit cargo to it.
- Check pulp temperatures on the cargo and record them.
- Switch the unit off for loading at an ambient dock, so it is not drawing warm humid air across the coil.
- Load fast. Minutes matter. Have the whole load staged before the doors open.
- Keep the T-floor clear and stay below the red line.
- Place data loggers with the product at the positions your risk assessment specifies, per data loggers and excursions.
- Close the doors, then start the unit and confirm the setpoint, vent setting and humidity are as booked, per reefer setpoints.
- Record everything: pulp temperatures, times, settings, and a photograph of the finished stow.
General guidance. Follow your own commodity protocol and any cold treatment requirement from the importing country, both of which override general practice.
