Reefer containers: setpoints, airflow and pre-trip inspection

Supply air, return air, and why the unit does not know the product temperature

The refrigeration unit sits at the front of the container and does one job: it pulls air in through a return-air grille, cools or heats it to match the setpoint, and pushes it back out as supply air through the top of the box, down the walls, and up through the T-bar floor channels underneath the cargo. The unit's control sensor sits in that supply air stream. It never touches a carton or a pallet. Every setpoint decision the machine makes is based on the temperature of the air leaving the evaporator, not the temperature of the produce or protein sitting on the floor.

This matters because a reefer holds temperature, it does not chill warm cargo down to setpoint in any useful timeframe. A unit rated to move a modest volume of air per hour across a coil sized for maintaining a steady state can pull down a partially warm load slowly, but loading product at ambient temperature and expecting the container to do the cooling in transit is the single most common cause of a rejected shipment on arrival. Product should already be at or near its target pulp temperature before the doors close. The unit's pull-down capability exists to correct minor variance and to recover from door-open events during transit, not to substitute for pre-cooling.

A worked example: stone fruit pre-cooled to 2°C and stowed correctly will typically show supply and return air within a degree or two of setpoint within the first hour. The same fruit loaded at 18°C ambient into a container set to 2°C may show supply air near setpoint almost immediately, because the unit is working flat out, while return air and the actual product core stay elevated for many hours, sometimes for the entire ocean leg. The data logger, discussed below, will show exactly this pattern if a claim is filed.

The T-bar floor, the red load line, and why the stow cannot sit flat on the deck

The T-bar (or T-floor) is the ridged aluminium flooring inside a reefer container, and the ridges are not a slip-resistance feature. They create a channel underneath the cargo so supply air delivered at the front of the box can travel the full length of the container under the load, rise up through and around the stow, and return to the unit at the back near the ceiling or through side-wall grilles depending on the container design. If cartons, bags, or a pallet base sit flat against the floor with no gap, or if plastic sheeting or dunnage is laid across the full floor width, that channel is blocked and the rearmost pallets get starved of airflow regardless of what the setpoint says.

The red load line marked on the interior walls, usually just below the ceiling, marks the maximum stow height. Cargo stacked above it blocks the return-air path at the top of the box, which has the same effect as blocking the floor channel: air recirculates locally around the top pallets instead of completing the loop back to the return-air grille, and the unit's control sensor reads air that has already cooled, even while pockets deeper in the stow sit warm. A container that looks fully and neatly loaded can still fail on arrival if the person loading it did not leave the floor channel and the top clearance open.

Practical rule: leave the floor channel completely clear front to back, never bridge pallets across it with loose cartons, and treat the red line as a hard stop, not a target. A container loaded to the line with even airflow beats one loaded higher with dead air pockets at the corners.

Delivery air versus return air, and which one the data logger actually records

Reefer units report two temperatures continuously: delivery (or supply) air, taken as the air leaves the evaporator before it enters the cargo space, and return air, taken as air comes back to the unit after passing through the stow. Delivery air responds fast and reflects what the machine is doing. Return air responds slowly and reflects what actually happened to the cargo, because it has picked up whatever heat the product, the container walls, and any door-open events added along the way.

A standalone data logger, if the shipment carries one separate from the container's own recorder, is usually placed inside the cargo mass or at a defined monitoring point specified by the buyer, not at the return-air grille, so it can diverge from both container readings depending on placement. The container's own telemetry, the record a carrier or insurer will actually pull for a claim, logs both supply and return air continuously through the voyage. When a dispute arises over whether a shipment was held at temperature, the return-air trace is the more meaningful record, since it is the closest proxy the unit has to what the cargo experienced, while the supply-air trace mainly shows whether the machine itself was functioning.

Anyone specifying monitoring in a purchase or freight contract should say explicitly which reading governs, and whether an independent logger is required. A contract that just says 'maintained at setpoint' without naming supply or return air, or the logger location, leaves room for exactly the argument a claim depends on.

Fresh air exchange, humidity, and controlled atmosphere

Produce that is still respiring after harvest, most fresh fruit and vegetables, consumes oxygen and releases carbon dioxide and ethylene gas for the entire transit. Left in a fully sealed box, CO2 and ethylene build up and accelerate ripening or cause chilling injury and off-flavours, so the reefer unit has a fresh air exchange vent, usually a manually set damper or an electronically controlled CO2-managed vent on newer units, that bleeds a controlled volume of outside air in and stale air out to keep those gases within a safe range for the specific commodity.

Frozen cargo and most non-respiring product need the opposite setting. The vent should be closed, because any outside air brought in only adds heat and moisture load the unit then has to remove, working against the setpoint rather than protecting the product. Leaving a fresh air vent open on a frozen load is a common and entirely avoidable cause of frost build-up on the evaporator coil and reduced cooling capacity over a long voyage.

Controlled atmosphere (CA) containers go further, actively managing oxygen and CO2 levels rather than just venting, typically used for commodities like avocados, asparagus or certain apple and pear varieties where extending the storage life meaningfully changes the viable shipping window. CA equipment and the gas-generation system add real cost per box and require a tighter seal check during pre-trip inspection, since any leak defeats the atmosphere control entirely. Confirm with the carrier well before booking whether a specific vessel string and container fleet actually offers CA capacity. Not every reefer slot does, and substituting a standard reefer at the last minute for a CA-dependent commodity can shorten shelf life meaningfully by the time the cargo reaches the receiver.

What a pre-trip inspection proves, and what it does not

A pre-trip inspection (PTI) is an automated diagnostic the reefer unit runs on itself before a container is released for loading: it checks the compressor, the evaporator and condenser fans, the defrost cycle, temperature sensor calibration, and the control system's ability to hold setpoint on an empty, closed box over a set cycle, typically running through several distinct test stages before returning a pass or fail code. A pass means the mechanical and electrical systems can maintain temperature under test conditions. It is a real and useful check, and a container should never be loaded without a current PTI pass on file.

What a PTI pass does not verify is anything about the load itself: it cannot confirm the cargo was pre-cooled to the right pulp temperature, that the stow leaves the T-bar channel and the return-air path clear, that the fresh air vent is set correctly for the commodity, or that the door seals will hold once the box is packed tight and stacked in a yard or on a vessel for weeks. A container can carry a spotless PTI record and still deliver a claim if the loading was wrong. Treat PTI as a necessary pre-condition for booking the container, not as a substitute for checking the load.

Most PTI failures that do occur trace to the same handful of causes: a defrost sensor out of calibration, a door seal that has degraded from repeated use, or a control board fault that shows up as an inability to reach or hold the test setpoint within the allowed window. Any of these should keep a container out of rotation until repaired, and a shipper receiving a unit for loading should always ask the depot or carrier for the specific PTI date and result, not just a verbal assurance that it passed.

Genset and plug-in power on the road leg

A reefer unit needs continuous power to hold setpoint, and on the ocean and rail legs that comes from the vessel's or terminal's reefer plug points, monitored by dedicated staff who log temperatures at set intervals. The road leg is where power continuity is easiest to lose track of. A container moving by truck either runs on a diesel generator set (genset) clipped to the chassis, or, for shorter moves between a cold facility and a port with no extended dwell, sometimes runs off the truck's own power take-off, though genset use is far more common for any leg of meaningful length or duration.

A genset has its own fuel tank and needs to be checked before departure for fuel level sufficient for the full estimated transit time plus a margin for delays, and for its own separate maintenance and inspection record, distinct from the container unit's PTI. A genset running out of fuel mid-transit is a full loss of temperature control for however long the gap lasts, and unlike a brief power interruption during a plug swap at a terminal, a fuel-out on the highway can run for hours before anyone notices, since telemetry connectivity on a moving truck is not guaranteed the way a monitored yard connection is.

For any road leg longer than a short drayage move, confirm the genset's rated run time against the planned transit time in the booking, not just its tank capacity on paper, since ambient temperature, the size of the pull-down required, and the commodity's tolerance for a temperature excursion all change how much margin is actually needed. A shipper moving a tight-tolerance product on a long inland leg should ask the trucker for the genset's fuel range in hours at the specific setpoint, not assume a full tank is sufficient.

Pre-load checklist

Confirm PTI status and date before the container arrives at the loading dock, and reject any unit without a current pass on file rather than loading first and querying it after the fact.

Verify product core or pulp temperature against the target setpoint before loading begins. A reefer corrects small variance, it does not substitute for pre-cooling.

Load leaving the full T-bar floor channel clear front to back, and stack no cargo above the red load line, checking both after the pallets are set, not just at the start of loading.

Set the fresh air exchange vent for the specific commodity: open and calibrated for respiring produce, fully closed for frozen or non-respiring cargo.

For CA-dependent commodities, confirm the booking is actually on a CA-capable container and vessel string, and have the seal integrity checked as part of the pre-trip process.

Confirm the monitoring plan in writing: which reading, supply or return air, governs any claim, and whether an independent data logger is required in addition to the container's own telemetry.

On any road leg, confirm the genset's fuel range in hours against the planned transit time before dispatch, not just its tank capacity, and check its own maintenance record separately from the container unit's PTI.