A refrigerated container is a temperature holding device, not a cooling device. Its machinery is sized to remove the heat that leaks through the walls and the heat the cargo itself generates, not to pull a warm load down to temperature. Load produce at ambient and it will still be warm in the middle when it arrives, with the data trace showing a perfectly correct setpoint the whole way.
That single misunderstanding is behind a large share of reefer claims. The other cause is a booking where the temperature settings were copied from the last shipment.
Setpoint, supply air and return air
Three temperatures exist inside a reefer and they are not the same number:
- Setpoint: what you asked for.
- Supply air: the air leaving the evaporator and entering the load through the T-floor, which is the coldest air in the box.
- Return air: the air coming back to the machinery after passing through the cargo, which is the warmest.
Which one the machine controls to depends on the cargo, and this is where disputes start:
- Chilled cargo, above freezing, is normally controlled on supply air, because chilled produce is intolerant of being frozen and the tight control point has to be the coldest air the cargo will meet.
- Frozen cargo is normally controlled on return air, because nothing is at risk from air a degree or two colder and what matters is the bulk temperature of the load.
When a claim is argued, the carrier will produce a data trace. Know which sensor it is reading before you accept that the box performed to specification.
Fresh air exchange
Fruit and vegetables respire after harvest. They consume oxygen and give off carbon dioxide, ethylene and heat. The fresh air vent lets outside air in to carry that away, and it is set in cubic metres per hour, typically anywhere from fully closed to around 250 m3/h depending on the commodity and the load size.
- Frozen cargo: vent fully closed. There is nothing to respire, and an open vent just lets in humid, warm air that forms ice on the evaporator.
- Fresh produce: vent open, to a commodity specific rate. Too little and carbon dioxide builds up. Too much and you are paying to chill outside air and you dry the cargo out.
Getting this wrong on frozen cargo is common, and it shows up as a heavily iced coil and a machine that cannot hold temperature by the second week.
Humidity
Standard reefers dehumidify as a side effect of cooling, and many can be set to a target relative humidity, which they reach by running the compressor harder. Most cannot add humidity back. If your cargo needs high humidity to avoid weight loss, say so explicitly and check the box has the feature rather than assuming it.
For dry cargo shipped in a reefer purely for humidity control, be clear that you are asking for a dehumidification setpoint, not a temperature one.
Controlled atmosphere
A controlled atmosphere unit actively manages oxygen and carbon dioxide levels, usually pulling oxygen down to a few percent, to slow ripening on long voyages. It is a separate, more expensive piece of equipment and it must be booked as such. It is not something that can be switched on at the port because somebody wrote it in an email.
Cold treatment
Some trades require the cargo to be held at a specified low temperature for a specified number of days in transit, as a quarantine measure against fruit fly. This is a regulatory requirement of the importing country, it needs calibrated probes placed in the pulp of the fruit and a certified record, and it fails if the temperature rises for even a short period. Do not confuse it with ordinary carriage temperature.
The booking checklist
- Setpoint, in degrees Celsius, stated to one decimal place.
- Whether control is on supply or return air.
- Fresh air exchange rate in m3/h, or explicitly closed.
- Humidity setpoint, or explicitly not controlled.
- Controlled atmosphere gas mix, if applicable.
- Cold treatment protocol and probe count, if applicable.
- Whether the cargo will be delivered pre-cooled to setpoint, and by whom.
That last line is the one that decides whether the shipment works. See temperature controlled freight, 9 things you must specify for the commercial side of the same conversation.
General guidance. Commodity specific carriage temperatures, vent rates and cold treatment protocols come from your shipper's own product specification and from the importing country's plant health authority.
