The 'temperature set point' is the temperature entered into the controller, or microprocessor, of a temperature controlled container. This determines the air temperatures supplied to the container. Nearly all refrigerated goods shipped in containers are full container loads, packed so that shipping line never has an opportunity to check the temperature of the goods. Accordingly, shipping line cannot warrant the temperature of the goods. To carry cargo at a single 'carriage temperature' is impossibility, as there will always be minor fluctuations.
The integral containers built since early 1995 can be set between -30 degree C and +30 degree C. Shippers must provide to the shipping company, at time of booking, written temperature settings, plus any fresh air ventilation requirements for their cargo.
However good the container, and however well cooled, packed, and stowed the cargo, there is of necessity a temperature gradient within the container, which is dependent on outside conditions. Such gradients are known and understood by container operators and the reason for temperature variations include: effects of ambient temperature, container thermal properties, air circulation rate, air flow patterns, refrigeration control system and loading temperatures.
Integral containers in chilled mode control the air temperatures via the supply air probe, and in frozen mode control air temperatures via the return air probe.
Modern integral units are fitted with dehumidifiers and in-built data-loggers measuring temperatures, relative humidity and events. Digital displays allow visual monitoring of temperatures. The software installed in these integrals also prevents fans from blowing warm, moist air into the container until the refrigeration system has restarted, and the evaporator coil has cooled. This helps maintain the integrity of the temperature chain.
Some units built before 1st January 1997 and all units built since, are fitted with a special 'bulb mode' switch and software that can adjust the defrosting system when carrying flower bulbs and similar cargos, thereby giving a 'soft defrost'. Drain holes in the container floors can be unplugged, an advantage for cargos which produce a large volume of water. All these containers can be fitted with USDA probes for the 'cold treatment' of fruit cargos if required. Integral reefers are not designed to condition cargo or commodities but only to maintain the required setting temperature. However by special arrangement this service can be offered for specific fruits and vegetables when mutually agreed between the shipper and the consignee.
Container refrigeration technology is constantly advancing with equipment manufacturers developing new equipment and ensuring we have the best available technology to meet our requirements.