A refrigerated container is a converted shipping container used in intermodal freight transportation and is refrigerated for transporting temperature sensitive cargo.
Though refrigerated containers will have a fundamental refrigeration unit, they generally rely on external power, from electrical power points at a land based site, a container ship or on quayside. When transported on road or on railroad wagon, they can be powered by diesel powered generators which are attached to the container whilst on journeys. Refrigerated containers are able to control temperatures ranging from -30o
C up to 30o
C, and 40o
Some refrigerated containers are equipped with water cooling systems, which can be used if the container is stored on a vessel without sufficient ventilation to remove the heat it generated.
Water cooling systems are relatively expensive, so modern vessels are more reliant on ventilation to eliminate heat from cargo holds, and the utilisation of water cooling systems is decreasing. Air and water cooling are typically combined. Air cooling eliminates the heat generated by the containers, while water cooling helps to reduce the heat rejected by the containers. The refrigerated containers utilise some heat exchangers that acts as water cooled condensers for water cooling.
A refrigeration system that is used at times where the journey time is short is total loss refrigeration, wherein frozen carbon dioxide ice (or liquid nitrogen) is used for refrigeration. The cryogenically frozen gas evaporates slowly, and therefore cools the container and is emitted from it. The container is refrigerated for as long as there is still some frozen gas available in the system. These have been used in railcars for several years, offering up to 15 days temperature regulation. As refrigerated containers are not very common for air transport, total loss dry ice systems are typically used. These containers have chambers that are loaded with solid carbon dioxide and the temperature are regulated by thermostatically controlled electric fans, and the air freight versions are designed to maintain temperature for up to about 100 hours.
Full size intermodal containers equipped with these “cryogenic” systems can maintain their temperature for the 30 days needed for sea transport. Since they do not require an external power supply, cryogenically refrigerated containers can be stored anywhere on any vessel that can accommodate “dry” (un-refrigerated) ocean freight containers.
Valuable, temperature-sensitive, or hazardous cargo often require the utmost in system reliability. This type of reliability can only be achieved through the installation of a redundant refrigeration system.
A redundant refrigeration system consists of integrated primary and back-up refrigeration units. If the primary unit malfunctions, the secondary unit automatically starts. To provide reliable power to the refrigeration units, these containers are often fitted with one or more diesel generator sets.
The influence refrigerated containers have on society is vast, letting consumers all over the world to enjoy fresh produce at any time of year and experience previously unavailable fresh produce from many other parts of the world.
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