A refrigerated container or reefer is an intermodal container

Intermodal container

An intermodal container is a standardized reusable steel box used for the safe, efficient and secure storage and movement of materials and products within a global containerized intermodal freight transport system...

 (shipping container) used in intermodal freight transport

Intermodal freight transport

Intermodal freight transport involves the transportation of freight in an intermodal container or vehicle, using multiple modes of transportation , without any handling of the freight itself when changing modes. The method reduces cargo handling, and so improves security, reduces damages and...

 that is refrigerated

Refrigeration

Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

 for the transportation of temperature sensitive cargo.

While a reefer will have an integral refrigeration unit, they rely on external power, from electrical power points at a land based site, a container ship

Container ship

Container ships are cargo ships that carry all of their load in truck-size intermodal containers, in a technique called containerization. They form a common means of commercial intermodal freight transport.-History:...

 or on quay. When being transported over the road on a trailer they can be powered from diesel powered

Diesel engine

A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

 generators

Electrical generator

In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

 ("gen sets") which attach to the container whilst on road journeys.

Some reefers are equipped with a water cooling system, which can be used if the reefer is stored below deck on a vessel without adequate ventilation to remove the heat generated.

Water cooling systems are expensive, so modern vessels rely more on ventilation to remove heat from cargo holds, and the use of water cooling systems is declining.

The impact on society of reefer containers is vast, allowing consumer

Consumer

Consumer is a broad label for any individuals or households that use goods generated within the economy. The concept of a consumer occurs in different contexts, so that the usage and significance of the term may vary.-Economics and marketing:...

s all over the world to enjoy fresh produce

Produce

Produce is a generalized term for a group of farm-produced goods and, not limited to fruits and vegetables . More specifically, the term "produce" often implies that the products are fresh and generally in the same state as where they were harvested. In supermarkets the term is also used to refer...

 at any time of year and experience previously unavailable fresh produce from many other parts of the world.

Cryogenic cooling

Another refrigeration system sometimes used where the journey time is short is total loss refrigeration, in which frozen carbon dioxide ice

Dry ice

Dry ice, sometimes referred to as "Cardice" or as "card ice" , is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue...

 (or sometimes liquid nitrogen

Liquid nitrogen

Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

) is used for cooling. The cryogenically frozen gas slowly evaporates, and thus cools the container and is vented from it. The container is cooled for as long as there is frozen gas available in the system. These have been used in railcars for many years, providing up to 17 days temperature regulation. Whilst refrigerated containers are not common for air transport, total loss dry ice systems are usually used. These containers have a chamber which is loaded with solid carbon dioxide and the temperature is regulated by a thermostatically controlled electric fan, and the air freight versions are intended to maintain temperature for up to around 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.

Redundant refrigeration

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 ISO container system consists of a standard ISO container (i.e. intermodal container

Intermodal container

An intermodal container is a standardized reusable steel box used for the safe, efficient and secure storage and movement of materials and products within a global containerized intermodal freight transport system...

), integral primary and backup refrigeration units, and integral primary and back-up diesel generator

Diesel generator

A diesel generator is the combination of a diesel engine with an electrical generator to generate electrical energy....

 sets.

The two sets of refrigeration units are mounted on one end of the ISO container used for intermodal shipping. This is a much more usable design than others which may try to have equipment on each end and load from the side of the container. The refrigeration units (and generator sets) will be electrically interlocked for automatic start and stop operation as required, such that only one can operate at a time to maintain the required temperature set points. Should the primary refrigeration unit malfunction, the secondary unit would automatically start. Refrigeration units with more highly reliable scroll compressor

Scroll compressor

A scroll compressor is a device for compressing air or refrigerant...

s can also be used in order to maintain the desired temperatures.

The two sets of fuel-powered generator sets will power their respective refrigeration unit whenever necessary. The primary generator set will start automatically based on the status of the cord-supplied electrical power. If the primary generator engine cannot start after a pre-set time, the secondary generator will automatically start.