10.3.1   The significance of interfaces for the cryptoclimate in the container
An examination of published incidents of loss due to climatic factors involving container cargoes reveals that such incidents affect the entire range of products with no particular class of product being disproportionately represented. On the basis of the published examples, losses caused by sweating are clearly the most striking. Sweating includes both that which occurs on the cargo itself (cargo sweat) and that which drips down onto the cargo from the upper surfaces of the container (container sweat). All classes of goods are affected by this type of loss. For example, reported losses range from nonhygroscopic gods, such as steel and steel products, canned foods, to hygroscopic goods, such as cocoa, coffee, millet, dried fruit, sago, pepper, milk powder, furs, textiles and rattan furniture.
In addition to the preponderance of losses due to sweat, a second problem is particularly noticeable, namely the care which is required to adapt the goods, loaded under the climatic conditions of the place of departure, to the climatic conditions of the destination while in transit, without causing damage to the goods or making such damage inevitable due to inadequate adaptation. The theoretical basis on which these issues are addressed resides in "interfacial" physics, which take account of the differences in heat and water vapor transfer at interfaces. The most important basic requirement in this connection is to prevent condensation of the water vapor present in the air at an interface, whether on the container wall boundaries, on the surface of the cargo, in air layers in the vicinity of interfaces or within cargo blocks, if the temperature of the interface falls below the dew point temperature of the surrounding body of air. This requirement in turn makes it necessary to adapt the temperature of the cargo to the anticipated air temperature at the destination. Abrupt changes in temperature or humidity or both occur at these interfaces.
The following types of interface in container transport may be distinguished on the basis of their thermal and hygroscopic properties:
  1. Container parts as 1st order interfaces
    These include interfaces which exhibit good heat transfer, are impermeable to water vapor and on which relatively large variations in temperature occur on exposure:
    • container walls and ceilings

  2. Container parts as 2nd order interfaces
    These include interfaces which, in addition to exhibiting good heat transfer, are also permeable to water vapor or actively interact with the water vapor in the container:
    • wooden dunnage
    • dunnage

  3. Cargo surfaces as 1st order interfaces
    Hygroscopic goods which release heat and water vapor into the container air. These include:
    • actively respiring goods of vegetable origin
    • goods of vegetable or animal origin or chemical products which, as a result of ongoing biological or chemical processes, have a tendency to undergo self-heating and are capable of exchanging water vapor with the air

  4. Cargo surfaces as 2nd order interfaces
    nonhygroscopic goods with surfaces having good thermal conductivity and a relatively large heat capacity of the individual package or stack:
    • unpackaged metallic surfaces
    • hygroscopic and nonhygroscopic goods packaged in metallic containers or metal foils as the surface

  5. Cargo surfaces as 3rd order interfaces
    Surfaces of hygroscopic goods capable of heat transfer and permeable to water vapor which exchange heat and water vapor with the container air without actively generating heat or requiring this exchange in order to retain service properties:
    • salt and fertilizer
    • sugar
    • hygroscopic minerals, ores and rock
    • lumber, furniture
    • general cargo packaged in wooden cases

Contact  |  Site Map  |  Glossary  |  Bibliography  |  Legal Notice  |  Paper version