Formation of container sweat in the event of a drop in air temperature due to sea route: voyage into temperate latitudes
One source of sweat is the proportion of the product which is constituted by water. goods of vegetable origin, such as coffee, rice, cereal products, lumber products together with citrus and other fresh fruit, release water vapor into the container air while in transit. On the basis of their water content and biotic activity, they establish an equilibrium moisture content. If, during the voyage, the container wall is cooled by the external air to below the dew point temperature of the internal air, sweat starts to form on the walls. The intensity of sweat formation depends, on the one hand, on the position of the moisture equilibrium, i.e. the air temperature/dew point difference in the container and, on the other, on the daily drop in air temperature, with nocturnal radiation from the container walls also being of particular significance. This process begins as soon as the ship leaves the subtropics in winter and may become particularly intense in frosty weather as the goods still store considerable quantities of heat which maintain thermal circulation, by means of which water vapor is constantly transported from the goods to the container ceiling.
Fig. 27 shows container sweat at low external air temperatures. In freezing weather, condensation may also be accompanied by immediate formation of ice or sublimation (transition from the solid to gaseous state without passing through the liquid state), so still further promoting the build-up of water on the ceiling because, when the temperature rises, instead of evaporating, the water drips down and causes wetting damage on the surface of the cargo.
TA - temperature, external
TW - water temperature
TL - cargo temperature
φA - rel. humidity, external
TR - hold temperature
A - dew point, external
R - dew point, hold
TS - temperature of side wall

Figure 27: Formation of container sweat in the event of a drop in air temperature due to sea route: voyage into temperate latitudes;
U. Scharnow [46]

Excessively rapid cooling of the outer layers of the stack may also result in wetting damage. In this case, condensation occurs within the stack on the cold, outer layer of the cargo stack. The effects of this are particularly disadvantageous for the goods. In bagged cargo, such damage is observed directly beneath the upper layer of bags, which appear to be dry from above. The damage is revealed when the first bags are lifted.

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