By: Pakarada Premtitikul
InterDry (Thailand) Co., Ltd.
Formation of condensation in container due to temperature changes by sunlight
Hygroscopic goods collected in an airtight container will lead to condensation in sunny weather.
During the day the container walls and ceiling heat up. In the evening they cool off leads to accumulation of condensation in the container.
During heating of the container walls and the internal air, the relative humidity drops. The container floor and the cargo will now dissipate condensation by which the absolute humidity in the air rises.
The water from the cargo comes from the upper layers of the load. This reduces the vapour pressure of the liquid inside the cargo which then migrates from the underlying layers of the cargo to the outer layers.
If the container warming has reached its peak, the temperature will drop with the result that the relative humidity increases. Part of the moisture will condense on the cooler container walls when their temperature gets below the dew point of the internal air. During the heating that occurs over the following days, the condensation of the vertical walls evaporate into the air so that the cargo needs to evaporate less condensation to achieve balance.
The condensation on the ceiling does not get into thermal contact with the circulating air, but will diffuse to lower drier air. This process is slower than the evaporation of condensation on the vertical walls by dry air. The condensation on the ceiling does not dry despite the heat. So day after day there is more and more because of moisture condensation from the cargo goods.
When the limit is reached, the condensation begins to drip. The right drawing in Figure 1 shows that the majority of the condensation is formed on the ceiling . This is important because condensation on the vertical walls do not directly cause damage but condensation from the ceiling drips into the goods.
The figure below shows the forming of condensation inside a container that was heated during the day. The moment the temperature of the container wall goes below the dew point of air inside the container, condensation forms in the container.
Figure 2: The daily temperature variations in a container: The temperature, relative humidity and the dew point of air, the temperature of the cargo and the container walls.