It might seem simple to pack cargo into an ocean container for transport around the world. However, care must be taken or the cargo may not arrive safely and unharmed at its destination.
The problems come with the nature of the container itself – metal manufacture, sealed, subject to massive temperature in transit and delays caused by congestion and storage on route.
After the container is loaded and its doors are shut and locked, subtle changes in the internal atmosphere begin. The degree of damage they cause can often be determined by the length of the voyage and the conditions it travels in.
Saturated Air
One of the first issues is saturated air. For any given atmospheric pressure, air holds more water vapour at higher than lower temperatures. The maximum amount of water vapour contained in a cubic metre of saturated air decreases with a decrease in temperature: at 40°C it can contain 51 grams, at 20°C 17 grams and at 0°C only 5 grams of water.
What is the Dew Point
The dew point of air is the temperature at which a sample of saturated air will condense. Warm air has more capacity to support water vapour within it than cold air. Condensation occurs when moisture-laden air releases its water vapour on surrounding surfaces in the form of water droplets. For condensation to occur the three conditions need to be present: a difference in temperature between the inside and outside of the container, moisture and a pathway.
So-called Container Sweat happens when the skin of the container is cooled to a temperature below that of the dew point of the air enclosed within the container, resulting in the formation of water droplets on the interior roof and side panels. This water then drips down on the cargo causing mould and water damage.
Cargoes that spontaneously heat from within can increase the problem.
For example, a cargo loaded in the tropical belt in warm conditions with high relative humidity, is transported to cold winter conditions in Europe. The outside temperature gradually cools down in transit until it experiences cold conditions in Europe. The steel container allows the chill to conduct from the outside of the panel through to the inside.
This means the situation inside the container is now one of cool side panels and warm moist air in the header space above the cargo and within the stowing. The temperature of the side panels is therefore below the dew point of the air inside the container and condensation occurs. Condensation will continue until the dew point of the interior air falls to that of the outside air.
Cargo Sweat
One solution, which is quite simple, is to ventilate the container by passing air through it and replacing the warm moist air with similar air to outside the container. This prevents the formation of cargo sweat when the surface of the cargo is cooler than the dew point of the air enclosed within the container.
Droplets of water then form on the surface of the cargo. An example is where a cargo of tinned goods is loaded in cold winter conditions in Europe and transported to the tropical belt. The container will gradually heat up during transit to the warmer moist climate however the cargo temperature will lag behind, slowly heating up and replacing the cold from loading.
If ventilation was allowed to take place the warm moist air from outside the container would condense on the cold cargo. In this case it is better to avoid ventilation during transit and allow the cargo temperature to gradually increase thereby restoring equilibrium between the cargo temperature and the outside air.
Road And Rail Transit
Problems can often occur during transit when a container lingers in storage. For instance, a non-insulated container with a load of bagged coffee is subjected to the radiant heat of the sun in the tropics. The air inside the container will become heated and absorb moisture from the coffee thus establishing high humidity conditions. Night cooling can cause the temperature of the container skin to fall below the dew point of the humid conditions within the container thus causing condensation.
The need to avoid radiant heat is emphasised in this example and late packing of coffee prior to export would also help. In addition long periods exposed at trans-shipment terminals such as Djibouti, in the summer, should be avoided.
At the other end of the journey it is important that early delivery is carried out for sensitive/hygroscopic cargoes arriving at terminals with near-zero temperatures. In such cases the cargo can experience the so-called cold shock of first night ashore. When acceptable to the shipper it is often prudent to crack a door open to ventilate the container.
Thanks to Compania Chilena de Navegacion Interoceanica for background information.
James Graham - I have been a professional journalist since the 1980s. I started on weekly newspapers in the English Midlands - working intially on one of ...
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