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Ventilation

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Ventilation. Cargo damage due to climatic conditions includes such effects as mould formation, germination of grain, corrosion and rust on metals, and wetting of sensitive materials such as leather. The general cause of such damage is condensation from various sources.

Ventilation of cargo spaces cools the cargo (or warms it if necessary) so that temperature differences between the cargo and the atmosphere are kept to a minimum. Ventilation also prevents the accumulation of moisture in the cargo space, whether this be a cargo hold or a container, thus reducing condensation within the space. The purposes of ventilation also includes dissipation of gases which can be flammable and/or noxious, removal of heat, and removal of taint which can damage odour-sensitive cargo. Considerable harm can be done to some cargoes by ventilating too much and to others by not ventilating at all. Some cargoes, which contain moisture, may give off this moisture on the sea passage. These are called “hygroscopic” cargoes. Moreover, if cargoes are loaded in a-cool climate and then the vessel travels to a warmer climate any incoming air may condense on the cargo. Cargo may then give off moisture. This is known as “cargo sweat”. If the ship loads in a warmer climate -and proceeds to a cooler climate, the cold sea temperature will cause the air in the hold to condense on to the ship’s structure and can also damage cargo. This is known as “hold sweat” or “ship’s sweat”.

For example, if canned goods are loaded in New York in winter for transit through the Panama Canal, the cans will not be much warmer on approaching the Canal than when they were loaded. However, the outside temperature will have risen, as will the “dew point”. (The dew point of the air is the temperature of a glass or metal object just cold enough to cause dew to appear on it when exposed to that air.) Ventilation will cause the outside air to enter the cargo space and deposit moisture on to the colder cans. This would be cargo sweat.

On the other hand, if bags of rice are loaded in Burma in December for discharge in Japan, as the vessel proceeds to the destination the vessel’s steelwork becomes cooler but the cargo retains much of its high temperature in the cargo spaces. Rice is a hygroscopic cargo. The warmth inside the mass of rice will cause an upward current of air from the cargo to condense underneath the cold decks or container tops and this moisture can be deposited on the cargo.

 

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