Sweat is formed when the water vapour in the air condenses out into water droplets when cooled below its dew point.
Sweat may be formed in two ways:
This forms when the cargo is at a lower temperature than the external air and where the air is moist and is admitted to the cargo holds. If such warmer, moist air is allowed to come into contact with the cargo the air is cooled and water droplets from the air are deposited on the surface of the cargo. Cargo sweat will form when the temperature of the cargo is less than the dew point of the air which surrounds it.
Cargo loaded in a cold climate and then transported to warmer regions will rise in temperature slowly in response to increasing sea and air temperatures. For as long as the temperature of the cargo remains below the dew point of the external air the cargo hold ventilator flaps should be kept closed and no ventilation take place. The cargo hold should be made as air tight as possible.
If the cargo hold hatches are opened to load further cargo whilst the existing cargo temperature is below the dew point of the external air then such air will enter the hold leading to the formation of cargo sweat. It is thus important to plan Cargo Stowage with this in mind.
Additionally certain cargoes themselves contain moisture (timber, tobacco, jute etc.) And will result in an increase of moisture within the hold increasing the chances of cargo sweat to form on existing cold cargo. Again in order to prevent such an occurrence Cargo Stowage plans should try to prevent the carriage of moisture laden cargoes with others sensitive to cargo sweat
A voyage from a warmer to cooler climate with cargo loaded warm and meeting cooler air should not cause the formation of cargo sweat. However in such circumstances there is the possibility of ship’s sweat. Ships sweat can occur when warm moist air comes into contact with the cooler steelwork. Ships sweat can form below the hold deck heads, on the undersides of hatch covers and on the side plating of cargo holds. This sweat may drip onto or come into contact with the cargo. Thus ships sweat can also cause damage to sensitive cargoes. This can be avoided by introducing cooler, drier air into the holds. Ships swear can form on the ships side when meeting a cold current even when the air temperature is high. Inspections of holds should be conducted to check for signs of this. Hold ventilation should be open at all times except when the external air has a high humidity and the dew point of this external air is higher than that of the air in the hold or adverse weather is causing spray in the vicinity of the ventilator intakes.