It is a very simple topic and as common as everyday event like WHY Water bottle from fridge gets water on the outer surface.
A large number of cargo claims are made for goods transported by ships which have been damaged in transit. Much of this damage is caused by either ‘ships sweat’ or ‘cargo sweat’ and could be effectively reduced by prudent ventilation of cargo spaces.
Cargo or goods carried at sea are exposed to the atmosphere which is high in moisture content. Due to the very nature of the trade, cargo damage due to moisture continues to be a cause for concern to all parties in the trade. Very heavy cargo claims due to damage to cargo caused by moisture continue to be made.
Between 1996 and the year 2000 claims accounted for in excess of 2 million US dollars in only one P&I club.
Cargo ventilation, as statistics indicate, appears to be the most neglected or least understood aspects of cargo care & cargo operations. It is also one such activity that does not attract the attention it deserves and more often than not it is too late by the time the consequences are realised.
Why is sweat caused onboard?
Sweat is caused by the condensation of saturated air on a cooler surface. It is formed when water vapor in the air within the hold condenses out into water droplets once the air is cooled below its dew point. The water droplets may be deposited onto the ship’s structure or onto the cargo.
Some definitions are useful in understanding sweat.
Is defined as the temperature at which air cannot absorb any more water vapour; i.e. it is saturated. In simple language if the air parcel falls below its dew point temperature, then the moisture within the air parcel will condense.
Means that a parcel of air has absorbed its maximum amount of moisture for that temperature, however raising the temperature will allow for more moisture to be absorbed.
Is the ratio between the actual moisture content and the maximum moisture content that the parcel of air can contain.
Is cargo, which can absorb or give off moisture. They are mainly of plant origin. Since they may retain or absorb water, excessive amounts of inherent moisture may lead to significant self-heating and moisture migration within the cargo. This will result in cargo getting caked or rotting.
Examples of Hygroscopic cargo: Rice, flour, grain, coffee, tea.
Is cargo, which does not absorb or give off moisture. Cargoes like steel are non-hygroscopic but are liable to damage if the atmosphere is moist.
This results when water droplets are deposited on parts of the cargo when the surrounding air comes in contact with a cool cargo. For this to happen, the temperature of the cargo must be lower than the dew point of the surrounding air.The condensation which forms directly on the cargo, when the temperature of the cargo is less than the dew point of the air in the hold.This is generally associated with incorrect ventilation. That is introducing warm moist air into the hold with cold cargo.It usually occurs when the voyage is from a cold region to a warmer place and the outside air has a dew point above the temperature of the cargo.Cold cargo cools the air in contact with it and condensation takes place.
Is the condensation which forms directly on the vessels structure when the temperature of the ship’s steel work is lower than the dew point of the air inside the hold. Condensation so formed on the structure will then trickle down to the cargo. This can occur when the ship moves to cooler climates (from a relatively warm area) and the ship’s sweat so formed can trickle onto the cargo or even accumulate at the bottom of the hold.