- This is the name given to condensation which occurs in a ship’s cargo spaces on the cargo (cargo sweat) or the steelwork (ship’s sweat).For sweat to occur, there must be moisture in the hold atmosphere and a difference of temperature between the air in the hold and the cargo or the ship’s steelwork
- The temperature difference usually occurs as the ship moves from a cold climate to a warm climate or vice versa or from a cold current to a warm current or vice versa
- The larger the change in temperature the more likely is the formation of sweat.
- This is the condensation which forms on the surface of the cold cargo when warm moist air comes in contact with it.
- It will form when the dew point of the air in the hold is higher than the temperature of the cargo and this is most likely to occur when the ship has loaded cargo in a cold region and air is admitted to the hold as the ship is traveling to a warmer region.
- In these conditions all ventilation should be stopped and the hold kept closed with the air unchanged as far as possible.
- The temperature of the cargo will rise very slowly to equal the external temperature, and so long as the hold remains closed, the air within it will gradually become warmer and able to hold more Moisture.
- With an inert cargo the dew point of the air in the non‐ventilated hold remains constant as it warms up.
- If the cargo contains moisture, it will give off moisture as it warms up.
- This is the condensation which occurs when warm moist air in the hold comes into contact with the cold steel work
- It forms when the dew point of the air in the hold is higher than the temperature of the steelwork and this is most likely when the ship has loaded in a warm region and is steaming towards cold climatesShip’s sweat caused by a low external temperature is most likely to be deposited first in the vicinity of the hatch coaming and the fore and aft ends of the holds amidships as the topside tanks provide an insulating layer which delays the penetration of cold from the external air to the plating forming the tank/hold separation. When a cold current is met, causing a low sea temperature, the side shell plating will be cooled, providing favourable conditions for the formation of sweat.
- When passing from a warm region to a cold region full ventilation should be continued whenever possible in order to withdraw moist air from the hold and replace it by drier external air
Sources of moisture in cargo spaces include:
- The cargo itself which may possess some natural moisture and create an atmosphere known as storage atmosphere in any compartment in which they are stored
- Rainfall during loading
If conditions were moist when the hold was closed on completion of loading.
- The amount of moisture in the air is measured by its dew point, which is the lowest temperature to which a mass of air can be reduced without condensation occurring.
- As ‘condensation’ is a bad thing, air with a high dew point is a bad thing
- Dew point is obtained from a table, entered with readings taken from a dry and wet bulb hygrometer.