Sweat is formed when water vapor in the air 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. In the former, it is known as ‘ships sweat’ and this may run or subsequently drip onto the cargo. When the water droplets form on cargo this is known as ‘cargo sweat’ and will occur when the temperature of the cargo is cold and the incoming air is warm.
To avoid sweat and its damaging effects it is imperative that ‘wet and dry’ bulb temperatures of the air entering and the air contained within the cargo compartment, are taken at frequent intervals. If the temperatures of the external air is less than the dew point of the air already inside the space, sweating could well occur. Such conditions give rise to ‘ships sweat’ and is commonly found on voyages from warm climates towards colder destinations. Similarly, if the temperature of the air in the cargo compartment (or the cargo) is lower than the dew point of incoming air, sweating could again occur, giving rise to ‘cargo sweat’. This would be expected on voyages from cold places towards destinations in warmer climates.
If cargo sweat is being experienced or likely to occur, ventilation from the outside air should be stopped until more favourable conditions are obtained. However, it should be noted that indiscriminate ventilation often does more harm than no ventilation whatsoever. It is also of concern that variation in the angles of ventilators away from the wind can cause very different rates of air flow within the compartment. The angle at which the ship’s course makes with the wind also affects the general flow of air to cargo compartments. In general, the greatest air flow occurs when the lee ventilators are trimmed on the wind and the weather ventilators are trimmed away from the wind. This is known as through ventilation (Figure 39).
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