Do you know about water droplets inside shipping containers?

1. Principles of Water Droplet Generation in Shipping Containers

The amount of water vapor that air can hold is limited. At a given temperature, the maximum amount of water vapor the air can contain is the saturated water vapor content for that temperature. Saturated water vapor content changes with temperature; the higher the temperature, the greater the amount of water vapor the air can hold. If the water vapor in the air exceeds the saturated water vapor content, the excess portion will condense into water droplets. Additionally, if air containing a certain amount of water vapor is cooled, it can also reach a saturation state. If the air temperature continues to drop further, the excess water vapor beyond the saturated content will also condense into droplets. The temperature at the saturation state is generally referred to as the dew point temperature for that condition. The amount of water vapor in the air is expressed as humidity. The mass of water vapor contained in each 1m³ of air is the absolute humidity of the air, expressed in g/m³. Knowing only the absolute humidity is insufficient, as it does not indicate whether the water vapor in the humid air is saturated. Therefore, relative humidity is used to indicate the degree of dryness or wetness of the air. Relative humidity refers to the ratio of the actual amount of water vapor contained in a certain volume of air at a specific temperature to the maximum amount of water vapor it can contain at that same temperature (i.e., in a saturated state). It indicates how close the air is to saturation, usually expressed as a percentage. This phenomenon, where excess water vapor in the air condenses into droplets due to changes in the temperature inside the container and the dew point temperature inside the container, is commonly referred to as "condensation."

2. Root Causes of Water Droplet Generation in Shipping Containers

Certain cargo loaded in the container, especially the packaging materials and cushioning materials used inside, generally contain a certain amount of moisture. Under the influence of temperature changes within the container and changes in water vapor pressure, this moisture will migrate to achieve equilibrium in the water vapor content of the air inside the container. Tests on empty containers show that even if the temperature inside the container fluctuates significantly, the humidity does not change drastically. This is because there is no large amount of hygroscopic material inside the container. Conversely, if dry cushioning and packaging materials are used inside the container, they can play a role in regulating the moisture levels within the box. The cushioning and packaging materials typically used in containers are made of wood. This wood, including the interior lining panels and the floor, has a moisture content of at least several percent. Regarding the specific properties of wood, for example, a living tree, once dried, even if allowed to absorb water again, will not return to its original high moisture content. Moreover, reabsorbed moisture is released quickly, rapidly returning the wood to a dry state closer to its treated condition. The wood used as interior lining panels for containers is treated using artificial drying methods and typically has a moisture content below 15%. In actual use, tests have proven that the moisture content of these interior panels ranges from about 9% to 15% and is trending toward even lower levels. However, the packaging and cushioning materials used inside the container, even if well-dried, generally have a moisture content of 20% to 30%. Therefore, if the temperature inside the container rises, these materials release significant moisture. This process raises the dew point of the air inside the container. When the dew point temperature is higher than the surface temperature of the objects inside the container, water droplets form on those surfaces.

3. Types of Water Droplets in Shipping Containers

Water droplets inside sea shipping containers are of two types: droplets on the inner walls of the container (including the roof, side walls, and front wall) and droplets on the surface of the cargo inside the container.

1. Water Droplets on the Inner Walls of Shipping Containers

The air temperature inside sea shipping containers rises around noon due to solar radiation, causing the relative humidity inside the container to decrease. Additionally, the rise in interior temperature prompts the cargo and packaging materials inside to release moisture, thereby increasing the water vapor content inside the container and raising its dew point. At night, due to drastic changes in external weather or sea conditions, the outer walls of the container rapidly cool. If the temperature of the walls and roof drops below the dew point temperature of the air inside the container, the water vapor in the interior air will condense into droplets on the inner surfaces of the walls and roof. Many general cargo containers do not have a lining installed on the inside of the roof, leaving the metal roof panel exposed to the interior. This further exacerbates the tendency for condensation on the ceiling. Consequently, when cargo with a relatively high moisture content, such as grain, is loaded in hot and humid regions and shipped to cold regions, the phenomenon of condensation becomes even more pronounced. Special attention must be paid to this aspect.

2. Water Droplets on the Surface of Cargo

If cooled cargo such as canned goods, machinery, steel materials, known as metal products, is loaded in cold regions and shipped to warm regions, the external temperature changes affect both the cargo and the air temperature inside the container. The temperature of the cargo changes more slowly than the temperature of the air inside the container. Accordingly, the surface temperature of the cargo is generally lower than the interior air temperature. On the other hand, as the air temperature inside the container rises, the relative humidity decreases. This process triggers moisture evaporation from materials such as wood packaging and cushioning elements, raising the dew point of the interior air. Consequently, the water vapor in the container air quickly reaches saturation. Then, if the interior air temperature increases further above the cargo's surface temperature, water droplets form on the cargo surfaces. Therefore, regardless of whether the droplets form on the inner container walls or on the cargo surfaces, they result from changes in external air temperature, the movement of heat and moisture in the interior air, and air exchange between the interior and exterior of the container.

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