We know that climate conditions in a sea container have a major impact on cargo during the transport of goods. Moisture problems, in particular, can lead to damage and therefore rejected loads. It is therefore important that those involved have sufficient knowledge of the risks when it comes to temperature and moisture. The decision to transport certain goods by standard sea container is determined by three important factors:
- External conditions (temperature, humidity, sun, rain, wind)
- Load (moisture content, temperature)
- Type of container (open/closed, ventilation/no ventilation, insulated/not insulated)
External climate conditions affect the climate conditions in a container. They are mainly determined by the transport route, the season, the time of the day and the weather. It is challenging to predict to what extent these factors will affect the climate in the container and the effect on the load. No situation is the same, but some awareness of the relationship between these factors is useful in assessing transport risks in advance.
For example, cocoa beans and spices are transported by sea container from warm Africa to relatively cold Europe. The transport goes through different climate zones, which means that there is a high risk of mould formation due to moisture. Insects are also often taken away with the freight. To prevent the risk of moisture and insects, it is crucial to protect the container with, for example, container insulation.
How is the temperature in the container determined?
How hot or cold it gets in the container is mainly determined by the heat exchange over the container walls. External conditions ensure that specific temperatures ‘penetrate’ into the container and are led through the walls. The air temperature, precipitation, snow and sun all influence the temperature inside the container. Because these factors are not stable, such as temperature fluctuations during the day and night, temperature changes will also occur in the container. Another external factor is the location of the container on the ship. When a container is placed below deck, there will be less risk of temperature fluctuations than a container positioned above deck. There is also a difference in the colour of a container. A brown-painted container attracts more heat than a white-painted container.
How is the humidity in a container determined?
The humidity in a container is determined by, among others, things, the hygroscopic properties of the cargo (goods, pallets, packaging material) and, for example, the moisture content in a container floor. Besides, containers can be damaged, as a result of which moisture in the form of rain or seawater can penetrate the container. What we often note is that the products that are transported are placed on pallets that were stored outside. Wood absorbs moisture, and this does not disappear once the pallets are loaded in the container. The moisture content in the container floor is also often high. This is because containers are cleaned before use. As soon as the temperature in the container reaches a certain point, this unvisible moisture is attracted to the air and condensation forms with all its consequences.
How does moisture develop due to temperature?
Depending on the temperature and relative humidity, each air mass has a specific dew point temperature. This dew point temperature is the limit value for the formation of condensation. When air is cooled to below the dew point (for example through cold container walls or other surfaces), condensation forms. No condensation is formed above the dew point. In general, there is always a risk of condensation when cold surfaces come in contact with too warm and humid air masses.
A simple way to illustrate the dew point is a can of soda. When it is removed from the refrigerator, condensation forms on the surface after a while. The temperature in the space outside the refrigerator is higher than in the refrigerator. The air then settles (at the dew point) on the surface of the bottle, creating the well-known drops on the can.
The container load
The load also determines the climate conditions in a container. The mass of the load, just like the surface, is essential here. Loads that can absorb a lot of moisture (hygroscopic) determine the water vapour balance in the container. The leading causes of condensation in closed containers are always the cargo, packaging, wooden floors and pallets. Water vapour can therefore also arise from the load. Because the temperature in the container rises, moisture is extracted from pallets that were stored outside, for example, or from the container floor, which contains moisture because it has been cleaned. The moisture traps on the container walls and ceiling, creating container rain.
For example, one of our customers, a logistics service provider in mainly personal care products, provided transport of soap blocks from Rotterdam to China. Soap has a natural tendency to evaporate (sweating), which means that moisture is released from the product itself. If this happens during transport, the chance of damage is very high. In this case, the application of container insulation, supplemented with additional desiccants, proved to be the solution.
It is good to take the following into account, especially when transporting a hygroscopic load:
- The temperature/dew point difference depends on the moisture content of the load.
- The lower the moisture content of the load, the higher the temperature/dew point difference and the lower the risk of condensation.
- The higher the moisture content of the load, the smaller the temperature/dew point difference and the higher the risk of condensation.
It is, therefore, essential that goods are loaded as dry as possible to limit the risk of condensation. However, it sometimes happens that loads must be moist, such as fruits and vegetables. It is desirable that the load itself is moist, but that no condensation may form on the products
The climate conditions in a container are also partly determined by the kind of container. The explanation so far mainly relates to standard closed containers. Other circumstances apply in open containers, on flat racks or in refrigerated containers.
Standard containers always contain moisture. Containers in use wear out, especially on the side of the doors, creating holes that allow moisture from the outside.
Reefer containers are sometimes used unconnected as regular containers. Temperature fluctuations will be less significant due to the thick walls, but there are risks involved. Certainly when it comes to longer transports. Heat or cold may be retained more than necessary, condensation will occur faster when the temperature in the container drops, and there is also much less space available in a reefer container. Besides, the handling costs are higher. It is therefore certainly worth considering whether the application of container insulation offers a solution.
In open-top containers, the climate mainly adapts to external conditions; these containers, therefore, offer less protection to the cargo, but also prevent a climate that is not suitable for storage. The climate in the container is comparable to the standard containers, but with more exceptional ventilation through the openings.
Prevent cargo damage
Insight into the temperature conditions in a container during transport is essential, but it is always difficult to predict what the conditions are going to be because it concerns a mix of various factors. It is nevertheless useful to have some knowledge of external conditions such as climate zones and weather forecasts, the impact of a hygroscopic cargo, and what type of container is used with or without protection. Above all, it is clear that moisture must be prevented in the container in order not to have cargo damage. At this moment it is essential to avoid large temperature fluctuations and not to allow the temperature to come close to the dew point. Hopefully this article has brought you more insight, but if there are any questions please contact us.