When deciding whether a certain container is suitable for transporting a certain cargo, it is vital for those involved to have sufficient knowledge about the anticipated climatic conditions in the container. The three major factors which have a decisive impact upon the cryptoclimate (microclimate) in the container are:
External climatic conditions
External climatic conditions have a decisive impact upon the climatic conditions inside containers. External climatic conditions are in particular determined by the transport route, season and time of day and the current weather (rain, sunlight etc.). Due to the diversity of these factors, it is not straightforward to predict how the container climate will change in transit.. It is not possible simply to transfer the experience gained from one transport operation to another, as the conditions prevailing in transit often vary greatly, but an awareness of how the factors interact is helpful in assessing transport risks. The factors influencing container climate are discussed in greater detail below:
1. Temperature conditions in the container
The temperatures encountered in containers are primarily determined by heat exchange across the container walls. Good heat-transfer properties, especially through the steel walls, and the relatively large ratio of container surface area to container volume have a favourable impact in this respect.
In addition to solar radiation, external air temperatures, wind and precipitation also have an impact upon temperatures. Due to the wide variation in levels of solar radiation over a day, considerable temperature variation also occurs inside the container. This particularly applies to the air layers located directly beneath the container roof, as this is where the effects of solar radiation are at their strongest and thus where the greatest heat exchange occurs. On exposure to precipitation, such as rain, the container roof likewise cools more rapidly than, for example, the side walls, and the underside of the roof thus cools down most readily. Overheating of the air inside the container, i.e. heating to above the external air temperature, may be considerable even under normal weather conditions. In contrast, the variations in temperature of the cargo inside the container are less marked.
2. Humidity conditions in the container
Humidity conditions in the container are primarily determined by internal factors, i.e. the prevailing conditions are largely determined by the hygroscopic characteristics of the cargo and its packaging. Hygroscopic auxiliary packaging materials, such as squared lumber for cargo securing, and the water content of the flooring may also play a significant part. Incoming outside air usually has no negative impact upon humidity. Since the temperature prevailing inside the container is generally higher than the outside temperature, incoming air would also reduce relative humidity.
Seawater or rain may penetrate damaged containers. This constitutes a considerable potential risk. If the container is packed in wet weather (snow, rain), additional moisture may get into the container.
Two typical indices for characterizing the humidity of air are absolute and relative humidity. Absolute humidity (f) is the quantity of water present in a specified volume of air. Absolute humidity is upwardly limited by the maximum (saturation) humidity (fmax) of the air, a value which varies with air temperature. The hotter is the air, the more moisture it may contain. Absolute humidity is stated in g/cm3. Relative humidity (U) is calculated from the ratio of absolute humidity to maximum (saturation) humidity. On this basis, it may be concluded that, at constant absolute humidity, relative humidity falls as air temperature rises. Relative humidity is stated in %. It is calculated as follows: U [%] = (f / fmax) * 100
3. Dew point temperature
Depending upon air temperature and relative humidity, any mass of air has a certain dew point temperature (td). This dew point temperature is the limit value for the formation of condensation. If air is cooled to below its dew point (e.g. by cold container walls or other surfaces), condensation forms. No condensation is formed above the dew point. As a general rule, there is always a risk of condensation whenever cold surfaces come into contact with excessively warm and moist masses of air.
4. Temperature/dew point difference The temperature/dew point difference (t – td) states the difference between the actual air temperature of a mass of air and its dew point. This difference indicates the severity of the risk of condensation; the smaller is the difference, the greater is the risk of condensation.
The cryptoclimatic conditions in the container are also determined by the cargo with which the container is packed, with both the mass of the cargo and the surface area of the cargo readily accessible to the container air being of significance. Hygroscopic cargoes largely determine the water vapour balance in the container due to their sorption behaviour (hygroscopicity) by adjusting the relative humidity of the residual air in the container to the cargo’s particular equilibrium moisture content. The largest sources of condensation in closed containers are always the cargo, its packaging, wooden flooring and any hygroscopic auxiliary packaging materials. Condensation is thus actually possible only if water enters the container via one of these sources. Rising air temperatures in the container drive water vapour out of the hygroscopic cargo. At constant absolute humidity, a rise in air temperature in the container results in a drop in relative humidity. However, since the cargo endeavours to establish the equilibrium moisture content, it releases water vapour into the container air. This water vapour may then condense, for example, on the cold container walls and ceilings (due to overnight cooling). Condensation is most severe on the container ceilings, such that, despite being strongly heated by the sun, the condensation does not dry out during the day (as often occurs on container walls), so resulting in a continuous increase in the quantity condensation. As a result, the water drips from the ceiling down onto the cargo. As already stated, the water content of the cargo plays a decisive part.
The following correlations may be established:
- The temperature/dew point difference is dependent upon cargo water content.
- The lower is the cargo water content, the greater is the temperature/dew point difference and the lower is the risk of condensation.
- The higher is the cargo water content, the lower is the temperature/dew point difference and the greater is the risk of condensation.
It is thus important for cargoes to be loaded as dry as possible in order to minimize the risk of condensation.
The type of container
Changes in climate within containers are determined not only by external climatic conditions and the cargo, but also by the type of container. The above explanations largely relate to closed standard containers. Other conditions prevail in open containers, on flatracks or in refrigerated containers. Standard containers should not be considered to be absolutely water vapour tight. The unavoidable wear and tear on the containers in service, especially in the door area, time and time again results in leaks. Every leak is a source of condensation.
Passively ventilated containers (“coffee containers”) are primarily used to transport cargoes from the hot tropics to European latitudes. Replacement of the warm, very humid air in the container cools the cargo and dissipates the moisture it releases. Since the temperature of the cargo is higher than the temperature of the air surrounding the container, the necessary circulation of heat is maintained. In open containers, the microclimate largely adapts to the external climatic conditions; these containers thus provide less protection to the cargo, but also prevent a cryptoclimate unsuitable for storage from developing. The open sides or the roof may be closed with tarpaulins, so immediately forming a cryptoclimate, similar to that described for standard containers, but with greater ventilation due to the openings always present with tarpaulin covers.