Moisture processes in the container

A container is a closed system with it’s own ”weather” inside. It differs from the warehouse in that the variation in temperature is much greater. It is not unusual to see containers wherein temperatures range from freezing to 60-70C during the course of a single voyage.

The central fact is that warm air can hold more moisture than cold air. That means that if warm air is cooled, it becomes more humid. And if it is cooled enough, some of the moisture must rain out – condense. That is exactly the same phenomenon that causes dew in the grass or fog on a cool autumn.

In a container a fast temperature change of 5-10C is often enough to cause problems. Water will condense on the coolest available surface, which is often the container ceiling or walls. From there it may drip down onto the cargo and cause damage – “container rain”. At other times it condenses on the cargo, – “cargo sweat”-, which is usually even more damaging

Even without any condensation, elevated humidity over a period of time is sufficient to cause damage. Many metals will corrode or discolor at a rather modest level of humidity , 60-70%. At higher levels of humidity, 80%-90%, moulds will grow, labels will peel and corrugated boxes will start to soften.

The Relative Humidity (RH) is a percentage measure of how much moisture the air holds as compared to the maximum mount of moisture air at that temperature can hold. That means that completely dry air has a RH of 0%. The RH can never be more than 100%, or any excess moisture will rain out. There is little danger of damage to anything if the RH is below 50% or so.

The Humidity Changes when the Temperature does

The important thing to realize is that the humidity of the air changes only as a result of the change in temperature. When air cools it becomes more humid, – even though the moisture content in the air remains the same.

The Humidity in a container will go up and down throughout the voyage, as a result of changing temperature only. If the temperature changes rapidly enough there is sure to be moisture trouble, even if the container may be fairly dry by reasonable standards.

In a container, moisture evaporates into the air during periods when the container is warm. The warm dry air can accept a lot of moisture. Or warm moisture containing air enters from the outside through “Container Breathing”. When the container cools down, that air becomes very humid. And it is then the troubles start.

But the temperature doesn’t have to vary in time to create a difference. It is equally bad when different parts of a container are at different temperatures. When warm air moves into a colder part it becomes humid and perhaps even condenses moisture. Tons of moisture can be redistributed within a container during a voyage through such processes. Mysterious patterns of damage may arise, such as mold only in certain parts of the cargo.

Temperature changes in a container may arise because one side of the container is exposed to the elements and another is not. Or it may arise simply as a result of a great thermal inertia in the cargo as outside temperatures change. It is common that it takes weeks for the temperature to equalize through a densely stuffed cargo.

It should be noted that all the basic processes outlined above are strongly nonlinear. A small difference in conditions may cause a grate difference in outcome. That is why the pattern of damage may seem unpredictable.

Where Does the Moisture in the Container Come From?
The moisture in the container:
* Is in the air when the container doors are closed
* Is contained in the cargo and packaging and is evaporated throughout the voyage
* Enters from the outside through so called container breathing.

The amount of air contained in the air at loading depends at the temperature and the humidity at loading. If loading at cool temperatures the amount is seldom significant, at most a few hundred grams. At loading in the tropics, however, the total amount of moisture could be a Kg or more.

Most cargo and packaging materials can both absorb and evaporate moisture. What happens depends on the temperature and how humid the surrounding air is. It is common that the cargo will evaporate during one part of the voyage and absorb during a different part.

No container is airtight. Moisture can move both into and out of the container as a result of temperature variations. Unfortunately, common circumstances will lead to a gradual build up of moisture within the container.

It could very well happen that you start with a very dry cargo, but at some later time the cargo has absorbed a lot of moisture which may be released in a very destructive way. If there is a temperature difference within the cargo, very substantial amounts of moisture may be re-distributed within the cargo. The moisture will always move from the warmer to the colder part.

Any absorbers put in the container are of course expected to be part of the solution and not the problem. Alas, that is not so. Unfortunately almost all kinds of absorbers, other than Absorpole and Absorbag, will re-evaporate moisture under some circumstances, usually in connection with a period of elevated temperature some time into the voyage.

Container Breathing
No container is airtight. If the seals are good and the vents are taped shut, air will move in and out more slowly, but any pressure differential between inside and outside will certainly be equalized in a matter of hours.

The air pressure outside a container will vary for metrological reasons over the course of a voyage. When the barometer falls, air and moisture will move out from the container, and when it rises the reverse will happen.

This effect becomes much more significant if the container is subject to repeated cycles of large temperature variations. When the container cools, the pressure inside it goes down. Air and moisture from the outside will move in until the pressure is equalized. When the container heats up, the reverse happens.

While moisture can move both in and out of the container, it is not a balanced process. Under very common circumstances, cycles of temperature variations will lead to a build-up of moisture within the container.

If the container contains absorbent packaging material, that build-up can be very significant indeed.

Moisture Exchange of Packages within the Container
A package is like a container in miniature. Even where it completely sealed, there could still be moisture damage inside as a result of temperature changes alone. In fact, most packages exchange a lot of moisture with the air inside the container. Almost all common plastics, except alu-foil, let moisture diffuse through to a significant degree as will coated or uncoated cardboard. The least mistake in sealing a plastic package will anyway leave it subject to “breathing” processes.

For a plastic wrapped package, including a pallet liberally shrink wrapped, the most important process of exchange is diffusion through the plastic. The diffusion rate is proportional to the surface area of a package. Thus it is important to note that a bigger package has a smaller surface area in relation to its volume, than does a small package. When you put many boxes into a pallet, or stuff many pallets closely together, you lessen the significance of moisture diffusion.

For a wooden crate, diffusion as such may be of less importance in the timeframe of a typical voyage, but the natural breathing of the wood may be a dominant mechanism. If not, the “breathing” will be the most important aspect. The breathing is proportional to the amount of free air inside the crate and it is exponentially dependent on the temperature outside at constant relative humidity.

It is worth noting that moisture will not only move into the packages, but also out of them if the container environment is sufficiently dry. In practice it is often found that it makes a more sense to install moisture protection in the container and leave the pallets open at top and bottom to breathe, than to attempt to seal out the moisture.

Climate in the container and climatic influencing factors

The significance of interfaces for the cryptoclimate in the container

An examination of published incidents of loss due to climatic factors involving container cargoes reveals that such incidents affect the entire range of products with no particular class of product being disproportionately represented. On the basis of the published examples, losses caused by sweating are clearly the most striking. Sweating includes both that which occurs on the cargo itself (cargo sweat) and that which drips down onto the cargo from the upper surfaces of the container (container sweat). All classes of goods are affected by this type of loss. For example, reported losses range from nonhygroscopic goods, such as steel and steel products, canned foods, to hygroscopic goods, such as cocoa, coffee, millet, dried fruit, sago, pepper, milk powder, furs, textiles and rattan furniture.

In addition to the preponderance of losses due to sweat, a second problem is particularly noticeable, namely the care which is required to adapt the goods, loaded under the climatic conditions of the place of departure, to the climatic conditions of the destination while in transit, without causing damage to the goods or making such damage inevitable due to inadequate adaptation. The theoretical basis on which these issues are addressed resides in "interfacial" physics, which take account of the differences in heat and water vapour transfer at interfaces. The most important basic requirement in this connection is to prevent condensation of the water vapour present in the air at an interface, whether on the container wall boundaries, on the surface of the cargo, in air layers in the vicinity of interfaces or within cargo blocks, if the temperature of the interface falls below the dew point temperature of the surrounding body of air. This requirement in turn makes it necessary to adapt the temperature of the cargo to the anticipated air temperature at the destination. Abrupt changes in temperature or humidity or both occur at these interfaces.

The following types of interface in container transport may be distinguished on the basis of their thermal and hygroscopic properties:

  1. Container parts as 1st order interfaces

    These include interfaces which exhibit good heat transfer, are impermeable to water vapour and on which relatively large variations in temperature occur on exposure:

    • container walls and ceilings
  2. Container parts as 2nd order interfaces

    These include interfaces which, in addition to exhibiting good heat transfer, are also permeable to water vapour or actively interact with the water vapour in the container:

    • wooden dunnage
    • dunnage
  3. Cargo surfaces as 1st order interfaces

    Hygroscopic goods which release heat and water vapour into the container air. These include:

    • actively respiring goods of vegetable origin
    • goods of vegetable or animal origin or chemical products which, as a result of ongoing biological or chemical processes, have a tendency to undergo self-heating and are capable of exchanging water vapour with the air
  4. Cargo surfaces as 2nd order interfaces

    nonhygroscopic goods with surfaces having good thermal conductivity and a relatively large heat capacity of the individual package or stack:

    • unpackaged metallic surfaces
    • hygroscopic and nonhygroscopic goods packaged in metallic containers or metal foils as the surface
  5. Cargo surfaces as 3rd order interfaces

    Surfaces of hygroscopic goods capable of heat transfer and permeable to water vapour which exchange heat and water vapour with the container air without actively generating heat or requiring this exchange in order to retain service properties:

    • salt and fertilizer
    • sugar
    • hygroscopic minerals, ores and rock
    • lumber, furniture
    • general cargo packaged in wooden cases

Storage temperatures in the container

If the correct decision as to the suitability of a container for transporting a product without causing damage is to be made, it is essential to have sufficient information about the anticipated climatic conditions in the container. Fig. 9 shows factors which have an influence on the cryptoclimate in the container.

Factors influencing container cryptoclimate

Figure 9: Factors influencing container cryptoclimate

The four decisive influencing factors are:

  • weather conditions during the voyage
  • the type of cargo with which the container is packed
  • the type of container
  • the container stowage space

Clarifying the complex thermodynamic processes occurring in containers, especially in containers exposed to radiation, was the objective of the hold meteorology study group at the Warnemünde-Wustrow University of Seafaring (Fig. 10), where cryptoclimate was investigated in two containers, both on a test rig and on board commercial vessels. The investigations were carried out on two standard containers, each of which was equipped with an air lock to prevent disturbing the cryptoclimate when monitoring and making measurements and a weather station. The containers were packed with hygroscopic goods, in particular sawdust in one case and packets of sugar wrapped in paper in the other.

Climatic conditions during the voyage are determined by the route, season and current weather events. Consequently, it is not entirely straightforwardly possible to transfer the experience gained from one voyage or one route to another as the stresses vary between the different routes and individual voyages. Solar radiation, air temperature and wind are of significance to thermal stress.

The temperatures encountered in containers are primarily determined by heat exchange across the steel boundary surfaces, with inward and outward radiant transfers predominating.

Hold meteorology study group of Warnemünde-Wustrow University of Seafaring

Figure 10: Hold meteorology study group of Warnemünde-Wustrow University of Seafaring, 1970:
container with air lock and weather station; Svenson

Good heat-transfer properties, especially through the metal walls, and the relatively large ratio of container surface area to container volume have a favourable impact in this respect (20′ container, approx. 1.80 m²/m³).

Influence of solar radiation on daily variation in container temperature – radiation classes

The average air temperature in the container and also the temperature of the cargo surface are, on a daily average, higher than that of the external air.

The daily variation in the individual temperatures is of great significance to maintaining quality.

In addition to radiant conditions, external air temperatures, wind and precipitation also have an impact upon temperatures. The great daily variation in overall radiation results in a marked variation in temperature within the container. This variation primarily affects the temperatures of the container air and in particular of the bodies of air in those areas exposed to radiation (e.g. under the container ceiling).

Overheating of the air inside the container, i.e. heating to above the external air temperature, may be considerable even under normal conditions.

For example, daily overheating on sunny summer days amounts on average to 20°C even in temperate latitudes and is still higher in the subtropics. This means that temperatures of > 50°C, to which the surfaces of the cargo are exposed, may occur in the upper part of the container.

Four radiation classes were defined to describe radiation conditions, the classes being calculated on the basis of the measured duration of sunshine and solar altitude for 10 day measurement periods. The classes may be described in words as follows:

Class A:

Little or no effect of solar radiation. Average maximum overheating is 2.0°C (less than three hours of sunshine per day with low solar altitude, no radiant input on several days of the 10 day measurement period).

Class B:

Weak effect of solar radiation. Average maximum overheating is 5.2°C (four to eight hours of sunshine per day, but without sunshine on each individual day of the measurement decade).

Class C:

Moderate effect of solar radiation. Average maximum overheating is 11.5°C (up to twelve hours of sunshine per day, but without sunshine on each individual day of the measurement decade).

Class D:

Strong effect of solar radiation. Average maximum overheating is 17.3°C (more than twelve hours of sunshine per day, in general on all days).

Class A primarily occurs in Central Europe during the autumn and winter months, class B in the autumn and spring, class C in the summer and class D in periods of radiation weather in high summer which are similar to subtropical conditions.

Fig. 11 shows air overheating at an upper measurement point in a stationary container. The values were plotted by radiation class. It should be noted that the overheating values shown in Fig. 11 are averages. Overheating of 20 – 25°C was measured at an upper measurement point in the container in

0.0% of class A,

0.8% of class B,

5.4% of class C,

25.5% of class D,

as a proportion of all measurements, with overheating in class D being in the 15 – 25°C range in 83.6% of all cases. At an external air temperature of 25 – 30°C, air temperatures within the container may accordingly rise as high as 50 – 55°C.

Average daily variation in overheating of the air inside a container
Figure 11: Average daily variation in overheating of the air inside a container,
plotted by radiation class; Svenson

Depth of penetration of temperatures

The influence of temperature variations of the container walls and of the air in the container on the temperature of the goods is a significant factor in storage.

Fig. 12 shows the daily amplitude in goods temperatures measured within the stack of a container packed with sugar on a sunny day in June: the daily amplitude in the interior of the stack is only 1.2°C, while that for the superficial layer is 6.3°C (see Fig. 12).

emperature differences over 24 hours within the stack of a container packed with sugar in sales packaging

Figure 12: Temperature differences over 24 hours within the stack of a container
packed with sugar in sales packaging; U. Scharnow

This means that the temperature in the interior of a stack of cargo adapts to changing external temperatures only very slowly. It is clear from these measurements how far the interior temperature of the goods lags behind changes in external air temperature caused by changes in weather conditions (see Fig. 13).

Measured air and goods temperatures at different times of day in a container packed with 16 metric tons of white sugar on a sunny day in June a) at 06:00, b) at 14:00 and c) at 18:00

Figure 13: Measured air and goods temperatures at different times of day in a container packed with 16 metric tons of white sugar on a sunny day in June a) at 06:00, b) at 14:00 and c) at 18:00; U. Scharnow

The exposure of the goods to thermal stresses is determined by the size of the stack and its internal compactness. Stacks which are so dense that the ambient air cannot freely circulate within the stack do not readily adjust to external temperatures and water vapour also cannot be dissipated. Considerable delays may be observed even with this comparatively small stack in the container. The daily variation in temperature of a packed cargo is less marked. Although none of the cargo is more than 1 m away from a boundary surface of the cargo stack, the daily variations in air temperature in the container have, as can be seen, only a very gradual impact on the daily variation in goods temperature in the interior of the stack.

A different temperature regime is to be anticipated in a container which is completely filled with goods than in an empty or partially filled container.

Fig. 15 shows, for example, the frequency distribution of overheating in two differently loaded containers. While M 11 was obtained in a container packed with 16 metric tons of sugar (see Figs. 12 and 13), M 2 was simultaneously obtained in a container packed with 1.75 metric tons of bagged sawdust (see Fig. 15) which occupied approx. 30% of the container volume. The differences in humidity were still more extreme. While the sugar container did indeed also constantly exhibit comparatively high relative humidities of 70 – 80%, no appreciable sweating occurred and, after 6 months’ storage, the sugar was unpacked again in perfect condition, whereas severe sweating was constantly observed in the sawdust container.

Frequency distribution of overheating in two differently loaded containers

Figure 15: Frequency distribution of overheating in two differently loaded containers; U.Scharnow/Svenson

Water vapour in the container

Water vapour conditions in the container are primarily determined by internal factors, i.e. the water vapour conditions are largely determined by the hygroscopic properties of the cargo inside. The quantity of water vapour contained in the air is small and does not generally result in sweat damage. However, it may lead to other damage, e.g corrosion of metal goods.

Relatively large quantities of sweat in closed containers are always attributable to the cargo or its packaging (and/or the container floor if wooden). Sweat is thus actually possible only if water enters the container with the cargo. High air temperatures in the container and the associated low relative humidity drive water vapour out of the hygroscopic cargo. This water vapour condenses on the container walls and ceiling which is cooled by nocturnal radiation. Investigations reveal among other things that, starting with a dry container ceiling and side walls, sweat coverage increases in stages and reaches a maximum after a few days.

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