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.


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InterDry Power Desiccant – Weight gain by absorption

We have been testing and tweaking our products for many years. I would like to have a look at one of those tests that we participated in together with 4 of our main competitors.

Below is the result of a test where the weight gain by absorption was calculated by a temperature of 38 degrees Celcius, 90% Relative Humidity, during a period of one month. Calculations were made by re-calibrating the initial weight to 1000 grams uniformly amongst all different brands of desiccant so that direct comparison was possible.

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All products showed initially a very high absorption in the first 7 days. Our competitors dropped out very quickly as you can see. Other Brand 2 started losing its absorption power after 5 days and even started leaking after 7 days. Other Brand 3 and Other Brand 6 started leaking on day 11.

Can you imagine what happens to your cargo if in the first week there is no protection from humidity inside your container?

InterDry Power Desiccant 1000 grams went on to absorb moisture resulting in a final weight of approximately 2800 grams.


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Does a container floor have any effect on the relative humidity in a shipping container?

By: Pakarada Premtitikul
General Manager
InterDry (Thailand) Co., Ltd.

Container Floor Tests

Speed of achieving a hygroscopic equilibrium.

In this experiment, a piece of plywood, used for container flooring at MSC, was conditioned at 23 ° C and 50% relative humidity. The piece of wood was placed in a climate test cabinet at 45 ° C and 75% RH where the wood absorbs moisture. Every day the wood was weighed and the moisture content of the wood was determined. With this procedure we have more knowledge about the speed at which wood reaches its hygroscopic balance.

The wood was protected with tape in places where in reality it is not subjected to the internal container air. Following pictures show how the wood was protected.

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The tested piece of wood top and bottom.

The initial mass of the wood was 3259g. Following graphs show the results of the test. The first chart shows the mass of absorbed moisture per day. The second chart shows the increase in the moisture content of wood per day.

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Absorbed moisture per day

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Increase in the moisture content of wood

Unfortunately, the hygroscopic equilibrium had not been reached before the trial was stopped. The moisture content remained almost constant after 3 days. Yet the wood continued absorbing moisture. Since it is plywood, that means that once the moisture content of the top layer remains constant, the moisture will move to the lower layers of wood.

Comparison of the climate of two containers at MSC.

In order to see the difference between climate change and the change in the moisture content of the container floor in a container with a desiccant and a container without desiccant, the following experiment was made.

Two containers at MSC were used for this test. One container (container U6472633 MSC) was provided with desiccant and the other container (U1089745 MSC) was left as it was. The ventilation holes in the two containers were sealed with tape.

A data logger was used in both containers to measure the changing climatic conditions. In container U6472633 MSC a probe was placed near the ceiling and a probe on the floor. In container U1089745 MSC a probe was mounted on the ceiling and a probe on the outside of the door to measure the outdoor conditions. Following figure shows the positions of the probes of logger 56,844 in container MSC U6472633 and logger 56,845 in container U1089745.

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Before the container doors were closed, the moisture content of the floors were measured. Following figure shows the moisture content of the container floor at various locations in the two containers before the start of the trial.

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The test was performed over a period of 28 days at the port of Antwerp, and below are the results of the data loggers.

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Data logger 56844:

The purple curve and the green curve are respectively the relative humidity near the ceiling and the relative humidity near the container floor.

It can clearly be seen that fluctuations in relative humidity in the vicinity of the ceiling are much higher than those in the vicinity of the floor.

The purple curve shows maxima of 85% and 90% and minima of 30% and 40%. The green curve shows maxima of 70% and 75% and minima of 55% and 60%.

The blue curve and red curve are respectively the temperature near the ceiling and the temperature near the floor.

The fluctuations near the ceiling were slightly larger than those near the floor. The minima of both are roughly equal at about 10° C. The maximum temperature at the ceiling was about 40 ° C and at the floor approximately 30° C.

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Data logger 56845:

The purple curve and the green curve are respectively the relative humidity near the ceiling and the relative humidity outside the container.

The fluctuations of relative humidity are not comparable because outside the container it was sometimes larger and sometimes smaller.

The blue curve and red curve are respectively the temperature near the ceiling and the temperature outside the container. Minima are comparable, but the maxima showed extreme differences. The maximum temperature at the ceiling was about 40 ° C and outside about 30 ° C.

For a better comparison between the climate of both containers, the temperature changes and changes in the relative humidity near the ceiling are shown in the figure below.

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The temperature and relative humidity within containers MSC U6472633 and MSC U1089745 and outside.

The temperatures were almost exactly the same. The relative humidity, however, showed some differences. The relative humidity in the container with the desiccant showed smaller maxima.

The explanation lies in the fact that an amount of moisture was absorbed by the desiccant resulting in less moisture in the air, resulting in a lower relative humidity.

After weighing the desiccant, it was discovered that a total of 4.465 kg of moisture was absorbed. Since the air in a container itself contains little moisture, this must have been moisture from the floor. The desiccant always causes a lower relative humidity in the air so the floor was looking for a new hygroscopic equilibrium. Therefore, the moisture of the floor in the container with desiccant should have dropped as well. Next figure shows that this indeed is correct.

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The moisture content of the container floor in MSC U6472633 decreased 2 to 5%, while the moisture content in container MSC U1089745 remained constant.

Climate control inside a container is a very difficult task.

There are so many variables to be taken into consideration that it is practically impossible to determine a precise figure on the number of desiccant needed within a container.

Nevertheless, significant conclusions can be made such as the fact that a container floor is one of the main sources of humidity.

Desiccant always absorb more at higher temperatures and higher relative humidity. High relative humidity provides a high absorption capacity, while a high temperature results in faster moisture absorption.
If the packing unit increases in mass, the rate of absorption decreases, but the active absorption time increases.

When comparing the different desiccant products two main groups are distinguished by their absorption properties, namely the rapid absorbers and those with a long effective absorption time. If a rapid moisture-absorption is desired, for example for a shorter sea voyage, the first group of desiccants holds perhaps the greatest advantage. If a constant fluid intake is desired, the second group is the most desirable.

As the products that continue to absorb also the ones that absorb the most, fewer units are needed for this type of desiccant which is a big advantage. It should be noted that the desiccants with faster absorption, are the clay based desiccants. The desiccants that ultimately absorb the most are the desiccant based on salt.
From the test at MSC can be concluded that desiccants work. Products succeed in absorbing the moisture from the wood to bring down the relative humidity in the air. Because the relative humidity is lower, a new hygroscopic equilibrium needs to be created. In the MSC container was that new hygroscopic equilibrium was created by the wooden floor expelling moisture.

Finally, it should be noted that the choice of desiccant is not easy, but it is determined by a diverse range of interacting conditions such as type of cargo to be shipped, the length of the voyage, the sea route, the season the container is transported and so on.

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