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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About moisture and condensation in shipping containers

Q: How do InterDry absorbers help to solve moisture problems?

InterDry absorbers contain Calcium chloride that very aggressively grab and absorb moisture from the air. They dry the air. When there air is dry, there are no moisture problems.

Q: Can InterDry absorbers solve all moisture problems?

Well, not all. Some cargoes may be so wet that any reasonable number of InterDry absorbers get overwhelmed. But InterDry absorbers can reliable protect even very difficult cargoes that may contain tons of moisture, such as coffee beans, wood products or paper.

Q: I load my container under dry conditions and it is very tightly sealed. How come I still experience moisture problems?

Your cargo or the packaging, including container floors, pallets and crates, contain moisture that is evaporated into the air during transport. Wet packaging material is the most common cause of unexpected moisture problems.

Q: I have shipped the same cargo for years with InterDry absorbers without any trouble, but now I have a lot of damage. Have you changed the poles?

Check your container and your packaging material. Did you just start to store your pallets outdoors? Does your forklift drive into the container with snow on the wheels? Did you just change supplier of crates? You can’t tell by looking whether wood are carton is dry. The moisture properties of wood and cartons have an exponential character. It makes a huge difference if your pallets moisture content should be 20% instead of 17%, say.

Q: I ship consumer goods in tubes/cans/jars etc that contain no moisture, yet I still have problems.

Consumer goods are often shipped with a lot of cardboard packaging. Even if the boxes seem dry they could literally hold tons of water.

Q: Each container of my cargo of peanuts/coffee/cocoa contains tons of moisture. What difference does it make that InterDry absorbers absorb a few litres during a voyage?

All the important things that happen have an exponential character. That means that a small change in circumstances can have a huge effect on the outcome. InterDry absorbers create circumstances that allow almost all of the moisture to remain in the cargo even while the level of humidity in the air is lowered by a crucial amount, sufficient to prevent damage. It is a question of ”leverage”.

Q: Does it make a lot of difference that my cocoa beans have a moisture content of 8% instead of 7%?

Yes, such a difference could be all the difference between no damage and disaster. The moisture behaviour of most agricultural products have a strong exponential character.

Q: My cargo of peanuts had suffered damage in the centre even though the outside of the cargo looked fine and there was were no signs of condensation?

Lots, if not most, damage to cargoes is caused by prolonged periods of elevated humidity without any condensation (Container rain, Container sweat, Super Saturation Event). It is common that cargoes loaded at cool temperature and then moved into warm condition suffer damage in the centre of the cargo as a result of a difference in temperature between the outside and the centre of the cargo. Warm air from the outside of the cargo becomes humid as it moves into the cooler centre. InterDry absorbers protect against this effect even though the absorbers are mounted on the container walls.

Q: I had damage to my cargo even though I used lots of silica gel and there was no condensation. Would it help to switch to InterDry absorbers?

Calcium chloride absorbs moisture even when the humidity is not very high. This protects the cargo against damage caused by prolonged periods of elevated humidity. Some kinds of steel start to corrode at 70% relative humidity, moulds can grow at 80% relative humidity and at near 90% relative humidity lots of things go wrong. Yet, InterDry absorbers are also at their most efficient protecting against condensation. Most other products, such as silica gels, are really effective only in very humid conditions and in protecting the cargo against condensation damage.

Q: What is so great about InterDry absorbers anyway?

Well, they will not fall off the wall, get punctured during loading and unloading, leave a wet puddle on the cargo or run out after half the voyage. They are installed in seconds without ladders and take up no cargo space. The capacity of each absorber is big, so fewer is required. The cost of an installation is very competitive, even against much inferior alternatives.

Q: How many absorbers do I need?

The number of InterDry absorbers required to protect the cargo depends on the cargo, the temperature conditions during the voyage, the length of the voyage – and just how safe you want to be. For some really dry cargoes e.g.. steel coils or household removals, 2-3 InterDry absorbers are enough. For a lot of ”normal” goods 4-6 InterDry absorbers is about right. Some cargoes with very difficult moisture properties on long voyages may require up to 16 InterDry absorbers.

Q: Do I need to line my container with kraft paper?

Lots of containers are lined with Kraft paper primarily for reasons of hygiene or to simply isolate the cargo from direct contact with the container walls. The liner will act as a kind of sponge, catching and absorbing any droplets of water and then re-evaporating the moisture into the air. If liner is used without InterDry absorbers it could contribute top a kind of pumping effect, drawing moisture out of the cargo. When used together with InterDry absorbers the liner will act as a buffer in extreme conditions, and will prevent any container rain from reaching the cargo. Much the same can be said for so called dew cloths.

Q: My container is absolutely filled with cargo. Will the InterDry absorbers still work?

Moisture diffuses very effectively, even through a seemingly compact cargo. Experience shows that InterDry absorbers will make a difference even to mould growth inside cartons in the cargo. It is, however, necessary that some free space is left in front of each InterDry absorbers. If some InterDry absorbers have collected less water than others inside a container, there may be a problem with air access to those absorbers.

Q: I have problems with mould growth inside my shrink-wrapped pallets. Will InterDry absorbers help?

Yes, so long that there is some access of air through the top and bottom of the pallets. If this is not possible, a spiked roller may be used to tear holes in the shrink wrap.

Q: My shipments of steel/galvanised components/aluminium/ machinery etc arrives corroded, stained or discoloured despite heavy packaging. Will InterDry absorbers help?

You can forget about your tectyl, coatings, oil-paper and plastic wraps that are expensive both to apply and remove. Your container can probably be equipped with a sufficient number of InterDry absorbers to protect against any damage at less cost than your present packaging.

Q: I got some brine on my hands while removing used InterDry absorbers . Is it dangerous?

No it isn’t. Calcium chloride is non-toxic and environmentally safe. It is the second biggest constituent of sea-salt and is liberally sprinkled over icy roads in cold countries. The brine is somewhat similar to very salty seawater, and may cause irritation and rashes if left to dry on the skin. We recommend that you wear gloves and goggles when handling used absorbers, but should you get splashed by brine just wash off immediately with lots of fresh water.

Q: Can I re-cycle my used absorbers?

The absorbers can not be re-used but can be disposed of environmentally friendly. It doesn’t contain any harmful chemicals.


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What’s the difference between absorption and adsorption and sorption?

The terms absorption and adsorption are used freely to indicate the same process of taking moisture from the air using desiccants, mostly because we don’t want to complicate the explanation of what our product does. But there is a difference.

Absorption generally refers to two phenomena which are largely unrelated. In one case, it refers to when atoms, molecules, or ions enter some bulk phase – gas, liquid or solid material. For instance, a sponge absorbs water when it is dry.

Absorption also refers to the the process by which the energy of a photon is taken up by another entity, for example, by an atom whose valence electrons make transition between two electronic energy levels. The photon is destroyed in the process. The absorbed energy may be re-emitted as radiant energy or transformed into heat energy. The absorption of light during wave propagation is often called attenuation. The tools of spectroscopy in chemistry are based on the absorption of photons by atoms and molecules.

Adsorption is similar, but refers to a surface rather than a volume: adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or, more rarely, a liquid (adsorbent), forming a molecular or atomic film (the adsorbate). It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution.  This is what happens to calcium chloride desiccants in container shipments.

Sorption encompasses both processes, while desorption is the reverse process.

* Taken from Wikipedia.

Wikipedia: Absorption (chemistry)

Wikipedia: Absorption (light)

Wikipedia: Adsorption

Wikipedia: Sorption

 

 Whats the difference between absorption and adsorption and sorption?

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