Facts to keep your container dry?

KEEP YOUR CONTAINER DRY

The Causes of Moisture Damage

Moisture damage is caused by:

• Condensation
• In the ceiling and the walls of the container (Container Rain)
• On the cargo (Cargo Sweat)
• Prolonged periods of elevated humidity
• Condensation takes place on the coldest available surface, often in the container ceiling
• Liners protect against container rain, but not against humidity

Moulds and Chemical Degradation
There are more than 20.000 kinds of fungi, moulds and bacteria that may grow in your cargo. By the time you have bad smells and visible fruit bodies (green, orange or black spots) the fungus is already well established.

Most fungi produce toxins. Rule of thumb: the more humid growing conditions, the more toxic the growth is.

Most fungi require a short period of humid conditions (70-80%) to start growing, but will thereafter continue to grow at much drier conditions (60-70%) and even at freezing temperatures.

Fungicides are mostly ineffective. If you kill one kind, another fungus will take over. Insects may appear as eggs hatch under moist conditions.

Chemical reactions degrade glue (which then let go), causes discoloration and bad smells.

The Moisture Balance in a Shipping Container

Moisture in the air in the container derives from the outside and from the cargo. Desiccants remove moisture from the air.

Temperature Changes Change Humidity

Warm air can hold more moisture than cold air. (Rule of thumb: 10g/m3 at 10 degrees, then double every degrees)

The relative humidity measures the humidity as a percentage of the maximum amount of moisture air can hold at that temperature

Dry Cargo

Moisture Containing Cargo

Even a dry cargo can have moisture problems, but when the cargo and packaging contains moisture the problem becomes greater.

Moist Cargo Balance Point Evaporation and Absorption

The cargo evaporates moisture and the World Drybag absorbs moisture. The balance point is at a low rate of evaporation => a slow & steady flow of water from cargo to Drybags.

Temperature Differences Within the Cargo Cause Problems

Temperature differences within a cargo may take weeks to equalize.

Different temperatures mean different humidity conditions in different parts of the cargo. RH differences of 10-20% within a cargo are not unusual.

Many cargoes will absorb moisture from air above a certain level humidity and evaporate moisture into air below that level. In such cases large amounts of moisture may move from a warm part of the cargo to a cooler part of the cargo.

Dry Cargo

Moist Ambient Conditions

When the temperature drops, the pressure in the container drops, and air with ambient moisture content enters the container (breathing-in). When the temperature goes up, the opposite happens (breathing-out).

Desiccants Remove Moisture from the Air

Dry Cargo

Container With Desiccants

World Drybags Protects Against Moisture

• Active substance Calcium Chloride
• High absorption capacity, more than 100 % of own weight
• High rate of absorption at all levels of humidity above 40 %
• Not prone to exhaustion at extreme events.
• Does not re-evaporate moisture once absorbed
• Non-toxic, safe with food products and environmentally safe.
• Easily mounted and mechanically protected
• Fits into wall corrugation
• Suitable for all applications in shipping containers

Desiccant Checklist

How many World Drybags do I need?

The Drybags must have

• A sufficient Activity (absorption rate) to protect against condensation and lower the humidity at the worst time
• A sufficient Capacity (amount of salt) to last the voyage

The number of Drybags required depends on

• The length of the voyage
• Temperature variations during the voyage
• Ambient conditions during the voyage
• The moisture content of the cargo and packaging
• The desired level of risk

Non-linear physics A small change in conditions have a great effect on the outcome

Are moisture problems unpredictable?

Just about all physical moisture processes are strongly non-linear, often having an exponential character. Some examples:

Regular Voyage Moisture Analysis

Determine the number of Drybags required from experience with similar cargo and voyages. Then analyse actual voyages. Increase or reduce the number of bages used accordingly. Keep systematic records, e.g. to allow adjustments between seasons.

Temperature and moisture conditions during voyage can be measured throughout the voyage with a data logger.

Moisture conditions may be indicated by using sensors or Moisture Indicator Cards and by observing the amount of water in the Drybags. This amount may vary a lot between different voyages but all Drybags in a container should collect about the same amount of water.

Moisture conditions are indicated by analyzing any kind of cargo damage and its location.

4-8 Drybags in a Dry Cargo 20 ft, But...

Make sure pallets, container flooring & crates are also dry.

Consumer goods cargoes usually contain a lot of cardboard packaging which may release moisture.

Some metal cargoes are at risk even at moderately increased humidity (>60 %)

Many anti-corrosive oils and films contain water which may release moisture.

A cargo with a small volume increases the volume of breathing.

Mind the loading temperature A cool cargo in warm surroundings may be a risk for weeks (even if the cargo is dry)

For a short voyage, the number of Drybags can be reduced by using faster Drybags (World Drybags 1400 grams)

6-12 Drybags in a Moist Cargo TEU

Examples of moist cargoes are seeds, beans, nuts, etc. Some wood cargoes, some paper cargoes etc. (or cargoes with moisture containing packaging).

The conditions at the center of the cargo may differ a lot from those on the outside.

Mind the loading temperature A cool cargo in hot surroundings (e.g. when crossing the equator) or a warm cargo in cool surroundings are at risk for weeks.

Daily temperature cycles may cause a pumping of moisture from the cargo and are a special risk.

Container breathing is less of a concern.

Checklist at Shipping

Close the vent holes with tape when you have a dry cargo. The container will still breathe, but more slowly. Moist cargo may be shipped with vent holes open or closed.

Check that the container floor, pallets and packaging are dry. Softwood packaging should have maximum 18-19 % moisture.

Cargo in pallets must be able to dry. Check that any shrink wrap or other plastic packaging does not cover completely.

Check that the World Drybags are not covered. There should be at least a few cm of free space in front of most of the surface area of the drybags.

Check that all bages are securely hung and that any World Drybags will not be crushed by the cargo.

Fundamentally there are two different kinds of condensation.

• Cargo sweat
  This occurs when moving cargo from a cold or temperate climate to a tropical one. The air containing moisture or water vapour condenses-out on the cargo or its packing.
  
  
• Container sweat
  This is water vapour in the air condensing or forming on the inside surfaces of the container when moving from tropical or temperate climates to cooler areas.
  
  
• Fungi, moulds and mildew do not usually cause any problems below approximately 55% Relative Humidity.
  
  
• The effect or onset of corrosion is a function of many variables where humidity and time are also present. A fixed humidity level cannot be stated because of the variables, however the lower the humidity the better.
  
  

The type of goods to be protected must be considered when determining the amount of water or relative humidity that needs to be removed. This in-turn will determine the quantity of World Drybag^TM desiccants required in a container to reduce and hold the humidity at a level that will not cause damage or deterioration to the products in the container.

This depends upon :

• The weather during the voyage
• The duration of the voyage
• Type of goods, commodities or materials in the container
• The condition of the container
• The sensitivity of the goods
• Moisture inherent in the floor and/or wooden or other types of pallets or packaging materials

The chart clearly illustrates the problem with moisture. If the container is stuffed in 90% relative humidity and 30 degree temperature, the air can then hold about 27 gram of water per m3. If the temperature is lowered to about 0 degree the air can then only hold about 5 gram of water. The differently 22 gram will fall out as water and potentially damage the goods.

Humidity or water in goods loaded into a container.

Generally the relative amount of moisture / water in the goods is a factor of the type of goods and the relative humidity of the ambient air at the time of loading. Obviously, tropical air will have a much higher relative humidity content than say, air in a temperate climate. Consequently, goods loaded into a container contain moisture or water and water vapour in the air. For example, if there is 5 tons of furniture in the container, the furniture by itself would usually have moisture content of between approximately 8 and 25% depending on timber-type and the drying process used. This equates to about 400 - 1,250 litres of water. This moisture, some of which will re-enter the air inside the container, is the main source of Condensation.

Humidity or water vapour in the air inside a container.

One 20-foot container has a volume of approximately 33m3. 1m3 of tropical Asian air of 90% relative humidity at 32oC contains 33 grams of water per m3. This is a total of 1.3 litres of water. (At 55% RH at the same temperature the air contains 28 grams per m3. By contrast, 1m3 of European air of 100% RH at 0oC contains 5 grams of water per m3, or a total of 0.2 litres. At 55% it would contain 3 grams of water per m3). Once the ambient air temperature surrounding the container drops, 1.2 litres of water can condense on the inside walls and other surfaces within the container, i.e. packaging and goods. A relatively high humidity level corresponds approximately to ambient air of 25oC at 90% RH (relative humidity). A low humidity level corresponds approximately to ambient air of 22oC at 50 - 60%RH.