EMC Calculator

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 'Super Dry' poles 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° temperature, the air can then hold about 27 gram of water per m3.
If the temperature is lowered to about 0° 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.

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