Saturated Air : For a given atmospheric pressure, air holds more water vapour at higher than lower temperatures.
The maximum amount of water vapour contained in a cubic meter of saturated air decreases with a decrease in temperature. At 40 degree C it can contain 51 grams, at 20 degree C 17 grams and at 0 degree C only 5 grams.
Dew Point : Is the temperature at which a sample of saturated air will condense. Warm air has more capacity to support water vapour within it than cold air.
Relative Humidity : Expressed in percentage terms is the ratio of water vapour present in a given sample against the saturated level. If the Relative Humidity (RH) is said to be 100% then the air sample is saturated.
Condensation : Occurs when moisture laden air releases its water vapour on to the surrounding surfaces in the form of water droplets.
For condensation to occur the following conditions need to be present :
Container Sweat : Occurs when the skin of the container is cooled to a temperature below that of the dew point of the air enclosed within the container. This result in water droplets forming on the interior roof and side panels that drips down on the cargo causing mould and water damage. Cargos that spontaneously heat from within can increase the problem.
An example is : A cargo loaded in the tropical belt in warm conditions with high Relative Humidity, is transported to cold winter conditions in Europe. The temperature outside the container gradually cools down in transit until it experiences cold conditions in Europe. The steel container allows the chill to conduct from the outside of the panel through to the inside. The situation inside the container is now one of cool side panels and warm moist air in the header space above the cargo and within the stow. The temperature of the side panels is therefore below the dew point of the air inside the container and condensation occurs. Condensation will continue until the dew point of the interior air falls to that of the outside air. Different solutions are discussed further in this procedure however the simple solution is to ventilate the container by passing air through it and replacing the warm moist air with similar air to outside the container.
Cargo Sweat : Occurs when the surface of the cargo is cooler than the dew point of the air enclosed within the container. Droplets of water then form on the surface of the cargo.
An example is : A cargo of canned goods is loaded in cold winter conditions in Europe and transported to the tropical belt. The container will gradually heat up during transit to the warmer moist climate however the cargo temperature will lag behind, slowly heating up and replacing the cold from loading.
If ventilation was allowed to take place the warm moist air from outside the container would condense on the cold cargo. In this case it is better to avoid ventilation during transit and allow the cargo temperature to gradually increase thereby restoring equilibrium between the cargo temperature and the outside air.
Radiation of Heat at Terminals and On Board Ship : Where possible cargos sensitive to condensation must be protected from the extremes of radiant heat and extreme cold as under:
At the other end of the journey it is important that early delivery is carried out for sensitive / hygroscopic cargos arriving at terminals with near zero temperatures. In such cases the cargo can experience what we call the “cold shock” of first night ashore. When acceptable to the shipper it is often prudent to crack a door open to ventilate the container.
Hygroscopic Commodities : Are those which are permeable to water and which retain moisture under certain conditions.
Timber, coffee, cocoa and most materials of organic origin are hygroscopic. Coffee for example can have a moisture content of 12% and container sweat is therefore an issue.
Rusty cans with peeling labels are the nightmare scenario for supermarkets. De-humidifiers are used in warehouses to create controlled conditions for goods and packaging prior to export. In very special cases de-humidifiers have been used in containers prior to loading to ensure a dry container is presented for loading.
The moisture content in the floors of containers is an important aspect of condensation control and maximum permissible amounts may be specified by customers, e.g 15-18 % moisture.
Desiccants & Absorbent Materials :
A basic desiccant is one which will absorb 27% (of its dry weight) of moisture when placed in an atmosphere of 50% Relative Humidity at 25 degrees C.
Desiccants can be very effective when used with certain cargoes (Steel Reels, Cars, Carton-ware). In the case of hygroscopic cargos they may only be part of the solution because of the high moisture content present within the cargo (e.g. Coffee 12%). At the other end of the spectrum, in certain situations desiccants can extract too much moisture from the cargo and where the cargo is sold by weight, this result in claims.
Silica-gel is probably the most familiar to us and others include activated alumina and activated clay. Brand names using desiccants include Dry Bag, Absormatic, Moisture-Grip and Grafo Therm.
Dry Bag: A Danish desiccant made up of roasted moler clay mixed with calcium chloride. Bags of kg are normally used in containers and the numbers utilized will be determined by the commodity. Dry-Bag is successfully used with Cars, Machinery, Milk Powder and many other cargos.
Absormatic: Uses water retaining poles in conjunction with a desiccant and are placed into the corrugation of the container side panels in order not to waste cubic space for cargo. Effective but vulnerable to forklift damage which can release moisture into the container.
Moisture-Grip: A product developed in Japan and is an absorbent polymer sheet taped to the container ceiling of the container. Sometimes used with cargos of tobacco.
Grafo Therm: A porous paint application applied to the container ceiling which absorbs moisture within the container. This product has been championed as the solution to condensation in coffee shipments. Where major temperature gradients are present, as with coffee entering a cold winter climate, this may not have the capacity to absorb all the condensation. Requires maintenance and may absorb dirt and contamination.
Insulation : Insulation blankets are normally used to clad the inside of a GP container with temporary insulation. They are used mainly to combat cold being conducted through steel GP containers to cargos such as red wine and liqueurs.
These are listed in this section to make the point that insulation can reduce extreme temperature gradients between inside and outside the container.
Vent-Containers having vent ducts along the side panels at top and bottom rails achieve this by convection effect. Warm moist air is expelled via the upper ducts and replaced with colder ambient air via the lower ducts. Plywood insulation on the interior of side panels is fitted only on part of the owned fleet and is preferable. High leasing costs are an issue with this equipment.
Fantainers are used successfully to carry onions and potatoes throughout the world. An extraction fan draws ambient air through the cargo to maintain equilibrium with the temperature outside the container. Control of ancillary equipment such as control boards and leads reduces the appeal of this method. The use of Fantainers for use with other cargos is currently being evaluated.
GP Containers: A large number of owned containers have four small passive vents in way of top corner castings with each providing air paths with at least 5 sq. cm. total cross sectional area.
The above provides a small amount of ventilation and it is important for sufficient free space to be left in the header space for it to be effective. The majority of dry cargos can be successfully carried in GP containers.
Taping off the passive vents in GP containers is also necessary when carrying hygroscopic cargos as under and also for cargos liable to spontaneous combustion.
The use of desiccants combined with lining the container with Kraft paper or fiber board are additional options according to the risk. (Details are available under coffee stowage). This amounts to parceling the cargo effectively to avoid contact with the steel container and placing desiccant bags on top of the stow to absorb moisture in the header space.