Goods carried can be said to be hygroscopic. ie: containing natural moisture (such as rice), or non-hygroscopic – eg: steel bars. Nevertheless, whether containing moisture or not, commodities are prone either to give rise to condensation and/or to be adversely affected by it, this problem being exacerbated by the varieties of outside temperatures through which a ship’s voyage may progress – see Figure 1.1.
Careful ventilation of a ship’s cargo compartments is therefore vital, and as a general cargo ship is loaded, avenues and passageways may require to be constructed so as to increase the circulation of air in these otherwise enclosed spaces. The use of wooden
divisions, termed dunnage. to facilitate ventilation is important.
Mats and dunnage made from such material as kraft-paper and plywood, are used to support goods and to keep them from touching steel decks and holdsides where condensation might take place.
In cargo handling however, as in most practical subjects, technological advances are continually made. An innovation has been the inflatable dunnage bag made from lightweight but strong weatherproof nylon textile, and capable of withstanding a high constant pressure or a shock loading, whilst being resistant to oils, chemicals, and salt water.
Speedily filled by compressed air, the bags can be located precisely where required and expanded as necessary to restrain quite substantial commodities from shifting. Where incoming air is cooler than the temperature in the cargo compartment (usually occurring on voyages from warm to cooler areas) water vapor condenses into droplets which form on the ship’s structure. These droplets may run or drip on to cargo. Where incoming air is warmer than the temperature in the cargo compartment (usually occurring on voyages from cool to warmer areas) water vapor condenses into water droplets which may form around the cargo.
The point at which water vapor condenses into droplets is termed the “dew-point” and it is on dew-point measurement readings that ventilation decisions should be based. Hold sides are usually lined with cargo battens or sparring spaced strips of wood slotted into brackets, serving the dual purpose of keeping non-bulk cargo off ship’s sides, whilst creating air-flow channels round the edges of the cargo. Unfortunately, battens and their retaining clips may easily be damaged and costly to replace.
Consequently, there is a modern tendency to replace battens by plywood and kraft-paper or by cargo-nets in certain circumstances.
Modern general cargo vessels most likely be equipped with electrical ventilation but older ships tend to rely upon natural ventilation to combat condensation, air being introduced via rotatable, cowled openings on deck, circulated around the cargo compartment, then expelled naturally. This system is termed Through Ventilation.
In certain cases there may be no need to ventilate, when for example, non-hygroscopic cargo is loaded dry and the cargo compartment contains no hygroscopic materials, whilst in other cases – eg: where a cargo may be liable to spontaneous combustion, ventilation in certain circumstances may even be dangerous.
Furthermore, poor ventilation can be worse than no ventilation whatsoever, and with some older ships, given certain climatic circumstances, it is very difficult to avoid condensation in cargo compartments. For this reason, the safe carriage of certain commodities – eg: jute, is greatly assisted by mechanical or electrical ventilation, under which systems outside air can be prevented from entering cargo compartments, and re-circulated air channeled around instead. If necessary passing through a dehumidifying unit to remove any moisture content.