Solid bulk cargoes
|Infobox on Solid bulk cargoes
|Example of Solid bulk cargoes
|Stowage factor (in m3/t)
|Humidity / moisture
Solid bulk cargoes
Shipment / Storage
Solid bulk cargoes (Ventilating cargo spaces)
There are two problems that frequently arise during the ocean transport of bulk vegetable products – ship’s sweat and cargo sweat. They must be controlled if the cargo is to out-turn in sound condition.
If the cargo is warm compared with the external atmospheric conditions (during shipments from (sub) tropical countries to Europe in winter), this could result in condensation forming on the deckhead producing ship’s sweat. This could drip onto the cargo surface and cause spoiling as a result of mould growth. If the ventilating air is cool, it will take up moisture vapour by diffusion from the intersticial air in the surface layers of the cargo, cooling the surface of the cargo, possibly below the dew point of warm air rising from the bulk. Water will then condense in the cooler surface layers of the cargo (cargo sweat) producing a wet ‘cake’ just below the surface in which microbiological spoilage will eventually occur.
Ventilation of general cargoes is normally achieved by surface or through ventilation or a combination of both. However, because of the closely-knit nature of solid bulk cargoes, there is little chance of obtaining through ventilation; indeed, such ventilation should be avoided. The best that can be done is to operate surface ventilation – either by natural or mechanical means – so that the warmer foul air is exhausted from immediately above the cargo, replacing it positively with fresh air from outside, so that warmer foul air may continue to ascend through the cargo, and be replaced with a controlled input of fresh air, and so on. Where cowl ventilators are available, positive surface ventilation is best achieved by trimming the leeward (and/or furthest aft) ventilators to face the wind, and to trim the windward (and/or forward) ventilators with their backs to the wind. In bad weather all vents would be either closed-off or trimmed back-to-wind.
Where fan-assisted mushroom or grill type vents are fitted, mechanical surface ventilation should be carefully monitored, and never used if there is an imminent risk of fire. It is not sufficient simply to admit fresh air to the cargo compartments without consideration of the difference between internal and external air temperatures, relative humidities and dew points; and to the sea surface and main-engine cooling water intake temperatures. The seafarers’ guiding rule, where only natural surface circulation is involved – and where the nature of the cargo itself demands – is stated simply in four parts.
- This first relates to the cargo surface temperature: Ventilate only when the dew point of the air being admitted to the cargo compartments is lower than the temperature of the cargo surfaces. The other three parts relate to the loading temperature of the cargo.
- In the early stages of a voyage a cargo, such as expellers – the normal fibre residues after pressing seeds, kernels and nuts for oil – (and even bagged rice as well as bulk) should not be ventilated for positive air change with outside air which is less than 3°C colder than the loading temperature of the cargo itself.
- Positive ventilation can be adopted when the outside air is more than 3°C cooler. Positive ventilation should be operated when the temperature difference is about 5°C or more, always providing that risk of fire is not imminent.
- There are limitations in stating things so simply, but, in the instance of many bulk cargoes, such general guiding rules could be expected to produce a satisfactory out-turn of cargo where such cargo was loaded in a sound carriage condition to start with.
Do not ventilate
Some bulk cargoes should not be ventilated at all if they are in a sound state at time of shipment, and only be ventilated with extreme caution when adverse cargo conditions are identified. For instance, maize and most hard grain cargoes should not be ventilated at all if the moisture content is at the correct shipping level, or below, at time of loading. Grain that consists of seeds grown in a dry climate has a comparatively low natural moisture content; the seed itself has a protective outer skin, which preserves the moisture content and prevents the seed drying out. Whole grain loses its moisture much more slowly than grain that has been milled or pulverised. If whole grain is shipped in a sound condition with a moisture content of less than 12%, it will remain more or less stable for months requiring no ventilation. In fact, uninformed ventilation may set in motion the very processes of moisture migration, which it is hoped to prevent. When grain is shipped with moisture contents in excess of 14%, moisture migration may well occur in any event; spoilage may then be increased by uninformed ventilation; again, lack of ventilation may assist in limiting the damage.