ventilation cannot significantly affect a heating process which is occurring more than about a metre below the surface. What can occur when the surface of a heating cargo is continuously cooled by ventilation, is that the vapour pressure differential between the interior of the cargo and the periphery is maintained and consequently the phenomenon of moisture migration is encouraged.
Stowage regulations
The irrelevance of surface ventilation to the carriage of grain is apparent from the stowage regulations in force in all major grain exporting countries, which insist that the vessel be stowed so that shifting of the cargo is impossible. Under these regulations, a ship’s grain carrying compartments are classified as either partly filled or full. Grain in partly filled compartments must be levelled and topped off with bagged grain or other suitable cargo, tightly stowed and extending to a height of 1-2m above the bulk. The bagged grain or other suitable cargo must itself be supported by a platform made either of close boarded wood, or strong separation cloths laid over the whole surface of the bulk cargo.
These regulations provide that in compartments totally filled with grain, the grain shall be trimmed so as to fill all the spaces between the beams, in the wings and ends. Further, to ensure that the compartment is maintained fully filled during the voyage, the compartment must be equipped with a feeder, from which grain can flow into the compartment if the cargo settles during the voyage. Alternatively the grain in the area of the hatch may be trimmed hard up to the deckhead beyond the hatchway to form a saucer. This saucer and the hatchway above is then filled with bagged grain or other suitable cargo extending to a height of at least two metres in the centre of the saucer. The bagged grain or other suitable cargo must itself be stowed tightly against the deckhead, and the longitudinal bulkheads, the hatch beams and hatch coamings.
The express purpose of the regulations, is to reduce to a minimum – and if possible to eliminate – the head space between the surface of the cargo and the overlying deck. With cargo stowed correctly in this way there is no possibility of effective surface ventilation.
Other cargoes where moisture migration is substantially more rapid
We took grain stowed in bulk as a first example because this probably represents a cargo in which moisture migration is the slowest compared with other cargoes which may be carried both in bulk and in bags.
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The rate of moisture migration and the amount of moisture moving in other cargoes may be higher because, on the one hand, of differences in voyage and loading temperature, and, on the other, of the physical nature of the cargo stowed.
A typical cargo in which rapid moisture movement can occur is bagged rice. This cargo is usually loaded at a high temperature and at a moisture content just below the critical level which is about 14%. If the cargo is stowed in a block, stow temperature changes in the external atmosphere and sea water may set up serious temperature gradients between the centre and peripheral regions of the stow, with the result that massive moisture movement occurs leading to the formation of both cargo sweat and ship’s sweat. This in turn results in part of the cargo becoming excessively wetted. Microbiological deterioration occurs in the wetted cargo.
In order to prevent or minimise this problem, bagged rice is normally stowed so that linked vertical and horizontal ventilation shafts are incorporated in the stow to facilitate moisture movement from the bulk to the external atmospheres.
Even with this form of stowage, when a rapid fall in external temperature occurs, as might be experienced with a vessel sailing to Northern Europe in the winter, serious sweat formation can result. This is well known to surveyors working in Northern European ports
A similar phenomenon also occurs with bagged cocoa shipped from West Africa to Northern Europe. Here, the cargo is artificially dried so that ventilation in the early stages of a voyage, i.e. before about the latitude of Dakar, can result in the cargo picking up moisture from the atmosphere and is not normally recommended.
After this, ventilation may be used to minimise sweat formation; but it must be borne in mind that cooling the surface of the cargo encourages moisture migration by increasing the temperature gradient between the bulk and the surface of the cargo, and may also result in the formation of cargo sweat. Thus, shock cooling of the surfaces of the cargo should be avoided and ventilation during the hours of darkness or during cold weather is probably best avoided.
It can be shown by calculation that, in any event, when cold conditions are encountered, the rate of emission of moisture from a normal cargo of cocoa can be substantially higher than the rate at which such moisture can be removed by a normal ventilating process, even assuming the ventilating atmosphere becomes saturated as it passes over the cargo. Thus,
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