from the inside of the stow to the outside, and there will be a gradual drop in the temperature of the air which moves and the grain in contact with it. Hence water vapour will be absorbed en route, lowering the dew point of the air moving towards the periphery of the stow. Thus it is not possible to make an exact prediction of what conditions are necessary for cargo sweat to occur.
Heating up If there is a temperature differential between the outside of the stow and the inside, then moisture migration will result from the mechanism previously described. Such moisture migration will also occur when one part of the bulk becomes heated-up for any reason, e.g. insect infestation, microbiological activity or proximity to a hot bulkhead. In all these circumstances moisture will migrate from the warmer region to colder parts of the stow.
Warmer to cooler We have illustrated, taking maize as an example, the reasons why moisture migration occurs. As with maize, the problem of moisture migration is most evident with exports of biological materials from warmer climates to cooler climates. Moisture migration can occur from many causes but, however the temperature differential comes about, the result will always be (where the moisture content is uniform) a movement of moisture from the warmer to the cooler parts of the cargo.
Moisture migration is observed in cargoes where ‘insect infestation’ occurs. Here, centres of heating arise from the respiratory heat from the insects and moisture migrates from these spots to form a wetter shell in the cooler cargo immediately surrounding the heated zone. As heating becomes progressive, the heating zone of course expands as the wetter shell moves outwards.
A second example is where ‘ship’s heat’ causes a localised rise in the temperature of the cargo in contact with the source of the heat – e.g. an uninsulated engine room bulkhead. Here moisture migrates from the warm cargo and forms a layer of increased moisture content in the cooler cargo adjacent to it.
Unfortunately, the straightforward pattern of moisture movement resulting from a vapour pressure differential is not the only phenomenon that results from temperature differential in a cargo. Where temperature differentials are present, convection currents are set up owing to the fact that warm air is less dense than cold air. Thus, if heating occurs within a cargo, there will be a tendency for moisture to migrate in all directions from the heating zone. But there will also be a tendency for
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hot air to rise from the heating zone, to be replaced by cooler air coming in from the sides and underneath. The warm air will carry moisture with it, so that the pattern of moisture movement will be distorted in a vertical direction. In fact, where a hot spot occurs in a cargo, moisture movement is greater in a vertical direction than either laterally or downwards, because convection currents reinforce the upward movement of moisture. Thus for grain loaded warm and subjected to peripheral cooling, the major amount of moisture movement will be in a vertical direction, i.e. more water will pass towards the top of the cargo than towards the sides. If it is not possible to remove the water migrating to the top region of the cargo by ventilation, a subject that is discussed later in this report, more damage may be anticipated in the top layers than at the sides.
The rate of moisture migration
Having established the causes and the pattern of moisture migration, we now consider the quantitative aspects of the phenomenon.
Difference in vapour pressure
The rate at which moisture moves from a warm to a cold region is dependent to a large extent on the difference in vapour pressure between the warmer and colder parts of the cargo. From Table 1 it will be seen that the vapour pressure of interstitial air of a cargo of maize at 14% moisture content does not increase directly with temperature. Thus an increase in temperature from 15°C to 25°C will give a vapour pressure increase of 9.2mm Hg, whereas a rise in temperature from 25°C to 35°C will give a vapour pressure increase of 15.2mm Hg. It therefore follows, that moisture migration will be greater, all other things being equal, when moisture is moving from cargo at 35°C to cargo at 25°C than when moisture is moving from cargo at 25°C to cargo at 15°C, although the temperature difference in both cases is the same. Thus, when considering rate of moisture movement within a cargo, not only is difference in temperature important, but also the ‘actual temperatures’. A further factor is of course the differential in temperature in relation to distance – thus moisture will move more rapidly from cargo at 25°C to cargo at 15°C if the distance through which it must travel is only 1m rather than 10m, because it will be obvious that the vapour pressure gradient is much greater in the former case. In this respect the ‘thermal conductivity’ of the cargo in question is of considerable importance; the lower the conductivity, the slower heat will move through a cargo, and hence the less the potential for moisture movement.
Initial moisture content
The initial moisture content is also important. If we consider a cargo of maize at 14% moisture content
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