loaded at 35°C with its periphery cooled down to 25°C, the equilibrium vapour pressures will be 31.5mm Hg and 16.3mm Hg respectively, giving a differential of 15.2mm Hg. Under the same temperature conditions, but with maize at moisture content of 11%, the equilibrium vapour pressures will be 22.4mm Hg and 11.6mm Hg, giving a differential of 10.8mm Hg. Thus the differential at lower moisture contents – therefore moisture movement – is less. In addition, (and this is of considerable practical importance), a much greater quantity of water can be absorbed by the cooler grain before the moisture content is raised to a level at which spoilage will commence.
Compactness
Because of the importance of convection currents in moving moisture, the more readily air can move through a cargo, the more rapidly moisture can be carried through that cargo in the moving air, so that all other things be in equal, there will be more rapid moisture movement through a cargo that is less compact (e.g. pellets) than through a cargo which is for example powdered, where the movement of air will be very limited.
The cargo itself Finally, when considering the rate at which moisture may move through a cargo, it is necessary to consider the nature of the cargo itself. Thus cargo such as grain, which consists of seeds grown in dry climate, has comparatively low moisture content and the seed itself has a protective outer skin, which is relatively impermeable to moisture. In fact, one of the main purposes of this skin is to prevent the seed from drying out, either during growth or subsequent to growth and prior to germination. Thus moisture is released rather slowly from seed products such as wheat and maize when compared with other products, particularly those grown under wet conditions in the tropics, where there is no natural necessity to conserve moisture. Similarly, whole grain will lose moisture much more slowly than grain that has been milled or pulverised in some way, where the natural protective coating is disrupted.
Quantitative data for the release of moisture from various products is scanty, and direct comparisons are particularly difficult. We have therefore not been able to give examples to illustrate the above.
When studying moisture movement there are two factors of interest. The first is the actual quantity of water moving from one place to another. The second is the rate at which the ‘zone of enhanced moisture’ moves forward. We have done some work on the latter factor with maize.
17
It was found that in 28 days, a zone of enhanced moisture had moved approximately 1m in a vertical direction (i.e. with convection currents reinforcing the moisture movement) from the hot spot. The temperature differential in this experiment was from 40°C to 21°C over a distance of approximately 1.25m. The actual quantities of water involved could not be accurately determined. There is no doubt whatsoever, that with other types of cargo both the rate of movement and the quantities of water moved would have been many times greater than was found with maize.
Therefore, when considering the significance of potential moisture migration in a cargo, it is necessary to consider the vapour pressure differential in relation to the distance between the hotter and colder zone, the temperature of the hotter material and the temperature of the colder material to which moisture is migrating, the initial moisture content, the nature of the cargo and the ease with which air may move through it.
Practical application To take simple practical illustrations of this, it is not unusual to shift bulk grain round the world in tankers (where of course there is no possibility of ventilation) and to store bulk grain in unventilated silos for long periods of time where considerable differences in temperature can occur between summer and winter. This is only possible because under normal conditions, the rate of moisture migration in bulk grain is low. When cargoes of cocoa or rice are considered, the rate of moisture migration is many times greater. It would of course be courting disaster to attempt to carry cocoa from West Africa to Northern Europe in tankers. Thus the quantitative aspects of moisture migration are of primary importance when considering the best method of carrying a particular cargo on a particular voyage.
No general rules
In the following section, we discuss ventilation in general terms in order to illustrate how the use of ventilation can assist in minimising the deleterious effects of moisture migration. Because of the many factors involved however, it would be unwise to attempt to formulate general rules for the carriage of cargo to minimise the effects of moisture migration.
Grain in bulk General
Vessels which carry grain in bulk vary in their capability for ventilating the cargo. Considerable quantities of grain are carried in tankers with no ventilation whatsoever. Sometimes grain is carried in vessels fitted with a sophisticated Cargocaire system of surface ventilation, which also has facilities for pre-conditioning
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34