16.3mm Hg. A change in the temperature of the maize will result in a change of the equilibrium relative humidity and in the vapour pressure. The table below shows equilibrium temperatures for maize at 14% moisture content. The temperatures at which saturation vapour pressure occurs (i.e. 100% relative humidity) are included in the table. These temperatures are known as the ‘dew points’.
Table 1. Temp / ERH / VP / DP – Relationship of maize at 14% moisture content
Temp Equilibrium RH Vapour pressure Dew point °C
%
15 20 25 30 35
60.0 64.4 68.0 71.5 75.0
mm Hg 7.1
11.2 16.3 22.9 31.5
°C 7.4
13.0 18.7 24.3 30.0
Thus, air at 25°C and 68% equilibrium relative humidity will have a vapour pressure of 16.3mm Hg, but if this air is reduced to a temperature of 18.7°C, then moisture will be deposited because the saturation vapour pressure will then be reached. If we assume that the ship carrying this maize of 14% moisture content and of temperature 25°C passes into a region of colder water, then the outside of the cargo will assume the temperature of the cold sides of the vessel, and if we assume this to be 15°C, it can be seen from the table that such maize will have an equilibrium relative humidity of 60% and a vapour pressure of 7.1mm Hg. The cooling process of the colder sea will not noticeably affect the maize in the centre of the bulk, since maize is a poor conductor of heat. Its thermal conductivity at normal moisture contents is less than five times as great as that of loose cork insulation and only one fifth the average value for concrete. Thus the maize in the centre of the stow will still have a temperature of 25°C and the interstitial air in this region will still have a vapour pressure of 16.3mm Hg.
A vapour pressure difference is therefore created, between the interstitial air in the maize in the centre and the interstitial air in the maize on the periphery of the stow. Consequently, there will be a flow of moisture vapour from the high pressure region to the low pressure region in order to equalise the pressure difference, and water will thus move from the centre towards the periphery.
This movement of water from the inner portion of the cargo will have the immediate effect of causing a reduction in the vapour pressure of the air there, but equilibrium conditions will be restored as result of more water moving from the grain into the interstitial air, so that the original vapour pressure of 16.3mm Hg will be
15 13
Argentine maize: The isotherm graph below shows equilibrium relative humidity plotted against moisture content.
17
16
15
14
12
11
10
9
8
30
40
50
60
70 Equilibrium relative humidity %
maintained. Consequently, there will be continuous flow of water vapour from the warmer part of the stow to the colder part.
Cargo sweat at periphery
In the example which we have given, the overall effect of this transfer of moisture vapour will be to cause deposition of physical water in the periphery of the stow in contact with the cold hull. This follows from the table, which shows that a vapour pressure of 16.3mm Hg at 25°C will have a dew point of 18.7°C. As this dew point is higher than the temperature of the cargo at the periphery, water will be deposited on the cargo. This illustrates the mechanism whereby ‘cargo sweat’ is produced3
.
The above example is an over-simplification of what happens in practice, since there is a tendency to set up a temperature gradient in the maize, along the route
3 Studies have shown that cargo sweat, i.e. the depositing of liquid water on the surface of maize grains is extremely unlikely, either as a result of the transfer of moisture from warm ventilating air to a cold surface layer of grain, or from warm moisture-laden air rising from a bulk of warm grain towards a cold surface layer. However, moisture transfer would still occur in either situation causing the moisture content of the cold grain to rise. The effect is therefore the same.
80
90
15 C o
20 C o
25 C o
30 C o
35 C o
Moisture content absolute %
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