Moisture


migration and surface ventilation


This article explains how and why moisture migration takes place and discusses to what extent surface ventilation can reduce or eliminate the damage to which moisture migration gives rise. The answer depends on the commodity; with grain in bulk, surface ventilation can do little or nothing; with rice or cocoa in bags, surface ventilation can do much more, but it cannot guarantee a sound outturn in all circumstances.


Movement of moisture


Moisture migration is the name given to the movement of moisture within a cargo. Thus a situation may arise where the total amount of water held in a cargo in a given space may be the same at the end of a voyage as it was in the beginning, but as a result of moisture migration, the moisture contents of various parts of this cargo have changed considerably (gains or losses being found). It is more usual, however, for part of the moisture that migrates to be lost to the external atmosphere as a result of ventilation, or to be drained off into the bilges.


Physical considerations


Vapour pressure (VP) and relative humidity (RH) Vapour pressure


The atmosphere comprises a mixture of nitrogen and oxygen in the proportion of 78% nitrogen to 20% oxygen; approximately 2% represents other gases and this includes water in the form of vapour. Pressure exerted by the atmosphere will partly be dependent upon the pressure exerted by the water in vapour form, and this proportion of the total atmospheric pressure is known as the ‘water vapour pressure’ of the air at that time.


Saturation vapour pressure Vapour pressure is measured in the same way as other gaseous pressures, i.e. in mm of mercury 1


. It will be


recalled that the normal atmospheric pressure at sea level is 760mm Hg.


1 Vapour pressure can also be measured in terms of other units – either in atmospheres or kilopascals. 1atmosphere (1 atm) = 760mm mercury (mm Hg) = 101.325 kilopascals (kPa).


13


As the quantity of water in the atmosphere increases, so the vapour pressure will increase proportionately. At a given temperature, the air can only hold a specific amount of water vapour, and the pressure exerted in the atmosphere when this limiting point is reached is referred to as the ‘saturation vapour pressure’ of the air at the particular temperature.


Super saturation


Any attempt to increase the water vapour in the air at this point will produce ‘super saturation’ and then water will be deposited from the air in liquid form, either as droplets to form fog or cloud, or on suitable surfaces in the form of water drops, e.g. as sweat in a ship’s hold.


Relative humidity


Under most circumstances, the vapour pressure of water in the atmosphere is less than the saturation vapour pressure. The percentage value of the actual vapour pressure in relation to the saturation vapour pressure is defined as the ‘relative humidity’ of the atmosphere. Thus, if the air only holds half its potential maximum amount of water in the form of vapour, then the relative humidity will be 50%, and at saturation vapour pressure the relative humidity will be 100%. Warm air is capable of holding more water vapour than cool air, so the actual weight of water that is required for saturation increases with increasing temperature. Thus for a given volume of air containing a constant weight of water vapour, the relative humidity will vary as the saturation vapour pressure changes with the temperature. If the temperature rises, the saturation vapour pressure will increase, so that the relative humidity will fall.


. However the saturation vapour pressure of air at 30°C is 31.8mm Hg. Therefore the relative humidity is 9/31.8 or 28.3%, i.e. by increasing the temperature 10°C, a fall in relative humidity of 23.2% has occurred. The reverse effect occurs if air containing a given quantity of water is cooled.


Temperature rises – relative humidity falls This phenomenon may be illustrated with an example. Let it be assumed that a given quantity of air at 20°C has a vapour pressure of 9mm Hg. The saturation vapour pressure of air at 20°C is 17.5mm Hg. Therefore the relative humidity is 9/17.5=51.5%. If the air is heated to 30°C, the quantity of water in the air remaining the same, then the vapour pressure of the air will still be 9mm Hg2


Relationship at different temperatures The graph shows the relationship between the vapour pressure and relative humidity at different temperatures, e.g.100% relative humidity at 10°C represents a water vapour pressure of 9.2mm Hg; at 20°C of 17.5mm Hg,


2 Actually, there will be a very slight rise in vapour pressure, but this may be ignored for the purposes of the example.


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