as moving stores which may pass through a variety of climates in a comparatively short period. A number of special problems are likely to arise in such circumstances.
There are only limited records of microclimatic conditions in railway boxcars or road vehicles which are relevant to the types of commodity supplied as food aid. However, technology now exists to use devices such as time-temperature integrators, data loggers and electronic chip monitors to generate the necessary data.
Ships’ holds
The temperatures of ships’ cargoes change very slowly during a voyage, and tend to remain at or close to the original temperature at the time of loading. On the other hand, the steel structure of the ship and the ventilating air admitted to the holds will have a temperature very close to that of the surrounding sea and air. Figure 6.1 illustrates the relationship between the temperature of a grain cargo, the temperature of the air in the hold, and the temperature of ambient air during a voyage from North
Effects of climate in stores and in transportation
America to West Africa. It is the difference between the temperature of the cargo and temperature of the air or the ship’s steelwork that can cause condensation within the hold. Condensation in ship’s holds is usually described in two categories.
• Cargo sweat is the term used for the consequences of condensation taking place directly on the cargo. It is brought about by loading the cargo into the ship at a low temperature and subsequently exposing it to warm moist air. A typical example is the shipping of food aid commodities in winter from a temperate climate to the tropics. As the vessel sails into the warmer regions the steel hull of the ship, in contact with the warm water and exposed to the higher ambient temperature and the radiant heat of the sun, will warm up. However, the temperature of the cargo is unlikely to change significantly, especially if it is carried in bulk or is tightly stowed. Under these conditions the temperature of the cargo may be below the dewpoint of the atmosphere, and moisture will condense directly onto it from the ventilating air during the voyage or on arrival at the tropical port, while the ship’s steelwork remains dry.
• Ship’s sweat is the term used to describe condensation on the cold ship’s steelwork of moisture originating from the cargo. Ship’s sweat is severe when a hygroscopic moisture-containing cargo, loaded into a ship’s hold at a high temperature (in a tropical port, for example) is carried into a region where the ambient and sea temperature fall quickly. The steelwork of the ship will cool quickly but the cargo will not, and it is this difference in temperature that may bring about the ‘sweating’. When the steelwork is cooled to below the dewpoint of the air in equilibrium with the cargo, condensation may form on the underside of the deck and on the cool sides of the hold, and this condensed water may then drip onto the cargo.
52
Effects of climate in stores and in transportation
40 Arrival at West African port
Unloading cargo
Temperature ˚C
Weeks
Figure 6.1 Relationship between the temperature of a grain cargo, the air in the hold and the ambient air during a voyage from North America to West Africa
The differences in temperature created by a voyage through various climatic zones can be further aggravated by temperature differences created by conditions within the ship. For example, a cargo stowed near the sides or bottom of the ship, or adjacent to a refrigerated compartment, is likely to be cooled. More serious perhaps is the warming of a cargo by heat from the engine room bulkhead, the propeller shaft, or the heating elements of fuel tanks. If such heating occurs the cargo may never cool sufficiently despite the operation of the ventilation system, and worse still it may cause the temperature of the cargo to rise to hazardous levels.
A further cause of high temperature within cargoes of durable commodities is
‘spontaneous heating’ due to the activities of insects and micro-organisms.
In Figure 6.2 some of the areas within a ship where warm or cool conditions are likely to prevail are shown, together with points where food residues may lodge and become the focus of insect infestation.
In conventional cargoes (bags, cartons, drums, etc.), the temperature effects can be reduced by keeping the commodities away from direct contact with warm or cool surfaces by means of wooden dunnage or fixed cargo battens. They can also be avoided by stowing non-hygroscopic cargoes in the hottest and coolest areas of the holds. Modern bulk carriers are usually designed so that the temperature effects – from the engine room bulkhead, for example – are likely to be minimal.
Older ships have cowl ventilators which take in or discharge air from the holds and provide a natural ventilation system, although in rough weather the ventilators are closed to prevent sea spray entering the holds. Modern vessels are equipped with forced ventilation systems, and the fans can be operated in any conditions to provide rapid changes of the hold air when necessary.
Effects of climate in stores and in transportation
Transverse beams
Figure 6.2 Areas within a ship where warm or cool conditions are likely to prevail
It is generally assumed that the risk of condensation in ships’ holds can be reduced by ventilation which either cools or warms the cargo and removes moisture.
However, cargo ventilation presents complex problems, and decisions as to whether or not to ventilate will depend on the nature of the cargo, its condition, and the prevailing climatic conditions during the voyage.
Such decisions are sometimes based on the measurement of the dewpoints of the air inside and outside the hold. It will normally be safe to ventilate if the dewpoint of the external air is lower than that inside the hold. However, even under these conditions, there might be occasions when it is unwise to ventilate. For example, during a voyage from a temperate to a tropical region the ship’s cargo will warm only very slowly and excessive ventilation might result in condensation forming on the exposed cargo surfaces.
The dewpoint inside a ship’s hold might be high when a warm hygroscopic cargo with a relatively high moisture content is carried from a tropical to a temperate region.
Ventilating with air of a lower dewpoint will tend to remove moisture from the hold, but the surface of the cargo might be cooled quickly if the temperature of the external air is much lower than that of the cargo. Under these conditions warm moist air rising from the warm cargo below could condense in the cooler surface layers and cause spoilage.
The British Standards Institution’s publication BS 4672-1971 provides some guidance on cargo ventilation, but there is often disagreement between experts on both the general aspects of ventilation and on the advisability of ventilation in specific circumstances. It is not surprising, therefore, to find that instances of ships’
sweat and cargo sweat still occur despite the use of modern ventilation systems and the vigilance of cargo officers.