Why is there a need for ventilation of cargo spaces?
Most cargoes are loaded in cargo holds which are closed before proceeding to sea to prevent any external elements like bad weather, water, heat, cold from affecting the cargo inside. But when the voyage is long, keeping the hatch totally closed, can cause various problems such as:
- Deterioration of cargo due to ship/ cargo sweat
- Cargo damage due to heating of cargo
- Hazards posed due to presence of a dangerous gases in the hold emitted by cargo (Toxicity, flammability etc)
- Accelerated corrosion of the ship's structure
The main objective of merchant shipping is to carry cargo safely from one place to another. The ship and hence the ship owner is liable if there is any damage to cargo during transit. For this reason you should know how best to ventilate the compartment to prevent any damage to the cargo and environment which is safe for crew members to enter and/or work.
Also, if we do not ventilate the cargo spaces there will be an increase in the temperature of the hold, especially when the ship is sailing from a colder to a warmer region as the deck which is exposed to the sunlight will heat up and by conduction the temperature of the air inside the hold will begin to rise. An increase in the sea water temperature will also have the same effect on the shipside plating and consequently on the hold temperature.
"Moisture damage" is the source of a significant number of cargo claims, often involving bagged or bulk agricultural products. Claimants typically allege that failure by the ship to ventilate correctly resulted in the development of condensation ("sweat"), causing the cargo to deteriorate.
The air at sea is very humid and it contains a large quantity of water vapour. When this humid air comes in contact with relatively colder surfaces, the moisture condenses on the surface. When the air in the hold comes in contact with bulkheads / decks which are colder than the dew point of the air, the moisture condenses on the bulkheads/ decks forming ship sweat. Also when the humid air comes in contact with the cool cargo (temperature lower than the dew point of the air) moisture condenses on the cargo forming Cargo Sweat.
The amount of water vapour contained in a parcel of air depends on the temperature of the air. The parcel is said to be saturated if it has the maximum amount of water vapour it can hold at that temperature. If this parcel is further cooled excess water vapour will condense in the form of "sweat"
Ships are fitted with either natural or mechanical ventilation systems. In addition to minimizing the onset and degree of sweat, ventilation may also serve to remove taint and disperse any gases which some cargoes may emit.
Cargo sweat refers to condensation which may form on exposed surfaces of the cargo as a consequence of large amounts of warm, moist air being persistently introduced into a hold containing substantially colder cargo.
Ship’s sweat refers to condensation which forms directly on a vessel’s structure when the air within a hold, made warm and moist by the cargo, comes into contact with cold surfaces as the vessel moves into cooler climates. Cargo may be damaged by overhead drips, by contact with sweat which has formed on the ship’s sides or by condensed water which may accumulate at the bottom of the hold.
The amount of water vapour that air may contain is highly dependent on its temperature. A given volume of air is said to be saturated when no more water can be absorbed. If the air temperature then falls, condensation will occur.
As air rises in temperature so does its saturation moisture content; its capacity to retain water climbs by ever-increasing amounts. Thus, when hot air is cooled, its potential for releasing water in the form of condensation is far greater when it is cooling from higher temperatures than when cooling from lower temperatures.
Apart from periods of fog or rain, ambient air is rarely saturated. Moreover, it will never be totally dry. Within these two extremes the amount of water retained by the air will vary according to the prevailing conditions.
Relative humidity is the actual amount of water vapour in the air compared with the saturation amount of water vapour in the air at the same temperature and pressure. The figure is usually expressed as a percentage, with saturated air having a relative humidity of 100%.
At main deck level, ambient sea air over the open oceans will normally have a relative humidity in excess of 80%.
Dew point temperature
When an isolated volume of air cools, relative humidity increases as the temperature falls. Once the temperature has descended, to the level at which saturation occurs, water begins to condense. This temperature is known as the "dew point".
Dew point temperature may be measured by a variety of methods. Ships generally use a traditional wet and dry bulb arrangement consisting of two identical mercury thermometers, one of which has a damp wick covering the bulb. These are normally housed in a protective marine screen.
The dew point temperature may then be determined by using a "Dew point Table". This figure is important when considering cargo ventilation requirements.
Wet and dry bulb thermometers
When using traditional wet and dry bulb thermometers, the accuracy of the dew point temperature will depend on the condition of the equipment. The muslin covering the wet bulb should be clean, the water in the reservoir should be distilled and the bulb itself should be wet.
In order to ensure that the readings are correct, the device should always be positioned clear of any exhaust vents, other draughts and all sources of heat.
Dew point measurement
Theoretically, all decisions regarding cargo ventilation should be based on dew point temperatures, comparing the dew point of the ambient air with dew point of the air inside the hold.
Given that most ships are customarily equipped with wet and dry bulb thermometers located close to the bridge, determining the dew point temperature of the ambient air is usually straightforward.
However, ascertaining the dew point temperature inside a cargo space is more problematic. One of the simplest methods is to use a "whirling psychrometer", swinging the instrument inside the hold until the wet bulb temperature has stopped falling and remains steady.
All readings should be taken well away from any air inlets, ensuring that only hold air is tested. Enclosed space entry procedures should always be observed. If access to the holds is impossible or undesirable, and provided there is no significant air flow, wet and dry bulb thermometers may be placed in the trunking of an exhaust ventilator or similar pipework leading from the compartment, allowing the device to be drawn out and read from above deck.
Once the above information has been obtained, the rules are simple;
Dew point Rule
VENTILATE if the dew point of the air inside the hold is higher than the dew point of the air outside the hold.
DO NOT VENTILATE if the dew point of the air inside the hold is lower than the dew point of the air outside the hold.
Three Degree Rule
In many instances it is impracticable to measure hold dew point temperatures accurately, or at all. In such cases ventilation requirements may be estimated by comparing the average cargo temperature at the time of loading with the outside air temperature several times a day. Ventilation may then be carried out on the following basis;
VENTILATE if the dry bulb temperature of the outside air is at least 3°C cooler than the average cargo temperature at the time of loading.
DO NOT VENTILATE if the dry bulb temperature of the outside air less than 3°C cooler than the average cargo temperature at the time of loading, or warmer.
In order to apply the Three Degree Rule, it will be necessary for the ship’s staff to take a number of cargo temperature readings during loading.
The rules for ventilation are simple and are easily illustrated by the following examples.
Cargo sweat: Consider the following case:
- A ship has loaded steel cargo at Antwerp in January for shipment to Singapore. Antwerp in January is a winter zone and the temperature is low.
- As the cargo spaces would be sealed after completion of loading, the cargo and the air temperature inside the hold will be cold.
- During the voyage to Singapore the outside atmospheric temperature will rise as the ship passes from winter to the tropical zone.
- Compared to the volume of air inside the hold, the volume of air brought into the hold through the ventilator ducts at any given time is small.
If ventilation is permitted - the warm moist air entering the cold cargo space will be cooled and if cooled below its dew point will condense on the cold steel cargo surface causing cargo sweat. This can cause the steel to rust.
Here the solution is to restrict ventilation. Do not ventilate when passing from a cold area to a warm area, always allow the air and cargo in the hold to warm up naturally.
Exactly the same phenomenon causes ship's sweat.
- Consider a ship having loaded cargo in a warm region is proceeding towards a cold region. For example: bagged rice is loaded in Bangkok, in November, for shipment to Inchon in South Korea.
- At the time of loading, the ambient temperature in Bangkok was 26 degrees C. The cargo and the air inside the cargo hold is warm and moist and would continue to remain so for some time after the hold is sealed.
- Air and seawater temperature starts to drop as the voyage progresses.
- As the seawater and the outside air cools the air inside the hold which is in close contact with the steel plates also cools.
- When this parcel (air inside the hold adjacent to the steel plates) is cooled below its dew point, water vapour is given out which condenses on the ship's structure forming "ship's sweat".
- This does not directly cause a cargo problem but when the moisture droplets run down the beams and drips onto the cargo, which it invariably does, it may result in cargo damage and subsequent claims.
To prevent this ventilate the compartment freely. The rule is to ventilate when travelling from a warm tropical zone to a cold winter zone.
"HOT TO COLD, VENTILATE HOLD, COLD TO HOT, VENTILATE NOT"
Precautions with respect to ventilation:
- It may not be always possible to restrict ventilation for sweat reason alone. Some cargoes give off odour or obnoxious gases which may damage or taint other cargoes. Here ventilation may become necessary. In such cases where ventilation has to be done - AIR SHOULD TO BE PASSED THROUGH DE-HUMIDFYING UNITS. This will ensure that condensation does not take place inside the hold. De-humidification is carried out by using chemicals like silica gel or activated alumina which have a high affinity for water and absorb all the moisture in the air. The same chemicals can be heated to dry out the absorbed moisture and can be reused.
- During periods of heavy weather, steps should be taken to prevent rain and spray from entering the cargo spaces. This may mean suspending ventilation until conditions improve. If so, the circumstances should be logged.
- It is important to appreciate that ventilation should also be carried out during the night if the readings indicate that ventilation is appropriate. Ambient temperatures are usually lower at night, therefore the risk of ship’s sweat developing is more likely during the hours of darkness.
- In addition to ventilating the holds according to the above regimes, it is important that regular inspections of each compartment are carried out where possible. This need not involve entry into the cargo space itself - for example ship’s sweat may be seen forming on the underside of manhole covers. In such instances, and especially at night, the cargo should be ventilated irrespective of the Dew point or Three Degree Rules, weather permitting.
- STOWAGE: Given the sensitive nature of many hygroscopic products and the possibility of sweat, efforts should be made to ensure that such cargoes do not come into contact with hold steelwork. This is particularly important in the case of bagged agricultural produce intended for human consumption such as rice, beans and flour.
For bagged cargo, rows of dunnage or bamboo poles should be laid in the direction of the bilges to aid drainage, not more than 20 cms apart. A second layer should be placed on top at right angles to the first before covering the whole area with matting.
If the cargo space is not fully fitted with cargo battens, bamboo poles or dunnage should be positioned crosswise against the frames to keep the bags away from the sides of the ship. Ideally, they should also be lashed together at the intersections to prevent them from becoming disturbed during loading. As an extra but not essential precaution, mats may be placed against this arrangement. In the same context, the top surface of the stow may be covered with thick kraft paper.
Expert opinion is that biologically stable bagged hygroscopic cargoes do not require ventilation channels, unless specifically demanded by the IMDG Code (e.g. some types of seed cake, fishmeal). Nevertheless, for certain commodities many charterers still require ventilation channels to be built into the stow. If so, the charterers should be asked for written instructions regarding the number and position of such channels, and these should be followed accordingly.
- Bunker tanks: Hygroscopic products may be damaged by localized sources of heat. Incidents have occurred where parts of parcels of grain have been scorched or have become discolored when lying against hot bunker tanks. As far as possible, the bunkers used during the voyage should be drawn from tanks situated well away from holds containing hygroscopic products. If impracticable, bunker tanks adjoining cargo spaces should be heated only when required, ensuring that the temperature does not rise above normal operational levels.
Ventilation Procedure of Cargo Space:
It should be noted that indiscriminate ventilation often does more harm than no ventilation whatsoever. It is also of concern that variation in the angles of ventilators away from the wind can cause very different rates of air flow within the compartment. The angle at which the ship’s course makes with the wind also affects the general flow of air to cargo compartments. In general, the greatest air flow occurs when the lee ventilators are trimmed on the wind and the weather ventilators are trimmed away from the wind.
- Through ventilation:
In this case air is passed through the entire area of the hold right down to the bottom. This can be achieved by means of ducts, double layering of the bottom dunnage and use of side battens to allow free flow of air. The air entering through one set of ventilators reaches the bottom of the compartment via trunkings, rises upwards through the cargo and finally exhausts out of another set of ventilators. In case of natural ventilation this is achieved by turning the leeward ventilator into the wind and the windward ventilator away from the wind, thereby achieving a greatest air flow rate which is required for through ventilation.
- Surface ventilation:
Certain cargoes like coal give off flammable gas and generate heat due to exothermic reaction. If the heat generated over a period of time is sufficiently high, then the presence of the flammable gas and the atmospheric oxygen in the right proportion could lead to spontaneous combustion. Hence it is important that the flammable gas is expelled but at the same time too much oxygen should not be injected into cargo. This is achieved by only ventilating the cargo on the surface known as surface ventilation.
This is achieved by trimming the windward side natural ventilator to the wind and the leeward side ventilator away from the wind giving a very easy flow to the wind. If the ventilator has trunkings which lead to the bottom of the hold - it should be kept shut. This type of ventilation also helps to control the rise in temperature of both the cargo and the hold spaces.
If the dew point temperature in the cargo compartment can be retained below the temperature of the ships structure, i.e. decks, sides, bulkheads and the cargo, there would be no risk of sweat forming. Such a condition cannot always be achieved without some form of mechanical (forced) ventilation from fans or blowers. There are several excellent systems on the commercial market which have the ability to circulate and dry the air inside the cargo holds. Systems vary but often employ ‘baffle’ plates fitted in the hold and tween decks so that air can be prevented from entering from the outside when conditions are unfavourable. Systems re-circulating the compartment’s air can also operate in conjunction with dehumidifying equipment to achieve satisfactory conditions pertinent to relevant cargo.
Some cargoes requiring special attention
- Coal cargoes: Most types of coal evolve methane gas on the surface thereby increasing the temperature of the hold. In combination with oxygen, spontaneous heating can also take place. Hence for coal cargo no through ventilation should be carried out. Only surface ventilation to be carried out at regular intervals.
- Grain cargoes: These are subject to germination, depending upon temperature and the presence of moisture. Through ventilation will therefore be essential.
- Coffee, tea, cocoa: There are very expensive and delicate cargoes likely to be damaged due to staining, tainting and odour. They should not be stowed with cargoes like sugar, oils of any kinds, copra or any other cargo which is very hygroscopic. Coffee is susceptible to moisture damage. Surface ventilation to remove moisture and odour from the compartment is advisable.
- Rice: This likely to suffer moisture damage. It heats up quickly and evolves carbon dioxide. Surface ventilation should be used. With mature grain less ventilation is required.
- Molasses: Molassesor green sugar ferments giving a rise in temperature. There is formation of ethyl alcohol, carbon dioxide, glycerol, aldehydes and various other acids.The acids could affect the ship's steel structure causing corrosion. Through ventilation is vital.
- Fruit cargo: These require extreme temperature control or else they will be damaged. Hence refrigeration is usually required with constant monitoring of temperature. In case of short voyages where fruits may be carried without refrigeration, surface ventilation for removal of any gases formed, should be adopted.
- Paper cargoes– paper may be carried in many forms from waste paper to newsprint. The compartment, in whatever form the paper is to be carried, must be in a dry condition and well ventilated. Newsprint is carried in rolls which are normally stowed on their ends to avoid distortion, preferably on double dunnage. A ship’s steelwork would normally be protected with waterproof paper to prevent ships sweat from damaging the rolls.
- Copra– dried coconut flesh. Liable to heat and spontaneous combustion. It could taint other cargoes and cause oxygen deficiency in the compartment.Requires good surface ventilation.
The ventilation requirements for various cargoes are readily available in Thomas stowage or other similar publications. For bulk cargoes, the shipper will provide information regarding the cargo and its moisture content in the form of a "Shippers Declaration".