Methods of ventilation used in general cargo ships

Methods of ventilation used in general cargo ships

Ventilation requirement
Not every commodity requires or even benefits from ventilation during transport. Cargo loaded in bulk carriers may only benefit from surface ventilation, i.e. a change of air between the underside of the deck or hatch above and the surface of the commodity. It follows that if there is a problem within the stow, such as pockets of moisture, ventilation will have little effect.

Ventilation of cargo may be necessary to remove heat, dissipate gas, help prevent condensation and/or remove taint. Heat may be generated by live fruit, wet hides, vermin, and commodities liable to spontaneous combustion. Gases which may require dissipation include inflammable and explosive gases such as those emitted by coal, vehicle exhausts in Ro-Ro ships, CO2 and ethylene from ripening fruit and vegetables.

Condensation appears in two basic forms, i.e. ship sweat and cargo sweat. The former appears as beads of moisture condensing on the ship's metalwork and, typically, might occur on the side of the hold when the sea temperature falls and reduces the adjacent metal temperature below the dew point of the surrounding air. Similarly, cargo sweat forms on the surface of the cargo when its temperature is below the dew point of the air adjacent to it.

The removal of existing or residual taint by means of ventilation may be carried out in conjunction with an ozonating unit and may be required to reduce the probability of sensitive cargo becoming affected by the taint, or to improve working conditions for labour.

The proper ventilation of holds therefore, is indispensable to the correct carriage of some goods, and may assist in the preservation of the ship's structure itself. Sometimes with coal cargoes and those which give off inflammable and explosive gases, or are liable to spontaneous combustion, it may be absolutely necessary for the safety of crew and ship.

A well designed system of ventilationnatural or mechanicalaims at inducing a constant circulation of air throughout the holds and the mass of cargo by which heat, moisture, fumes, vapours, gases and odours given off by the cargo are discharged into the open air and replaced by air of a dew point suitable to the prevailing conditions. Three systems of ventilation are in general use:

a. The natural ventilation of ordinary cargo compartments, supplemented in certain cases by modified mechanised air circulating systems.
b. The temperature control system of circulating air in insulated compartments, controlling its temperature and the carbon dioxide content given off by certain commodities .
c. Mechanical ventilation in cargo spaces and control of humidity in the compartment.

The principle of most of these types of mechanical ventilation is to reduce the dew point of air in ships' compartments by de-humidification to the point at which ships' sweat and cargo sweat cannot occur. No attempt is made to change the temperature condition in the holds, the whole object being to control the dew point of the air surrounding the cargo.

Correct ventilation can keep cargo in good condition in the great majority of circumstances if properly employed. No system of ventilation can prevent damage if the ship's officers do not know when to and when not to use it. Also, of course, stowage of the cargo must be correctly planned with regard to the various classes of commodities of which it is composed, together with proper dunnaging and positioning to ensure that the ventilation reaches all the necessary cargo.

Open-top, open-sided and flatrack containers may sometimes be stowed with cargo requiring ventilation while on board, e.g. fruit, vegetables, spices, etc. If stowed on deck then it must be ensured that adequate ventilation can reach the contents while properly protecting them from rain and spray. Additionally, the container should be stowed on deck in such a position that the tilt is protected against damage from wind and waves during the passage. Under deck, the stowage position should be so situated as to take maximum advantage of any mechanical ventilation and care should be exercised against cross-taint, e.g. a container of wet hides might leak strong-smelling brine.

When to Ventilate Some Guidelines

Ventilate if the dew point of the air is lower than the dew point of the cargo space. The changing air will ventilate the cargo space and there should be little danger of condensation.

Restrict ventilation if the dew point of the outside air is higher than the temperature of the cargo. To ventilate under such conditions would cause danger of condensation on cargo as the outside air with higher dew point comes into the cargo space.

When the vessel is fitted with a mechanical ventilation system, the air should be re-circulated within the cargo space when the introduction of external air is restricted. Despite this guidance special attention must, of course, be given to conditions where cargoes are giving off fumes, etc., where according to certain circumstances ventilation may be necessary despite the possibility of condensation. The aim should always be to avoid stowage of cargo requiring different ventilation treatment within the same compartment. Similarly two compartments with different ventilation requirements should effectively be sealed off from each other.

It is often required that cargo normally of a very dry nature, e.g. milk powder, casein, etc., has to be stowed in compartments adjacent to hard frozen chambers. Experience has shown that the most effective method of carrying cargo under these conditions is to restrict the ventilation completely during the passage. In some refrigerated vessels, decks or bulkheads are fitted with thermal injection. The principle of such being to provide a heat input equal and opposite to the heat loss from the refrigerated compartment. The fitting of this equipment is of considerable value in ensuring safe out turn of cargoes stowed under such conditions, but does not warrant departure from the basic principles of ventilation.

The importance of maintaining a dry ship and dry cargo cannot be over emphasised. The cargo should be protected from all unfavourable exposures of temperature and moisture from the point of origin to the delivery and the shipowner, for his own protection, should know thoroughly the history of the products delivered to his charge and of the condition in which they are received. Typical of this category is the carriage of perishables in open top and open side containers. It is often the practice for the tilts of these containers to be left open while stowed below decks to improve the ventilation.

These covers should be replaced and secured at time of (or prior to) discharge. Similarly, when loaded, the ship's staff should check the appropriate covers are drawn back to take maximum advantage of under deck ventilation. When terminal staff have adjusted covers for the same purpose during the container dwell time at the terminal, it is important that ship's staff and terminal staff liaise to maintain the weathertight integrity of these containers while at the same time affording the contents maximum ventilation.

The responsibility of the terminal operator includes protection of the cargo on the wharf awaiting loading, in so far as practical conditions permit. Stowage, subject to the same limitations, should provide the most advantageous condition in the ship. Through an understanding of what takes place within a loaded ship, the officers can reduce sweating hazards and provide intelligent operation or sealing of ventilation systems.

There are two options in meeting the problem of cargo sweat: the control of the dew point in all parts of the loaded cargo space or the tempering of the cargo to keep it above the dew point of the air to which it may be exposed while at sea or at warm ports of call or at its destination. As an example, canned condensed milk is sometimes shipped in a warm condition out of temperate latitudes so that cargo sweat has less chance of occurring during the passage through warmer climates.

Ship sweat may be prevented by air conditioning systems efficiently designed and efficiently operated. Air conditioning plants are not the panacea for all ills and unless the cargo being served by them is delivered in temperatures above the dew point of the air at the port of delivery, there is no guarantee that condensation will not occur.

The temperature of the air both outside and in different cargo compartments should be regularly taken and recorded. It is of utmost importance that attention to the ventilation of cargo be clearly and regularly recorded in the Deck Log with special mention made of any opening of hatches, etc. The absence of such records may result in large claims for alleged neglect.

In most ships a combination of natural and mechanical ventilation is provided in the accommodation and machinery spaces. Mechanical supply is common, and a natural exhaust may be permitted in a number of compartments; but where fumes, etc., are present, for example from galleys, a mechanical exhaust is required. This is also the case with many public rooms, often crowded and where smoking is permitted. The mechanical supply is by means of light steel sheet trunking, with louvres at each outlet. Fans may be of the quiet running centrifugal type with a separately mounted motor.

The holds of most dry cargo ships are ventilated by a mechanical supply and natural exhaust system. Here the object is to reduce the hold temperatures if necessary and prevent large amounts of condensation accumulating on the hull and cargo. Often the cargo hold fans which are of the axial type are located in the mast houses, although they have been positioned in derrick posts where these posts are used to ventilate the tweens and holds .

Dry cargo ships may also be fitted with de-humidification facilities, controls being provided so that each hold can be supplied with dry air or outside air. If dry air is desired when the weather dew-point approaches or is above the temperature in the hold, the air supply or recirculating air may be drawn through a conditioning plant where it comes into contact with a moisture absorbing solution. A dry air fan then passes this de-humidified air to the cargo hold ventilation system. Air conditioning is a common feature in the accommodation of modern ships. Room temperatures are controlled by a thermostat, heated or chilled air may then be supplied as required and humidity control is also provided.

Cowl ventilators

Natural Ventilation method

In natural ventilation method , cowls are trimmed into the wind to take in outside air, and trimmed back to wind to allow the air circulation an exit from the hold. Fans may be incorporated into this cowl ventilator system especially for the lower hold regions where fans assist delivery and air extractors assist the exhaust system. Cowls may also be fitted with manually operated closure flaps.

Forced Ventilation method

More recent developments in ventilating systems have led to air being predried before entering the hold. In some cases the temperature of the air as well as its humidity may be controlled before entering the compartment .This artificial or forced ventilation, has become increasingly popular because, when properly used, it can almost prevent any sweat damage to cargo.

Summarized below some more details on general cargo ship cargo handling procedure and operational info:

Cargo handling procedure for general cargo ship

Methods of ventilation used in general cargo ships

Machinery system main info pages