English Maritime Terminology on Naval Engineering

The cargo holds of most dry cargo ships have either natural or mechanical ventilation systems. A natural ventilation system, as the name suggests, is based on nature’s law of air circulation. Air can enter and leave the hold through the vents located above the deck level. This allows cool, heavier air from the outside atmosphere to replace the lighter, rising warm air inside the cargo hold. Some older designs of dry cargo ships were fitted with ventilator shafts leading to the lower parts of the cargo hold. In these cases, air circulation in the cargo hold was achieved by trimming the ventilators (cowl direction) with respect to the wind direction. This was more effective on general cargo ships carrying bagged or breakbulk cargoes requiring through ventilation. Through ventilation is when the air is forced into the body of the cargo. It is generally not required for solid bulk cargoes and is difficult to achieve. When a hold is loaded with a solid bulk cargo, the ventilation provided is usually surface ventilation, with air flowing over the surface of the cargo from ventilators. On newer designs of dry cargo ships (particularly bulk carriers), the hinged-door type ventilators are often fitted on the sides of the hatch cover. These can then be opened depending on the relative wind direction to provide adequate surface ventilation within the cargo hold.

Mechanical ventilation, in contrast, is a system where the movement of air is forced using electrical fans within the ventilator shafts. All type of fans can be present in cargo holds (exhaust, supply or reversible). With some systems, it is possible to vary the speed of the fans to further control the ventilation. An open-circuit system draws atmospheric air into the hold by fan induction and exhausts it from the hold by fan extraction. The capacity of a ship’s hold ventilation fans is normally expressed in the number of air changes that can be achieved in an empty hold per hour. When cargo is carried, the number of air changes per hour increases, as there is less air in the hold. It is important to note that if a wrong decision has been made to ventilate cargo on ships with means for mechanical ventilation, when in fact the cargo should not have been ventilated, the resulting damage to the cargo would be much greater than the cargo damage caused by improper ventilation on a ship provided with natural ventilation only, due to mechanical ventilation having much greater air exchange capacity than natural ventilation.

It is important to be aware that on some voyages the only way of removing moisture from the cargo hold atmosphere, thereby eliminating the possibility of condensation, is through air-conditioning of the hold. A closed-circuit system uses a dehumidifier and temperature control system so that each hold can be supplied or recirculated with dry air. The air is dried by drawing it through a conditioning plant with a moisture absorbing solution before it is passed into the cargo hold ventilation system. Alternatively, portable dehumidifiers can be fitted in the cargo holds to remove moisture from the hold air and control the dew point. These dehumidifiers utilize a desiccant material to reduce moisture content in the air. Typical systems may include a fluted or corrugated media configured in a rotating mass. As the air passes through the flutes of the material, it contacts the desiccant and gives off its moisture.