1.2 Types of ventilation (natural and mechanical ventilation system)
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.