3. Ventilation
Ventilation is provision of supply of fresh untreated
air through a space.
The aim of ventilation should be replace air in the
hold with drier air which has a lower dew point and
so prevent the formation of sweat as far as possible.
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4. Why do we require ventilation on
board?
To avoid sweat which is forming on cargo and
shipside.
To reduce temperature of air in compartment and
bring it on par with ambient temperature.
To expel toxic, flammable or unwanted gases given
out by cargo.
To make compartment sufficiently safe for any
person to enter.
To prevent depletion of oxygen content of air inside
the compartment.
5. 5
To prevent damage to cargo which have following
properties:-
Susceptible to heat.
Subject to combustion.
Likely to contaminated by odours of other gases.
For the removal of taint or smell of previous
cargoes.
To supply the fresh air to the “live”cargoes.
6. Sweat
Sweat is formed when water vapour in the air
condenses out into water droplets once the air is
cooled below it’s dew point.
When these water droplets deposited on the ship’s
structure then it is called as “ship sweat”.
When the water droplets deposited on exposed
surface of the cargo then it is called as “Cargo sweat”.
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7. Types of cargo affected
Hygroscopic Cargoes – Hygroscopic products have
natural moisture content and are mainly of plant
origin. They may retain, absorb or release water
vapour and may lead to self heating and moisture.
e.g. Grain, Rice,Cotton,Tea, Sugar.
Non-hygroscopic cargoes - Non-hygroscopic
products have no water content. However certain
products {e.g .Steel} may be damaged if stowed in a
moist environment. Other may be harmed if
packaged using hygroscopic material
.e.g.Paper,Wood
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8. Three Degrees Rule
The three degrees rule may be applied when it is
impraticable to measure hold dew point temperature
accurately. In such cases ventilation requirements
may be estimated by, several times a day comparing
the average cargo temperature at the time of loading
with the outside air temperature. ventilation then
carried out on following bases.
“VENTILATE” if dry bulb temperature of outside ‘3
degree c’ cooler than average temperature of cargo at
the time of loading.
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9. 9
DO NOT VENTILATE if the dry bulb
temperature of outside air is less than 3 c cooler than
average temperature of cargo at the time of loading
or warmer.
10. Types of ventilation
As per construction ventilation has two types:-
1. Natural Ventilation.
2.Forced Ventilation.
As per area covered in ventilation there are two type:-
1. Surface Ventilation.
2.Thorough Ventilation.
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11. Natural Ventilation
This is carried out with help of cowl ventilators. They
are fitted on top of mast house or deck. The mouth of
ventilators can be turned around in appropriate
direction depending on the use.
It has following disadvantages :-
It is dependent on the direction of wind. Ventilation is
not possible there is no wind or if it is blowing from
abaft of the beam.
It reasonably effective for surface ventilation but not
enough for through ventilation unless the wind force is
high.
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12. 12
The air entering the compartment is expected to force
harmful gases out of the exhaust ventilators. However
this is not always effective and very often it merely
helps in diluting the gases inside the compartment,
unless wind force is sufficiently high.
If ventilation of cargo is very important, the cowls have
to be constantly trimmed in the correct direction
whether by day or by night with every shift of wind
direction . Any default on part of duty officer in this
regard could damage the cargo.
17. Mechanical/Forced Ventilation
This method overcomes the disadvantages of natural
ventilation system .
Electrical blowers are fitted inside ventilator to either
inject air into or exhaust the air from the compartment.
If only surface ventilation is required or is harmful gases
have to be expelled, then blowers of ventilators on one
side of compartment are operated in the exhaust mode.
As air with gases is expelled from compartment,
automatically fresh air enters through ventilators on the
opposite side without help of blowers this process
enables gases to be physically dragged out of
compartment and be replaced by fresh air.
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18. 18
1. Because of the shape of the ventilators, it cannot be used
for natural ventilation. Unless the blowers of atleast one
set of ventilators are operated, ventilation is not possible.
2.The biggest disadvantage of this system is the electrical
blower itself. If flammable gas is being expelled from the
compartment then due to sparking of motor or any
electrical junction , the gas could ignite.
20. 20
As blowers of atleast one set of
ventilators will be operated at a
time, these should be periodically
inter-changed during voyage to
avoid over-stressing the same
blowers.
22. Surface
Ventilation
In this type ventilation of only the surface of the
cargo is carried out without allowing too much air to
permeate within the lower levels of cargo. To provide
extra surface ventilation, the hatch cover may be
partially opened, provided the weather conditions
permit the same.
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23. Thorough Ventilation
In this type the air entering through one set of
ventilators reaches the bottom of the compartment
via trunking, rises upwards through the cargo and
finally exhausts out of another set of ventilators.
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24. Other Methods of ventilation
1. Cargo Battens = The purpose of wooden cargo
battens, which can be fitted horizontally or vertically
,is to keep the cargo off the ships inner steel hull. This
arrangement produces an air gap of about 230m.m.
between steelwork and cargo surface which reduces
the cargo sweat damaging cargo parcels .
2. Dunnage = The purpose of dunnage is that dunnage
provide air gap to underside of cargo. This is allow
ventilation around all sides of cargo stow and it
effectively reduce the risk of sweat damage to cargo.
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25. 25
Tank top ceiling :- A wood sheathing which covers
the steelwork of the tank top, in the way of the
hatchway in the lower hold. This timber not only
protects tank tops but also assists drainage of any
moisture migration in the space.
Separation of cargoes :-The idea of separation is to
ensure that the cargo parcels are not allowed to
become inadvertently mixed and the temperature of
cargo is changed.
26. BIBLIOGRAPHY :
IGNOU CARGO WORK BOOK
CARGO WORK BOOK
By DJ HOUSE
www.google.com
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