Methods of refrigeration used in general cargo ships
Why refrigerate?
Refrigeration is essentially the removal of heat through
the process of evaporation. We choose to refrigerate
commodities such as fruits and vegetables because we
want to prolong their “practical shelf life” – the time from
harvest until the product loses its commercial value.
Humidity is also a very important factor in the conservation
of many fruits and vegetables. By shipping products in our
Controlled Atmosphere containers, the humidity inside the
containers can be increased, thus minimising any weight
loss in the fresh produce.
Temperature is not the only important part of the
equation… but it is the key element to successful
refrigerated transport.
Technically speaking, the internal biological and chemical
processes of fresh produce, such as respiration, will continue
after harvesting. This means that the product absorbs oxygen
(O2) and releases carbon dioxide (CO2) and ethylene (C2H4).
This is a process that generates heat.
Lowering the temperature reduces the respiration, and
consequently the heat, considerably. Therefore, temperature
is the most important factor when prolonging the practical
shelf life. As high concentrations of CO2 and ethylene can
deteriorate the commodities, these gases must be removed
and replaced with fresh air through the ventilation system.
Ethylene production is especially high in fresh produce such
as apples, peaches, apricots, avocados and pears.
Many perishable cargoes are carried in refrigerated compartments on dry
cargo ships, and there are an appreciable number of vessels specifically
designed for carrying refrigerated cargo only.
Stowage of refrigerated cargo
Chilled meat cargo is hung
from the strengthened deck stiffening members, and the tween deck height is
arranged to provide space below the hung carcasses for the circulation of
air. Frozen meat is stacked in the holds of the ship. Fruits and vegetables
are stowed in a manner which permits an adequate flow of air to be maintained
around the crates, etc.
As a rule the refrigerated rooms in general cargo ships are made rectangular
to keep down insulation costs.
Refrigeration systems on board
Brine made by dissolving calcium chloride
in fresh water will have a freezing point well below the desired temperatures
of the refrigerated compartments. Cold brine may be pumped at
controlled rates to give the correct working temperature, and it is led from
the evaporator of the refrigerating machine to pipes at the top of the cold
compartment. The brine absorbs heat from the compartments and returns
to the evaporator where it is again cooled and recirculated.
Air must be continually circulated where fruit is carried to disperse any
pockets of carbon dioxide gas given off by the ripening fruit. The brine is
then led into grid boxes and air drawn from the bottom of the compartments
by fans is blown over the brine grids into the compartments via
trunking arranged along the ceiling.
Refrigerated ship midship section
Methods of insulation
As the steel hull structure is an excellent conductor of heat, some form of
insulation must be provided at the boundaries of the refrigerated compartments
if the desired temperatures are to be maintained economically.
Cork, glass fibre, and various foam plastics in sheet or granulated form
may be used for insulating purposes, also air spaces which are less efficient.
Glass fibre is often used in modern ships as it has a number of advantages
over the other materials; for example, it is extremely light, vermin-proof, and
fire-resistant, and it will not absorb moisture. On the decks and particularly
at the tank top the insulation must often be load-bearing material, and cork
might be preferred, but fibreglass can be supported by tongue and grooved
board linings and wood bearers.
The thickness of the insulation depends on
the type of material used and the temperature to be maintained in the compartment.
However the depth of stiffening members often determines the final depth. Insulating material is retained at the sides by galvanized sheet
steel or aluminium alloy sheet screwed to wood grounds on the frames or
other stiffening members .
Insulation on the boundaries of oil tanks, e.g. on the tank top above an
oil fuel double bottom tank, has an air space of at least 50mm between the
insulation and steel. If a coating of approved oil-resisting composition with
a thickness of about 5mm is applied the air gap may be dispensed with.
Suitable insulated doors are provided to cold rooms in general cargo
ships, and in refrigerated cargo ships the hold and tween hatches may
be insulated. Patent steel covers or pontoon covers may be filled with a
suitable insulating material to prevent heat losses.
A particular problem in insulated spaces is drainage, as ordinary scuppers
would nullify the effects of the insulation. To overcome this problem brine
traps are provided in drains from the tween deck chambers and insulated
holds. The brine in the trap forms an effective seal against ingress of warm
air, and it will not freeze, preventing the drain from removing water from the
compartment .
General Cargo in Refrigerated Chambers
The most important consideration when stowing general in refrigerated chambers is that of avoiding
damage to insulation, air-trunking and gratings. Where practical, soft packages and cases should be
stowed in the hatch wings while bulky or unprotected heavy cargo should be stowed in the hatch square.
These latter should be clean lift as dragging out from the wings and ends can cause damage to insulation.
Advantage follows if the floor or ceiling in squares of hatchways is protected by means of stout boards
or sheathing. Sizeable packages, having flat surfaces (cases in preference to bales), should be selected for
stowing against the pipe battens at sides and ends, so as to minimise the risk of cargo protruding into the
grids and, by exerting pressure on the piping, cause the same to leak at the joints.
The permanent battens should be in good order or, in the absence of such, battens should be provided for
the protection of the piping and cargo, such to be placed vertically, close spaced and "stopped" in position.
When bale, bag or other goods liable to damage from sweat or rust stains are carried, the piping as well
as other ironwork should be covered with kraft paper, etc.
Goods or liquids such as creosoted materials, aniline oils, essential oils, cargoes with a phenolic base,
chemicals, etc., which leave behind strong odours, which cannot safely be dissipated by ventilation, should
not, on any account, be stowed in refrigerated chambers. Some of the most serious claims for damage and
consequent costs for getting chambers again fit for refrigerated cargoes have been through phenol taint.
All dusty cargoes liable to choke air passages should be avoided. Heavy packages should be stowed on the ceiling with dunnage or bearers in order to avoid damage to
ceiling and insulation by evenly distributing the weight over the maximum surface.
it is essential that all products are treated correctly prior
to stuffing. Even though the temperature, ventilation and
humidity are all optimal during the entire voyage, products
will only arrive in perfect condition if the pre-treatment has
been performed correctly. Successful shipping begins at the
product sourcing area. More .....
On deck refrigerated containers are generally serviced by clip-on air cooled electric motor drive cooling units. The units are plugged into the
ships electrical system by way of suitable deck sockets. More .....
Proper packaging procedures will help protect frozen cargo
during transport. Frozen products do not require air holes in
the top and bottom of the cartons. Air flowing around the load
is sufficient to remove heat that has penetrated the container.
The cartons should be stacked directly on top of each other to
take advantage of their strength in the corners. More .....
Packaging plays an important role when it comes to protecting the cargo. The packaging material must be able
to support a stacking height of up to 2.4 metres (7’10’’).
The material should be able to withstand humidity without
collapsing, and should allow the passage of an adequate
vertical airflow through the cartons in order to maintain the
desired temperature.. More .....
Goods should be well stowed within the package,
evenly distributed and properly secured. Items
completely filling the case or carton contribute to
the strength of the whole package. Items which do
not completely fill the package must be cushioned
against shock or vibration. More .....
Proper ventilation of fresh, chilled products is necessary to
remove the heat, carbon dioxide and other gases produced
by the cargo. Heat is removed by continuously circulating
the internal air, whereas carbon dioxide and other gases are
removed by replacing the internal air supply with cooled
fresh air.. More .....
Refrigerated cargoes include meat carcases, carton (packed) meat, fruit, cheese, butter, fish and offal. Ships are specifically designed for their carriage, with separate spaces in holds and ’tween decks, each fitted with suitable insulation and individual control of ventilation. Ordinary general cargoes may be carried in the spaces at other times, the temperature being regulated accordingly for the type of cargo being carried. More .....
classification societies publish rules and regulations which are principally concerned with the strength of the ship, the provision of adequate equipment, and the reliability of the machinery . More .....
To maintain the assigned class all steel ships are required to be surveyed and examined by the Society’s
surveyors at regular periods.
The major hull items to be examined at these surveys only are discussed
here.. More .....
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