Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means.
Refrigeration has many applications, including, but not limited to: household refrigerators, industrial freezers, cryogenics, air conditioning, and heat pumps. Transporting refrigerated cargo in reefer containers can be classified as
food or non-food items and as chilled
or frozen items.
Fig:Frozen meat products
Stored below minus 10°C, these are
`dead' cargo, ie no chemical processes
or reactions take place within the
product and no gases or heat is
It is a general misconception
that for all refrigerated cargoes,
the lower the temperature, the
better they are preserved.
For chilled cargoes, the
temperature must be maintained
very close to the set-point. If the
temperature falls below the set-
point, the cargo can be damaged
Frozen cargoes are not damaged
even when the temperature falls
below the set-point for prolonged
periods of time. If, for example,
the temperature of ice cream is
maintained at minus 25°C instead
of minus 20°C, there is no damage
or deterioration. Because there is
no heat produced by a frozen cargo,
it is much easier to maintain the
When there is a power outage or
a planned shut-down of the ship's
power generation, it should be
remembered that chilled cargoes
are more vulnerable to damage and
deterioration due to heat generation
and build-up inside the container.
Where possible, chilled cargo reefer
containers should be clustered
together and connected to one
power source or circuit breaker. Power
should be restored as soon as possible
to chilled cargo containers, before the
frozen cargo containers. The frozen
cargoes are not as easily damaged
since they do not produce heat and
the temperature rise is very slow.
Chilled Cargoes and chilled operation mode
During chilled operation the circulating fans run at maximum rpm and the intake air temperature is regulated constantly.
Chilled cargoes, also known as
perishable cargoes, are stored above
minus 10°C. Some chilled cargoes
are live and respiring, with chemical reactions and processes taking place
within the product and a continual
output of gases and heat.
Fig:Chilled Fruit products
Chilled cargoes are temperature
sensitive. The heat some chilled
cargoes generate through chemical
reactions needs to be removed
outside the cargo space quicker than
it is released to prevent heat build-up
and a rise in temperature. Some
cargoes require that the gaseous
products of the chemical reactions
also be removed to avoid damage
and deterioration in the quality of
the product.Chilled cargoes include fruit,
vegetables, dairy products,
chilled meat, flower bulbs,
electronic equipment and
The temperature of a chilled cargo
must be maintained within a very
narrow band of ±0.5°C around the
set-point. Temperatures outside this
can cause the cargo to deteriorate.
Chill damage is caused when the storage temperature goods of vegetable origin drops below the specified limits. This process means that fruit suffers irreparable metabolic damage which renders it incapable of ripening.
Type of packaging & stuffing methods
Fig:Packaging requirement of chilled and frozen cargo
Each commodity has different airflow requirements. Inside a
reefer container the airflow is influenced by the type of packaging
and the method of stuffing used. In the case of pre-cooled
frozen goods, air only has to flow around the cargo, since no
heat has to be dissipated from the goods themselves. Only the
heat which penetrates the insulation from outside has to be
removed. When transporting chilled goods such as fruit and
vegetables, however, air also flows through the cargo, as it
generates respiration heat internally which has to be dissipated.
There are two standard loading patterns for perishable products
in reefer containers:
block stow of break bulk cargo (e.g. loose cartons), and
palletised cargo stowage (e.g. cartons on a pallet).
With both stuffing patterns, cargo must cover the entire T-floor
to ensure proper distribution of refrigerated air. This is possible
to arrange in most cases when, for example, loose cartons are
stuffed into the reefer container.
However, not all palletised
cargo or, say, drums, can be stuffed in this way. In that case,
when the cargo does not cover the entire T-floor, heavy
cardboard or dunnage must be placed where no cargo is
stuffed. This will avoid short-circuiting the circulating air and
ensure proper refrigerated air distribution in reefer containers
with bottom air supply. Improper stuffing, and thus respective
by-passing of the circulating air, initiates a larger spread of
different temperatures within the cargo and can lead to severe
The height of the cargo must not exceed the red cargo load
line, which shows maximum allowed cargo height, so that
ample free space is left above the stow to ensure proper air
circulation around the load.
Stuffing chilled products
The significant difference when stuffing chilled products such as
fruit and meat is that refrigerated air must be circulated through
the entire load. This is because heat in the reefer container is
not generated just from the outside, but may also be produced
by the cargo itself. The respiration process of fruit and vegetables,
for example, requires air circulation both around the
commodity and through the load to remove respiratory heat,
water vapour and gases such as carbon dioxide and ethylene.
Correct cargo packaging is essential in maintaining product
quality during transportation and marketing. In addition to protection,
packaging in the form of bins, boxes, crates, etc. serves
to enclose the product and provide the means of handling.
The packaging must withstand:
- rough handling (stuffing and unstuffing)
- compression from the accumulated weight
of stacked packages
- impact and vibration during transport
- high humidity during pre-cooling, transit
The most commonly used types of packaging are cartons,
crated boxes and bags. The material used for this packaging
depends on the product, packing method, pre-cooling method,
strength and buyer’s specification.
Cartons for fresh fruit and vegetables require airflow holes
in the top and bottom so that when stacked they align with
adjacent cartons. The number, placement, size and shape
of the air holes are determined by the product being
packaged. Wax-impregnated cardboard or other materials
that will not lose strength in high-humidity environments
are to be applied. The strength of a carton is its corners.
Stacking cartons directly on top of each other is recommended
to minimise crushing of the cartons below.
If loading cargo on pallets, the cartons on the pallets should be
placed so that air fl ows up into the cartons unrestricted. The
corners of each carton should be supported directly by the
pallet, and if pallets are wrapped in plastic to provide stability,
the bottom and top of the pallet/cartons must not be covered.
Stuffing frozen products
If frozen cargo is pre-cooled to the correct carrying
temperature as prescribed, it is only necessary for air
to circulate around the periphery of the load. A block
stow, i.e. one that has no deliberate spacing between
any of the packages or pallets, is all that is required. It
is, of course, necessary to ensure that air can circulate
under, over and to each side and end of the stow.