CONTROLLED ATMOSPHERE:

Controlled atmosphere is an inert gas system used to extend the storage life of seasonal perishable products and has been used for many fruits and vegetables.

For successful storage of fruits for long periods, the natural ripening of the produce has to be delayed without affecting the eating quality. To achieve this, temparature has to be brought down to lowest level and to control oxygen, carbon - dioxide and relative humidity inside the chamber to a certain level. A typical controlled atmosphere would be expected to have flexibility to control gas levels within the following ranges :

O₂    :   < 1 - 8%

CO₂ :   0-15%                                                                       V

RH    :    40 - 90%

1)    The oxygen level can be decreased by :

a)  injecting pure nitrogen as a gas or liquid from bottles or tanks ;

b)  burning propare in an open flame burner, or a burner with a catalyst; generating gas (nitrogen with a low oxygen level) on board from compressed dry and clean air, using high pressure membranes etc.

2)    The carbon - dioxide level can be increased by :

a) injecting carbon dioxide gas ;

b) fruit respiration ;

3)    The carbon dioxide level can be decreased by :

a) fresh air or gas injection ;

b) hydrated time ;

c)  carbon scrubbers ;

d) water scrubbing etc ;

4)    Relative humidity can be increased by :

a)  injecting water mist;

b)  steam ;

c)   evaporating water etc.

INSULATION:

Traditionally, cargo spaces of reefers have been insulated with rock wool or fibre glass slabs on ship sides and bulkheads and high density polyurethane slabs on decks and tank tops. Timber supports, suitably protected against rot are placed between steel structures and inner face lining, such as aluminium, stainless steel, PVC or marine plywood to eliminate heat bridges. In recent years, the trend was to apply sprayed in situ polyurethane to ribbands, corners and complicated areas to reduce the heat transfer. In some modern design polyurethane is sprayed on steel sides and frames before glasswood slabs are pinned to the sides. These slabs are covered by mesh wire and sprayed with polyurethane again to a thickness of 50 mm before plywood lining is applied.

In other designs, plyurethane is simply injected between the ship side and plywood lining or prefabricated polyurethane panels 60-100 mm thick are fitted with mineral wool between the frames. In installations where mineral wool is not fitted, pressure equalising valves are applied to prevent the panel's collapse in the event of rapid cooling of chambers or temparature fluctuations in the void space between the ship side and the panels.

A overall coefficience of heat transfer for a modern reefer vessel, regardless of the type of insulation fitted varies between 0.45 to 0.52 W/m² °C, with a tendency towards lower figure.

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