Refrigerated Cargo Ship

Overview

A refrigerated cargo ship, or reefer, carries perishable cargo: bananas, citrus, deciduous fruit, fish, meat and dairy, held at a controlled temperature from load port to discharge. The classic reefer is a conventional ship with insulated holds cooled by a central plant, not a box ship carrying refrigerated containers. It is fine-lined, fast and relatively small, around 150 m and 11,000 cubic metres of insulated Refrigerated Hold space across several low tween decks. The cold comes from a central Refrigeration Plant, the carriage life is extended by a Controlled-Atmosphere System, and a medium-speed Medium-Speed Main Engine drives the ship at around 20 knots because the cargo cannot wait. The hull is built from around 40 prefabricated Hull Blocks.

Insulated holds

The defining feature of a reefer is its insulated cargo space. Each Refrigerated Hold is lined throughout with polyurethane Insulation Panels and divided into low tween decks, because palletised fruit is stacked only a few high and the deck spacing suits forklift handling. Cold air is delivered by Hold Fans that blow chilled air up through removable Floor Grating channels and over the stow, returning to the Air Coolers where chilled brine pulls the heat out. Even temperature throughout the stow is what matters: a warm pocket spoils the fruit around it, so air distribution is engineered as carefully as the cooling itself. Temperature Sensors log delivery-air, return-air and cargo temperatures continuously, the record that proves the cold chain held when the cargo is sold.

Refrigeration plant

The Refrigeration Plant is the heart of the ship. Several Refrigerant Compressors raise refrigerant pressure, Condensers reject the heat to seawater, and Brine Chiller evaporators cool a secondary brine or glycol circuit that Brine Pumps distribute to the hold air coolers. Using a secondary brine loop rather than piping refrigerant straight to the holds limits the refrigerant charge and contains any leak in the machinery space. Redundancy is built in: multiple compressors and pumps mean one failure cannot lose a hold full of fruit, and the Refrigeration Control System system trims compressor capacity and brine temperature hold by hold to hold each cargo within a fraction of a degree of its setpoint.

Controlled atmosphere

Plain refrigeration slows ripening; controlled atmosphere slows it further. Fruit goes on respiring after harvest, burning sugar and oxygen and giving off carbon dioxide and ethylene. By lowering the oxygen and managing the carbon dioxide in a gastight hold, the Controlled-Atmosphere System puts the fruit into a near-dormant state and adds days or weeks to its carriage life, enough to ship avocados or bananas across an ocean and still land them green. A Nitrogen Generator displaces oxygen with nitrogen, a CO2 Scrubber strips the excess carbon dioxide the fruit produces, and Gas Analysers watch the oxygen and carbon dioxide in each hold while CA Valves hold the target mix. The holds are made gastight by Gastight Seal boundaries so the controlled atmosphere does not leak away.

Cargo handling

Loading speed matters because every hour the holds are open lets heat in. The Cargo Gear uses insulated Insulated Hatch Covers with a foam core so the weatherdeck openings do not break the insulation, Deck Cranes for palletised cargo and stores, and on many reefers Side Cargo Ports: insulated doors in the side shell through which pallets are loaded horizontally by conveyor or forklift, served by internal Pallet Elevators. Side-port loading keeps the weatherdeck hatches shut and the cold in, and lets the ship work several decks at once for a fast turnaround.

Propulsion and power

Reefers are among the faster merchant ships because their cargo is high-value and time-sensitive: a banana ship races the ripening clock across the Atlantic or Pacific. The Medium-Speed Main Engine is a medium-speed four-stroke diesel through a reduction gear to a Controllable-Pitch Propeller controllable-pitch propeller, giving around 20-22 knots and the thrust control that frequent berthing needs. Each Cylinder Unit runs a Piston in a Cylinder Liner under a Cylinder Head, fed by the Fuel Injection System. Power passes through the Shaft Line to the propeller, steered by a Rudder with a Bow Thruster for berthing.

Power and accommodation

The refrigeration plant is a constant heavy electrical load at sea and in port, so a reefer carries large Auxiliary Gensets, more generating capacity for its size than a dry-cargo ship. The crew of around 20 lives in the aft Deckhouse in Crew Cabins served by an HVAC Plant, with the Bridge Console carrying reefer-plant alarms alongside navigation so a cooling fault is caught at once, and a Lifeboat Station meeting SOLAS. Conventional reefers have lost ground to refrigerated containers on many routes, but on dedicated high-volume trades such as bananas and citrus the purpose-built reefer still moves cargo faster, colder and at lower handling cost than a container alternative.