Refrigeration system and controlled atmosphere of a banana container.

Sketch and description of the reefer container system

A line diagram shows: an integral/ (or clip-on) refrigeration unit at one end with a refrigerant circuit composed of compressor, condenser (air-cooled with fans), expansion valve and evaporator coil, arranged in a closed vapour-compression loop. The refrigerant (e.g. R-134a or R-404A/R-407C) is compressed, condensed rejecting heat, expanded through the TEV and evaporates in the evaporator cooling the airstream. An air circulation system forces cooled air through the cargo: a thermo-controlled fan delivers air, and floor T-bar ducts/vents distribute it; the air flows up through the banana boxes/stow, returns through the ceiling, and is drawn back over the evaporator coils. A thermostat or temperature sensor controls compressor cycling to hold the set point, usually 13-14 C for bananas in the green/ripening state, with the heater available to hold temperature if ambient is low. Some units use two-speed fans and a ventilation/flushing damper to draw in outside air and expel ethylene.

Controlled atmosphere (CA) - how it extends green life and shelf life

Bananas are a climacteric fruit that produce ethylene gas as a ripening trigger. In a CA container the atmosphere is modified by reducing oxygen (commonly down to about 2-5%) and/or enriching carbon dioxide, and by controlling ethylene. Lowering oxygen slows the climacteric respiration of the fruit, reduces metabolic heat and the rate of ethylene-driven ripening, so the fruit stays green and firm for longer and shelf life is extended. Exposing the cargo to low oxygen and/or removing ethylene retards colour change and softening, and reduces over-ripening and spoilage in transit, so bananas can be shipped green and ripen to a controlled stage at the destination. The CA system monitors O2/CO2/ethylene, uses a nitrogen generator or flush/semipermeable membranes to maintain the low O2, adds a small measured ethylene (optional) and scrubs CO2 as required; a ventilation phase flushes the space before discharge.

How the airflow is designed

The airflow is designed for efficient cooling and uniform temperature:

• Air is drawn over the evaporator by the unit fans and discharged through the floor T-bar (bottom-air delivery) so it flows longitudinally under the cargo.
• Banana boxes are double-vented and stacked so that air rises through vertical vent channels between the boxes (with air circulation space left between each box/stack), reaches the ceiling and returns to the return-air duct/evaporator.
• The stow is arranged to leave clear air lanes; cartons are arranged to allow the air to pass around every box, avoiding blockages.
• Return air is sensed so the unit controls on return-air temperature, and the fans provide sufficient air changes per hour to remove heat of respiration.
• Even air distribution, correct dunnage and stowage pattern, and adequate spacing at the unit end are critical to avoid hot spots and condensation.