There are three goals for air circulation:

•   Refrigeration through heat extraction or insulation
•   Controlling humidity
•   Gas exchange

If your cargo has been pre-frozen and you are loading the container under controlled environmental conditions, you will want to conserve the temperature of your product. Nothing is better for this than block stowage, where the air circulates the entire perimeter of the cargo around the walls, roof, and floor of the container, preventing any outside heat from reaching the cargo.

The situation is very different for fruits and fresh vegetables. Here, the air must circulate throughout the cargo to absorb heat generated by product "breathing," as well as any ambient heat the cargo may have absorbed during consolidation.

Remember that air, like water, takes the path of least resistance. Air that circulates in a container often returns too quickly to the refrigeration system. To prevent this, canvas ducts or baffles are used, as well as stowing the cargo to form air channels.

As opposed to refrigerated warehouses, where air generally circulates horizontally, the air in refrigerated containers circulates vertically, from the bottom upwards. You should keep this in mind when planning your packing, or at least when stowing your cargo.