A typical arrangement of the refrigeration system and air distribution in a
refrigerated semi-trailer is shown in Figure 1. In these vehicles, top air delivery is
predominately used. Refrigeration unit fans cause temperature-controlled air to
circulate around the inside of the vehicle roof, walls, floor, and doors to remove heat
which is conducted and infiltrated from the outside, returning to the cooling coil via
the floor or space under pallets as shown in Figure 2. For chilled cargoes horizontal
channels are required between rows of cartons to allow good return airflow through
the load, Some of the air should also flow through and between the cargo, particularly
when carrying fruit and vegetables, where heat of respiration may be a significant
proportion of the heat load. To facilitate air return to the evaporator coil, some trailers
are fitted with a false bulkhead wall with metal grill or holes in the lower part for
return air passage.