Figure 15: Air flow in porthole container: the cold air is blown into the lower, supply air opening via the attached refrigeration unit (ship’s refrigeration system, "clip-on unit" or terminal refrigeration system). It is distributed over the entire container length via the gratings and rises upwards through the cargo. The air is then drawn off from the container’s upper return air opening via the air channel left clear beneath the container roof and cooled in the attached refrigeration unit. If the maximum load height is exceeded, there is not enough space left for the air to circulate properly.