Porthole containers
Porthole containers do not have an integral refrigeration unit. They operate using the Conair system, in which the insulated containers are accommodated in specific slots on board ship and an entire stack is supplied with cold air from a single refrigeration duct via connection ports or portholes (see Fig. 85).
Figure 85: Air flow in Conair refrigeration duct for porthole containers

Cold air is blown in at the bottom and the "warm" air is removed at the top (see Fig. 86).
In port or during transport ashore, these containers must continue to be cooled via these refrigeration portholes. External refrigeration may also be provided by clip-on units (mobile refrigeration units which can be attached to the container).

Figure 86: Air flow in a 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.

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