Refrigerated containers without their own refrigeration unit, also called insulated or porthole containers. They are reliant on an external supply of cold air. This is achieved by refrigeration units of various types, permanently installed on the ship, permanently installed in the terminal or clip-on units for individual containers. Porthole containers are thermally insulated and have two sealable openings on the end walls (the portholes) through which cold air can be blown into the container and warm air can be extracted. The cold air is forced through the lower opening into the container, then distributed throughout the load via the T-bar floor, and subsequently flows through the load to the top of the container and is extracted through the upper opening