Modes Of Refrigerated Transport

Temperature-controlled cargo carriage is by intermodal containers and conventionally refrigerated ships. Some conventional refrigerated ships also carry integral containers (see Fig. 1). The world's container capacity is about the same as conventional ships and continues to expand. Some trades also use porthole-insulated containers in ships with central refrigeration systems.

Containers manufactured to international standards (ISO) provide a secure door-to-door delivery system with landside movements by road, rail, or barges (7). This in-

Evaporator fan Evaporator coil •

Figure 3. View of a porthole container with a clip-on unit.

Figure 1. Cutaway view of a loaded integral container. Source: Carrier Transicold, reproduced with permission.

Road Trailers

Most temperature-controlled trailers have mechanical refrigeration units powered by their own diesel engine. Major design advances have occurred in recent years due to noise restrictions, moves to more environmentally friendly refrigerants, microprocessor controls, and the need for smaller turning circles. They have resulted in better air temperature control, less defrosting, and lower power demands. Many countries have introduced less restricting maximum weights as road systems have improved, and vehicles have more axles.

Many trailers use refrigerated air blown over the top of the cargo (top-air delivery) and returned along the floor (see Fig. 4). In temperature climates, flat floors predominate, with return air passing through the pallet bases to the refrigeration unit (see Fig. 5). T-section floors are available for use in warmer climates. Air chutes, mounted under the roof, are sometimes used to provide better air distribution along the length of the unit (9). Some haulers use liquid nitrogen or carbon dioxide to provide refrigeration. These systems are almost noiseless in operation but need cargo close to the carriage temperature for efficient use.

termodal chain depends on a supply of electricity from electrical mains or generators. Containers operate with air supplied from the refrigeration system along the T-section floor (see Fig. 2). It circulates around the cargo and in the space between the cargo and the doors. The air then returns along the space between the roof and the top of the cargo to the machinery section at the front (8).

In the chilled mode (warmer than — 10°C), a sensing probe in the supply air controls the air temperature to the container. In the frozen mode (colder than — 10°C), a sensing probe in the return air of the refrigeration unit controls the air temperature.

Porthole containers have air delivered through couplings to their bottom portholes. It passes around the container and returns through the top portholes. This older system continues to operate well, especially when high volumes of cargo are being carried. Clip-on units (see Fig. 3), towers, or total loss refrigerants such as liquid nitrogen or carbon dioxide provide refrigeration while ashore. This system is restricted to certain trade routes and ports.

Evaporator fan Evaporator coil •

Condenser fan

Condenser coil Compressor

Figure 2. Schematic layout of a refrigerated container.

Figure 3. View of a porthole container with a clip-on unit.

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