In the history of the development of freezer shipping containers, there are five types of refrigeration methods: water ice refrigeration, dry ice refrigeration, freezing plate refrigeration, liquid nitrogen refrigeration and mechanical refrigeration.
In addition, freezer shipping containers can also include jacketed reefer containers, modified atmosphere reefer containers, low-pressure reefer containers, liquid air reefer containers and liquid carbon dioxide reefer containers.
There are two types of refrigerated container transport ships in the development process, and there are two types of freezer shipping containers corresponding to this.
One is that the container itself does not have refrigeration equipment but has a thermal insulation structure. The container ship is centrally refrigerated. The cold air sent from the refrigeration equipment on board the vessel enters the container through the air inlet, cools the cargo through the air inlet, and returns opening at the container’s end. After that, it returns to the refrigeration equipment through the return air outlet and the return air duct.
Another type of freezer shipping container transport ship is equipped with a power socket on the cargo bulkhead. The container comes with a refrigeration unit and is equipped with a cable more than 10 meters long. Insert the cable plug into the power socket on the ship’s bulkhead to start the refrigeration unit. Perform refrigeration.
The first type of freezer shipping container can only be used as a thermal insulation box after it is separated from the refrigerated container shipping vessel, significantly limiting its use scope. Judging from the current development situation, most refrigerated containers adopt the second type, namely mechanical refrigeration.
From the perspective of the heat load source of the refrigerated container, the refrigerated container has to withstand a variety of heat loads during the transportation process.
From the perspective of a refrigerated container’s loading and transportation process, its thermal load should include two parts, namely the cold consumption inside the box and the heat exchange between the box and the outside.
In actual transportation or storage using freezer shipping containers (sometimes refrigerated containers are used as temporary cold storage), the main consumption factors are also different due to the other goods being transported and the initial conditions of the transportation.
The cold consumption inside the freezer shipping container includes:
- The cold consumption of the goods in the box to cool down. (Generally, there is no mean consumption when transporting frozen goods because the temperature of the goods is shallow when they come out of the cold storage, and the goods themselves can become a cold source).
- Cold consumption for cooling. Before the refrigerated container is cooled, the temperature is the same as the outside temperature at the time. After cooling, the inner wall and the inner temperature of the box are the same, while the outer wall is the same as the outside temperature at the time. Thus, the average temperature of the box is the average of the inner and outer walls.
- Cold consumption of cargo breath. Generally speaking, goods such as fresh vegetables and fruits will release breathing heat during transportation.
- The cooling consumption of the circulating fan. When the evaporator fan is running, it will bring heat to the cargo in the box.
- Other cold consumption, such as defrosting, mean consumption, etc.
The heat exchange between the freezer shipping container and the outside world includes:
- Cold consumption for heat transfer.
- Cold consumption due to heat leakage. When there is a temperature difference or pressure difference between the inside and outside of the refrigerated container, it will leak heat through holes, holes, and gaps that are not tightly sealed. The amount of heat leakage is related to external weather conditions.
- Extra heat transfer and cold consumption for particular surfaces. The temperature of some surfaces of the refrigerated container is quite different from other surfaces. This surface’s heat transfer and mean consumption should be compensated, for example, the extra cold consumption of the surface directly exposed to solar radiation.
- Cold consumption of ventilation. When the freezer shipping container transports certain goods (such as uncooked fruits, vegetables and fresh eggs), ventilation is required. If the outside temperature is higher than the temperature inside the box during ventilation, the outside air will bring heat into the box. The air needs to drop to the temperature inside the box, and in the other hand, some water vapour may condense (assuming it falls to the original temperature level in the box). Thus,), the sum of these two parts of heat is the cold consumption of the ventilation of the freezer shipping container.