There are three refrigeration methods for refrigerated vehicles, namely water ice and salt ice refrigeration, dry ice refrigeration and cold plate refrigeration.

Water ice and salt ice refrigeration:

Under atmospheric pressure, the melting point of ice is 0 ℃. The endothermic value of ice melting is 334.8kj/kg. Adding salt to water ice can reduce its melting point. In a certain range, the more salt in water ice, the lower melting point. The experimental results show that when the mass of salt is 29% of the mass of water ice, the melting point of the mixture can reach the lowest value - 21.2 ℃. If the salt content is increased, the melting point will not decrease. It is usually based on the transport temperature of refrigerated goods to choose different components of salt ice. For example, using salt ice with 22% salt content, the temperature in the car can be maintained at - 18 to - 13 ℃.

The investment of water ice refrigeration device is small and the operation cost is low, but the heat absorption per unit mass of common water (salt) ice is small and the cooling in the car is limited. In addition, the melting of salt ice will pollute the environment and food, corrode the carriages and damp the goods. Therefore, water (salt) ice refrigeration is mainly used in the refrigerated transportation of fish and other aquatic products.

Dry ice refrigeration:

Under an atmospheric pressure, the sublimation temperature of dry ice (solid CO2) is low (- 78.9 ℃) and the sublimation heat absorption is large (573.5 kJ / kg). Therefore, it can not only obtain a lower temperature (generally lower than - 20 ℃) but also obtain a larger cooling capacity when it is used as the car cooling source. Therefore, the refrigeration method is suitable for the transportation of frozen food.

Dry ice refrigeration device is simple, low investment and operating costs, easy to use, goods will not be affected by moisture. Coz gas produced by dry ice sublimation can inhibit microbial reproduction, slow down fat oxidation and weaken the respiration of fruits and vegetables. However, dry ice sublimation is easy to cause frosting; Too much CO2 will lead to fruits, vegetables and other refrigerated materials breathing difficulties and necrosis; The temperature in the compartment is difficult to adjust; The cost of dry ice is high, and the consumption is large, so the practical application is less.

Cold plate refrigeration:

The principle of cold plate refrigeration is to use the cold storage capacity of refrigerant after freezing. Before transportation, the cool storage agent in the cold plate in the compartment is charged to cool and freeze, and then the cool storage agent in the cold plate is used to melt and absorb heat during transportation, so as to keep the temperature in the compartment within the suitable temperature range of the transported goods. Therefore, the cold plate is also called "cold storage plate".

The structure of cold plate refrigeration device is divided into integral type and split type. The integrated power unit, refrigeration unit and cold storage plate are all placed on the vehicle; The split type is only equipped with refrigeration unit and cold storage plate. When parking, the ground solid power unit is used to drive the refrigeration unit to charge the cold storage plate. The latter is often used in practical application. The common refrigerant is eutectic solution with low melting point, and its melting point is about 10 ℃ lower than the suitable temperature in the chamber. When the suitable temperature of the transported goods changes, the composition of the eutectic solution will also change.

The cold plate device itself is heavy and bulky, occupying a certain volume of the car, and the cold plate can only work for 8-15h at a time. Therefore, cold plate refrigeration is suitable for medium and short distance transportation of medium and light refrigerated vehicles. In recent years, with the increasingly prominent problems of energy and environmental pollution, the application of cold plate refrigeration has developed rapidly, and has become a refrigeration mode second only to mechanical refrigeration.

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