Refrigerator vehicles have a variety of cooling methods, the following five are more common cooling methods

1 water ice and salt ice refrigeration

At atmospheric pressure, the melting point of ice is 0°C. When the ice melts, the heat absorption is 334.8kJ/kg. Adding salt to the water ice can reduce its melting point. Within a certain range, the more salt in water ice, the lower the melting point. Experiments have shown that when the mass of added salt is 29% of the mass of water ice, the melting point of the mixture can reach a minimum value of -21.2 °C. If you add more salt, the melting point no longer declines. The salt ice of different compositions is usually selected based on the proper temperature of the refrigerated cargo. For example, using salt ice with 22% salt content, the temperature in the compartment can be maintained at -18~-13°C.
The water ice refrigeration device has a low investment and a low operating cost, but ordinary water (salt) ice has a small amount of heat absorption per unit mass, and the cooling in the vehicle compartment is limited. In addition, melting of salt ice will pollute the environment and food, corrode the car and the value of goods damp. Therefore, water (salt) ice cooling is mainly used for refrigerated transport of fish and other aquatic products.

2 Dry ice cooling

At an atmospheric pressure, the dry ice (solid CO2) has a low sublimation temperature (-78.9°C) and a large sublimation heat absorption (573.5 kJ/kg). Therefore, it is used as a compartment cold source to obtain not only lower temperatures (generally lower At -20°C ), and larger cooling capacity can be obtained. Therefore, this refrigeration method is suitable for the transportation of frozen foods.

Dry ice refrigeration unit is simple, low investment and operating costs, easy to use, the goods will not be damp. The COZ gas produced by the sublimation of dry ice can inhibit microbial growth, reduce fat oxidation, and impair the respiration of fruits and vegetables. However, dry ice sublimation can easily cause frost formation. Excessive CO2 gas will lead to difficulty in breathing of fruits and vegetables and colds. The temperature in the cabin is difficult to adjust. Dry ice is expensive and consumes a large amount, so it is less practical.

3 cold plate cooling

The principle of cold plate cooling is to use the amount of cold stored in the coolant to perform refrigeration. Before the transportation, the cold storage agent in the cold plate in the compartment is “cooled” to cool it, and then the cold storage agent in the cold plate is used to melt and absorb the heat during the transportation, so that the temperature in the compartment is maintained in the proper temperature range of the transported cargo. Inside. Therefore, the cold plate is also called "storage plate."

The structure of the cold plate cooling device is divided into integral type and split type. Integrated power units, refrigeration units, cold storage panels, etc., are all placed on the dry truck; split-type vehicles are equipped with only refrigeration units and cold storage panels. When parking, the solid-state power unit drives the refrigeration unit to “cool” the cold storage plate. The latter is often used in practical applications. The commonly used coolants are eutectic solutions with low melting point, and their melting points are usually about 10 °C lower than the appropriate temperature in the compartment. When the proper temperature of the goods to be transported changes, the composition of the selected eutectic solution must also change.

The cold plate device itself is heavier and has a larger volume and occupies a certain volume of the compartment, and the cold plate can only be cooled continuously for 8 to 15 hours 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 cooling has developed rapidly and has become the second only to mechanical refrigeration. the way.

4. Liquid nitrogen refrigeration

Liquid nitrogen refrigeration is the use of liquid nitrogen vaporization heat absorption for cooling. At atmospheric pressure, the boiling point of liquid nitrogen is -196°C and the latent heat of vaporization is 200 kJ/kg. The specific heat of nitrogen is 1.05 kJ/(Kg.°C), so when it is vaporized and heated to -20°C per kilogram of liquid nitrogen, the absorbed heat is about 385 kJ, the boiling point of liquid nitrogen is low, and it is a by-product of oxygen production. Has been more widely used.

The liquid nitrogen refrigeration unit is simple in structure, reliable in operation, free from noise and pollution, and has large liquid nitrogen refrigeration capacity and rapid refrigeration, and is suitable for quick freezing. Liquid nitrogen vaporization does not make the cabin damp, and nitrogen is good for keeping food fresh and preventing drying. In addition, liquid nitrogen refrigeration temperature control is accurate (±2 °C). However, the liquid nitrogen has a high cost and needs to be filled frequently, so the promotion is limited. Similarly, other low-temperature vaporized liquid gases can also be used as refrigerants, such as liquid carbon dioxide (CN2).

5. Mechanical refrigeration

Mechanical refrigeration methods include: vapor compression, absorption, and steam injection. At present, the vapor compression type is the most widely used. Here's how the cooling works.

At a certain pressure, the liquid will boil at a certain temperature (boiling point). When the liquid boils, it absorbs the latent heat of vaporization and produces a phase change, which turns into saturated vapor. At the same pressure, the boiling points of the different liquids and the latent heat of vaporization are different. For example, at an atmospheric pressure, the boiling point of water is 100 °C, the latent heat of vaporization is 2256.7 kJ / kg, and the boiling point of freon 12 (R12) is -29.8. °C, the latent heat of vaporization is 165 kJ/kg. The liquid used for boiling refrigeration is called a refrigerant or refrigerant. In refrigeration technology, the boiling of refrigerant is called "evaporation", its boiling point is called "evaporation temperature", and boiling refrigeration is called "evaporative cooling."

Vapor compression refrigeration is evaporative cooling. The refrigerant is placed in a closed system. The liquid refrigerant vaporizes and absorbs heat in the evaporator, releases heat in the condenser and recondenses to a liquid state. Under the drive of the compressor, the refrigerant continuously circulates.

The main reason for the widespread adoption of mechanical refrigeration devices is that the refrigeration units can both cool and heat, which expands the scope of use; the temperature inside the cabin can be controlled automatically, the temperature regulation is precise and reliable, and the range of temperature adjustment is wider. It can adapt to various refrigerated goods. Transportation. Despite the complex structure of mechanical refrigeration devices, high purchase and operation costs, and high operating noise, mechanical refrigeration has so far been a reliable and effective method of refrigeration.