Factors affecting the boiling heat transfer of refrigerant liquid

Factors affecting the boiling heat transfer of refrigerant liquid

In the evaporator, the heat of the cooled medium is transferred to the refrigerant through the heat transfer wall, so that the liquid refrigerant absorbs heat and vaporizes. The change in the state of the refrigerant in the evaporator is actually a boiling process, which is customarily called evaporation. The heat transfer effect in the evaporator is also the same as the condenser, which is affected by factors such as the heat transfer coefficient on the refrigerant side, the thermal resistance of the dirt on the heat transfer surface, and the heat transfer coefficient on the cooled medium side.

　　The influence of the latter two factors is basically the same as that of the condenser, but the heat transfer coefficient of liquid on the refrigerant side is essentially different from the heat transfer coefficient of gas during condensation. The heat transfer temperature difference in the evaporator of the refrigeration device is not large, so the boiling of the refrigerant liquid is always bubbly boiling. Many bubbles are generated on the heat transfer surface during boiling, and these bubbles gradually become larger, detach from the surface and rise in the liquid. After they ascend, they continuously produce bubbles there. The boiling heat transfer coefficient is related to factors such as bubble size and bubble velocity.

　　Here is mainly to analyze the factors affecting the boiling heat transfer of refrigerant liquid.

　　 1. The influence of refrigerant liquid physical properties

　　 The thermal conductivity, density, viscosity and surface tension of the refrigerant liquid have a direct influence on the boiling heat transfer coefficient.

　　 A refrigerant with a larger thermal conductivity has a smaller thermal resistance in the heat transfer direction and a larger boiling heat transfer coefficient.

　　 Under normal working conditions, the temperature difference between the refrigerant in the evaporator and the heat transfer wall is generally only 2~5℃. The intensity of the convective heat transfer depends on the convective movement of the refrigerant liquid during the vaporization process. During the boiling process, the movement of bubbles in the liquid causes the liquid to be disturbed, which increases the possibility of contact between various parts of the liquid and the heat transfer wall, making it easier for the liquid to absorb heat from the heat transfer wall and the boiling process is faster. Refrigerant liquid with lower density and viscosity is stronger by this disturbance, and its convective heat transfer coefficient is larger.

The greater the density and surface tension of the refrigerant liquid, the larger the diameter of the bubbles during the vaporization process, the longer the time for bubbles to leave the heat transfer wall, the less bubbles are generated per unit time, and the smaller the heat transfer coefficient. .

　　 Generally speaking, the thermal conductivity of Freon is smaller than that of ammonia, and its density, viscosity and surface tension are larger than that of ammonia, so its boiling heat transfer coefficient is smaller than that of ammonia.

　　2. The influence of liquid wetting ability of refrigerant

　　 If the refrigerant liquid has a strong wetting ability on the heat transfer surface, the bubbles generated during the boiling process have fine roots, which can be quickly separated from the heat transfer surface, and the heat transfer coefficient is also larger. On the contrary, if the refrigerant liquid cannot wet the heat transfer surface well, the roots of the bubbles formed will be very large, reducing the number of vaporization cores, and even forming a gas film along the heat transfer surface, which significantly reduces the heat transfer coefficient.

　　 Several commonly used refrigerants are all wetting liquids, but the wetting ability of ammonia is much stronger than that of Freon.

　　3, the influence of the boiling temperature of the refrigerant

　　 During the boiling process of the refrigerant liquid, the more bubbles generated per unit time on the heat transfer wall of the evaporator, the greater the boiling heat transfer coefficient. The number of bubbles generated per unit time is related to the length of time the bubbles are generated until they leave the heat transfer wall. The shorter the time, the more bubbles are generated per unit time. In addition, if the diameter of the bubble when it leaves the wall surface is smaller, the time from generation to departure of the bubble will be shorter.

　　 When the bubble leaves the wall, its diameter is determined by the balance of the buoyancy of the bubble and the surface tension of the liquid. Buoyancy forces bubbles to leave the wall, while liquid surface tension prevents bubbles from leaving. The buoyancy of the bubble and the surface tension of the liquid are also affected by the density difference (the density difference between liquid and vapor) at the saturation temperature. The buoyancy of the bubble is proportional to the density difference. The surface tension of the liquid is proportional to the fourth power of the density difference.

Therefore, as the density difference increases, the rate of increase of the liquid surface tension is much larger than the rate of increase of the bubble buoyancy. At this time, the bubble can only rely on volume expansion to maintain balance, so the diameter of the bubble when it leaves the wall Big. The density difference is related to the boiling temperature. The higher the boiling temperature, the smaller the density difference at the saturation temperature, the faster the vaporization process, and the greater the heat transfer coefficient.

　　 It is explained above that when the same refrigerant is used in the same evaporator, the heat transfer coefficient increases with the increase of the boiling temperature.

　　 4. The influence of evaporator structure

　　In the process of liquid boiling, bubbles can only be generated on the heat transfer surface, and the effective heat transfer surface of the evaporator is the part in contact with the refrigerant liquid. Therefore, the size of the boiling heat transfer coefficient is related to the structure of the evaporator. The experimental results show that the boiling heat transfer coefficient of the finned tube is greater than that of the smooth tube, and that of the tube bundle is greater than that of a single tube. This is because after the fins are added, under the conditions of the same saturation temperature and the heat load per unit area, the conditions of bubble generation and growth, finned tubes are more advantageous than smooth tubes. Due to the increase in the number of vaporization cores and the decrease in bubble growth rate, the bubbles are easily separated from the heat transfer wall. The experimental results also show that the boiling heat transfer coefficient of the finned tube bundle is greater than that of the smooth tube bundle. According to some information, at the same saturation temperature, the boiling heat transfer coefficient of Rl2 in the finned tube bundle is 70% larger than that of the smooth tube bundle, and R22 is 90% larger.

　　 According to the above analysis, the structure of the evaporator should ensure that the refrigerant vapor can quickly leave the heat transfer surface. In order to effectively use the heat transfer surface, the vapor generated after the liquid refrigerant is throttled should be separated from the liquid before entering the evaporator, and in the operation management, the evaporator should maintain a reasonable refrigerant liquid flow rate.

　　 In addition, the oil in the refrigerant also has a certain influence on the boiling heat transfer coefficient, and the degree of its influence is related to the oil concentration. Generally speaking, when the refrigerant oil content is not more than 6%, this effect can be ignored. When the oil content is larger, the boiling heat transfer coefficient will decrease.