【Daily Question】Chemical Engineering Principles 124: Heating by Boiling (July 30)

nuclear boiling-------------------------

Film boiling. The heat transfer efficiency is optimal at this time

Large-volume saturated boiling can be divided into the following three stages: ① Surface vaporization； ②nuclear boiling ； ③Film boiling. In the various stages of saturated boiling of the aforementioned liquid, in what state should industrial boiling apparatus operate? It would be better if you could explain the reasons for the choice. Large-volume saturated boiling curves: Experimental observations show that, as the temperature changes during large-volume saturated boiling of any liquid, three different types of boiling states occur: surface vaporization, nucleate boiling, and film boiling. As a specific example, we take the boiling of saturated water at atmospheric pressure on the surface of a platinum heating wire. ⒈ Surface vaporization: , increasing slowly. At this point, the superheat of the liquid in close contact with the heating surface is very small, no bubbles are formed, and heat transfer between the heating surface and the liquid occurs through natural convection. At this stage, vaporization occurs only at the liquid surface. ⒉ Nuclear boiling: When this occurs, bubbles are formed on the heating surface and rise rapidly. This is because the formation and detachment of bubbles cause increasingly severe disturbances to the liquid near the heating surface. ⒊ Film boiling: When it increases to a certain value, the number of bubbles formed on the heating surface **increases; at this point, the rate at which bubbles are formed is greater than the rate at which they escape from the surface. In this way, the bubbles connect before breaking away from the surface, starting to form an unstable vapor film that can rupture at any time to turn into larger bubbles that leave the heating surface. As increases, the bubbles tend to stabilize; since the thermal conductivity of gas is much lower than that of liquid, the heat transfer coefficient decreases instead. This stage is unstable film boiling. When a stable gas film forms on the heating surface, it completely separates the liquid from the heating surface. However, due to the high wall temperature, radiation heat transfer plays a significant role, causing the heat transfer rate to increase rapidly; this stage is characterized by stable film boiling. The turning point from nucleate boiling to film boiling. The heat flux density and critical temperature corresponding to the critical point. The difference in temperature is referred to as the critical heat load. It can be seen that nucleate boiling has the advantages of a high heat transfer coefficient and a low wall temperature; therefore, industrial boiling systems are all operated under this condition. Nuclear boiling is actually achieved by controlling it to be no greater than its critical value.

Nuclear boiling condition, to prevent the equipment from being damaged.

Nuclear boiling condition should be avoided to prevent equipment damage. From which two aspects can the enhancement of boiling heat transfer be approached? Improve the heating surface to provide more vaporization cores ； Additives are added to the boiling liquid to reduce surface tension

②nuclear boiling； Vaporization from within the medium

Large-volume saturated boiling can be divided into the following three stages: ① Surface vaporization； ②nuclear boiling ； ③Film boiling. In the various stages of saturated boiling of the aforementioned liquid, in what state should industrial boiling apparatus operate? ②nuclear boiling