Types of Boiling
Muhammed Zakiy K
Mechanical Engineer with 12 years of experience in the Conventional Power Generation Industry
Boiling Process
When water is heated, steam generation starts in the form of bubbles at the heated surface. These bubbles are lighter and hence replaced by the heavier water and the circulation continues. This is called boiling. There are two types of boiling as follows
Nucleate Boiling
When the water adhered to the wall of the container receives heat, it is converted into steam bubbles and gets detached from the wall as the size of the bubbles increases. These bubbles move through the water body due to low density. Hence other water particles replace their space and this process continues. This process of individual bubble formation is called Nucleate Boiling. The inner wall of the container is continuously in contact with the water and temperature remains at saturation point corresponding to the temperature, causing no damage to the container.
Film Boiling
If the heat flux (heat added per unit area per unit time) is increased at a faster rate, more steam bubbles will be formed which will obstruct the water droplet and bubbles generation. Hence a thick steam film will be formed on the inner wall of the container. This prevents the heat flow flow to the water and gets super heated and the metal temperature increases. This phenomenon is called Film Boiling.
The deviation of boiling process from Nucleate Boiling to film boiling is called Departure from Nucleate Boiling (DNB). Occurrence of DNB in steam generation process leads to overheating of the metal.