What is film boiling?

Asked by: Rich Gallow

Answer

Water boiling on a pan get its heat as a result of conduction from the pan's surface to the water. In film boiling the process of heat transfer is quite different.

Water exposed to atmospheric pressure boils at approximately 100 degrees Celsius. Water below the surface is not directly exposed to the atmosphere and can remain a liquid as it superheats above this temperature. At a sufficiently high temperature the water below would vaporize and form a gas layer that supports the body of water above. As a result heat transfer from the heated surface to the water occurs mainly through radiation. This process is known as film boiling.

If you take a pot of water and heat it on a stove in your kitchen, film boiling would be impossible to achieve. You may have more success on a laboratory burner. But the phenomena can be observed in your kitchen.

If you heat a pan sufficiently and sprinkle water on it you may see the drops of water bead up and 'dance' around on the surface. This occurs because as the drop hits the pan a layer of vapor is formed underneath and this supports the water droplet. The vapor continually escapes to the sides and is replenished by the water drop above until it disappears.
Answered by: David Latchman, B.Sc. Physics, University of the West Indies