What occurs when the entire liquid is at or above saturation temperature for the existing pressure?

• A. Bulk Boiling
• B. Local sub-cooled boiling
• C. Film Boiling
• D. Condensation

Answer: (A)

When heat is applied and the entire liquid is at or above its saturation temperature for the given pressure, boiling happens throughout the whole liquid mass, not just at the heated surface. In this situation every region is at the point where liquid and vapor can coexist, so vapor generation occurs in the bulk of the liquid. This is what we refer to as bulk boiling.

If only part of the liquid were at or near the saturation temperature (while the rest remains cooler), boiling would be localized near the hot surface—local sub-cooled boiling. A vapor film forming between the surface and the liquid, which is film boiling, requires a much higher surface temperature than the bulk Tsat and creates a separating layer of vapor. Condensation is the reverse process, where vapor turns back into liquid.