Which type of heat exchanger involves phase change of the fluids?

• A. Two Phase
• B. Direct Contact
• C. Indirect Contact
• D. Parallel Flow

Answer: (A)

Phase change in heat exchangers occurs when heat transfer happens while one of the fluids changes state, such as liquid to vapor or vapor to liquid. This is the defining feature of a two-phase heat exchanger, where latent heat drives the energy transfer. For example, in boiling or evaporating sections, liquid absorbs heat and becomes vapor; in condensation sections, vapor releases latent heat and becomes liquid. The temperature during these transitions stays near the saturation point, allowing large heat transfer even with relatively small temperature differences. Direct contact and indirect contact describe how the fluids exchange heat (whether they touch directly or are separated by a wall) and do not by themselves require a phase change, while flow arrangement like parallel flow concerns direction of movement rather than phase behavior. Therefore, the type that inherently involves phase change is the two-phase heat exchanger.