How does a direct expansion (DX) evaporator operate?

• A. Refrigerant boils directly inside the evaporator coils to absorb heat from the process.
• B. The evaporator is flooded with liquid refrigerant.
• C. The evaporator condenses refrigerant inside the coils.
• D. It uses external cooling water to remove heat.

Answer: (A)

Direct expansion evaporators cool primarily through the refrigerant boiling inside the coil. The refrigerant is throttled to a low pressure and enters the coil as a liquid. As it sits in the coil at that low pressure, it reaches its boiling point and begins to vaporize, absorbing heat from the process material—this heat is taken away as the refrigerant changes from liquid to vapor. The result is effective cooling driven by the latent heat of vaporization, with the vapor then returned to the compressor. There’s no flooding of liquid in the coil, no condensation happening inside the coil, and external cooling water isn’t needed to remove the heat—the refrigerant’s own phase change does the work.