In a DX evaporator system, the refrigerant enters the evaporator as

• A. completely vapor
• B. densely subcooled liquid
• C. pure liquid
• D. a mixture of liquid with some flash gas

Answer: (D)

When a DX evaporator uses an expansion device, the liquid refrigerant is throttled from condenser pressure down to the much lower evaporator pressure. That sudden drop in pressure means the liquid can no longer stay entirely in the liquid phase at the new conditions, so a portion of it flashes into vapor. The result is a two-phase stream—liquid plus some flash gas—entering the evaporator. This mix is essential because the liquid portion continues to absorb heat and vaporize inside the evaporator, while the vapor portion is already present to carry away energy as it moves toward the compressor.

So the refrigerant entering the evaporator is not completely liquid, not completely vapor, and not subcooled; it’s a mixture of liquid with some flash gas.