In a direct expansion evaporator system, the refrigerant entering the evaporator is best described as

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

Answer: (B)

Direct expansion systems use a metering device that lowers the refrigerant pressure before it enters the evaporator. That sudden drop causes part of the liquid to flash into vapor at the evaporator pressure. So, the refrigerant entering the evaporator is a saturated two-phase mixture: some liquid remains and some vapor (flash gas) is present. This mixture is essential because the vapor portion will continue to evaporate inside the coil, absorbing latent heat, while the liquid portion also absorbs heat and vaporizes. Therefore, it is not all liquid or all vapor, but a mixture of liquid with flash gas.