When an expansion valve is operating properly, at the end of the evaporator will contain what?

• A. Only superheated vapor
• B. Only saturated liquid
• C. Saturated vapor
• D. Mixture of vapor and liquid

Answer: (A)

When the expansion valve properly throttles the refrigerant, it drops to a low pressure so the refrigerant entering the evaporator begins boiling as it absorbs heat. Inside the evaporator, liquid refrigerant boils off and becomes vapor. By the time the refrigerant exits the evaporator, all of the liquid has boiled away and the vapor is heated above its saturation temperature for the evaporating pressure—that is, it is superheated. This is why the outlet is intended to contain only superheated vapor: it prevents liquid from reaching the compressor and helps ensure smooth, safe operation. If liquid were present at the outlet, it would indicate incomplete vaporization and potential damage from liquid slugging.