In a reciprocating refrigeration system, the refrigerant at the inlet to the compressor is typically in which state?

• A. Liquid
• B. Saturated liquid
• C. Superheated vapor
• D. Mixed vapor and liquid

Answer: (C)

The key idea is that the compressor in a vapor-compression system is designed to compress gas, not liquid. The refrigerant leaving the evaporator is generally a low-pressure vapor, but to protect the compressor and ensure smooth operation, some superheating is provided in the suction line so the refrigerant entering the compressor is fully vapor. Superheated vapor means the vapor temperature is above its saturation temperature for that pressure, so there’s no liquid present. Liquid at the inlet can cause slugging, poor lubrication, and potential damage, while a mixture of vapor and liquid would still risk liquid entering the compressor. Saturated liquid would be even worse for a compressor designed for vapor, so the practical inlet condition is superheated vapor.