What is the state of refrigerant entering the compressor on a reciprocating machine?

• A. Saturated vapor
• B. Liquid
• C. Superheated vapor
• D. Mixture

Answer: (C)

Superheated vapor is what enters the compressor. After the refrigerant leaves the evaporator, the system is designed so the gas is kept slightly above its saturation temperature at the suction pressure. This ensures the inlet to the reciprocating compressor is dry vapor, not liquid. Liquid entering a compressor can cause severe damage from slugging and hydraulic lock, while vapor that’s only partially superheated can still contain liquid droplets. A touch of superheat keeps the suction gas safely vapor, protects the pistons and valves, and helps maintain proper lubrication and clearance. If it were saturated vapor, there could be liquid present; if it were liquid, it wouldn’t be compressible and would damage the machine; a mixture would also risk poor compression.