A typical shipboard domestic multi-box refrigeration system operates with one compressor and condenser. What is the purpose of the sensing line connected to the thermal bulb at the evaporator coil outlet?

Correct Answer: Option B — To transmit the bulb pressure (proportional to the coil temperature) to the thermal expansion valve diaphragm.

The capillary sensing line transmits the saturation vapor pressure corresponding to the bulb temperature directly to the top of the thermal expansion valve diaphragm. This pressure signal acts in unison with system superheat spring forces to accurately control liquid refrigerant metering.