CONDENSATION IN VENT LINES OF SHIPBOARD WASTE OIL EVAPORATION SYSTEMS: A HEAT TRANSFER AND OPERATIONAL ANALYSIS

Waste-oil evaporation is widely used onboard ships to reduce water content and sludge volume prior to incineration. Repeated onboard observations indicated that part of the evaporated water returned to the waste-oil tank despite continuous vapor generation, suggesting vent-line condensation. This study investigates vent-line condensation in a commercial bulk carrier using onboard observations combined with a simplified thermal-resistance and energy-balance approach. The system consisted of a waste-oil service tank connected to an approximately 30 m uninsulated DN80 vent line routed through the engine room. Condensate returning through the vent-line drain was collected and measured during a representative 12-hour evaporation operation. The analysis showed that approximately 428 L of water was evaporated, whereas approximately 96 L condensed within the vent line and returned to the waste-oil tank, corresponding to nearly one quarter of the evaporated water. Sensitivity analysis demonstrated that condensation was governed primarily by heat loss from the vent pipe to the engine-room environment, with external natural convection representing the dominant thermal resistance. The predicted condensation agreed closely with onboard observations. Vent-line condensation may reduce evaporation effectiveness and introduce uncertainty into onboard waste-management records because returned condensate is not measured separately. Applying a 30 mm mineral-wool insulation layer reduced the predicted condensation from approximately 96 L to approximately 20 L. These findings identify vent-line condensation as an operationally significant but often overlooked phenomenon and demonstrate that vent-line insulation provides a simple and effective means of reducing condensation-related return flows and improving process reliability.