USCG Exam PracticeThermodynamics and heat exchangers
A lube-oil cooler aboard a vessel has been in service for several months without cleaning. The lube-oil outlet temperature has risen above normal limits even though seawater flow rate and inlet temperatures are unchanged. What is the most likely cause, and what property does it affect?
- A. Increased seawater salinity has raised the specific heat of the cooling water, reducing heat absorption
- ✓ Fouling on the tube surfaces has added thermal resistance, lowering the overall heat-transfer coefficientCorrect
- C. Counter-flow has shifted to parallel-flow as scale accumulates at the inlet, reducing the log-mean temperature difference
- D. Air in-leakage has raised the partial pressure of the seawater, reducing its latent heat capacity
Why B is correct
DOE-HDBK-1018 Vol.1 §2-1 identifies fouling — scale, sludge, or oil films — as the most common reason a cooler gradually loses capacity, because it adds thermal resistance and lowers the overall heat-transfer coefficient. DOE-HDBK-1012 §2-3 confirms that anything adding resistance lowers the overall coefficient and heat transferred.