Fouling on the tube surfaces of a lube-oil cooler gradually reduces its cooling capacity. Which combination of factors governed by the overall heat-transfer coefficient explains this degradation?

1. A. Fouling increases surface area and raises the log-mean temperature difference, reducing net heat flow.
2. ✓ Fouling adds thermal resistance, lowering the overall heat-transfer coefficient and therefore reducing total heat transferred for a given surface area and temperature difference.Correct
3. C. Fouling reduces shell-side turbulence only, leaving the tube-side film resistance and wall conduction unchanged.
4. D. Fouling raises the absolute temperature of the tube wall, increasing radiation losses and reducing convective transfer.