When placing a shell-and-tube heat exchanger with a non-removable tube bundle such as a lube oil cooler with fixed tube sheets into service, to avoid thermal shock what statement represents the correct operating procedure?

Correct Answer: Option C — Introduce both the cooling water and the lubricating oil simultaneously and gradually to avoid thermal shock.

Introducing both fluids simultaneously and gradually ensures uniform thermal expansion across the fixed tube sheets. This prevents the differential expansion that leads to tube-to-sheet joint failure. Maintaining a balanced temperature rise is critical for the longevity of non-removable bundle designs, as they cannot accommodate significant thermal stress.