In a heat exchanger startup, which side is introduced first?

• A. Cool side
• B. Hot side
• C. Either side
• D. Both sides

Answer: (A)

Start with the cool side. The reason is to minimize thermal shock and control the temperature ramp across the exchanger during startup. Introducing the cooler flow first lets the exchanger warm up gradually, reducing differential expansion between the shell and tube sides, and protecting gaskets, seals, and welds from sudden heating. If the hot side were brought in first, the hot surfaces would heat the exchanger rapidly once flow on the other side starts, increasing the risk of mechanical stress, leaks, or damage. Starting with both sides or the hot side first isn’t standard practice because it doesn’t provide that controlled, gradual temperature rise needed to protect the equipment.