In a large petroleum refinery, a critical process involves cooling a hot hydrocarbon mixture after it leaves a distillation column. The stream is at 150°C (302°F) and must be cooled to 50°C (122°F) before it can be sent to storage tanks. For this duty, a shell and tube heat exchanger is selected, and 16 BWG tubes are specified for the tube bundle.
Why 16 BWG is the Ideal Choice:
Optimal Balance of Strength and Efficiency: The hydrocarbon stream is at a moderate pressure (15 bar). A 16 BWG tube, with a wall thickness of 1.65 mm (0.065 inches), provides excellent mechanical strength to contain this pressure without risk of deformation or failure. At the same time, the wall is not excessively thick, allowing for efficient heat transfer. A thinner tube (e.g., 18 BWG) would have better heat transfer but less pressure capability, while a thicker tube (e.g., 14 BWG) would be stronger but less efficient.
Erosion and Corrosion Resistance: The hydrocarbon stream may contain small amounts of corrosive elements like sulfur and water vapor. Cooling water on the shell side often contains dissolved chlorides and oxygen. The 16 BWG wall thickness provides a generous corrosion allowance. This means the tube can withstand a certain amount of material loss over its 20+ year design life without developing leaks or compromising pressure integrity.
Fouling Management: Periodic cleaning (e.g., hydro-blasting) is required to remove carbon deposits (fouling) from the inside of the tubes. The robust wall of a 16 BWG tube can withstand this mechanical cleaning process much better than a thinner-walled tube, reducing the risk of damage during maintenance.
Vibration Resistance: The flow of fluids on the shell side can cause tube vibration, leading to fatigue and failure at the support baffles. The inherent stiffness of the 16 BWG tube helps to dampen these vibrations, increasing the reliability and longevity of the entire heat exchanger.
In this application, the tubes would typically be made from ASTM A179 Carbon Steel due to its good thermal conductivity and cost-effectiveness for this service. The 16 BWG specification ensures the heat exchanger operates safely, efficiently, and with minimal downtime for maintenance, making it a dependable workhorse in the refinery's operation.
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