Shell and Tube Heat Exchangers
Shell and tube heat exchangers are the most versatile and widely used type. Learn about construction, tube arrangements, baffles, and how to maintain them.
Construction
A shell and tube heat exchanger consists of a cylindrical shell containing a bundle of tubes. One fluid flows through the tubes (tube side), the other flows across the tube bundle (shell side). Heat transfers through the tube walls.
The tube bundle is held in place by tube sheets at each end. Baffles on the shell side direct the shell fluid across the tubes in a zigzag pattern, improving heat transfer and supporting the tubes.
Tube Arrangements
- Fixed tube sheet: Tubes are permanently fixed to both tube sheets. Simple, low cost. But the shell side cannot be cleaned mechanically (no access to the tube bundle). Thermal expansion must be accommodated with an expansion joint.
- U-tube (hairpin): Tubes are bent into a U-shape, entering and exiting through the same tube sheet. The bundle can be removed for cleaning. One end is free to expand. Good for high temperature differentials.
- Floating head: One tube sheet is fixed, the other floats (moves with thermal expansion). The bundle can be removed for cleaning. Most versatile design, but more expensive. Standard for refinery and petrochemical applications.
Baffles
Baffles are plates installed on the shell side, perpendicular to the tubes. They serve two purposes: they direct the shell fluid across the tubes in a zigzag pattern (improving heat transfer) and they support the tubes (preventing sagging and vibration).
Baffle spacing, cut, and orientation affect both heat transfer and pressure drop. Closer baffles improve heat transfer but increase pressure drop. Typical baffle spacing is 20 to 50% of the shell diameter.
Advantages and Disadvantages
- Advantages: Handles high pressure and temperature, robust construction, can handle dirty fluids (with removable bundle), versatile configurations, well-understood design methods, easy to repair (individual tube plugging).
- Disadvantages: Lower heat transfer coefficient than plate type, larger footprint, heavier, more expensive for small duties, shell-side cleaning is difficult (fixed tube sheet type).
Which side for which fluid?
Put the hot fluid on the tube side if possible (tube side is easier to clean if scaling occurs). Put the high-pressure fluid on the tube side (tubes are stronger than the shell for pressure). Put the corrosive fluid on the tube side (only tubes need corrosion-resistant material, not the shell). Put the fouling fluid on the tube side if the bundle is removable (easier to clean).
Tip: Monitor the approach temperature (the temperature difference between the outlet of one fluid and the inlet of the other). An increasing approach temperature indicates fouling. When the approach temperature rises 2 to 3 degrees C above the clean baseline, schedule cleaning.