Shell and Tube and Plate Heat Exchangers
Most process heating with steam or hot water happens in one of two kinds of exchanger. A shell and tube exchanger puts one fluid in a bundle of tubes inside a shell; a plate exchanger squeezes thin fluid channels between corrugated plates. They suit different pressures, fluids and spaces.
You'll learn
- Describe how each type is built
- Compare them on pressure, temperature, size and fouling
- Choose between them for steam and liquid duties
- Know their maintenance needs
Before this: U-value, LMTD and fouling
Two ways to bring fluids together
In a shell and tube exchanger, one fluid flows through a bundle of tubes and the other flows around them inside a cylindrical shell, guided back and forth by baffles. With steam heating, steam usually condenses on the shell side and the liquid being heated flows in the tubes.
In a plate exchanger, a stack of thin, pressed metal plates forms narrow channels; the two fluids flow in alternate channels. The corrugations make the flow turbulent, so heat transfers very well for the size, and capacity can be changed by adding or removing plates.
How they compare
| Shell and tube | Gasketed plate | |
|---|---|---|
| Pressure and temperature | High; limited by shell and tube design | Moderate; limited by gaskets |
| Size for a given duty | Larger | Compact; high U |
| Close temperature approach | Harder | Easy |
| Fluids carrying solids or fibers | Tolerates dirty fluids better | Narrow channels can clog |
| Steam condensing | Common and robust | Possible within gasket limits |
| Cleaning | Tube side by rodding; shell side harder | Opened and plates cleaned individually |
| Changing capacity | Fixed | Add or remove plates |
Brazed and welded plate exchangers remove the gasket limits at the cost of being harder or impossible to open, and suit clean fluids at higher pressures.
Steam-side design and maintenance
- Steam enters at the top of the shell side; condensate leaves at the bottom through a trap sized for the exchanger’s duty and its lowest differential pressure.
- An air vent at the point farthest from the steam inlet removes air that would blanket the tubes.
- A vacuum breaker on the steam space stops it pulling a vacuum when the control valve closes, which would otherwise hold condensate back.
- Tube vibration and erosion at the steam inlet are prevented by an impingement plate.
- Differential expansion between hot tubes and cooler shell is handled by U-tubes, a floating head, or an expansion bellows on the shell.
A tube leak lets one fluid into the other. With steam on the shell side and product in the tubes, product usually leaks into the condensate, which is why condensate from process exchangers is monitored before it returns to the boiler.
Key takeaways
- Shell and tube: tubes in a shell; robust, high pressure, tolerates dirty fluids.
- Plate: thin corrugated channels; compact, high U, close approach, gasket-limited.
- Steam side needs a correctly sized trap, an air vent and often a vacuum breaker.
- Tube leaks contaminate condensate: monitor it.