What Is a Shell and Tube Heat Exchanger?
Shell and tube heat exchangers are the most widely used and are among the most effective means of heat exchange. A shell and tube heat exchanger is a device where two working fluids exchange heat by thermal contact using tubes housed within a cylindrical shell.
The temperature difference between the fluids inside the shell and tube serves as the driving force for heat transfer. Shell and tube heat exchangers are designed to handle a wide range of temperatures and pressures. They feature a compact design, simple construction, and ease of maintenance, making them highly efficient for heat exchange applications.
Shell and tube exchangers have a number of tubes mounted inside of a shell. One fluid flows through the tubes (tube side) while the other flows over the outside of the tubes (shell side). Baffles are used on the shell side to increase turbulence, prevent stagnation and improve the heat transfer rate.
Fouling: The Enemy of Heat Exchangers
Fouling is a significant issue for many heat exchangers. When scale builds up on the tubes, the efficiency of the unit drops sharply. As fouling worsens, heat transfer performance deteriorates. Early detection of fouling can be achieved by monitoring pressure drops or any decreases in equipment performance.
Fouling manifests in several forms, particularly in hygienic processing systems. The four main types of fouling include:
1. Incrustation: The accumulation of a crust or coating of processed fluids, minerals, or cleaning agents on the surfaces of heat exchanger components.
2. Scaling: A form of incrustation caused by the deposition of substances like calcium carbonate, calcium sulfate, and silicates.
3. Sediment: Composed of corrosion products, metal oxides, silt, alumina, microalgae, and their excrement.
4. Biological Growth: Biofouling can originate from bacteria, nematodes, protozoa, and other microorganisms.
Clearing Tube Blockages
Regularly clearing blockages from the tubes is essential to maintaining the efficiency of the unit. The frequency of cleaning will vary depending on the liquids circulating through the shell and tube heat exchanger.
For slime or algae buildup, using high-pressure water (also known as hydro blasting) can be effective in clearing the obstruction. For larger deposits or more stubborn blockages, mechanical cleaning methods may be required. This typically involves wire brushes, scrapers, or even "rodding" for smaller heat exchangers.
When using mechanical cleaning, it's important to apply caution to avoid damaging the tubes with excessive force.
In cases of particularly stubborn deposits, chemical cleaners or detergents can be used. Always ensure the chemical cleaning agents are compatible with the tube material to avoid causing damage.
The Visual Inspection
After cleaning the tubes, conduct a thorough visual inspection to check for any signs of leaks, cracks, or corrosion in the system. A common method for detecting leaks is by using dye, which makes it easier to spot problem areas. Alternatively, you can pressurize the shell side of the system, which will force air through any leaks in the tubes.
If sacrificial anodes are used in the system, be sure to inspect these as well during the visual check. A general guideline is to replace the anode plates if more than 50% of the plate has been eroded.
Repairing Leaks
Leaks in shell and tube heat exchangers can occur for various reasons, often leading to cross-contamination of fluids. Thermal fatigue, resulting from extreme temperature differentials between the shell and tubes, is a common cause. Here are several methods for addressing leaks in these systems:
Tube and Tube Sheet Repair
If the tube seal to the tube sheet becomes loose, the first step is to attempt re-rolling the tube. If this is not possible due to expansion limits, alternative methods like beading or welding may be required. Leaks found on the tube sheet itself generally need to be addressed by welding.
Plugging Tubes
Plugging a tube is a temporary solution to prevent leakage from affecting the rest of the system. However, this method alters the overall flow rate and reduces system efficiency. While plugging can be effective for short-term fixes, it is advisable to limit the use of plugs.
Re-tubing
As tubes age and become worn, replacing them with new ones may be necessary. This process, however, requires extended downtime for the unit, making it a more involved and time-consuming repair option.
Shell and Tube Heat Exchanger Maintenance Checklist
- Depressurize the unit
- Clean out the headers and scrape down tube sheets
- Clear tubes of any blockages (may involve acids, high-pressure water, drill rods, scrapers, calcite cleaners, or scale removers)
- Visually check for leaks
- Replace gaskets
- Verify pressure gauge and thermometer accuracy
- Check and clean strainers if present
- Check operating safety devices
- Clean exterior surfaces as needed
- Verify operation of sump tank
- Tighten all mechanical connections to recommended torques
Advantages and Disadvantages
Advantages:
- Cheaper than plate heat exchangers.
- Simple design and easy to maintain.
- Suitable for higher pressures and temperatures than plate heat exchangers.
- Lower pressure drop (ΔP) compared to plate heat exchangers.
- Leaking tubes are easy to locate and isolate.
- Tubes can be double-walled to prevent cross-contamination between shell side and tube side fluids.
- Easy to install sacrificial anodes.
- Less prone to fouling than plate heat exchangers.
Disadvantages:
- Less efficient than plate heat exchangers.
- Requires more space for tube removal and maintenance.
- Cooling capacity cannot be increased as easily as with plate heat exchangers.
