By Eugene le Roux, FSAIRAC, and Eamonn Ryan
Heat exchangers are fundamental components in a wide variety of thermal systems, ranging from industrial refrigeration and HVAC systems to automotive engines and chemical processing plants. This is Part Two of a two-part series.
The goal in design is to maximise heat transfer while keeping the required surface area and cost in check. Image by Upklyak | Freepik.com
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Serviceability
In real-world conditions, heat exchangers are prone to fouling – an accumulation of unwanted materials on heat transfer surfaces. In water-cooled systems, biofilms, scale and mineral deposits can significantly reduce thermal performance. To counter this, designers must ensure that heat exchangers are serviceable.
Shutoff valves are often installed around the exchanger to isolate it from the system for maintenance without disrupting the entire process. In shell and tube exchangers, the tubes are designed to be more accessible than the shell for cleaning and replacement. As such, fluids with higher fouling potential (like water) are usually passed through the tubes.
In finned air heat exchangers, commonly used in HVAC and automotive applications, dust, debris and moisture can clog the fins and reduce airflow. When moisture condenses between the fins – especially in humid environments – it can further restrict air passage and promote corrosion. Regular cleaning and proper filtration are crucial to maintain efficiency in these systems.
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Corrosion and material compatibility
Corrosion is another major concern, particularly when the fluids involved are chemically reactive with the materials used in the exchanger. A well-known example is ammonia, a highly effective refrigerant, which reacts aggressively with copper and its alloys. Using incompatible materials can lead to rapid deterioration, leaks and potential system failure.
To prevent such issues, materials must be selected based on compatibility with the working fluids. Common materials include stainless steel, titanium and various high-grade polymers, depending on the specific application. Protective coatings, corrosion inhibitors and sacrificial anodes may also be employed to enhance durability.
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Flow configuration
The arrangement of fluid flow within the heat exchanger greatly influences its thermal performance. The three primary configurations are:
- Parallel flow: Both fluids move in the same direction. This is simple to design but generally less efficient in transferring heat.
- Counter flow: Fluids move in opposite directions, maximising the temperature gradient and therefore the heat transfer. This configuration is most efficient and is commonly used where performance is critical.
- Cross flow: Fluids move perpendicular to each other. This arrangement is often found in automotive radiators, where air passes across the tubes carrying engine coolant.
In some designs, a mixed flow configuration is used, combining elements of the above to optimise performance within space and operational constraints. A shell and tube condenser, often used in industrial refrigeration, is a typical example of such a mixed flow system.
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Effectiveness and temperature difference
The efficiency of a heat exchanger is largely driven by the temperature difference between the two fluids. Engineers often refer to either the log mean temperature difference (LMTD) or the arithmetic mean temperature difference (AMTD) to evaluate performance. While AMTD offers a rough estimate, LMTD provides a more accurate representation of heat transfer potential, especially in counter-flow arrangements.
The goal in design is to maximise heat transfer while keeping the required surface area and cost in check. In a chiller, the typical temperature difference between refrigerant and water might be around 10°C, while in an automotive radiator, the difference between engine coolant and air could be as high as 50°C. This large temperature gradient, sometimes referred to as the cooling constant, reflects the greater driving force for heat transfer in the radiator.
Although LMTD is more precise, engineers may use AMTD for quick diagnostics or preliminary calculations, particularly during troubleshooting, where thermal inefficiencies may be inferred from unexpectedly high temperature-difference values.
Conclusion
Designing a heat exchanger requires a careful balance between safety, efficiency, serviceability and cost. From high-pressure safety mechanisms and material compatibility to thermal fatigue and fouling prevention, each consideration plays a critical role in ensuring the exchanger performs reliably over its service life. By understanding and accounting for these factors, engineers can develop heat exchange systems that not only meet performance requirements but also offer longevity and ease of maintenance in a wide range of applications.
