Emerging Technologies: Design Innovation for High-Temperature and Pressure Heat Exchangers

Industries such as oil and gas, fertilizer, and chemical processing manage operations that occur within extreme temperature and pressure conditions. These severe conditions pose significant challenges to the equipment’s design, fabrication, and maintenance. Heat exchangers are one of the most important parts of these processes, and they are essential for efficient thermal management. Conventional approaches to designing and operating heat exchangers under these conditions can lead to catastrophic failures.

This article discusses the various challenges associated with heat exchangers that operate at high temperatures and high pressures and provides various design solutions for heat exchangers operating above ~300 °C and ~70 bar (g).

By Rashik Bansal – Sr. Technical Professional Leader, KBR; Arjun Gupta – Associate Technical Professional, KBR; Sonali Padhi – Associate Technical Professional, KBR

Manufacturers continue to face difficulties with malfunctions during operation despite the widespread use of heat exchangers. With increasing severity of operations, heat exchangers face many problems that may affect their effectiveness, including corrosion, fouling, and temperature gradient.¹ These phenomena occur at high temperatures, rapidly decreasing the lives of heat exchangers. Studies show that at the steam inlet temperature of 649 °C the heat exchanger’s effectiveness dropped to 62%.² Fouling is another significant phenomenon that occurs when the temperature in the system increases, especially with compact heat exchangers.³ Traditional aluminum plate and frame heat exchangers (PFHE) have an operating temperature of less than 200 °C and can tolerate pressures of up to 12 MPa. The maximum temperature and pressure restrictions for employing a PFHE with stainless steel are around 800 °C and 8 MPa, respectively.⁴ Material selection is also critical in ensuring that components can withstand thermal and mechanical stresses without degradation over time. Leakage is another challenge with high pressure heat exchangers. Samantray, J. S., et al. observed a leakage rate of 0.7% on the shell side for an exchanger operating at 132 bar of shell side pressure.⁵

1. Challenges

1.1 Challenges Associated with High Temperature

A number of physical phenomena impact the integrity and efficiency of heat exchangers that are subjected to high temperatures, including materials that become softer and lose mechanical strength after exposure. Materials that are not made to resist thermal expansion and frequent thermal cycling are especially vulnerable to this.

When heated to high temperatures, metals undergo thermal expansion, which can cause changes to dimensions and create stresses. Creep is the gradual deformation of materials caused by continuous tension at high temperatures. Heat exchangers are especially vulnerable to thermal strains when there are temperature gradients across their surfaces. Rapid temperature changes that occur, particularly at startup or shutdown, invite stresses that may cause cracking and deformation. Heat exchangers experience fouling and scaling more at higher temperatures than at lower temperatures, which decreases the heat transfer surfaces and lowers efficiency. Frequent maintenance in such cases is necessary. Stream leakage is more likely to occur at high temperatures because the components expand at joints and gaskets.

1.2 Challenges Associated with High Pressure

High-pressure conditions introduce a distinct set of challenges that necessitate rigorous material selection and mechanical design. Heat exchangers operating at high pressures must withstand significant tensile stresses. Materials must be selected for their ability to maintain strength under repeated loading and unloading cycles, known as fatigue resistance. High-pressure systems are prone to failure if materials lack sufficient fatigue resistance, leading to cracks and, ultimately, failure. Ensuring the structural integrity of high-pressure heat exchangers is complicated by the difficulty of inspecting components for defects. Non-destructive testing techniques such as ultrasonic and radiographic inspection are essential to detect flaws before they lead to failure. As operating pressures increase, the size and weight of the heat exchanger also increases due to the need for thicker walls and stronger materials. This necessitates careful consideration of material costs, transportability, and ease of installation.

1.3 Impact on Physical Properties and Heat Transfer

The increase in temperature and pressure has a profound effect on the physical properties of the fluids passing through the heat exchanger.

• For gases, as temperatures rise, viscosity and thermal conductivity increase in most cases, which improves convective heat transfer. However, a significant decrease in gas density at high temperatures can negatively impact convective heat transfer. Conversely, liquids generally benefit from the reduction in viscosity at high temperatures, enhancing their convective heat transfer properties
• Under high pressure, gases exhibit increased density and thermal conductivity, leading to improved heat transfer, especially in dense-phase gas flows. However, liquids, being largely incompressible, show relatively minor changes in their heat transfer properties with increasing pressure.

2. Design Considerations

To overcome the challenges posed by high temperatures and pressures, various design innovations and materials suited for these applications must be incorporated in the thermal and mechanical design of these heat exchangers.

2.1 Material Selection

High-temperature heat exchangers often require the use of advanced alloys such as Inconel 625, Haynes 282, and Sanicro 25 due to their high mechanical strength, resistance to oxidation, and ability to withstand thermal cycling. These materials, while more expensive, significantly outperform conventional carbon steels in high-temperature environments. For example, Inconel 625 retains its mechanical properties up to 700 °C, making it a popular choice for heat exchangers used in chemical and nuclear applications.

2.2 Thermal Expansion Management

When designing for high-temperature applications, different materials exhibit varying levels of thermal expansion. To mitigate this, designers often use U-tubes or floating heads, which allow components to expand without causing mechanical stress. Expansion joints are another solution that absorbs thermal expansion without introducing stress into the system.

2.3 Gasket and Sealing Technology

One critical factor in high-pressure systems is minimizing the risk of leakage. The use of welded joints instead of gasketed joints is recommended for high-pressure environments. Welded diaphragms or expansion bellows can further reduce the risk of leakage while maintaining flexibility in the system.

2.4 Heat Exchanger Type Selection

Different heat exchanger designs, such as Tubular Exchanger Manufacturers Association (TEMA) standards, offer specific advantages. For instance, TEMA C-type exchangers are preferred for high-temperature, high-pressure applications because of their reduced likelihood of leakage, compared to TEMA B-types. TEMA D-types with screw plug exchangers allow for custom arrangements and easier maintenance.

2.5 Safety and Reliability

Startup and shutdown conditions often subject heat exchangers to rapid changes in temperature, causing thermal stresses. The use of rupture disks and other pressure-relief devices can prevent catastrophic failures in the event of overpressure.

3. Possible Solutions

3.1 Use of Appropriate Design Temperatures

Designing heat exchangers with different temperatures for various components can significantly reduce material costs and improve efficiency. Instead of designing all components for the maximum operating temperature, parts like pass partition boxes, tubes, and channel covers can be engineered for lower temperatures where possible. Similarly, hot and cold tubesheet can be designed with different temperature tolerances, optimizing material usage without sacrificing safety.

3.2 Metallurgy of Components

The selection of materials for heat exchangers operating at high temperatures is vital for their longevity and performance. Nickel-based alloys such as Inconel 740H and Haynes 282 offer excellent strength and oxidation resistance at temperatures up to 800 °C. Ferrous alloys, such as stainless-steel grades 316 and 347H, are also widely used for lower temperature applications due to their cost-effectiveness. Hastelloy is frequently used when chemical aggressiveness and high temperatures are combined, as it can resist extreme corrosion while maintaining mechanical integrity.

3.3 Design at Differential Pressure

Heat exchangers operating under differential pressure scenarios often experience lower pressure gradients compared to the maximum operating pressures of individual fluids. For example, a tube side operating at 100 bar and a shell side at 90 bar create a differential pressure of just 10 bar. Designing for the differential pressure, rather than the full pressure, significantly reduces the required tube wall thickness, lowering material costs and weight while preserving safety margins.

4. Case Study

4.1 Advantages of Using Differential Design Temperatures for Heat Exchanger Components

In high-temperature, high-pressure heat exchangers, the use of uniform design temperatures across all components can lead to excessive material costs and increased equipment thickness, as each component is designed to withstand the highest potential temperatures. However, by applying different design temperatures (DTs) tailored to specific components — based on their actual thermal exposure — engineers can optimize material use, reduce costs, and improve heat exchanger performance. This case study explores the practical benefits of differential DTs in the context of the shell-and-tube heat exchanger handling gas on the shell side and superheated steam on the tube side.

When using uniform design temperatures across all components, the overall dimensions of the heat exchanger tend to increase, which consequently raises the total weight of the equipment. This added weight leads to higher wind and seismic loads, necessitating more robust supporting structures and increasing the overall cost of installation. Additionally, as the tubesheet thickness grows, there is a slight reduction in the effective tubeside heat transfer surface area, which may impact thermal performance. Furthermore, due to the elevated operating temperatures, a material upgrade to 2¼ Cr (chromium steel) may be required, as temperatures around 454 °C bring certain components into the creep range, where long-term deformation under stress becomes a concern.

5. Conclusion

High-temperature and high-pressure heat exchangers are critical for improving energy efficiency in various industrial processes, from oil refineries to power generation. Addressing the challenges posed by extreme conditions requires the careful selection of materials, robust design solutions, and advanced safety mechanisms. Implementing the solutions discussed and outlined in this article provides insight to ensure that heat exchangers remain reliable, durable, and efficient in operation, ultimately reducing maintenance costs and extending the life of the equipment.